Printer Friendly

Control of polymer properties by melt vibration technology: a review.

1. INTRODUCTION TO MELT VIBRATION TECHNOLOGY: REVIEW OF SCIENCE AND TECHNOLOGY

Defects such as weld lines, sink marks, and warpage are caused by melt fronts collision, unbalanced flow, uneven cooling, nonuniform internal stress and inhomogeneous nucleation and growth of crystals as the part solidifies. Varying the processing parameters can result in the modification of the molded part outlook and physical properties, but the modifications are often slight and not quantified, and they also rely, to a large extent, upon the expertise of the operator who uses his experience and art to determine the processing parameters. Unfortunately, the conventional wisdom for a good processing window involves increasing the clamp tonnage, sometimes as high as 150-300 Mpa of pressure, which substantially contributes to the price of the molding equipment.

Some new molding techniques have surfaced that make use of specific pressure {and shear induced) profiles prior to or during molding, to control the flow pattern and/or the internal structure and morphology of the plastic as it is being shaped. One can distinguish processes that make use of vibration and oscillation of melts from those which simply control, in a specific way, the filling and packing and cooling parameters. For example, in the nonvibration process category, Chang (1) discloses a method and apparatus for controlling haze and crystallization in thermoplastic materials, such as polyethylene terephtalate (PET), by controlling the pressure during cooling, from high to low pressure, to induce a greater amount of crystallization, and from low to high pressure, to obtain a significant lower amount of crystallization.

The technique of injection-compression molding, as described by Maus and Galic (2), should be treated among the category of nonvibrational processes. The process uses an "adaptive" mold cavity with a predetermined resiliency within the moldset, to automatically control melt pressure and densification during mold packing. This results in the control of the final quality and performance of the molded article, e.g. the internal stresses and resulting birefringence of compact disks.

Zachariades and Economy (3) described a process for producing "super strong' polymers in planar directions by combining injection or compression with rotational flow during molding.

There are three categories of patented processes using vibration to modify the molding process and/or the properties of molded materials:

1. The common practical feature among the patents of the first category is their use of mechanical shaking/oscillation or ultrasonic vibration devices to homogenize and increase the density of the material molded, either in the liquid stage or in the solidifying stage, either at a macroscopic or microscopic level (4-7).

2. The second category of patents and processes using vibration is based on the fact that material rheology is a function of vibration frequency and amplitude in addition to temperature and pressure. This can be put to practical use to influence diffusion and rate sensitive processes which depend on viscosity and relaxation kinetics, such as nucleation and growth of crystals, blending and orientation (7-19).

3. In a third category, vibration is essentially used to generate heat locally by internal friction (20) or to decrease surface stresses at the wall interface between the melt and the barrel or the die to increase throughputs (21-28). The heat generated locally by pressure pulsation can be significant enough, in injection molding, as to avoid the premature freezing of the gate, resulting in a significant reduction of the shrinkage in the final part (20).

The significant reduction at the wall interface of the friction coefficient increases the throughput of melt flow through vibrating dies (21-25) and reduces orientational birefringence (26-28).

In the first category of processes and equipment making use of vibration (4-7), the technique is one of shaking or local micro shaking in the case of ultrasonic vibration. In some patents of this first category, the initial states of the materials treated are granules or pellets and the vibration is applied to this state. The result of applying vibration is to compact the granules and powders and to combine the effect of heat and mechanical shaking so as to avoid the trapping of bubbles between the granules and thereby obtain a more homogeneous product with better mechanical properties. The shaking can also be applied to the melt, to decrease the number of bubbles in the melt. This can be practically used to eliminate or strengthen weld lines such as shown by Allen and Bevis (6, 7). This process relates to an injection molding technique claimed to eliminate part defects and increase strength, particularly with fiber-reinforced materials, by use of multiple gates and injection pistons to oscillate the melt back and forth in the mold to achieve desired orientation effects across weld lines. In its simplest form, the Allen and Bevis process splits the melt into two identical feeds. Each feed is equipped with its own packing chamber and piston, and is capable of supplying pressure to the cavity independently of the other. During the molding cycle, the molten polymer is injected from the barrel into the mold through one or both piston channels in the processing head, depending on the desired program. Once the mold is filled the pistons are actuated in a selected sequence. The piston action first develops fluctuating melt pressure that moves and shears the melt across the weld across the weld line in the cavity and gate areas. This results in actually shaking the melt fronts to increase the interface area and eliminate the air trapped between the two colliding fronts. The pistons are moved to apply compression-decompression forces to the melt. New material is introduced to compensate for shrinkage and voids. Variation of the Allen and Bevis process makes use of the same system of reciprocal pistons acting on any part of the melt, not necessarily across a weld line such as their patent require: This could make their process belong to the second category, but such a process is protected and described by other patents (8), which will be discussed in section 3.1.

Another application of melt oscillation to create orientation effects in melts containing fibers (or LCPs) is used by Klockner Ferromatik Desma, the so-called Push-Pull technique, which is apparently not covered by any patent. This process is also based on oscillation of the melt in the mold to improve part quality, and uses a molding machine with twin injection units, such as are used in multicomponent or two-color work, along with software modifications to control movement of the injection units. After filling a two-gated mold, one injection screw advances while the other retracts, creating the oscillation of melt in the cavity. Substantial benefits of the technique are said to be enhanced fiber-reinforcement effect and reduction of defects from internal weld lines.

For processes using ultrasonic energy, the objective is to alter the kinetics of nucleation and growth of crystals in the melt to obtain more homogeneous solidified parts. Lemelson (4) described an apparatus and method for controlling the internal structure of plastics in a mold by application of ultrasonic energy to the solidifying material to effect beneficial control of the crystalline structure formed thereof upon solidification. Ultrasonic vibrations may be utilized per se or in combination with other forms of energy applied to the solidifying material so as to orient or otherwise control the grain or crystalline structure thereof for improving the strength and other physical characteristics of the solidified article. Pendleton (5) also describes an ultrasonic process to modify the structure of crystallizable thermoplastic materials. In one embodiment of Pendleton's invention, air-jet mono-whisles, consisting of a resonant chamber and an exponential horn, are directed at a tubular bubble being blown extruded as it passes the area above the annual orifice and below the frost line. The objective is to break down the large spherulites by ultrasonic energy to improve clarity of the blown films.

In general, for the vibrational processes of the first category, the frequency and amplitude of vibration is not changing during the treatment, and this feature distinguishes them from the second category of vibrational treatment referred to in Ref. 8. There is an exception: for those patents of the first category that wish to optimize the effect of vibration during molding, the frequency of vibration is varied for the system to stay at resonance. The reason is simple: the mold and the material constitute a mechanical system in which the amplitude of vibration transmitted is a maximum when the frequency is the frequency of the mechanical system. This latter quantity is a function of the stiffness of the system and hence of its temperature. Mechanical shaking may induce a certain degree of heating in the material being treated, or the change of mold temperature as it cools change the stiffness of the mechanical system. The result is the variation of the resonance frequency of the system and therefore a decrease of the effect of the compacting process. Some of the patents remedy this situation and vary the frequency of vibration during treatment in order to constantly remain on the resonance frequency of the mechanical system in order to optimize the effect of the treatment. For our concern these patents are still referred to in the first category, the shaking category, although the frequency actually varies during the solidification in the mold.

The seminal patent for the second category provided a method and apparatuses for transforming the physical characteristics of a material by controlling the influence of rheological parameters (8). In particular it provided a method for molding by vibration to control or modify the physical properties of the molded materials, notably their mechanical and optical properties. The process uses vibrational means in order to influence and/or tailor a change in state, either a transitional state (melting transition, glass transition) or a relaxation state, i.e., the internal friction related to viscosity and orientation. This invention is based on the fact that material rheology is a function of vibration frequency and amplitude in addition to temperature and pressure. Low frequency (1-100 Hz) and high amplitude vibrations affect plastic materials properties in much the same way as lowering the temperature does, which influences the viscosity at a given temperature: It either increases it if the viscosity is Newtonian, or decreases it if shear thinning occurs. As illustrated in section 2.1 and in Refs. 9-11, changing the frequency or the amplitude, and coupling it with traditional cooling treatments, allows one to simulate fast quench rates, which can be readily applied to control crystallinity and boost orientation effects, which are known to benefit the material properties.

The process allows one to kinetically freeze in plastic materials specific nonequilibrium states that benefit the physical properties. This is accomplished by crossing phase transitions at a chosen viscosity determined by the rate of change of melt rheological parameters (8-9). This results in the modification of the thermal mechanical history characteristics of a moldable plastic so as to produce a shaped article that shows, e.g., increased tensile strength and stiffness, increased flexural strength and flexural stiffness, increased impact strength, increased weld line strength, increased dimensional stability, and a decrease in the number of voids and sink marks.

For the third category, vibration of wall surfaces for extrusion and runners allows lower processing temperature and lower pressure. The throughput in annular dies can be increased by 30% to 50% by the effect of wall vibration (21, 23-25). Schramm also demonstrated, as early as 1976, that vibrating the nozzles of injection molding machines (20) resulted in higher throughputs and parts with less shrinkage. He attributed the absence of shrinkage to the delay in freezing the gate caused by friction heating resulting from the pressure pulses, allowing the part to solidify under continuous compensating pressure.

