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Processing - the powerful role of innovation.


Although there is a strong trend toward standardization of new plastics processing machines, the challenges of specific applications can inhibit off-the-shelf solutions. It often turns out that innovation, beyond simple material and processing parameter adjustments, becomes necessary before molded products can be successfully brought to market.

But even before fundamental innovations are incorporated in the processing cycle, consistency of material and basic machine functions must be controlled in what is usually a complex operating environment.

Bruce Whipple, senior process specialist, and Jon Newcome, process specialist, Mobay Corp., Plastics and Fibers Division, emphasize that from the equipment perspective, part-to-part variations are not necessarily associated with a particular brand or type of machine. "A bank of identical molding machines running the same part can differ vastly in performance," they say, "and certain machine components can contribute significantly to part-to-part variation. Valve wear and inconsistent valve seating, for example, have been found to be major causes of variations in molded part weight."

But, they continue, part-to-part variation need not be caused only by mechanical problems. In machines with closed-loop control logic, calibration of the hydraulic and electrical functions can be critical in establishing consistency between identical machines and within a given machine. This has been demonstrated in optical applications molding.

Where regrind is used to improve molding economics, the granulate size distribution can overshadow virgin material variations; this becomes more important as blade wear in the grinder promotes an even broader size distribution. Whipple and Newcome say that changes in the bulk density of the virgin-to-regrind feed, due to such particle size distribution, can have a dramatic effect, since most regrind-to-virgin proportioning is done by volumetric feeding. Thus even a sophisticated machine control system could have difficulty navigating a course through such granulate size variation.

Process troubleshooting

While the quest for material and machine consistency is fundamental and unceasing, the need also often arises for basic process innovations so that the part objectives can be met precisely. Ask GE Plastics, for one. Their year-old 165,000 [ft.sup.2] Polymer Processing Development Center (PPDC), in Pittsfield, Mass., feeds on manufacturing problems spawned by inadequacies in typical processing procedures relative to specific applications. Key PPDC development areas include injection molding, low pressure injection molding, industrial blowmolding, packaging, thermoforming, and extrusion. The giant Alpha 1 machine's multiprocessing technology also casts a long shadow of non-traditional opportunity.

At GE, Jack Avery's title, "manager, International Programs & Technical Commercialization," points up the myth of standardized solutions in many new applications. Fierce global completion is rarely overcome simply by the swat of a standardized solution. More often, with innovative design, the task must be handed to imaginative processing.

Plastics converters are realizing that processing flexibility gives more value to their clients. They are learning that today's tradition could be tomorrow's albatross. Winners will use and, if necessary, modify, the "best" available process, not necessarily the typical or most familiar one.

Some customers are looking to molders to deliver more than just an acceptable, or even a perfect, plastic part. Avery says today's customer shopping list can include "black box" development; design assistance; in-house tooling capability; inventory planning and coordination; forward integration into decorating, assembly, packaging, and shipping; and flexible process capabilities that can focus on specific applications and markets.

One customer asked GE's help in producing a thinner automotive lens, with less distortion than it was getting with standard injection molding. Another needed more dimensionally stable exterior body panels, with better surface and ability to take Class A painting temperatures.

One solution may be injection-compression molding, in which material is injected and then compressed and cooled to form the molded part. Because the mold cavity is larger than in a standard injection process, injection pressure and clamp tonnage can be lowered and machine costs reduced.

Precise control of compression during the injection stroke evens pressure distribution throughout the part and minimizes distortion. Compression, which completes mold filling and compensates for material shrinkage, may be simultaneous with the injection stroke, beginning when a percentage of the resin has been injected; or, after the total shot has been injected into the mold cavity. The nontraditional process improves flow and reduces lens birefringence by 50%, and the thinner lens also uses less material.

In the body panel, injection-compression molding appears to eliminate lifter lines that were transferred to the parts in standard injection molding, thus improving surface appearance. The panel's thinner walls reduce weight while retaining required impact resistance and dimensional stability.

Extrusion coating of magnet wire, rather than conventional dipping, solved another GE customer problem. Magnet wire typically is coated, in a multi-pass dip tower, with an epoxy resin dispersed in a solvent. Regulated by the U.S. Environmental Protection Agency, the solvent emissions are driven off in curing ovens. Industry suppliers wanted an alternative method.

