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IM alternatives produce performance advantages.

Numerous advantages, including greater freedom of design, resulted from using reinforced thermoplastics with less conventional methods of injection molding.

This article examines, in addition to conventional injection molding, the following alternative processing, sandwich molding, fusible core injection molding, multiple live feed injection molding, and push-pull injection molding.

Hollow Gas Injection Molding

The "Friedrich" patent gives Michael Ladney of Detroit Plastics Molding the exclusive right in the U.S. to inject gas into a machine nozzle by means of a sprue break (see Fig. 1). On the other hand, Cinpres Ltd., Tamworth, U.K., owns the right to inject gas into a mold cavity and runner system, as shown in Fig. 2. The injection of gas into the mold cavity provides a higher level of control and permits the use of hot manifolds without the problems that result from mixing of the gas with the melt. It also allows for film gates, production of larger parts, and the use of multicavity or multigated parts because gas flow can be separately controlled. CINPRES, which was first introduced as a low pressure process in 1985, overcomes the main limitations of injection molding and structural foam while combining most of the advantages of both processes.

Hollow gas injection moldings are produced by the controlled injection of an inert gas, into the molten polymer melt. The gas does not mix with the polymer, but instead forms continuous channels through the less viscous thicker section of the polymer melt; it maintains pressure throughout the molding cycle. During the cooling phase, the gas ensures positive contact between the polymer and the surface of the molding tool, and overcomes the major shortcoming of structural foam: Gas bubbles do not escape to cause swirl marks on the surface of the molded part.

A component that is molded by the process of hollow gas injection can combine thick and thin sections without sinking or warping. Gas channels (e.g., hollow ribs) are incorporated into the design so as to allow uniform transmission of gas pressure. The hollow channels formed by the gas act effectively as box sections, which further increase the stiffness/weight ratio of the molded part. Wall sections can be as thin as 0.007 in. The internal gas pressure normally eliminates sink marks, which are a frequently occurring problem in molded parts with varied cross sections.

During the cooling cycle of hollow gas injection molding, the polymer has little or no laminar flow. Thus, molded-instress and post-molding warpage can be significantly reduced. Pressure within a hollow gas injection molding can be as low as 15% of that needed for an equivalent injection molded component, resulting in much lower required press clamp forces. A significantly reduced investment in molding machinery, or an uprating of the effective clamping force of an injection molding machine by a factor of six to seven, is possible.

The cycle of hollow gas injection molding is usually slightly shorter than that of conventional injection molding, and is often half that of structural foam molding.

The hollow gas injection molding process has been employed successfully with most grades of thermoplastic polymers.

At ICI, neat and reinforced amorphous and crystalline polymers have been successfully hollow gas injection molded. ICI technical personnel have enjoyed great success in combining the process of hollow gas injection molding with the recently introduced Verton pultruded long fiber reinforced thermoplastic composites. Because the composites offer strength, modulus, impact resistance, and surface finish properties superior to those of short fiber reinforced thermoplastic analogs, the marriage of the two new technologies offers the engineer greater freedom of design. The designer can more readily incorporate thick and thin sections without fear of warpage or distortion. The low molded-in stress, greater stiffness/weight ratio, reduction in weight and cycle time, and excellent surface finish offered by the hollow gas injection molding cycle also greatly increase the designer's freedom.

In summary, the process of hollow gas injection molding offers many advances, which include the following: the capability of producing curved hollow sections; weight reduction; cycle time reduction; increased stiffness and functionality; elimination of sink marks; elimination of warpage; good surface finish; low molded in stress; and long narrow hollow sections.

For hollow gas injection molding, areas of application include the coring of ribs in rigid panels such as crates, tables, frames, base plates, mounting plates, and covers. They also include the hollowing out of entire parts such as tennis racquets, handles, wheels, pedals, support brackets, bent tubes, and toilet seats.