The viscosity at the wall surface of extruders or runners is the result of shear frictional forces, which slow down the passage of the melt of the molten plastic. Throughputs can be increased by increasing pressure, which require more powerful and more expensive equipment, or by increasing the temperature of the melt which reduces viscosity. Experiments with vibrated walls and vibrated dyes show that the slip-stick mechanism responsible for melt instability in annual dyes under high pressure is altered, allowing a large increase of the throughputs (21, 23-25) and/or a reduction of the required pressure or temperature at identical throughputs. This can be beneficial in the case where a temperature reduction results in a decrease of degradation by oxidation or by chemical decomposition.

The remainder of this article will illustrate the benefits brought about by the second category.

2. FUNDAMENTALS OF THE MELT VIBRATION METHODS

2.1 Influence of Shear Vibration on Viscosity

Shear thinning of plastic materials is well known and is used practically to lower the viscosity of melts during the filling stage of injection molding by increasing the speed of the injecting piston. This is particularly useful in the case of thin wall injection molding where considerable forces are required to fill the mold when the viscosity of the melt remains quasi-Newtonian.

Rheologists essentially use two types of instruments to characterize the flow behavior of fluids: capillary rheometers and rotational shear viscometers. In the latter, either a true rotational motion or an oscillation is imparted to the melt, leading to the knowledge of either the steady shear viscosity or the complex viscosity, [[Mu].sup.*], Cox and Merz (29) made the observation that plots of the complex viscosity, [[Mu].sup.*], vs. [Omega], the angular frequency, were similar to plots of viscosity vs. shear rate, the so-called Cox Merz's rule (29-31). While some rheologists object to the validity or the foundation of that rule (32), it is clear that shear thinning can be obtained, at a given temperature, by either increasing the shear rate or the frequency of oscillation of the melt. Figure 1a shows the decrease of the viscosity of polymethylmethacrylate, PMMA, at 239 [degrees] C as a function of radial frequency, [Omega] = 2[Tau]f, where f is the oscillation frequency in Hertz. The initial Newtonian viscosity, corresponding to ([Omega] = 0, drops from 130,000 Poises (13,000 Pa-s) to 20,000 Poises (2000 Pa-s), more than six times, when the melt oscillates at relatively low frequency, [Omega] = 100 rad/s (16 Hz). Further viscosity reduction is difficult to obtain by increasing frequency further, as shown in Fig. 1a by the relative fiat region between [Omega] = 100 and 500 rad/s. In conclusion, the efficiency of shear vibration in lowering the viscosity by shear thinning is obtained at the lowest frequencies, which is very favorable practically. Figure 1b displays the effect of vibration frequency at constant amplitude at constant temperature on the melt viscosity of a grade of polyethyleneterephtalate, PET, from Eastman, which is used for blow molding bottles. The same large decrease of viscosity is observed as the result of shear vibrating the melt at low frequency. This result is general and applies to all polymeric melts.

Viscosity is also a function of temperature. Figure 2 displays the effect of vibration frequency on the flow curves for a low molecular weight high flow polycarbonate grade from GE Plastics used to mold compact disks. The complex viscosity [[Mu].sup.*] is measured at decreasing temperature, constant frequency of oscillation to, in a RDAII (parallel plate configuration) from Rheometries. The numbers near the curves refer to the value of to in rad/sec:1, 10, 100, 251,500. Curve 1 is obtained under very low frequency of oscillation, which makes it coincide almost entirely with the Newtonian flow curve, usually well fitted by a WLF equation. One sees that the departure from the Newtonian behavior is quite large at 251 and 500 rad/sec and that it increases as temperature decreases. This is due to shear thinning. For instance, the viscosity increases 10 folds at 200 [degrees] C between [Omega] = 500 and [Omega] = 1. This means that if one would switch the vibration frequency between 500 and 1, while cooling and as the temperature reaches 200 [degrees] C, the viscosity would suddenly increase ten times, corresponding to a very fast quench at that temperature. This simple example of the use of vibration frequency to control viscosity can be put into practice to control the morphology of blended phases, especially when one of the phases can crystallize. For example, the viscosity of the amorphous polymer can be lowered by vibration and increased at a selected temperature chosen to avoid crystallization of the other phase, which is dispersed in the viscous medium. There are many variations to the above example with important practical applications.

Figure 3 illustrates another important characteristic of the use of shear thinning by vibration to lower viscosity. This Figure displays the log of viscosity [[Mu].sup.*] vs. the log of angular frequency [Omega] for various temperatures between 158 [degrees] C and 193 [degrees] C. This is a classical plot demonstrating that, because of shear thinning, viscosity is approximately the same at high frequency, regardless of the temperature difference, which is here 35 [degrees] C. When a part is molded or extruded, many defects such as flow marks, warpage, uneven shrinkage etc., come from the difference in viscosity created by the local pressure and/or temperature gradients. A gradient of viscosity is clearly visible at zero frequency in Figure 3, for temperatures between 158 [degrees] C and 193 [degrees] C, which is practically eliminated by vibrating the melt above 200 rad/sec (32 Hz).

2.2 Effect of Hydrostatic Vibrating Pressure on the Melt Properties

Shear vibration does not produce the same results as pressure vibration, for the same reasons that in static mode shear forces have the tendency to decrease the potential energy barrier of interaction or diffusion, while hydrostatic pressure increases it. Pressure increases the melt viscosity, shear stresses decrease it. Yet, the two effects are not independent because of the elasticity of the melt, which is responsible for normal stresses and swell.

The increase of viscosity with pressure can be correlated with the decrease of free volume due to packing (33-37). Shear viscosity can be measured under pressure at various temperatures and strain rates (38-40). The measurements indicate that the non Newtonian character of viscous behavior due to shear thinning is described by the same formula, under high pressure, than under atmospheric conditions, provided the value of [T.sub.g] is taken to reflect its pressure dependence, [T.sub.g](P), which can be determined from P-V-T experiments (41). For instance, for an atactic polystyrene d[T.sub.g]dP = 32 [degrees] C per 100 Mpa. The viscosity - strain rate relationship of a melt essentially depends on the value of its relative temperature with respect to its [T.sub.g], i.e. (T-[T.sub.g](P). This is illustrated in Fig. 4 which displays the calculated effect of hydrostatic pressure, with no vibration, on the viscosity of PET during cooling. In this simulation the formula proposed by Hieber (39) is used with d[T.sub.g]/dP = 14 [degrees] C per 100 Mpa. Using the Cox-Merz's rule (29), one can reasonably assume that, under pressure, the same result applies to the relationship between viscosity and frequency of a vibration, i.e. that shear thinning under high pressure occurs starting at a lower frequency (at constant temperature) or at higher temperature on cooling at constant frequency.

There is no data presently available to confirm experimentally the above prediction, although attempts have been made to study the effect of pressure on the viscoelastic behavior in the linear viscoelastic range (42). Piche et al., using ultrasonic measurements under high pressure (42), demonstrate that the effect of pressure on the value of G[prime], G[double prime] and K[prime], K[double prime] can be scaled with [T.sub.g](P).

Since the influence of the hydrostatic pressure on the glass transition temperature is well known (41), vibration patterns can be combined with pressure profiles during molding to tailor the viscosity without recourse to additives or a change of grade. Combined pressure profiles and vibration allows one to work at lower frequency to induce shear thinning. Hydrostatic pressure increases the sensitivity of the effect of vibration on melt viscosity, allowing viscosity differentials potentially as large as 100 to 1000 depending on the absolute pressure used. The same result applies to semicrystalline polymers.

Figure 5 is a simulated plot, using Hieber's formula (39), of the effect of vibration on the viscosity of PET under 10 Mpa hydrostatic pressure (assuming no crystallization). The viscosity differential is more than 3 decades, on a log scale, at 100 [degrees] C, which is the temperature at which maximum orientation would occur at this pressure under fast cooling rate conditions. This can be used to enhance quench rate effects. Combined pressure and vibration history can also be used to optimize orientation in a molded part, as explained in another section below.

Crystallization of polymers depends on the possibilities of nucleation and growth (43). The performance of the solidified melt is a strong function of the texture and the morphology resulting from the kinetics of vitrification or crystallization. High viscosity hampers crystallization, sometimes to such a degree that during cooling a solid of an amorphous structure, a glass, is formed. The overall rate of crystallization of a supercooled liquid is determined by the rate of formation of nuclei and the rate of growth of such nuclei to the spherulites. For polymer molecules the temperature region below the equilibrium melting temperature, [T.sub.m], is a metastable zone where the nuclei do not form at a detectable rate, but in which crystals, once nucleated, can grow. The effect of pressure on [T.sub.m] is dictated by the Clapeyron equation, which predicts a linear increase of [T.sub.m] with pressure. As [T.sub.m] increases with pressure, so does the degree of supercooling, ([T.sub.c]-[T.sub.m](P)), which controls the rate of nucleation. This simply explains the influence of pressure on the kinetics of crystallization. Both nucleation and growth show maxima in their rates, because at higher temperatures the driving force (supersaturation) decreases and at lower temperatures the rate of mass transfer is strongly decreased by the high viscosity. Homogeneous nucleation followed by growth of crystallites can only occur in the temperature range where the two curves overlap. This overlap can be influenced or even controlled by melt vibration.

