Advances in injection molding of fiber-reinforced thermoplastics during 1991—Part I: Materials and processing

SPECIAL REPORT

Advances in Injection Molding of Fi ber-Rei nforced Thermoplastics During 1991 -Part I: Materials

and Processing

Paul G. Kelleher New Jersey Polymer Extension Center at Stevens Institute of Technology

Castle Point Hoboken, New Jersey 07030

ABSTRACT

This is the first part of a survey of developments reported in the open literature during 1991 on the subject of injection molding of fiber-reinforced thermoplastics. Topics discussed include materials, polymers, reinforcing fibers and composite properties, advances in the injection molding process, fiber breakage, shrinkage, and warpage, as well as alternatives to the injection molding process. 0 1992 John Wiley & Sons, Inc.

INTRODUCTION

This review is an update of reports published previ- ously on this subject'.* and is restricted to work reported in the open literature during 1991 and late 1990. The patent literature was not surveyed. The subjects discussed comprise materials, including polymers, reinforcing fibers, and composites, as well as advances in injection molding process parameters, fiber breakage, shrinkage, and warpage. Also included are alternatives to the injection molding process.

MATERIALS AND PROPERTIES

Polymers

Heat-Resistant Polymers

In this review, injection moldable thermoplastic polymers that are capable of use as composite matri-

Advances in Polymer Technology, Vol. 11, No. 4, 305-313 (1992) 0 1992 by John Wiley & Sons, lnc.

ces at elevated temperatures are of particular interest because of the temperature requirements of military applications. In this section, information on the neat polymers is considered. Gardner et studied the structure, crystallization, and melting behavior of polyaryletherketoneketone and found two crystal structures. Details on structural dimensions were given. The form 2 structure can convert to the form 1 structure after melting. Hsiao et al.4 examined the crystallization and melting characteristics of a family of polyaryletherketones. Jensen and Hergenr~ther~ determined the effect of molecular weight on the properties of polyarylenetherketone. Petraccone et a1.6 studied the crystal and molecular structure of poly- aryletheretherketone. Arzak et al.' explored the effect of annealing on the properties of polyetheretherketone (PEEK). Results are explained as a consequence of both crystallinity and crystalline perfection. Medellin- Rodriguez and Phillips* reported on the crystallization

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ADVANCES IN INJECTION MOLDING I

behavior of PEEK and observed spherulitic growth over extensive crystallization ranges. Arzak et discussed the effect of processing on the properties of PEEK. Details of the effects of load level on the fatigue fracture of PEEK were presented by Brillhart and Botsis.Io Maeda" discussed the effect of pressure on the thermal behavior of quenched PEEK and polyethylene terephthalate (PET). Roovers et al.12913 in two articles presented data on the viscoelastic and other properties of polysulfone (PSU). Cheung et al.l4.l5 reported on the thermal characterization, phase morphology, and mechanical behavior of PSU- polyphenylene sulfide (PPS) blends.

Other Polymers

Wood16 reported on the designations and properties of new commercial grades of heat-resistant polymers such as polyphthalamide, nylon 4,6, polyarylamide, liquid crystal polymers, and PPS. In additional articles dealing with polyamides, Harschnitz et aI.l7 wrote about polyamide 4,6 as a design material; Shuren and YanxiangI8 studied structural reorganization in nylon 10,lO during thermal treatment; and Cojazzi et aI.l9 investigated phase transitions in nylons 8, 10, and 12. Bremner and RudinZ0 reported melt flow index values and molecular weight distributions of commercial thermoplastics. Roberts2' published work on the strain limits and maximum service temperatures for thermoplastic materials.

Reinforcing Fibers

Fiber Reinforcement: General

Ihm et aLZ2 reviewed the subjects of fibrils in natural materials viz. natural cellulose and collagen, as well as fibrils in synthetic materials such as drawn polymers, liquid crystalline polymers, and hard elastic materials. Structure and morphology were discussed in each case, followed by a general exposition of the microfi- brillar state. Several articles appeared on the advantages of liquid crystalline polymers (LCPs). Bafna and BairdZ3 showed that the inclusion of LCP as a viscosity modifier in a PPS composite promoted the penetration of the viscoelastic polymer matrix into the fine spaces between the reinforcing fibers. Blends of two LCPs demonstrated substantial increases in impact and tensile properties compared with the pure L C P S . ~ ~ Mithal and TayebP augmented the concept of in situ composites as alternatives to fiber-reinforced composites in a study of polyblends of LCP and PET. Sun et a1.26 reported studies of in situ formation of composites of LCP and polyetherimide (PEI).