When using hollow gas injection molding, the designer should be aware of some general considerations of design. Such considerations include the need to highlight, by marking a model or drawing, the thick sections to be cored out by the gas. They also include the following needs: to determine polymer gate location; to determine location of the gas nozzle(s), taking into account the filling pattern of the part; and to determine, on the basis of part geometry, material, and gate location, the number of gas nozzles required. Because the gas will follow the path of least resistance, tool design and nozzle location should be such that sections do not fill prematurely, gas is not forced to flow against the polymer flow, air is not entrapped between wall sections of varying thicknesses, and overflow wells are provided to uniformly core out the end sections.

Sandwich Molding

Production processes for moldings with a foam structure at the core have been well known for many years. The low pressure structural foam molding process, which was developed during the 1960s, made it possible to produce low-density moldings with minimal frozen-in stresses free of sink marks. However, unsatisfactory process related surface quality is the disadvantage of this molding technique.

Today, this process is used almost exclusively for moldings with a structured surface or in which surface quality does not play a dominant role.

A further development of the conventional structural foam molding process is the gas counterpressure process, which was developed in the early 1970s. It permits production of moldings with smooth surfaces that are considerably better than those produced with the structural foam molding process. In many cases, the surfaces are scarcely inferior to the surface qualities of moldings produced by means of conventional injection molding techniques.

The disadvantage of the process, which can be performed on conventional injection molding machines, is the need to use airtight molds; the result is that the molds cost considerably more than conventional structural foam molds.

The sandwich molding process represents the final step in the development of structural foam molding techniques. Schloemann-Siemag, and later, Battenfeld, developed the coinjection process to its present-day level. The sandwich molding process, which is based on patents held by ICI, can be used for thick-walled moldings with a foamed core and for thin-walled moldings. In the latter case, a conventional injection molding technique is used to produce the skin and core material. Figures 3 through 6 illustrate various stages of the sandwich molding process. Specific product characteristics can be achieved by using combinations of raw materials.

However, past results have shown that the processes listed here, together with the well-known conventional injection molding process, are still not always able to provide the optimum solution for many moldings.

Thick-walled moldings, for example, should not only be free of sink marks, they should also be light. Thin-walled moldings should be free from internal stresses. They should not warp, and should have no sink marks.

Cycle times play a significant role in determining cost effectiveness because the production machine is a major factor in the cost calculation. Processes that permit the use of injection molding machines, which demand considerably lower investment costs, offer obvious advantages.

Sandwich molding offers the plastics processor a technically and economically interesting solution for a wide range of commercial applications, including canoe paddles, toilet seats and cisterns, computer housings, copier parts, cash register covers, television escutcheons, audio cabinets, garden chairs, boxes and containers, shoes and soles, paint brush handles, automotive parts such as handles and knobs, and music center mainframes.

Advantages of sandwich molding that have favored its use in these and other applications include good surface quality over foam cores, without sink marks, silvering ("splash marking"), blistering, or outgassing. Colorability eliminates or reduces painting costs on structural foam moldings; it also allows inexpensive achievement of two-color effects by selective "one coating," which involves masking part of the self-colored skin and applying a paint of a different color.

Another advantage is its use of reground scrap or second quality material for the core. Because the usual core/ skin ratio is 1:1 or more, at least 50% of reground or second quality materials can be used, provided that they are compatible with the skin material.

Still another advantage is the selective use of two materials for skin and core to achieve particular engineering effects, such as a soft surface over a strong rigid core, or a lightweight composite structure with stiff skin and foam core. Fusible Core Injection


The practice of using fusible cores to injection mold parts with cavities that can not be formed or released otherwise has been known for some time. The use of the fusible core technique to injection mold intake manifolds in glass fiber reinforced nylon for automobile engines has been shown to be an economical and technologically interesting application. Because of the high temperatures in the engine compartment, thermosets were originally considered to be suitable materials. In the meantime, however, practical experiments have shown that thermoplastics, especially glass fiber reinforced nylon 6/6, exhibit noticeably better long term performance while retaining adequate rigidity and dimensional stability at high temperatures. Thermoplastic intake manifolds for automobile engines represent the optimization of the fusible core technique.