As far as the action of vibration is concerned, one must distinguish low frequency vibration, i.e. melt oscillation, from ultrasonic vibrations. Ultrasonic vibrations in the melt, in the supercooling temperature zone, probably enhances the capability to form nuclei and/or breaks the growing crystals into smaller pieces, which then themselves can act as nucleating sites for further growth. The growth kinetics actually modulates the nucleation process in that case. The result is a large increase in the number of small crystals leading to better clarity and a much greater strength.

The influence of low frequency melt vibration on the shear viscosity of melts has been discussed in the previous section. For low frequencies around 1 to 5 Hz, its application to control the kinetics of nucleation and growth is conceivably as simple as substituting the vibration (and pressure) dependent viscosity function into the equations for nucleation and growth.

At higher frequency - but still below 100 Hz - and larger amplitude, the coalescence of local nanometric free volume clusters into microcavities could generate high frequency phonons capable of acting as nucleating agents: The microcavities are small voids in the liquid that open in negative pressure regions. When the cavities collapse, high local pressures result. These high pressures change the melting temperature according to the Clapeyron equation. The change in melting temperature produces sufficient undercooling for initiating homogeneous nucleation.

2.3 On the Use of Melt Vibration to Control the Degree of Orientation: Orientation vs. Relaxation

If an isotropic molten or rubbery plastic is subjected to an external shear stress, it can undergo orientation (43). In amorphous polymers, this is associated with a rearrangement of the randomly coiled chain molecules, which extend in the direction of stress application (molecular orientation). In crystalline polymers, the phenomenon is even more complex: in addition to orientation effect in the amorphous regions between crystallites, crystallites may be reoriented or completely rearranged and oriented crystallization may be induced by the stresses applied. Orientation is accomplished by deforming the polymer at or just above its glass transition temperature. Orientation is frozen-in if the stretched polymer is cooled to below its glass transition temperature before the molecules have had a chance to return to their random orientation. The faster the cooling across [T.sub.g] the higher the remaining orientation.

The effect of orientation on the physical properties is considerable (43). Orientation results in increased tensile strength and stiffness. Orientation can convert brittle polymers such as polystyrene and polymethyl methacrylate into ductile materials.

However, not all polymers are sensitive to orientation, or, to be more accurate, to the orientation benefits: strength, stiffness, impact resistance. This is because relaxation takes place concurrently with orientation when stretching above [T.sub.g], and for some polymers, relaxation dominates over orientation. Relaxation corresponds to the unstretching of the macromolecules, or part of them, back to a random coiled configuration. The center of gravity of the macromolecule might rotate in the relaxation process, repositioning bonds in the direction of pull, but the potential energy giving the population statistics between the trans, gauche, and cis conformers remains unperturbed during relaxation, hence there is no orientation benefits to the mechanical properties. Stretching just above [T.sub.g] while quenching, to lower the temperature below [T.sub.g], favors orientation and its benefits. Slow cooling allows relaxation to take place, which cancels out the benefits of orientation.

Melt vibration technology allows a substantial increase of the orientation potential of polymers by reducing the amount of relaxation taking place when orientation effects are desirable. The distribution of free volume and, coupled with it, the modification of the conformational state of the bonds (cis, gauche, trans) can be completely altered by the stretching process, provided relaxation does not take place at the same time. The amount of orientation vs. relaxation taking place when a melt is deformed is related to the two components G[prime] and G[double prime] of the complex shear modulus [G.sup.*], which are a function of temperature, pressure, frequency, and amplitude of vibration. The ratio of G[prime]/[G.sup.*] can be monitored during processing and controlled by varying the pressure and the vibration frequency as the part is cooled. To optimize orientation effects, pressure and frequency history profiles can be combined during cooling to bring and keep the viscoelastic state of the melt in its optimum range to avoid relaxation: increasing G[prime]/[G.sup.*] to its maximum should be the goal. Conversely, if relaxation should be optimized, to eliminate internal stress for example, the material should cool under pressure and frequency history patterns that favor relaxation: G[prime] should remain as low as possible in this case.

Traditional processes of orientation work on the principle of elongating the full macromolecular chains, thus de facto creating anisotropicity in the material, which results in a decrease of performance in the unstretched direction(s). To boost the benefits of orientation for polymers that do not orient well (e.g., polycarbonate), a melt may be vibrated uniformly at a low percentage of deformation (5% to 30%) while it is sheared.

Pure hydrostatic vibration can also produce viscoelastic conditions, which favor what we call conformational orientation in the material, providing molded parts that produce the mechanical characteristics of a highly oriented plastic once cooled. The complex bulk modulus can also be decomposed into its elastic and loss components, K[prime] and K[double prime]. The ratio of the two can be modified by varying the frequency of vibration and the value of the mean pressure at a given temperature. An example of such a vibration treatment to improve the stiffness and strength of polystyrene is given in the section below dealing with mechanical improvements imparted by melt vibration. However, the efficiency of pure hydrostatic vibration to produce favorable viscoelastic effects, either a state of optimum orientation or optimum relaxation, is much reduced compared to shear vibration, and the best is often to combine pressure profiles with vibration patterns.

2.4 Vibration to Simulate Cooling Rate Effects Across Transitions

As a general rule, the rate of cooling materials from the liquid temperature to the solid temperature has a tremendous effect on the material's morphology, the structure, and the type and number of defects (sink and flow marks, warpage, bubbles, etc.). All the properties of polymers are influenced by the rate of cooling. For semicrystalline polymers, the number, size, type, and distribution of crystallites produced under specific cooling conditions, dictate to a large extent the degree of crystallinity, density of tie molecules, amorphous regions, and the overall morphology, which determines the performance of the finished products.

In the case of amorphous polymers, the physical properties in the glassy state can vary considerably with the rate of cooling through [T.sub.g] (alternatively with the subsequent annealing treatment below [T.sub.g]), as shown for the impact resistance of polycarbonate. Of all the unmodified amorphous polymers, polycarbonate has one of the highest impact resistance. Yet, it has been reported for PC (44) the possibility to lose completely its impact strength when the polymer is aged 20 to 40 degrees below its glass transition temperature. This same drastic loss of the impact characteristics of PC would of course be observed by cooling it slowly through its [T.sub.g] to permit relaxation under nonisothermal cooling conditions.

Since the properties of condensed glass cooled under isobaric conditions are considerably influenced by the rate of cooling through [T.sub.g], and since [T.sub.g] is pressure and frequency dependent, [T.sub.g](P, [Omega]), it is possible to correlate and superpose the effects of the rate of pressure application or the rate of frequency variation with the rate of temperature variation, then it seems conceivable to condense a glass at any chosen "apparent" cooling rate provided the pressure and vibration frequency variation can itself be fully monitored and programmed. This subject is fully described in other publications (9-16).

If we choose a temperature T, this temperature alone does not characterize the viscoelastic state of the polymer at that temperature, i.e., its viscosity. One need also specify the frequency of vibration (at constant amplitude) and the value of the glass transition temperature. So, the important variable is not temperature alone, but (T-[T.sub.g]). This is a direct application of the frequency-temperature equivalence. In order to decrease (T-[T.sub.g]) at a given rate, and thus control the viscosity, one can either cool the mold at constant frequency (i.e. practically at constant [T.sub.g]), and therefore (T-[T.sub.g]) decreases because T decreases, or one can increase the vibration frequency at constant amplitude, which increases [T.sub.g] with a rate imposed by its frequency dependence, or finally one can play simultaneously on both the mold temperature and the vibration frequency to achieve the desired cooling pattern. As (T-[T.sub.g]) decreases, the Newtonian viscosity increases, but shear thinning becomes more favorable. Pressure can also play a role to vary the rate of (T-[T.sub.g]) at will.

This general principle is used to create in situ super-quench rates by melt manipulation by varying simultaneously pressure and vibration frequency to induce instantaneous changes of viscosity.

2.5 Pressure and Vibration Profiles

Figure 6 illustrates how vibration and pressure can vary and combine during an injection molding cycle to optimize the performance of the molded part. LVDT (Volts) represents a variable proportional to cavity pressure. The x coordinate in Fig. 6 is the time during injection, from the filling of the mold cavity, through packing and cooling. A piston vibrates the melt at different stages during cooling, according to modes of vibration programmed in a computer attached to the piston controllers (see section 3.1). Figure 6 shows three modes of vibration, Each mode can be specifically designed to alter one type of melt behavior or another while temperature of the melt decreases in the mold cavity. In this example, pressure is vibrated during filling to reduce viscosity, which is schematically represented by the slanted lines on the initial ramp to 8 V, then vibration ceases and pressure remains constant for a certain time. Vibration resumes according to mode 1, which could be a certain profile of vibration favoring the creation of nuclei in the melt. Vibration resumes again at mode 2, then at mode 3 after another cessation in between. Meanwhile, the mean pressure is also varied synchronously, decreasing before mode 2, and increasing rapidly to reach mode 3. These modes could be implemented to significantly reduce growth of crystals and produce orientation effects, for instance.