GE's solution is to extrusion-coat Ultem polyetherimide resin. Solvents and downstream incineration are eliminated, and system costs are lowered with a ten-times-faster process.

Now, a 0.0015-inch-thick, high-viscosity thermoplastic is applied to a copper substrate. Concentricity and flexibility, with plastic adhesion to the substrate, is maintained at high line speeds and temperatures. Numerous processing trials established proper drying conditions, tooling, copper preheating, temperature profile, and quench. Wire samples tested by GE Motors and Essex Wire show extruded wire performance equals that of the traditional system. GE anticipates that the strong bond created between the Ultem resin and the copper substrate also could reduce shorts and faults.

Another GE customer wanted to extrude Noryl PKN 4717 low density foam for lamination to various acoustic-absorbing materials and cosmetic cloths for automotive headliners. Automotive specifications require passing sag tests in an environmental chamber at--20 [degrees] C to 85 [degrees] C. GE says the new headliners easily retain their stiffness at 90 [degrees] C.

Although a converter base was available for processing expanded polystyrene foam at about 470 [degrees] F, processing the higher viscosity Noryl at 530 [degrees] F to 600 [degrees] F decomposed the physical blowing agents.

Screw modifications enhanced mixing and melting so that the Noryl could be processed in the low 500 [degrees] F range, thus ensuring the thermal stability of the blowing agents. Rheological testing of the higher viscosity material also led to modified extrusion die designs, including increased clearances at the exit lips and a lowering of the pressure drop, and avoided the need to increase the extruder's torque capacity.

In a microwaveable tray program, 3-layer, A-B-A structures, in which the A, or cap, layers are a composition of polyphenylene oxide and high-impact polystyrene, screw design was critical. The screws must provide a compositional and thermally homogeneous melt when the cap layers are being tailored through on-line compounding by dry blending of additional resin. Minimal cap layer thicknesses, for improved economics, depended on the material grade and the flow system in the combining feedblock, with different conditions needed for two different makes of feedblock (Cloeren and Welex). Studies provided samples for performance testing, defined limits for economic targeting of the structures, and pinpointed processing information to aid GE's technical personnel in the event of field problems.

Eliminating porosity

Until recently, notes Ken C. Rusch, sales and engineering manager, Bumpers and Structural Systems, The Budd Co., Plastic Division, the most successful approach to controlling surface porosity in sheet molding compound (SMC) has been to inject a thin urethane coating into the mold cavity, in effect priming the part while it is still in the mold. The resulting sealed, porosity-free surface can then be topcoated without difficulty. All the SMC body panels on GM's new APV minivans use the in-mold coating method.

Rusch comments, however, that the method may not work well on some parts with a deep vertical section and, besides, this is "a |Band-aid' solution, since it does not eliminate the root causes of surface porosity."

Recent work by Budd has demonstrated that vacuum compression molding (VCM) will consistently eliminate surface porosity on SMC body panels. The 1991 Ford Taurus SHO hood is the first major horizontal body panel produced with VCM, achieving Class A surfaces without using in-mold coating or requiring extensive hand refinishing. All new molds at Budd are now being designed to accommodate VCM.

In the VCM process, a vacuum seal is built onto the compression mold, and the press is equipped with a large vacuum reservoir (accumulator). The SMC charge is placed in the mold, and as it is closed, the vacuum seal is engaged before the cavity side of the mold contacts the SMC material. At the point of full seal engagement, the mold cavity is evacuated rapidly to about 0.1 atmosphere, using the large vacuum reservoir. The SMC then is compressed, filling out the mold cavity while it is under vacuum. The cavity is vented shortly before the mold is opened. There is no significant increase in total cycle time.

Reduced porosity also is important for applications where sealing of liquids is essential. Engine covers or oil pans have been difficult in SMC because of permeability resulting from porosity. Using VCM, engine covers can now be molded from SMC at a significant cost and weight saving versus steel or aluminum. SMC valve covers, molded by VCM, were recently introduced on the Ford 4.6 liter engine.