The synergistic combination of a specially formulated engineering thermoplastic and the advanced fusible alloy core molding process has produced automotive inlet manifolds resulting in significantly improved engine performance.

To gain performance advantages for their fuel injected engines, both Volkswagen and Porsche have switched from diecast aluminum to the advanced manifolds. Major improvements of the thermoplastic manifold include a reduction in weight of 50% from that of the aluminum part, and a smooth interior surface finish, which greatly increases flow efficiency of the fuel-air mixture.

Maranyl XA 455S, a glass reinforced, heat stabilized, nylon 6/6 thermoplastic resin that is resistant to hot oil and grease, was developed specifically for use in molding the inlet manifolds.

Development of the fusible alloy core molding process has made technologically possible the injection molding of parts, such as manifolds, that have complex internal geometries. The alloy cores are die cast with a tin-bismuth alloy, which has a low melting point and is suitable for plastic overmolding. The core is then inserted into an injection mold and overmolded with the polymer to form the manifold. The manifold part containing the alloy core is removed from the injection molding machine, and then induction heated to melt out the low-temperature metal alloy, leaving the completed manifold.

The manifolds were extensively tested in the laboratory and in engine field trials. Tests included long term aging in hot air, exposure to petroleum, hot oil, and grease, and alternate cold water submersion and heating to 200 degrees C. Several thousand hours of testing were conducted and thousands of miles covered by vehicles with the thermoplastic parts.

Automotive engineers have long attempted to improve the interior finish of manifolds in order to increase fuel-air flow. Use of the fusible core technique to mold the complex, curved part produced the desired high-quality, smooth interior surface. On the basis of tests that evaluated the thermoplastic manifolds, Porsche found that the air resistance coefficient of a 90-degree bend was reduced by more than 50% in going from a rough to a smooth surface. The fusible core process provided the method necessary to allow molding of the hollow inner spaces of the curved manifold.

The combination of the correct material and the fusible core molding process has significantly improved the economics of manifold design and manufacture. While the diecast aluminum parts required extensive post-machining and assembly operations, the polymer composite provided the freedom of design that was required in order to consolidate several manifold components into one, and thus greatly reduced the costs of assembly and finishing.

Multiple Live Feed Injection Molding It is now possible to produce injection moldings without weld lines, voids, cracks, or microporosities, and to orient the fibers of fiber reinforced materials to obtain strengths greatly beyond the capabilities of hitherto conventional processing.

These possibilities are the result of work performed by Professor Mike Bevis, Dr. Peter Allan, and Dr. Peter Hornsby at Brunel University, UK. The technique represents the fruition of work performed at Brunel five years ago; it is significant because it permits fiber management," which had previously been impossible. Fiber management is the ability to obtain parallel fiber alignment in any direction and, if required, in alternate layers throughout a molding.

All of the fibers in a layer of material are laid out unidirectionally and parallel; directions of fiber alignment of the adjacent layers may be at an angle to each other. Fibers can be aligned circumferentially if a ring is molded, thus greatly enhancing hoop strength.

The technique is equally applicable to thin and thick section moldings. But to provide the necessary time for it to be performed, mold temperature must be increased and cycles extended by approximately 10%.

The desired directions of fiber alignment are decided during component design, and the molds are gated accordingly. For example, the fibers of the outer layers of a simple rectangular component may be required to run lengthwise; those of the inner layer, crosswise. The result is a good combination of flexural strength characteristics that resist bending forces in two directions.

The mold is thus provided with four gates comprising two opposite pairs. One pair is at the top and bottom and the other is at each side. Each pair is channeled to a device known as a double live feed, two of which are mounted in front of the nozzles of an injection molding machine that has two injection units.