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KO et al.27 reported the influence of continuous stabilization on the production of activated carbon fibers produced from polyacrylonitrile. Donnet and Guil- painz8 wrote a research report on the surface characterization of carbon fibers. Evaluations of the intrinsic mechanical properties of carbon fibers were conducted by Krucinska and Stypka.29.30

Hol13' discussed organic fibers as reinforcements for plastics. An update on short fiber reinforcements including glass, carbon, aramid, mineral, and ceramic fibers was presented by Galli.32 Peijs et a1.33.34 published two articles on composites containing both polyethylene (PE) and carbon fibers and on impact- resistant structures. Klunzinger et al.35 commented on the negative high coefficient of thermal expansion and nonlinear elasticity of high-performance fibers of rigid rod polymers.

Fiber-Matrix Adhesion

A number of authors dealt with methods of increasing adhesion between polymers and reinforcing fibers. Londschein and M i ~ h a e l i ~ ~ modified the surfaces of carbon and PE fibers with a cold mixed gas plasma while Pitt et a1.37 investigated an argon plasma technique for treating Kevlar and glass fibers to improve the adhesion to polycarbonate. Rearick and Harrisod8 etched carbon fibers using a nickel-catalyzed oxidation procedure for enhanced adhesion. Yuan et al.39 used plasma surface treatments on carbon fibers and determined the effect on mechanical property and interfacial shear strengths of composites. Irzhak et al.40 reported work on finish coating of carbon fibers in the production of carbon fiber-reinforced composites. Larena et al.41 performed a thermal and Fourier trans- form infrared (FTIR) study of E-glass short fiber func- tionalization with chloromethyl dimethyl chlorosilane. N e m o ~ ~ ~ discussed wettability and compatibility of reinforcing fibers for composite materials.

Composites, Properties, and Characterization

Not all of the following references deal with injection-moldable composites but the information is presented as useful background information.

Texts and Reviews

Baer and M ~ e t ~ ~ edited a text comprising the following subjects: introduction to performance polymers; structural hierarchy in semicrystalline synthetic polymers; structure-property relationships in liquid crystalline polymers; distortional plasticity of organic glassy polymers; ultrasonic analysis of mechanical

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deformation processes; carbon-carbon composites; microlayer composites; mechanical properties of polymeric composite materials; high-performance organic fibers for polymeric composites; and a review of mi- crofiber systems.

Mallick and Newman4 edited a text on composite materials technology that discussed primarily fiber-reinforced thermosets. Two chapters, however, dealt with thermoplastics, viz. long fiber thermoplastic composites and injection molding of thermoplastics composites.

The American Society for Testing and Materials (ASTM) published two recent books on composites. The first monograph4’ presented 38 articles on the testing and analysis of fatigue and low-velocity impact of composite materials. The second book46 covered testing and design and included sections on mechanical properties and failure criteria, quality assurance, process control, damage, flaws, and repair methods.

Kim and Mai47 considered high-strength, high-fracture toughness fiber composites with interface control. By proper control of the physical and mechanical properties of the fiber-matrix interface, high-strength characteristics can be combined with high toughness. The authors reviewed the existing theories of fracture toughness of fiber composites and the various methods for improving the fracture toughness through interface control. Conclusions and generalizations were drawn from the literature and areas needing further research were discussed. Ahmed and Jones48 examined particulate reinforcement theories for polymer composites.

Heat-Resistant Polymer Matrices

A number of articles on composites with heat-resistant polymer matrices have been published. Rogers49 discussed the properties and applications of poly- etherketoneketone (PEKK), providing data on mechanical and thermal properties of PEKK reinforced with either 20% carbon or glass fibers. Mishra and SchultzSo reported the crystallization kinetics of short glass and carbon fiber PEEK composites under melt strain conditions. The environmental effects on the water absorption and mechanical properties of carbon fiber-reinforced PPS and PEEK composites were studied by Ma and Y u ~ . ~ ’ The loss of mechanical properties under hot-wet conditions was relatively low. Ma et al.52 also investigated the mechanical properties and morphology of carbon fiber-reinforced PEEK and PPS composites on impact loading and immersion in water at 80°C. Strong et al.53 used electron beams to crosslink glass and carbon fiber-reinforced PEEK and PPS. This procedure had some advantages in poten- tially improving the physical properties of these com-

ADVANCES IN POLYMER TECHNOLOGY

posites. Eduljee et al.54 wrote about the application of micromechanics to the prediction of macroscopic thermal residual stresses in short fiber-reinforced PEEK. Desio and RebenfeldSS presented results on the iso- thermal crystallization of PPS and PPS composites containing glass, carbon, or aramid fibers. The crystallization kinetics of PPS and carbon fiber PPS composites were measured by Kenny and Maffe~zoli.’~ Com- parisons were made with the behavior of PEEK and PET. Caramaro et al.57 described the influence of fibers on the crystallization kinetics and matrix morphology of a carbon fiber PPS composite. It was possible to optimize some mechanical properties of the composite by adequately controlling the molding conditions and matrix morphology.