Increased in-plane shear strength and enhanced flexural fatigue is believed to result from elimination of air trapped between plies of SMC before compression. "With conventional SMC molding," Rush says, "air entrapment between the layers of material is a problem, causing areas of interlaminar weakness. This is always the case if the SMC contains continuous oriented glass fibers, as in a bumper beam. The VCM process now permits SMC to compete for structural applications, such as bumper beams, chassis cross members, or radiator supports." Thus, innovative application of VCM could open new horizons for SMC in automotive. Typical SMC, inherently low cost, is only 25% organic resin, 29% glass fiber, and 46% calcium carbonate.

Complex air duct

Application of multidimensional blowmolding solved a complex problem in the processing of a clean air duct. Typically one of the last parts to be designed, the air duct must provide airtight seals, resist collapse under engine vacuum, and withstand under-the-hood temperatures. One of the first visible parts when the hood is opened, its appearance must also complement the vehicle's styling.

The Subaru Legacy's clean air duct is an extreme case of a hollow shape that must follow a path curving through several intersecting planes. It must also be flexible enough to accommodate engine "roll" (movement), in relation to the air cleaner, during acceleration.

Applying the patented technology of Japan's Excel Corp., MES Corp., a U.S. custom molder, uses two different resins for soft and hard sections, with virtually no flash, and with the ability to vary wall thicknesses and layer one material on the exterior or interior surface. Using Santoprene thermoplastic elastomer from Advanced Elastomer Systems, the MES equipment extrudes a parison programmed with special software to attain the specific movement, the pre-blow, the wall thickness, and the materials required to shape the parison in three dimensions. The hot, preshaped parison is horizontally supported during extrusion by the bottom half of one of two cavities in the mold. When the parison is fully formed, the table rotates, the top half of the first mold cavity closes and is clamped in place, and blowing proceeds through an injection pin. While the duct in the first cavity is being blown to shape, a second parison is being extruded to shape into the bottom half of the other cavity. The process of rotation of the table, closing the cavity, and blowing is repeated in a sequence similar to that of injection blowmolding. The result is a blowmolded shape with virtually no flash to be recycled. To achieve a similar shape by conventional molding, an oversize parison would have to be pinched off over much of its length, generating flash that might weigh as much as the finished part. In addition, the wall thickness could not be controlled, and the hard and soft segments would have to be manufactured separately and clamped together. Some shapes would be too complex to mold conventionally.

In the Subaru Legacy air duct, the multidimensional process permits the use of Santoprene rubber to form the part's flexible bellows and sealing cuffs, while its tubular sections, with many inserts, utilize polypropylene. Robert L. Arnold, Advanced Elastomer Systems, L.P., and Ram Mehta, MES president, say the part could not have been made by conventional blowmolding, and it would have been costly to assemble from separately molded hard and soft parts held together by clamps. On the other hand, injection molding would have required complex collapsible cores, in a slow and labor-intensive sequence, and would also require assembly of separately molded hard and soft parts.

Low pressure solid molding

Hettinga Equipment Inc.'s low pressure solid injection molding system for integrally molding plastic with diverse materials, such as textile, film, metal wiremesh, and even electronics, in a single step, is another example of a moving away from standard techniques to solve a problem. Hettinga maintains that based on the maintenance of a fully plasticized and uniform melt front that remains intact after gentle injection into the mold cavity, the low pressure method avoids a need for high pressure packing, and thus avoids molded-in stress.

European and Asian manufacturers have applied the process for a number of years, but American industry has been slow to incorporate the domestically developed technology. Panasonic, the Japanese electronics manufacturer, had a problem with static electricity. The company was packaging its products in a flocked case. While the flocking (spraying a glue over the mold cavity and then electrostatically impressing loose fibers, or fluff, into the glue) was satisfactory from an appearance standpoint, residual static charges were creating problems with the Panasonic components, particularly the discs and magnetic tapes.

Hettinga's solution was to mold-in a textile liner, using low pressure solid injection. A velour fabric was placed in the mold cavity, and polypropylene resin was injected to create a finished structure. The method solved the static electricity problem. Eliminating glue used in the original flocking process permitted less costly polypropylene in place of ABS, labor was reduced, and special ventilation and safety measures were not needed. Panasonic saved almost 50% in the cost of producing the packaging.

