Plastics processing technology: opportunities for the next decade in blow molding.
Blow molding is the process of forming a hollow object by placing a softened or molten thermoplastic preform or parison inside a cooled metal mold, inflating it against the mold until the plastic retains the mold shape, and [TABULAR DATA FOR TABLE 1 OMITTED] then removing it and trimming the formed part from the non-part flash or trim.(1-3) Although most polymers can be blow molded in one fashion or another, polyethylene represents about two thirds of the estimated 10 billion lbs consumed in the U.S. in 1996. PET consumption represents another 2 billion pounds, with PVC, PP, PC, and ABS representing other identifiable polymers.
Blow molding markets are frequently divided into disposable containers, representing about 85% of the market with an estimated growth rate of about 4% per year, and industrial or structural products representing the rest, with an estimated growth rate of at least 12% per year. The growth in disposable containers is sustained by the continuing development of retortable plastic containers. The growth in industrial products is primarily driven by newer process developments, such as multilayer parison extrusion, sequential extrusion, and nonaxisymmetric blow molding.
The process provides a unique way of producing extremely complex, irregularly shaped hollow objects having walls that curve inwardly (re-entrant walls). Because blowing pressures are relatively low (0.2 to 1.0 MPa), cast aluminum molds are used, resulting in considerable savings when compared with conventional injection molding. The process limitations are the limited polymer resin palette, residual stress in parts, nonuniform wall thickness, and relatively high levels of trim for industrial products. In blow molding, only one surface of the polymer is pressed against the mold, as is the case also with thermoforming and rotational molding. As a result, part wall tolerance is usually broader than that for injection molding and extrusion. Table 1 compares some of the attributes of blow molding, thermoforming, and rotational molding.
The general categories of blow molding are given in Table 2. Typically, continuous extrusion processes are very high-volume processes used to produce parts weighing a few grams to 500 grams. Intermittent extrusion processes are used to produce industrial parts that weigh from a few kilograms to 500 kg.
Injection blow molding is a two-step [TABULAR DATA FOR TABLE 2 OMITTED] process. A test-tube-like preform is injection molded. The preform, still on its core rod, is then indexed to a blow station where the preform is inflated against a split mold wall. Although there is no technical reason to restrict the size of the part manufactured this way, most parts weigh less than 500 grams.
In the early 1970s, the desire to produce a rigid barrier plastic container to compete with glass bottles for the carbonated beverage industry led to the development of injection stretch blow molding. The earliest injection stretch blow molding techniques used an extruded parison or injection molded preform that was reheated and mechanically stretched prior to and during inflation. This technique is sometimes referred to as reheat blow molding. More recently, the process has appeared more like injection blow molding, with the exception that the core rod doubles as the mechanical stretching device. In addition, the technique of thermoforming a preform also appears feasible, particularly for widemouth containers. Again, there is no technical reason to restrict the type of polymer or the size of the part produced this way, but the market is dominated by the ubiquitous 2-liter bottle of PET, weighing less than 100 grams. Typically, in container extrusion, injection, and injection stretch blow molding, the body of the parison or preform resides completely within the mold cavity prior to inflation. For the production of planar articles, such as door panels or toy wheels, the parison is trapped by the closing mold, then inflated. As a result, the final part has peripheral flash, not unlike that generated by twin-sheet thermoforming.
The growth rate of the blow molding industry has been spurred by developments in machinery and materials, which are detailed below. The container industry has also focused on the development of recyclable containers, to the point where the HDPE milk bottle and the PET beverage bottle represent nearly 80% by weight of all post-consumer recycled plastics. And the industry continues to improve recyclability of these products, by replacing metal caps and paper labels with plastics and by replacing the beverage bottle and separate HDPE basecup with the one-piece petaloid bottle.