The challenge, as summarized in section 6, is to understand the several and separate effects vibration can have on the morphology, orientation, viscoelasticity, and solidification process in general, as well as on friction, wall slippage and fatigue; and use this knowledge to design specific vibration and pressure profiles that improve, in a well-characterized way, the quality and the performance of the molded articles.

Figure 7 illustrates how a vibration can be used to either increase or decrease the amount and rate of relaxation taking place in a molded plastic part. The molded part is a compact disc of polycarbonate injected at a hot temperature and submitted to high pressure as the mold cools slowly across the [T.sub.g] of the polymer. Pressure is here the average hydraulic pressure to keep the thickness of the disc constant at a temperature slightly below [T.sub.g] (the pressure curves are normalized by the initial injection pressure: P/[P.sub.o]).

Figure 7 shows that pressure drops because the material internally adapts to the deformation by relaxing. The rate of relaxation can be determined by the vibration profile exerted during relaxation: Curve (G) has no vibration and is used as a reference. A 1 Hz sinusoidal vibration wave of small amplitude is imposed onto the compact disc for curve (H). A more complex vibration mode (designated H12), which is designed to include specific harmonics, produces curve (F). One sees that the degree of relaxation is a strong function of vibration mode, and that depending on the type of vibration imposed, one can either slow down relaxation (curve (H)), even to the point of creating an internal stress, or accelerate it (curve (F)), which can be used to quicken the cancellation or removal of thermal and internal stresses resulting from fast cooling.

Figure 8 further demonstrates the effect of vibration on the relaxation of polycarbonate below [T.sub.g] for two vibration frequencies, 1 and 10 Hz, same amplitude, but slightly different temperatures, 131 [degrees] C and 133 [degrees] C. Unexpectedly perhaps, relaxation takes place at a slower rate for the 133 [degrees] C curve: This is due to the effect of the higher frequency on the viscoelastic state, effectively increasing (T-[T.sub.g]) by increasing [T.sub.g] because of its frequency dependence. The relaxation occurring at 10 Hz is, in fact, proceeding at a slower rate because its viscoelastic temperature is further away from its glass transition temperature.

3. APPLICATION OF THE MELT VIBRATION TECHNOLOGY TO INJECTION MOLDING, EXTRUSION, BLOW MOLDING, ETC

3.1 Injection Molding Machines

Melt vibration hardware for injection molding machines is described in several patents (6, 8) and is made available in several versions from various manufacturers: Solomat Partners L.P. has introduced two types of "Rheomolding" (45) melt vibration equipment, which are named "Rheojectors" (45). Rheojector I, shown in Fig. 9a and 9b, is an add-on device that fits between the barrel of the injection press and the mold. The accumulator and the piston are located between the mold and the screw of the injection molding machine (8, 9-16). This actually corresponds to a two-stage injection process, with plastication to the piston housing assembly occurring separately from the injection vibration stage. Figure 9b provides the general layout of the injection molding unit with the controllers.

Alternatively, the mold can be modified to incorporate the pistons directly into the cavity to impart changes exactly where they are needed, such as weld lines or the handle of a critical part: This is what Solomat offers as its Rheojector II [ILLUSTRATION FOR FIGURE 10 OMITTED].

Scortec Inc., a subsidiary of the British Technology Group, displayed for the first time at Interplas '90 its Multi-Live-Feed Molding technology, later Designated Scorim (6, 7, 46), which was developed at Brunel University in England in 1982, and is shown in Fig. 11 (46).

Scorim heads split molten plastic into two or more streams, and provide a path for each branch of the molten stream to enter and fill the mold through its nozzles, gate, and runner in the mold. The heads also allow each melt stream to pass through a chamber, where independently controlled pressures may be alternately applied and removed to each branch of the split plastic stream [ILLUSTRATION FOR FIGURE 11 OMITTED]. Each head is equipped with two double-acting hydraulic cylinders. These cylinders, with their attached pistons, provide the forces that dynamically move the molten plastic in the mold. Up to four pistons can be used in one process head.

Klockner Ferromatik Desma unveiled its "Push-Pull" injection molding process at the K'89 show. The Push-Pull machine is displayed in Fig. 12 (47). The Scorim and Push-Pull processes have in common the use of multiple gates and injection pistons to oscillate the melt back and forth in the mold to achieve their desired orientation effects. The Push-Pull process is also said to have potential in powder injection molding of metals and ceramics.

3.2 Extruders

Melt Vibration for the Pipe Industry and the Compounding Industry

Versions of melt vibration hardware for the extrusion industry are developed and described in the literature (8, 21, 22, 48, 49) but are yet to receive, to the best of our knowledge, any commercial application. The purpose of the extrusion by melt vibration is to address the need of the industry to extrude plastic tubes, which are converted to either ultra-oriented pipes, pellets, or sheets.

References 17 and 27 describe an extruder operating under melt vibration, The vibration takes place in several zones located after a cross-head die. Each zone is connected to a servo valve, controlled by a computer, which vibrates hydraulic oil circulating inside hermetically sealed concentric metallic foils. Plastic melt from the extruder flows between the inner and outer metallic foils of the different zones. Temperature, pressure, and vibration parameters (frequency and amplitude) of the zones are computer controlled and synchronized from zone to zone.

The most immediate effect of low frequency vibration is to lower viscosity, thus increase throughput. Small amplitude lateral contractions of the melt occur at different frequency and amplitude in these zones, appropriately chosen to accommodate the changing melt temperature and viscosity of the extrudate as it passes from one zone to another. The frequency and amplitude in the passing zones can even be designed to go beyond mere viscosity control, imparting a high degree of longitudinal orientation, even under low extrusion pressure. The orientation simply comes about when the frequency, amplitude, and phase shift of the sinusoidal oscillation functions in the zones are programmed to increase the swelling ratio [i.e. the normal stresses, which relate to G[prime]/[G.sup.*]) to such an extent that it extrudes faster than the throughput calculated from viscosity alone. In other words, the zones can be vibrated synchronically to increase the elasticity of the melt and the normal stresses, lowering viscosity by shear thinning and increasing both throughput and orientation.

A typical configuration may include three or four zones in which low amplitude lateral vibration occurs with an increased frequency from zone to zone, before the extrudate is placed in a sizing dye and cooled. In another possible configuration, the vibrating zones of the extruder work, on the contrary, to compress the extrudate in the low temperature zone by decreasing the frequency and the amplitude of the lateral contractions from zone to zone, which increases viscosity. The extrudate, brought into a pressurized pasty semisolid state in that zone can be cold drawn by radial contraction as to impart a high degree of radial orientation. The result is improved mechanical properties and better tolerances in the extrudate produced. Mechanical properties such as tensile strength and modulus of elasticity are significantly increased by the combined effect of radial and longitudinal orientations, which can easily be produced by combination of the means described in the configurations above.

The vibrating extruder described in Ref. 22 might be used to extrude vibrated pellets at faster throughput. The homogeneity of the melt is much improved and the dispersion between phases is also accrued with help of vibration. The applications for the compounding industry are listed in section 5.5.

3.3 Blow Molding and Gas Assist Molding

Air or nitrogen can be used to impart vibration and/or pressure pulses to a melt (50). Air is already used to blow preforms and parisons inside of molds (51), and to core out hollow articles in a process known as Gas Assist Injection Molding (52), particularly useful to facilitate the flow of plastic melt into the fine details of complex molds, as well as to reduce the content of plastic because of the hollowness created by the gas bubble.

It is clear that low frequency pressure pulses can easily be produced by existing gas control equipment (53), and directed to impart viscoelastic changes in blown articles such as bottles or bumper beams. The effect of gas vibration to increase G[prime]/[G.sup.*] or K[prime]/[K.sup.*] to enhance orientation in blow molded parts is the same as the effect of melt vibration described in section 2.3. The same should be true for the effect on crystallization control. The key is, once again, to match the varying rheological state of the material with a vibration-pressure gas treatment, which is known to produce characterized benefits to the morphology and performance. A new recently introduced gas assist process (50) attempts to add the flexibility of gas vibration to the technology of melt vibration and melt manipulation described in this paper. The new process seems to be particularly well adapted to injection molding.

3.4 Thermoforming

The first attempts to apply vibration during the cooling of a melted plastic part actually occurred (9, 10) on thermoformed samples. One half of the mold was vibrated as the other half remained fixed. The movable half was rigidly bolted to a vibrating table, transmitting pressure oscillation to the melt as it was cooled rapidly. Some results on PS, using such a lab vibrating mold, are described in the next section. The level of vibration control is easily achieved in this method, and the pressure or shear variables easily separable, but the costs associated with the energy to vibrate a mold half under pressure are perhaps not practical. The samples vibrated were, indeed, rather small in the experiments, which were conducted by this thermoforming-vibrated device (9-16). Yet, the improvements on the mechanical properties are quite spectacular, as demonstrated in section 4.2.