Other Polymer Matrices

A number of articles on other fiber-reinforced matrices were published. H a y e P discussed the advantages of thermoplastic composites over those of thermosets, including simpler formulation, better shelf life, and resistance to damage. He also reported on trends in these materials and their prospects for the future. Lauke et al.59 discussed theoretical considerations of the toughness of short fiber-reinforced thermoplastics. Gennaro and Castoldim studied the anisotropy of the flexural properties of short glass fiber-reinforced polyamide 6 in single- and double-gated specimens under dry and wet conditions. Bian et a1.61 investigated the effect of water absorption on the stress transfer- ability across the E-glass fiber/nylon 6 interface using the embedded single-fiber composite technique. Only a slight permanent deterioration of this property was observed in the treated samples. VoelkeF2 discussed the low-temperature impact properties of long fiber thermoplastic composite molding materials, including nylon 6, polypropylene (PP), PET and polystyrene- maleic anhydride. The fiber-dominant performance of the long fiber materials was evidenced by increasing impact values with increases in fiber content. No ap- parent ductile/brittle transition was observed for these compositions.

Additional articles dealt with other fiber-reinforced polymers. Janzen and Ehrenstein‘j3 characterized short glass fiber-reinforced polybutylene terephthalate (PBT) under dynamic loads. The authors concluded that from the characteristic values of fatigue crack propagation it was possible to infer the fatigue behavior of molded parts. Bowen and Johnson@ discussed long fiber-reinforced thermoplastics for the injection molding process. DucketP investigated short fiber composite materials. TermoniaM studied the tensile strengths of discontinuous fiber-reinforced compos-

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ites. Har~eI-6~ measured the compressive behavior of glass-carbon fiber hybrid composites. Peterson et a1.@ determined the static and high strain rate response of a glass fiber-reinforced thermoplastic. Hornberger and Mal10y~~ evaluated the mechanical properties of glass fiber-reinforced polycarbonate (PC) foam. Stiffness increased with increasing glass content and decreased with reduction in density. Quinn and Beal170 studied blends of low melting glasses with polyetherimide (PEI) and polyethersulfone (PES). The glass-polymer blends were injection moldable and showed increased modulus and thermal stability.

Chemical Resistance

Two articles contained information on chemical resistance of composites. Yeager et al.7i reported the effects of various automotive fluids on glass or carbon fiber-reinforced PPS, PES, PEI, PBT, and nylon 66. The superior resistance of long fiber-reinforced thermoplastics vs. conventional fiber-reinforced materials was demonstrated. Khan et al.72 described a fluorocar- bon resin reinforced with carbon fibers for resistance to corrosive environments. The long fiber-reinforced composition had better creep and mechanical properties than short fiber or particulate-filled fluorocarbons.

Orientation and Fiber Distribution

Pipes et al.73 developed a constitutive relationship for an ordered assembly of discontinuous fibers in a viscous fluid. The effective viscosities were functions of fiber aspect ratio, fiber volume fraction, and matrix fluid shear viscosity. The models may serve as a guide in establishing the influence of the fiber assembly geo- metric parameters and effect of the fluid properties on the anisotropic viscosities of the assembly. Schwartz et al.74 measured the spatial orientation of short fibers in injection-molded specimens of reinforced nylon 66. Polished cross sections were examined using a scan- ning stage and special image analysis software. A model was used to calculate elastic properties and thermal expansion properties. Calculation of thermal warping was an application of the study. O'Connell and D ~ c k e t t ~ ~ described a technique for accurately measuring the spatial variation of fiber orientation av- erages in injection-molded plaques of short carbon fiber-reinforced PEEK composites. Details are given of the estimated uncertainties of the orientation aver- ages to establish that the technique has sufficient accu- racy to characterize quantitatively the skin-core-skin structures of the composite. Thiery and San~chagr in~~ studied fiber orientation and microvoid content. Fiber orientation was influenced by fiber content, injection rate, and sample thickness. The importance of each

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parameter could be evaluated by image analysis instru- mentation.

Knott and H e r a k ~ v i c h ~ ~ developed analytic expres- sions for four of the five effective elastic constants of a transversely isotropic composite material containing circumferentially and radially orthotropic fibers. The composite properties were compared with those hav- ing isotropic fibers. The fiber morphology had a signifi- cant effect on the composite properties and stress distributions in a composite cylinder. Dzenis and M a k ~ i m o v ~ ~ determined the effects of fiber length distribution and orientation of short fibers on the dynamic viscoelasticity of a polymer composite. Shanker et al.79 discussed the influence of nonhomogeneous flow fields on the orientation distribution and rheology of fiber suspensions. The Stokes-Burger model was used to approximate the forces and moments on the fiber for the calculation of fiber angular velocities for nonhomogeneous cases where the velocity gradients vary over the fiber length.