Consider first production of a parison that is subsequently melt blown. As the polymer passes from the extruder to the mold, it first experiences high shear ([10.sup.3] [s.sup.-1]) through the die, then substantial extrudate swell and sag but nearly zero extensional rate ([10.sup.-4] [s.sup.-1]) as a hanging parison, and finally moderate to low extensional rate ([10.sup.1] [s.sup.-1]) as it inflates against the mold. Excessive die swell translates to wasted material. Excessive sag leads to nonuniform wall thickness from part top to bottom, or in extreme cases, unformability. In order to select an appropriate blow molding polymer, both shear and extensional viscoelasticity must be understood.(4,5)
High-density polyethylene (HDPE) has been used as the prototypical polymer, first, because it is very thermally stable, thus allowing extensive research efforts without measurable property changes, and second, because it is the most widely used blow molding polymer. Despite decades of characterization, the exact interaction of polymer material properties and temperature-, pressure- and shear-rate- dependent process conditions is not fully understood, even for HDPE. Through copolymerization and blending, some progress is being made in the development of continuous extrusion blow molding grades of polyamide 6, polypropylene, and even PET.
Intermittent parison blow molding is ideally suited for the fabrication of structural panels and large parts. Many potential applications require the properties of engineering polymers such as FR-ABS, RPVC, mPPO, and PC. Extrusion grades of these polymers have limited high-temperature thermal stability; there are few grades designed specifically for the long melt residence times required for large part blow molding in conventional equipment.
Although PET dominates the reheat blow molding area, as a monolayer it suffers from less-than-adequate water vapor, oxygen, and essential oil vapor permeabilities. And PET must be heat-set to provide a modicum of stability for retortable and hot-fill container applications.(6) PEN/PET blends and copolymers and multilayer parison blow molding, where oxygen-barrier and moisture-barrier resins such as EVOH and HDPE are layered with PET and appropriate tie layers, provide improved permeabilities and thermal stability, albeit at added material costs. Multilayer blow molding of HDPE and nylon 6, as an example, is also being developed for shaped fuel tanks.(7) The largest obstruction to economic viability of multilayer structures is the difficulty in processing the mixed regrind.
There have been significant improvements in the machinery needed for container production. Some newer developments include improved infrared heating techniques for reheat blow molding; very high speed rotary extrusion presses, particularly for the milk container industry; mold indexing on shuttle presses to allow for offset spout formation; multilayer continuous extrusion blow molding for barrier containers; and the continuous production of heat-set PET bottles by carefully controlling orientation crystallization and thermal crystallization, preform and mold temperature, and blow pressure and residence time in the mold.
Industrial blow molding has received the greatest attention from machinery builders, even though it remains a minor segment of the overall blow molding market. The need for complex, tortuous ductwork in transportation spurred the development of off-axis extrusion blow molding, incorrectly called 3D or three-dimensional blow molding. Conceptually, the task is quite simple. As the parison is extruded, it is partially inflated and lies against one half of the mold; either the extrusion head or the mold moves in a programmed two-axis or three-axis fashion. When the sausage-like tube has reached the mold periphery, the second mold half closes against it, severing it from the parison following, and inflating it against both mold halves.(2) The difficulty occurs when massive molding machinery that has very great inertia is asked to respond rapidly with accuracies of less than 10%. Figure 1 shows a schematic of one scheme. As mentioned, multilayer blow molding is used to manufacture barrier containers. A variation of continuous multilayer blow molding uses a valving system to switch from one polymer to another during extrusion. Unique products having alternating rigid and flexible segments are the result.(8)
When very large parts - such as fuel oil tanks or automotive instrument panel skins - are produced, cooling under pressure in the mold cavity governs the process cycle. Although there have been attempts to continuously extrude very thick, large-diameter parisons for the several minutes needed to cool the part in the mold, the preferred method is to accumulate the melt in a vessel ahead of the extrusion screw, and then extrude the parison at a relatively high rate to ensure minimum variation in parison wall thickness owing to sag and extrudate swell. Accumulator heads have been used for many years, primarily with HDPE [ILLUSTRATION FOR FIGURE 2 OMITTED]. The heads were not designed for rapid color changeover or for accumulator extrusion of thermally sensitive polymers such as FR-ABS, mPPO, and RPVC. Recently, the heads have been reengineered to provide rapid disassembly for cleanout,(9) and the rheological characteristics of polymers have been coupled with computerized flow channel analysis to produce streamline channel flow for thermally sensitive polymers.(10) Many parts produced on accumulator blow molding machines are flat panels that are destined to be foam-filled. To minimize the thinning in edge corners and 3D corners of the part, the lay-flat dimension of the extruded parison is usually greater than the part width. As a result, the newer accumulator head designs tend to be squat with disproportionately large-diameter dies and divergent die flow geometries.