4. INFLUENCE OF MELT VIBRATION ON THE PROPERTIES OF POLYMERS

4.1 Influence of Melt Vibration on the Mechanical Behavior of a Semicrystalline Polymer, Polypropylene

As illustrated for polypropylene in this section, melt vibration results in an extraordinary variety of improvements. Table 1 summarizes the results of the use of vibration during compression molding of a commercial polypropylene homopolymer on the elongation at break, hardness, and creep resistance. Polypropylene disks (3 mm thick, 32 to 70 mm in diameter) were heated in the mold, melted and vibrated during cooling (9, 10). It is clear from Table 1 that vibration melt manipulation brings a great deal of flexibility to the design engineer who can specify a certain treatment during molding instead of specifying different resin grades for achieving properties. Some of the improvements match or surpass the results normally brought about by the addition of additives. It is believed that the improvements already obtained in the analysis presented in Table 1 are far from being optimum because of the limited number of tests performed.
Table 1. Other Change of Properties for Polypropylene Specimens.

Percentage at break:

Normal samples: 500% to 900%
Vibrated samples: 20% to 1700%

Hardness (D scale, Shore):

Normal PP: 70.5
Vibrated: 68.5-83.0

Creep resistance

(Hardness % decrease after 5 sec relaxation):

Normal: 11%
Vibrated: 6.5%
Improvement: 69%

Stress drop after yield point [Delta][[Sigma].sub.c]

Normal: 9.9 N[mm.sup.2]
Vibrated: 0.0 N/[mm.sup.2]


Figures 13-15 shows the tensile stress-strain curves for polypropylene injection molded specimens vibrated with Rheojector I [ILLUSTRATION FOR FIGURE 9 OMITTED]. In these Figures only the vibration mode [ILLUSTRATION FOR FIGURE 6 OMITTED] is varied, while all other processing parameters remain the same. What also changes is the temperature at which are activated vibration modes of different profile. It is seen that a broad variety of mechanical behavior results from changing the vibration profile. While some samples showed a large increase of modulus and yield strength, some others were softened by the vibration treatment to the point that they would not break, even passed 1200% elongation [ILLUSTRATION FOR FIGURE 15 OMITTED]. The hardness and impact strength (at room temperature) also showed an incredible variety of response depending on the vibration mode chosen during injection.

Figures 16 and 17 show the DSC traces (heat capacity vs. temperature for a heating rate of 10 [degrees] C/min) for vibrated-injection molded polypropylene samples using Rheojector I and a reference (no vibration during packing). The crystallinity of the vibrated sample in Fig. 16 is 24% greater than the crystallinity of the reference nonvibrated sample. In Fig. 17, two melting peaks are observed for the vibrated specimen. The higher melting peak is much narrower, sharper and located closer to the thermodynamic melting temperature of polypropylene. This is probably an indication that a greater number of smaller, more perfect crystals have formed under the vibrational treatment at high temperature (mode 1 of vibration in [ILLUSTRATION FOR FIGURE 6 OMITTED]) and that imperfect spherulites (responsible for the lower peak) have had less opportunity to grow under the subsequent increased viscosity because of low temperature vibrational mode (mode 2 in [ILLUSTRATION FOR FIGURE 6 OMITTED]). Vibration also increments the [T.sub.g] of the amorphous regions in the interspherulitic range from 0 [degrees] C to 35 [degrees] C, presumably as a result of orientation of the molecules in this region.

Out of hundreds of tests performed, varying the melt vibration conditions, the crystallinity for the molded polypropylene samples increased from 45% average for the reference nonvibrated polypropylenes up to 63% for the melt vibrated samples. The onset of melting ([T.sub.i]), indicating the spread of spherulitic sizes, which typically starts around 115 [degrees] C average for the untreated samples, can be raised to 128 [degrees] C to 130 [degrees] C for melt vibrated samples.

In the case of injection-molded PP vibrated with Rheojector II [ILLUSTRATION FOR FIGURE 10 OMITTED], the melt is highly sheared and the nucleation and growth mechanism is altered by the elongation of the macromolecules in the flow direction between the reciprocating pistons. The texture of the spherulites formed and the orientation of the interlamellar and interspherulitic ties has a tremendous impact on the mechanical properties and on the clarity of the vibrated specimens. The region located between the two reciprocating pistons remains transparent after molding, which is remarkable in view of the rather large thickness of 3 ram. The mechanical properties of this transparent PP are much different from the conventionally molded PP and from the PP vibrated with Rheojector I [ILLUSTRATION FOR FIGURES 13-15 OMITTED]. Figure 18 shows the tensile curves for a reference (no vibration) and two transparent samples. For the top curve (molding 2) the tensile yield strength is increased from 4000 psi (27.2 MPa), for a reference sample (no vibration treatment during molding), to 6200 psi (42.2 MPa), an increase of 55%. The modulus of elasticity is increased by more than 64%. Compared with samples vibrated with Rheojector I, the specimens vibrated using Rheojector II do not form necking after the yield point. The stress remains at the same level after the yield point for one of the treated specimens (molding 1), without exhibiting the classical 1200 psi (8.2 MPa) drop observed for the reference sample, until the sample breaks for a strain equal to 25% to 50% of strain (compared to 500% for the reference). The broken sample elastically snaps back as it breaks and recovers some of the extended strain before rupture.

If the vibrated samples using Rheojector II are heated for a short period above 115 [degrees] C, their mechanical behavior changes as illustrated in Fig. 19. The elongation at break increases with the annealing time, but necking still does not occur. The annealing treatment can be adjusted along with the molding conditions to somewhat modulate the mechanical behavior and obtain a stress-strain curve "upon demand."

The influence of vibration on the crystal sizes and degree of crystallinity has already been reported by Pendleton (5) in the case of blown polyolefin films submitted to ultrasonic energy in the melt. The use of low frequency vibration to apparently generate the same type of results - an increase of the degree of crystallinity and almost total clarity in the case of thick polypropylene samples (12) - seems a greater challenge to understand.

Duval et al. (54) investigated the effect of the molding conditions on the semicrystalline structure of vibration-injection molded PP sample prepared with Rheojector II. These authors used low frequency Raman scattering and wide angle X-ray scattering to elucidate the ordering of the crystalline and amorphous phases brought about by shear vibration. They observed that external layers on each side of the central zone of the dog-bone ASTM specimen have a perfectly oriented crystalline structure - with no spherulites - at complete variance with the traditional behavior illustrated for the reference (no vibration) PP. Duval and his collaborators report the crystal structure, the orientation, the size of the crystallites and the effect of annealing, showing that annealed vibrated samples have a more ordered structure than non-annealed samples (54).

4.2 Effect of Melt Vibration on the Mechanical Behavior of an Amorphous Polymer, Polystyrene

Table 2 provides a summary of the improvements observed for the tensile properties of melt vibrated polystyrene samples submitted to compression vibro-molding. These results are also obtained with the same vibration equipment used to produce the results on polypropylene referred to in Table 1 (9, 10).

Improvements for the modulus of elasticity of up to 120%, for the breaking stress of 90% (or better), and for the energy to break (i.e., area under the stress-strain curve) of up to 150% are reported in Table 2. Figure 20 is a stress-strain curve for a reference polystyrene and a treated [Rheomolded (45)] polystyrene. The samples start to stretch after the initial slack, so the real strain starts when the stress increases.
Table 2. Improvements for Polystyrene.

Modulus:

Normal PS:

Mean value: 12,500 kg/[cm.sup.2] (177,000 PSI)
BEST value: 16,000 kg/[cm.sup.2] (227,000 PSI)

Vibrated PS:

Mean value: 22,500 kg/[cm.sup.2] (319,000 PSI)
BEST value: 16,000 kg/[cm.sup.2] (497,000 PSI)
 Improvement: 120%

Breaking stress:

Normal PS:

 450 to 520 kg/[cm.sup.2] (6400-7400 PSI)
Best value: 520 kg/[cm.sup.2] (7,400 PSI)

Vibrated PS:

Mean value: N/A kg/[cm.sup.2] (varies with treatment)
Best value: 850 kg/[cm.sup.2] (12,064 PSI)
 Improvement: 90%

Energy to break:

Normal PS:

 7 to 12 kg-cm
Best value: 12.5 kg-cm

Vibrated PS:

 15 to 25 kg-cm
Best value: 25 kg-cm
 Improvement: 150%


The Rheomolded vibrated samples display tensile strength and Young's modulus characteristics comparable to or significantly better than biaxially oriented samples quoted in the literature (43). It should be noted, however, that a difference between the Rheomolded and biaxially oriented PS specimens is the strain at break, which remains at 3%-4.5% for the Rheomolded PS (substracting the slack), whereas it is 8% - 18% for the biaxially oriented PS. This is perhaps the signature of melt vibrated orientation, which, in this case, can be compared to a three-dimensional orientation process.

Increase of Free Volume by Vibration. Increase of Activation Entropy and Enthalpy at [T.sub.g]

Analysis by Thermal Stimulated Current (TSC) is particularly well suited for investigation of the effects of thermal history and vibration on free volume (55). Figure 21 displays the TSC curve for vibrated and reference PET, clearly showing the changes occurring for the [T.sub.g] and [T.sub.g, p] peaks. The [T.sub.g] peak is associated with the depolarization of activated dipoles, and the second peak, [T.sub.g, p], originates from the discharge of space charges delocalized in the free volume of the structure (55). The analysis of the characteristics of the [T.sub.g, p] peak can help determine the effect of melt vibration on the free volume amount and free volume distribution in the sample by comparing the intensity of the peak for a reference and a vibrated sample, as illustrated in Fig. 21 for PET. Different vibrational treatments produce different patterns, even sometimes complex patterns (55), yet, in general, the effect of pure hydrostatic melt vibration is to increase the width and the intensity of the [T.sub.g, p] peak, i.e. increase the free volume in the sample. This is clearly demonstrated in Fig. 21.