Hettinga asserts that the "typical" molder is changing. Also, textile, wood, and metal fabricators are becoming molders themselves and adding value by altering their traditional products to take advantage of plastics technology. For example, Urban Co., a German fabricator of wood products, now uses the Hettinga process to mold window components from low cost resins and its plentiful wood fiber waste. HP Chemie Pelzer, a large German textile concern, and Eybl Durmont, a large Austrian textile concern, produce parts for auto and airplane interiors by molding-in their woven and nonwoven textiles. Fireproof interior cladding is being made by Durmont for the Fokker aircraft, and Aerobus currently is testing Durmont parts with molded-in fireproof textiles. The Japanese firm Kodama Chemical Industry Co., Ltd., a traditional metal fabricator turned molder, makes office floor and wall coverings with molded-in metal shielding to protect computers.

Hettinga says activity in Detroit for the process is picking up, notably for interior parts. Within the past two years, the Stolle Division of Alcoa Industries, Andover Industries, and ASAA Technologies, Inc., have installed low pressure solid injection molding equipment. Stolle combines chrome and plastic to mold bumper trim and bumper blocks, reducing weight and eliminating welding, for GM's Cadillac and Buick Riviera and Ford's Lincoln Town Car.

By using polypropylene and eliminating the need for gluing, Andover, a traditional molder, is able to cut production costs by 25% or more. ASAA, a wood fiber company, combines wood stock and plastic to make a variety of low cost parts, such as door liners for Ford vehicles. Low pressure molding enables ASAA to make deeply contoured door liners with molded-in textiles and bosses.

Molded-in film may be another important automotive application for the low stress/low pressure process. Now working with Hettinga, 3M predicts that 25% of its film sales will be for molded-in uses, possibly for auto cladding, trim, lenses, and even luxurious upholstery.

Two parts to one

Many automobile outside rearview mirror housings have thick wall sections, up to about 25 mm, in the area where the housing is attached to the door. In addition to requiring very long cycle times, these thick sections can cause severe, unacceptable sink marks.

Battenfeld of America says that this problem has previously been solved by either of two manufacturing techniques. One is a two-piece shell design. Two parts, with uniform wall thickness of 2.5 to 3.0 mm, are injection molded conventionally and welded or mechanically assembled. In the second method, a separate plastic component is produced in one mold. It then must be manually placed, as an insert, into a second mirror housing mold in the area of the necessary thick wall section. The mirror housing is then molded, encapsulating the insert. The final mirror housing does not exhibit any sink marks, and the cycle time has a reasonable value.

Two advanced processing techniques have provided more "elegant" solutions. For more expensive automobiles, the mirror housing is produced using coinjection molding. The skin is a nylon 6/6 with glass fiber reinforcement minimized in order to enhance surface appearance. The core material, also nylon, contains 30% glass fiber reinforcement and an endothermic chemical blowing agent to prevent sink marks. While the housing is relatively heavy, this property is desired for the more expensive cars in order to prevent or reduce vibrations.

For less expensive cars, the mirror housing is produced in polypropylene by means of Battenfeld's Airmould gas-assisted injection molding technique. Nitrogen is injected via an injection module (needle) directly into the thick-walled region of the mold. This creates a void, with a resultant reduction in weight and cycle time. Sink marks similarly are prevented.

Both solutions have a common characteristic. Parts requiring no prior, or subsequent, manufacturing steps are obtained using the basically conventional, but modified, injection molding machine.

Expanding role of sensors

For years, plastics processors have been using signals from the injection molding process to obtain a measure of the process consistency or for diagnostic purposes. Molders know that even subtle differences in variables, such as feed throat temperature, can mean the difference between successful processing and defective molded parts. Good molders take full advantage of the information that can be obtained from standard machine instrumentation, such as barrel temperature, ram position, and hydraulic pressure sensors.

Professor Robert Malloy, Dept. of Plastics Engineering, University of Lowell, says that newer, more sophisticated molding machines are equipped with additional information-generating signals that are manipulated using on-board micro-processors. Hydraulic injection pressure transducers typically have been used to control or monitor peak packing pressure and holding pressure values, which strongly correlate to part quality and dimensions. Additional information is obtained from these sensors by integrating the area under the injection pressure versus position (or time) curve during mold filling, which reflects the energy required to fill the mold and which is strongly influenced by the melt viscosity. Changes in injection energy or melt viscosity could be associated with material lot variations, temperature differences, regrind level, or even material degradation resulting, for example, from insufficient drying of moisture-sensitive polymers. Direct-measurement nozzle melt pressure sensors can be substituted for the hydraulic sensors.