Each channel is bifurcated in the live feeds; a venturi piston, the movement of which is freely programmable, acts upon each of the four channels. A separate hydraulic power pack drives the pistons. The "cylinders" in which the venturi pistons move are venturi channels that are angled to the main melt flow channels, hence the term "venturi pistons."

Each path leads to a gate to the cavity, and the gates are arranged such that each live feed delivers to two opposite gates. The two pairs of gates are arranged so that alternate layers of fiber may be at angles (normally a right angle) to each other.

During the molding sequence, the mold is first filled in the normal manner. It is usually filled through only one gate, and the piston is withdrawn from the channel in order to permit the flow of material. The molding then starts to gradually cool from the outside and inward, and the interface between solid and molten material moves progressively toward the center.

The Brunel technique, when used with thermoplastic materials, is applicable at this interface and its movement toward the center during cooling.

In the case of the molding concerned, it is desirable for the outer fiber to run lengthwise and the inner layer to run across. During the first part of the cooling stage, the live feeds for the top and bottom of the cavity are activated and those at the sides are shut off.

The pistons that act upon the channels leading to the top and bottom gates are then caused to move alternately in a "boxer" fashion: As one strokes down its channel and drives materials toward the cavity, the other withdraws, permitting a displacement of material that corresponds to the movement of the first piston (see Fig. 7).

The effect of the movement is to draw the melt across the solid/melt interface. In turn, this aligns the fiber in the direction of the flow of the melt.

Push-Pull Injection Molding

A new system that has been developed by Klockner provides a conventional twin-component injection system and a two-gate mold. A master injection unit pushes material through one gate, causing the mold to overfill. The overfill flows through the other gate and into the secondary injection unit, where the screw recoils 10 to 15 mm to make room for the material. This phase of the cycle is then reversed, the master injection unit receives the overfill, and the cycle is repeated. Figure 8 illustrates the principle of push-pull injection.

As the material flows back and forth through the mold, the outer layer freezes on the mold surface, and still-fluid material is pushed through the core of the mold until it solidifies-this causes a high degree of orientation in the finished part. Although ten repetitions are standard, the push-pull sequence reportedly can be repeated as many as forty times in the production of one part, and the ram speeds for both injection units can be adjusted for different mold pressures.

Injection cycles for the push-pull process are considerably longer than for conventional molding. However, because the material is cooling and freezing throughout the cycle, the holding pressure phase is less important for controlling shrinkage and warping. The injection phase and holding pressure phase virtually merge into one.

Highly anisotropic test parts that exhibit a maximum tensile strength of 37,700 psi (approximately 7250 psi higher than the published maximum strength for the material) have been produced by this method with the use of glass reinforced LCP. Flexural modulus was approximately 2.75 x 101 psi, as compared with 2.17 x 10' psi for most LCP parts. And because the material flows through the mold, converging flow paths can knit together more easily-external weld lines are effectively flushed out. Any weld lines that may exist within individual layers of material are dispersed throughout the part, thus minimizing their effect.

For production of isotropic parts that exhibit favorable tensile strength along more than one axis, Klockner has configured two injection units with a four-gate mold. In this process, the first sequence is as described above. Before the beginning of the second sequence, the two original gates are closed and two new gates are opened, providing a different material flow on a new axis. The gates are alternated for each sequence. Other developments in progress in push-pull technology include a degassing procedure to prevent material degradation, variations of cycle time and sequence to compensate for cooling gradients, and hollow parts.

Advantages of multiple live feed injection molding and push-pull injection molding include elimination of weld lines, controlled alignment of fiber reinforcement, and elimination of voids, cracks, and microporosities in large cross-section moldings.
COPYRIGHT 1991 Society of Plastics Engineers, Inc.
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Title Annotation:injection moulding
Author:Theberge, John
Publication:Plastics Engineering
Date:Feb 1, 1991
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