Melt vibration of amorphous polymers during molding increases the degree of cooperativity, i.e., widens the spectrum of relaxation below [T.sub.g] and above [T.sub.g], as demonstrated in Figs. 22 and 23. In these Figures, the specimen is a compress-molded 2-mm-thick polystyrene rapidly cooled to room temperature after 22.5 Mpa of vibrating platen pressure has been applied at 122 [degrees] C prior to and during cooling. Figure 22 is the TSC depolarization trace, and Fig. 23 is a DSC run. Three TSC peaks are clearly visible in FIG. 24: [T.sub.g] at 97 [degrees] C, [T.sub.g, p] at 112 [degrees] C, and another peak at 150 [degrees] C. The complexity of the relaxation kinetics above [T.sub.g] reveals the coupled nature of the interactions between the dipoles and the free volume (55). The DSC trace also shows strong cooperative relaxation above [T.sub.g] (bottom curve of [ILLUSTRATION FOR FIGURE 23 OMITTED]). Compare the top and bottom curves if Fig. 23, the top one serving as a reference trace (obtained by cooling the same sample in the DSC cell after the first run). The difference between the heat capacity of the two curves is preponderant just above the [T.sub.g] peak, and until a certain upper temperature is reached. The thermal history of the vibrated sample, revealed during the first run, creates a thermal activity above [T.sub.g] clearly similar to what is observed by TSC [ILLUSTRATION FOR FIGURE 22 OMITTED]. The thermal history is erased as the sample is heated in the DSC cell, and both reference and treated DSC traces are identical above a certain upper temperature.

As explained in Ref. 55, a strong cooperativity between conformers belonging to the macromolecules leads to an increase in stiffness and strength. It also raises the value of [T.sub.g] and the enthalpy and entropy of activation at [T.sub.g], [Delta][H.sub.g] and [Delta][S.sub.g], which all transcribe the coupling characteristics between the different relaxation modes.

For the reference and vibrated polystyrene samples of FIG. 20:

For the reference:

[T.sub.g] = 102.6 [degrees] C [Delta][H.sub.g] = 58.4 kcal/m [Delta][S.sub.g] = 90.9 cal/m [degrees] C

and for the vibrated polystyrene:

[T.sub.g] = 117 [degrees] C [Delta][H.sub.g] = 138.5 kcal/m [Delta][S.sub.g] = 287 cal/m [degrees] C

The large increase of [T.sub.g], [Delta][H.sub.g] and [Delta][S.sub.g] for the vibrated PS produced under hydrostatic vibration in the melt is remarkable: [T.sub.g] is increased by 15 [degrees] C; the values found for [Delta][H.sub.g] and [Delta][S.sub.g] are more than twice those of the reference, and comparable to the best values found for high performance polymers such as PEEK (55). These results suggest that the effect of melt vibration by controlled and monitored oscillatory hydrostatic pressure during cooling creates a high degree of coupling, which substantially enhances the mechanical characteristics of the frozen-in glass (Table 2). Additionally, the free volume is increased in the vibrated specimens.

4.3 Effect of Melt Vibration on the Mechanical Behavior of a Two Phase System, High Impact Polystyrene (HIPS)

High impact polystyrene is a two-phase system of rubber particles dispersed in a polystyrene matrix. The segregation of the rubber phase induces a large increase in impact strength through a mechanism of local yielding called crazing. Figure 24 provides the stress-strain curve for an injected HIPS sample, which is either melt vibrated [Rheomolded (45)] or not (reference) during packing. The yield stress is increased, the tensile modulus is increased and the area under the stress-strain curve up to breaking is increased by 90%. These results seem to point toward a better dispersity of the rubber phase in the polystyrene phase, with a retardation of the craze initiation and its stabilization to increase the strain at rupture.

5. ECONOMIC OUTLOOK: EXAMPLES OF APPLICATIONS OF VIBRATION MOLDING FOR THE MOLDING INDUSTRY

The following applications of melt vibration are either commercially available or in an advanced state of development.

5.1 Melt Vibration to Increase Throughputs, Lower Processing Temperature, Pressure

In the blow molding industry, for instance, in order for PET to increase its market share, processors must learn how to minimize AA (acetaldehyde). During preform molding, a small portion of the molten PET breaks down into acetaldehyde (51). Higher temperatures, often caused by shear or frictional stresses during plastication and injection, increase the rate of AA buildup exponentially. The use of vibration during filling of the preform and of vibrated hot runners can be used to reduce the viscosity by vibration-induced shear thinning, allowing a decrease of the hot runner and the melt temperature (56). This also translates into shorter cycle times.

5.2 Melt Vibration to Control Internal Stress

The material presently used for injection molding compact disks and CD-ROMs is optical grade polycarbonate. While polycarbonate has some properties that make it suitable for CD-ROM manufacture, there are disadvantages. The major disadvantage is high cost. The cost of the special optical polycarbonate grades required is many times that of less expensive transparent plastics such as polystyrene, PMMA, or amorphous polyolefins (APO). PS and PMMA cannot be used because of the formation of birefringence during the injection molding process. Birefringence results in distortion of light transmission, causing improper reading of the CD-ROM. The effect of vibration and the mode of vibration on the acceleration of the relaxation process is well demonstrated in Refs. 18 and 19 and in FIG. 8. Melt vibration technology has been explored at Solomat (12) for the fabrication of a cheaper, optically improved (birefringence free) CD or CD-ROM out of PS. Preliminary results have demonstrated the substantial impact of vibration treatment on the birefringence in PS (12, 17).

The design of specific vibration profiles applied during cooling and/or annealing can favor accelerated relaxation (12), and Fig. 7. The vibration wave is usually made up of a mixture of low frequency and high harmonic signals specifically blended to optimize the relaxation taking place.

Higher frequency of vibration, with low amplitudes, generating substantial internal heating, can also be used to counter thermal gradients across the thickness during cooling (15, 16). The expected effect is a better pit reproduction and depth and less clouding, two improvements which might be needed in the manufacturing of the new generation of digital video disks (DVD).

5.3 Melt Vibration to Eliminate Defects Due to Molding

Melt-flow vibration improves part properties by eliminating defects (6 - 8, 12, 57). In plastic product design, oversimplified assumptions are sometimes made: The plastic product is thought to be a homogeneous, isotropic, stress-free part capable of enduring long use for its particular application. The reality is that it is nonhomogeneous, anisotropic, with internal stresses occurring as a result of molding conditions, complex melt flow, and the uneven and nonuniform heat transfer from the mold surfaces.

In one possible implementation of melt vibration for injection [ILLUSTRATION FOR FIGURE 10 OMITTED], a pair of small pistons is inserted in the mold at critical locations, providing means to shear vibrate or pressure vibrate the melt locally, in effect enhancing strength and/or modulus at that particular place. The shear pressure aligns the fibers or the molecules in the flow axis direction, and the hydrostatic component of the stress tensor also modifies the morphology, the viscoelastic behavior and the amount of free volume, inducing strength, and increasing stiffness. The same results are claimed when the pistons are located outside the mold (6-8, 57). The vibrated shear and pressure effects provide the ability to eliminate or strengthen weld lines and to enhance the properties (by as much as 65% to 120% according to [7, 57]), giving melt vibration's users the possibility to design their molded parts with less expensive materials. It also permits the use of less material to produce a thinner part, a technology often called "thin wall" molding.

5.4 Extrusion of Biaxially Oriented Pipes. Higher Throughputs

The melt vibration process can be applied along the length of a tube, as it extrudes (49). Biaxial orientation can be induced by vibration as explained in section 2.3. For pipes, the radial and longitudinal orientation result in improved bursting strength, oxidation resistance, weather decoloration resistance, etc. The melt vibration technology used for the fabrication of pipes represents a classic biaxial orientation process in the quasi-solid-liquid state, which optimizes the degree of orientation (12, 13, 49). The vibrated barrel, which has several vibrating sections operating under different controlled frequency and amplitude, also operates as a pump that pushes plastic melt outward with greater throughput as a result of the reduction of the melt viscosity under vibration and of the wall surface's friction coefficient.

The improvements can be used to reduce the amount of plastic needed to achieve given specifications or reduce the operating melt temperature. When additives, pigments, fillers etc are added to the melt, the vibration intimately disperses the melt and produces a better mixing of the ingredients. The mixing under vibration leads to a better dispersity of the phases present in the melt (13, 48). It can also be used to blend raw material with recycled material, thus saving costs and satisfying environmental concerns. Vibration also reduces viscosity of the mix, allowing greater throughputs and/or the possibility to operate at lower temperatures.