Plastication energy, obtained by using torque transducers or hydraulic pressure transducers at the screw drive motor, can provide an indication of material quality. Plastication instrumentation can easily be added to the molding equipment, to provide another processing signal. Plastication torque, energy, or even time can be extremely useful when working with moisture-sensitive polymers, regrind, or difficult-to-feed materials.

The molder has the option of including any number and type of "add on" to his process for either control or monitoring purposes. Since instrumentation can be expensive, the key is to use only effectively placed sensors that are specifically needed to provide data that can be conveniently analyzed and stored. Hydraulic injection pressure sensors, for example, should be placed close to the injection cylinder, rather than in a manifold block some distance away from the event.

The use of clamp pressure sensors has solved problems for molders using more quickly produced "soft" spray metal or alloy tooling, notably for prototypes. Since these tools can be vulnerable to damage from even moderate injection and clamp pressures, tie rod or mold-strain sensors can be useful. Providing an accurate indication of clamp tonnage, and even injection force, they can assist in setup and in minimizing the potential for serious and costly tool damage.

In-mold process sensing, including variables such as cavity pressure, and mold and mold water temperature, are important process parameters and relate to part quality. In-mold sensors are especially useful where molding is done on a custom basis. A tool owner can find it difficult to relate part quality to machine variables when the mold is run at a new molder or on a different machine. The tool-based signals can be invaluable in setting up the tool on another machine or as a measure or the molder's process capability.

Malloy predicts that the use of sensors will increase, and that molders who make full use of the sensor outputs and find novel ways to use them will have a competitive and quality advantage.

Total process control

Kurt Fenske, vice president, sales, Engel North America, says that "more is involved than just directing the sequence of steps that characterize the machine's cycle or hardware performance. True in-cycle control ties the entire process to a specific product and involves process design, monitoring and evaluation, and correction." In the real world of plastics processing, innovation also is not far from the action center, as customer requirements are increasingly demonstrating.

PHOTO : Facing page: To eliminate sink marks, original automotive mirror housing required a separately molded insert, which was manually inserted in the tool. The housing then was molded around the insert in a second operation to produce the final part. Above: With the coinjection method, the mirror housing is produced in a single molding operation. The combination of nylon and a blowing agent provides thick solid walls and foamed core without sink marks. Below: By means of the gas-assist Airmould process, a polypropylene mirror housing is produced in a single cycle for less expensive cars. A separate "kit," attached to a standard machine, with a gas nozzle located at the tool, provides sink-free parts with the necessary wall thickness.

PHOTO : Facing page: Stress reduction achieved by injection-compression molding, versus standard injection molding, is evident in parts shown under cross-polarized light. Optical distortion from molded-in stress, in an injection molded polycarbonate automotive part (left), was reduced about 50% in an injection-compression molded part (right). GE Plastics. Right: Multidimensional extrusion blowmolding process produces clean air duct for the Subaru Legacy. The system combines bellows and sealing cuffs of Santoprene thermoplastic elastomer and tubular sections of polypropylene, with virtually no flash. Advanced Elastomer Systems and MES Corp.

PHOTO : Hollow, one-piece intake manifold of Ultramid nylon 66 resin, injection molded for BMW's new 4-valve/6-cylinder engine, uses the core melt technique. A core of tin and bismuth alloy, conforming to the manifold's channels, is inserted in the mold cavity. The nylon is molded around the insert. Placing the part in a special hot bath melts the core, which flows out of the part. The manifold features smooth inner surfaces and close tolerances. BASF Plastic Materials.

PHOTO : An automotive back shelf, with a nonwoven fabric backing, is produced by low pressure injection molding. "A" shows cross section of wood-filled polypropylene and fabric. "B" is without fabric. Hettinga Equipment.
COPYRIGHT 1991 Society of Plastics Engineers, Inc.
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Title Annotation:plastics industry
Author:Wigotsky, Victor
Publication:Plastics Engineering
Date:Jun 1, 1991
Previous Article:1990 annual report of Society of Plastics Engineers.
Next Article:A novel nonreactive HALS boosts polyolefin stability.

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