Parison programming has been available for decades. The object is to thin the parison where it will stretch the least (as in the neck region), thicken the parison where it stretches the most (as in the body of the container or in edge corners), and provide a wall thickness transition in a region such as the chime or shoulder. The earliest programming used cam-driven mandrels. Currently, microprocessor-controlled, servo-driven die lips and/or mandrels are used. Parison wall thickness control tends to be oversold, since extrusion speed, melt temperature at the die, melt temperature uniformity (both radially and axially), extrudate swelling, sag, and general variations in the rheological properties of the polymer influence the flow characteristics of each segment of forming parison the instant it is passing through the programmed die gap. Since all segments are attached, the general result is slight but discernible lot-to-lot shifting in the local wall thickness.
In order to minimize material usage for given product design criteria, the designer needs to predict wall thickness for both preforms and parisons, as well as the final part. As mentioned, most of the work on parison wall thickness prediction focuses on viscoelasticity of the polymer. The prediction of test-tube-like preform wall thickness that optimizes wall thickness for a given barrier application depends on the level of crystallinity in the preform, the temperature-dependent elastoplastic stress-strain characteristics of the polymer, and the level and distribution of frozen-in stresses resulting from the injection molding process.
Typically, inflation of the preform is similar to inflation of a rubber balloon, with stretching beginning locally by forming a nearly constant-diameter aneurysm that progresses the length of the preform.(11) Complete mathematical prediction is still lacking. The mathematical prediction of parison inflation follows finite element analysis, assuming that at inflation high strain rates, the rapidly cooling polymer responds hyperelastically rather than viscoelastically. With this assumption, General Electric's PITA program was developed in the early 1980s for both thermoforming and blow molding. Today, programming efforts have diverged as more has been learned about the differences in these two disciplines. The parison blow molding software must incorporate nonuniform parison wall thickness, parison pinch off and closure around the blow pipe, preinflation of the parison before the mold closes onto it, inflation control and pinch off in handle areas, and the capture of the parison edge during closure in the case of structural blow molding. As with the thermoforming wall prediction software, blow molding software packages are in the early stages of development.
1. N. Lee, ed., Plastic Blow Molding Handbook, Van Nostrand Reinhold, New York (1990).
2. A. Garcia-Rejon, "Advances in Blow Moulding Process Optimization," Report 82, Rapra Review, Reports, 7:10 (1995).
3. D.V. Rosato and D.V. Rosato, eds., Blow Molding Handbook: Technology, Performance, Markets, Economics. The Complete Blow Molding Operation, Hanser Publishers, Munich (1989).
4. G. Ajroldi, "Determination of Rheological Parameters From Parison Extrusion Experiments," Polym. Eng. Sci., 18, 742 (1978).
5. K.L. Stanfill and A.J. Giacomin, "Measurement of Non-Linear Viscoelastic Shear Properties of Programmed-Parison Blow Molding Resins," SPE ANTEC Tech. Papers, 37, 1425 (1991).
6. J. Myers, "Process Technologies Expand Markets for Stretch Blow Moulded Bottles," Mod. Plast. Int., 23:12, pp. 30-34 (December 1993).
7. J.H. Schut, "Auto Gas Tanks: The Great Barrier Grief," SPE Blow Molding Division Conference, pp. 21-31, Itasca, Ill. (Sept. 1992).
8. J.G. Schwaegerle and W.R. Deaton, "Sequential Extrusion Blow Molding: A Newly Accessible Technology," SPE ANTEC Tech. Papers, 39,1289 (1993).
9. R.A. Slawska, "Rule of Thumb Guidelines for Selecting Industrial Blow Molding Machines," SPE ANTEC Tech. Papers, 39, 1284 (1993).
10. J.L. Throne, C.I. Beal, and M.M. Balasko, "Blow Molding Extrusion Head," U.S. Patent 5,284,434, assigned to B.F. Goodrich Co.
11. M. Cakmak J.L. White, and J.E. Spruiell, "Dynamics of Stretch Blow Molding of Polyethylene Terephthalate," SPE ANTEC Tech. Papers, 30, 920 (1984).
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|Author:||Throne, James L.|
|Date:||Oct 1, 1998|
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