5.5 Melt Vibration Pelletizer

A pelletizer add-on can be adapted at the end of the pipe extrusion section - still hot - which transforms the vibration hardened molten extrudate into very fine pellets (12). The micropellets produced have better blending capabilities and faster melting cycles. Alternatively, a vibrating die can also produce pellets with a higher throughput and operate under lower temperature conditions. Furthermore, vibrating pelletizer dies allow the preparation of micropellets with a high degree of interpenetration between compounding constituents, between polymers and additives or pigments during masterbatching. The extrusion temperature profile is significantly lower than for traditional processes, enhancing the quality of the final product, and allowing compounding of thermally unstable additives and organic pigments.

5.6 Summary of Melt Vibration Applications and Claims

The manipulation of the viscosity during the molding stage can be applied industrially to important problems of the industry:

The melt viscosity can be substantially reduced during molding by vibration induced shear thinning. This should have a major impact for thin-wall applications and to ease processing of hard-to-process metallocene resins. The decrease of viscosity translates into lower processing temperatures, shorter cycle times, lower pressure, etc.

Melt vibration control is capable of improving mechanical performance (stiffness, strength, hardness) of existing resins to enlarge their markets. An advantage of melt vibration processing is that it can be specifically programmed to induce improvement in a given characteristic of the molded part, e.g. strength, modulus or transparency, etc., in addition to the benefits of a better consistency for the overall properties. This is done by the choice of specific vibration profiles [ILLUSTRATION FOR FIGURE 6 OMITTED], which are designed to either increase modulus and strength (by crystallization and/or vibration induced orientation effects) or ductility by pure shear orientation effects. For a given resin or blend, a software program can be developed for each application and mold and could be controlling the processing equipment to achieve the optimum results.

Melt vibration reduces the amount of new raw material entering a part by blending with regrinds from recycling processes (which are cheaper). Normal blends of raw/regrinds display a loss of mechanical performance, which has impeded the growth of the recycling industry. Vibrated blends would not only be more compatible blends, but their performance could be boosted back up to the level expected for normal usage.

Melt vibration should have an important impact on the industry of additives/compounding/masterbatching, which includes nucleating agents, thermal and UV stabilizers, antioxidants, flame retardants, plasticizer, etc. Vibration is known to allow a better mix, an increased blendability between the resin and the additive, allowing a better and more efficient masterbatch. In some instances, melt vibration alone has been shown to be capable of replacing the effect of an additive (nucleating agents), in others to enhance its effect (dyes, pigments).

Vibration of melts including blowing agents should allow one to control cell diameter and cell densities for injected and extruded microcellular foams.

The control of viscosity during nucleation and growth of semicrystalline polymers is of particular value to obtain much higher crystallinities and more ordered or well-defined morphologies; such vibrated melts are expected to yield molded parts with better properties such as: Better oxidation resistance, better UV stability, and better barrier to gas (PET bottles applications).

Because melt vibration process equipment requires the use of electronic and computer control technology, it also provides the injection molding plastic industry with new means to control the amount of defects in injected plastic parts, and results in greater performance consistency. Melt vibration parameters are programmed to overcome the effect of mechanical and/or thermal fluctuations from one injection cycle to another, resulting in a much narrower dispersion of the test data (12, 17).

6. FUTURE RESEARCH AND CHALLENGES

In conclusion, materials produced by melt vibration can be made substantially different and much improved. The hardware to achieve these improvements is relatively simple and adaptable to existing molding equipment. Melt vibration can readily be implemented in injection molding, extrusion, bottle blow molding, and pelletizer line. The design of any new melt vibration application project can use the same basic modular ingredients found on all projects: hydraulics and electronic controls and software. Only the mechanical parts (mold and die design) are different. The know-how relates to the determination of the best melt vibration processing parameters for a given resin. It could conveniently be packaged in the form of insertable cartridges in the electronic controls, one cartridge per resin.

Research should be directed toward a fundamental understanding of the effect of vibration parameters on the viscosity and the elasticity of the melt. Vibration effects include shear heating, shear thinning, and viscoelastic stiffening (modulus increases).

The effect of vibration amplitude on [T.sub.g], [T.sub.m], crystallization kinetics, and the viscoelastic behavior needs to be elucidated. The amplitude of melt vibration is another rheological processing variable that needs to be factored in as much as frequency. The effect of frequency and amplitude of vibration on shear thinning must be separated (58, 59). We need to further analyze the dependence of vibration amplitude on viscosity and pseudoplasticity. This study has to be carried out in the nonlinear visco-elastic range, with care taken to eliminate self-heating, wall slippage (60), and fatigue effects.

The effect of vibration on the crystallization kinetics must be investigated. The X-ray and low frequency Raman work of Duval et al. (54) on shear-vibrated polypropylene must be extended to other vibratory conditions and to amorphous materials. FTIR characterization should determine the effect of vibration variables on the conformation statistics. Systematic density measurements must be performed after the samples are molded under vibration and the aging stability of the vibrated materials must be determined.

Computer simulation models of polymer flow in molds and dies have become commercially available and are quite sophisticated (61, 62). The challenge is to incorporate in the constitutive equations and in the parameters of the simulation software the effect of vibration variables. For injection molding, for instance, the finite element analysis predicts, for a given part geometry, the flow and pressure-volume behavior during filling and packing. The incorporation of vibration during filling and packing requires the use of new non-Newtonian viscosity and PVT equations, which take into account the influence of the vibration parameters on the viscosity and the volume. The influence of cooling rate on the viscoelastic states and on shear thinning should also be incorporated into the model to be able to adequately predict internal stress and shrinkage (59, 63). A new rheometer capable of accurately describing in steady state conditions the separate effects of vibration frequency, amplitude, pressure, temperature, and cooling rate on viscosity and volume should be designed. The ultimate challenge (64) is to predict why the properties of a molded part, at usage temperature, depend on the thermal-mechanical history of the polymer during the molding operation, and how it can be modulated by coupled pressure, cooling and vibratory means. If and when the model is capable of integrating all the effects of vibration on viscosity and morphology, at macroscopic and microscopic levels, it will be possible to determine the optimum melt vibration processing conditions without recourse to an excessive number of experiments.

REFERENCES

1. L. F. Chang, U.S. Patent 4,150,079 (1979).

2. S. M. Maus and G. J. Galic, Patent PCT/US90/00843 (1990), (also refer to Modern Plastics, June 1988. pp. 38-40).

3. A. E. Zachariades and J. Economy, Super Strong Polymers in Planar Directions, Polym. Eng. Sci., 23, 266 (1983).

4. J. Lemelson, U.S. Patent 4,288,398 (1981).

5. J. W. Pendleton, U.S. Patent 3,298,065 (1965).

6. P.S. Allen and M. Bevis, U.S. Patent 4,925,161 (1985). Also R. A. Malloy in Plastic Part Design for Injection Molding, Hanser/Gardner Publications (1993), pp. 59, 60.

7. P.S. Allan et al., Composites Manufacturing, The Wolfson Center of Materials Processing, Brunel, The University of West London, Uxbridge, Middlesex, pp. 80-84 (June 1990),

8. J.P. Ibar, FR Patent No. 79 06532 (1979), 80 02620 (1980), 8004252 (1980), GB Patent No. 2046167 (1990), U.S. Patent No. 4,469.649 (1984), "Method and Apparatus For Transforming The Physical Characteristics of Material By Controlling The Influence of Rheological Parameters.", FR Patent No. 8616835 (1986), EP Patent 0273830 B1 (1991), FR Patent No. 8616834 (1986), US Patent No. 07.882,754 (1990), U.S. Patent No. 4,919,870 (1988), EP Patent 87402864.0 (1987), U.S. Patent No. 07/880,926 (1993), U.S. Patent No. 5,254.298 (1993), U.S. Patent No. 08/124,147 (1993), US Patent No. 08/183,673 (1993), CA Patent No. 1,313,840 (1993), EP Patent No. 0 274 317 (1993).

9. J.P. Ibar, ACS Polym. Prep., 21(1), 215 (1980), Vibro-Molding: A New Process to Mold Polymeric Materials.

10. J.P. Ibar, Polym.-Plast. Technol. Eng., 17(1), 11 (1981).

11. J.P. Ibar, Polymer-Communications, Vol. 24, 331 (1983), "Instability in the Rubbery State Revealed by DSC of Rheomolded Polystyrene Samples."

12. J.P. Ibar, Modern Plastics. Vol. 25, No. 1, 85 (1995).

13. J.P. Ibar, Polyblends '95 Preprints, SPE RETEC on Polymer Alloys and Blends, 208 (1995).

14. J. P. Ibar, SPE ANTEC 1996 Conference Proceedings (CD-ROM), Injection Molding, H4-New Technologies and Developments, Part I.

15. J.P. Ibar, SPE ANTEC 1996 Conference Proceedings (CD-ROM), Marketing and Management Division, H10-Economic and General Issues, "Potential Markets for Vibration Assist Molding."

16. J.P. Ibar, SPE ANTEC 1997 Conference Proceedings, Toronto, Injection Molding Division, "Melt Viscosity Reduction of Plastics by Vibration During Filling in Injection Molding."

17. A. Kikuchi and R. F. Callahan, "Quality Improvements Resulting from Rheomolding," SPE ANTEC 1996 Conference Proceedings (CD-ROM), Injection Molding, H4-New Technologies and Developments, Part I.

18. G. L. Slonimskii, et al., Vysokomol. Soyed: A16, 1, 232 (1974).

19. S. N. Nurmukhametov, et al., Mekhanika Polimerov; No. 4, 579 (1976).

20. H. A. Hengesbach, K. W. Schramm, D. Woben, R. Sarholz, 'Ausrustung von Spritzgiessmaschinen (Equipping of Injection Molding Machines), Report II-1 from IKV, at the Rhineland-Westphalian Technical University (RWTH) in Aachen (1976). Also, K. W. Schramm, "Injection Molding of Structural Parts Consisting of Plastic Molding Materials Utilizing Forced Oscillating Flow Processes," Doctor-Engineer Thesis, Rhenish-Westphalian College of Technology (1976).

21. J. Casulli, J. R. Clermont, A. Vonziegler and B. Mena, "The Oscillating Die: A Useful Concept in Polymer Extrusion," Polym. Eng. Sci.; 30, 1551 (1990).

22. C. M. Wong, C. H, Chen and A. I. Isayev, "Flow of Thermoplastics in an Annular Die under Parallel Oscillations," Polym. Eng. Sci.; 30 (24), 1574 (1990).

23. B, Mena, O. Manero and D. M. Binding, "Complex Flow of Visco-elastic Fluids through Oscillating Pipes: Interesting Effects and Applications," J. of Non-Newtonian Fluids Mechanics; 5, 427 (1979).

24. B. Mena, O. Manero and D. M. Binding, Rheol. Acta; 16, 573 (1977).

25. B. Mena, O. Manero and D. M. Binding, Rheol. Acta; 17, 693 (1978).

26. L. R. Shmidt and J. L. Maxam, "Injection Molding Polycarbonate Compact Disks: Relationship between Process Conditions, Birefringence and Block Error Rate," SPE ANTEC Technical Papers, 34, 334 (1998).

27. L. R. Shmidt and J. L. Maxam, "Injection Molding Polycarbonate Optical Disks Using an Oscillatory Boundary Condition," SPE ANTEC Technical Papers, 38, 447 { 1992).

28, A. E. Zachariades and B. Chung, "New Polymer Processing Technologies for Engineering the Physical and Mechanical Properties of Polymer Products," Adv. Polym. Technol. 7{4), 397 (1987).

29. W. P. Cox and E. H. Merz, J. Polym. Sci., 28, 619 (1958).

30. S. Onogi, H. Kato, S. Ueki and T. Ibarragi, J. Polym Sci., C15, 481 (1966).

31. S. Onogi, T. Masuda and T. Ibarragi, Kolloid-Z, Z. Polym., 222, 110 (1968).

32. L.A. Uracki, Private Communication.

33. S. K. Bhateja and K. D. Pae, "The Effects of Hydrostatic Pressure on the Compressibility, Crystallization, and Melting of Polymers," J. Macromol. Sci.-Revs. Macromol. Chem., C13(1), 77 (1975).

34. L. A. Utracki, "Temperature and Pressure Dependence of Liquid Viscosity," The Canadian Journal of Chemical Engineering, 61,753 (1983).

35. L. A. Utracki, "Pressure Dependence of Newtonian Viscosity," Polym. Eng. Sci., 23, 446 (1983).

36. L. A. Uracki, "A Method of Computation of the Pressure Effect on Melt Viscosity," Polym. Eng. Sci., 25, 655 (1985).

37. L. A. Utracki, "Correlation Between PVT Behavior and the Zero-Shear Viscosity of Liquid Mixtures," J. Rheology, 36(4), 829 (1986).

38. P. D. Driscoll and D. C. Bogue, "Pressure Effects in Polymer Melt Rheology," J. Appl. Polym. Sci., 39, 1755 (1990).

39. C. A. Hieber and H. H. Chiang, Polym. Eng. Sci., 32, 931 (1992).

40. S. E. Kadijk and B.H.A.A. Van Den Brule. "On the Pressure Dependency of the Viscosity of Molten Polymers,' Polym. Eng. Sci., 34, 1535 (1994).

41. P. Zoller, and D. Walsh, Standard Pressure-Volume-Temperature Data for Polymers, Technomic Publishing Co., Lancaster-Basel (1995).

42. F. Messines, L. Piche, C. Lacabanne, "Ultrasonic Characterization of Polymer Viscoelasticity," Makromol. Chem., Macromol. Syrup., 23, 121 (1989). Also: A. Sahnoune, F. Massines, and L. Piche, "Ultrasonic Measurement of Relaxation Behavior in Polystyrene," J. Polym. Sci., B,34, 341 (1996).

43. D. W, Van Krevelen, Properties of Polymers, Elsevier Publishers (1972), Chapters 19, 20.

44. D. G. Legrand, J, Appl. Polym. Sci., 13, (1969) 2129.

45. "Rheomolding"[R] and "Rheojectors"[R] are registered Solomat Partners L. P. Trademarks,

46. R. Malloy, G. Gardner and E. Grossman, SPE ANTEC Tech. Papers, 39, 521 (1993),

47. W. Michaeli and S, Galuschka, SPE ANTEC Tech. Papers, 39, 534 (1993).

48. J.P. Ibar, U.S. Patent 7,880,926.

49. John De Gaspari in Plastics Technology, March 1994 issue, "Technology News."

50. J.P. Ibar, U.S. Patent and PCT Application, "Method and Apparatus for Controlling Gas Assisted Injection Molding to Produce Hollow and Non-Hollow Plastic Parts and Modify Their Physical Characteristics" (1995).

51. Plastics Blow Molding Handbook, Norman Lee, editor, Van Nostrand Reinhold, New York (1990).

52. P. J. Zuber, "Relationship of Materials and Design to Gas-Assist Injection Molding Application Development" in Molding '95, ECM Fifth international Conference and Exhibit, March 27-29. 1995, New Orleans.

53. B. Miller. "Closed-Loop Controller Aids Gas-Assist Control,' Plastics World. July 1995, p. 13.

54. A Minardi, M. Boudeulle, E. Duval, and S. Etienne, "The Effect of The Molding Conditions on the Semi-Crystalline Structure of Rheomolded Polypropylene," accepted for publication in Polymers (1996).

55. J.P. Ibar, Fundamentals of Thermal Stimulated Current and Relaxation Map Analysis, SLP Press (1993).

56. Tests are underway at Husky's AMC (Toronto, Canada) on PET preforms,

57. John De Gaspari in Plastics Technology, October 1992 issue, p. 19 ("Technology News").

58. Rheometrics RDAII Instrument, RHECALC Users' Guide (902-00107 Revision C), Appendix A. p. 22, Fig. 16 (1994).

59. J. P. Ibar, "A New Formulation and Interpretation of Shear-Thinning of Polymeric Melts. Effect of Pressure, Strain, Cooling Rate and Vibration Parameters," submitted to Polymers.

60. D. W. Adrian, and A. J. Giacomin, "The Transition to Quasi-Periodicity for Molten Plastics in Large Amplitude Oscillatory Shear," Transactions of the ASME, 116, 446 (1994).

61. C-Mold, AC Technology, 31 Dutch Mill Road, Ithaca, NY 14850 USA. Also: H. H. Chiang, C. A. Hieber, and K. K. Wang, Part I and Part II, Polym. Eng. Sci., 31, 116 & 125 (1991).

62. Moldflow, MoldFlow Pty Ltd, Cochester Rd, Kilsyth Vic. 3175 Australia.

63. J. Ortat, A. L. Apitchi, J. P. Ibar, APS meeting, DHPP, Symposia on Entanglement Effects in Polymer Processing, Kansas City, March 1997, "Manipulation of Dynamic Entanglements During Processing. A new understanding of Shear-Thinning."

64. J. P. Ibar, "Do We Need a New Theory in Polymer Physics?", accepted for publication, J. Macromol. Sci.Revs. Macromol. Phys. (1997).
COPYRIGHT 1998 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1998 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Ibar, J.P.
Publication:Polymer Engineering and Science
Date:Jan 1, 1998
Words:13119
Previous Article:Analysis of nonlinearly elastic cantilever snap beams for assembly of plastic parts.
Next Article:Residual stresses, shrinkage, and warpage of complex injection molded products: numerical simulation and experimental validation.
Topics:


Related Articles
Melt-flow oscillation improves part properties.
Powder extrusion: fundamentals and different applications.
Preparation and characterization of composites of polypyrrole.
Morphological and Mechanical Properties of Polypropylene [PP]/Poly(Ethylene Vinyl Acetate) [EVA] Blends. I. Homopolymer PP/EVA Systems.
The effects of melt vibration blending on the subsequent crystallization and melting behavior of polypropylene/ultra-high-molecular weight...
Supercritical [CO.sub.2] welding of laminated linear low density Polyethylene films.
Vibration welding of nylon 6 to nylon 66.
Experimental studies on radial extrudate swell and velocity profiles of flowing PS melt in an electro-magnetized die of an extrusion rheometer.
Multiplex shear stress-induced nucleation in dynamic microcellular foaming process.
Effect of molecular weight and molding conditions on the replication of injection moldings with micro-scale v-groove features.

Terms of use | Privacy policy | Copyright © 2019 Farlex, Inc. | Feedback | For webmasters