Other Factors Other aspects of composites have been reported.

For example, Benveniste and Milohso wrote about the effective thermal conductivity of coated short fiber composites. Aboudis' determined the microfailure criteria for coated fiber composites. Wendt and Hornigg2 investigated the damage response of short glass fiber- reinforced thermoplastics. Slattery and Hacketts3 did a computational simulation of the creep rupture process in filamentary composite materials.

The properties of carbon fiber-polymer interfaces were discussed by Liao and Tung.&l Sorina and Gunyaevg5 studied the rate dependence of the strength of carbon fiber-reinforced plastics. Fang and Chung86 investigated the addition of tin-lead alloy particles for improving the fatigue resistance of carbon fiber composites. HarpeP7 reported the compressive behavior of glass-carbon hybrid composites. The friction and wear behavior of graphite fiber-reinforced composites were measured by Shim et a1.88 Brockmuller et aLg9 discussed the effect of countercomponent roughness on the wear behavior of unreinforced PEEK, as well as composites made with carbon or short glass fiber reinforcement.

Fan et a1.90 examined interfacial effects on stress distribution in model composites. Fiber-matrix interface property determinations were done by Marshall and Price.9i Zhu and A c h e n b a ~ h ~ ~ examined the effect of fiber-matrix interphase defects on microlevel stress at neighboring fibers. Mader and Freitag93 discussed interface properties and their influence on short fiber composites. Miller and T ~ r q u a t o ~ ~ investigated the effect of polydispersity of the fiber radius of the elastic

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and transport properties of fiber-reinforced composites. Hamoush and Salamig5 proposed an analytic model for predicting the ultimate tensile strength of fiber-reinforced composites when the failure is gov- erned by fiber debonding. Aboudi% predicted microfailure of the strength of composite materials under combined loading. Fisher et aLg7 studied failure characteristics in a thermoplastic composite material at 121°C.

Sell and Lepeniotisg8 compared two commercial software packages for designing fiber-reinforced for- mulations of two crystalline thermoplastics and gave details of the advantages and disadvantages of each.

pointed out that in many applications a small amount of a specialty silicone can dramatically improve processability, impact strength, and temperature limits of a composition. Pape and Plueddemannlm reported that silane coupling agents can improve mois- ture resistance and high-temperature properties and reduce the effects of degradation during processing of reinforced polymers.

White et

Property Data Bases

Breuer et a1.I0' reviewed the early stages of develop- ment of the CAMPUS data bank and gave extensive details of its organization and functions. In a later article, WeisslerIo2 discussed the 3 years of experience with the CAMPUS data bank and made several cau- tionary statements: There is a danger in the careless use of characteristic data; it is necessary to analyze thoroughly the requirements for each molding; and there is a need for determining the relationship between the properties of the test specimen made from the molding compound with the specific performance requirements of the molded part. The author pointed out the importance of processing effects on mechanical and thermomechanical properties in particular. The effects of anisotropy in both unreinforced and reinforced plastics cannot be accounted for in the data base.

Ahari et a1.Io3 wrote an article on an intelligent system for plastic materials selection. They pointed out that many computer data bases are used without con- sideration of any inference from the knowledge. Con- sequently, guidance by a materials expert is still necessary. For a material selection to be applied solely by a design engineer, one must be able to access and ma- nipulate a vast amount of data using informed expert logic intelligently. The authors described the application of an intelligent knowledge-based system software package with a data base of plastics. Michaeli and Mohr-Matuschekl" developed material models and a data base interface for finite element analysis.


INJECTION MOLDING

Molding Process

Review

Richardson and LatifIo5 reviewed the injection molding process, discussing the process, mechanism governing the material output from reciprocating screw machines, and the effects of processing conditions on properties of the molding (60 references).

Processing

Truckenmuller and Fritz'" compared extruded and pultruded materials with direct addition of roving strands in the injection molding of long fiber-reinforced thermoplastics. This method has the advantage of eliminating the compounding step now used with short and long glass fiber-reinforced plastics and the attendant heat history of the polymer. Positive results were achieved with state-of-the-art techniques. Blohm et al. lo7 discussed factors affecting the processing behavior and mechanical properties of glass fiber-reinforced nylon 6. Friesenbichler et al.loS reported on a method of injection molding without holding pressure. Rapid filling and compression are followed immedi- ately by constant volume cooling of the melt in the sealed mold cavity up to the moment of ejection. Ad- vantages of the method are listed. Boey'Og described the reduction in void content and its variability in polymeric fiber-reinforced composite test specimens using a vacuum injection molding process. BozzeliIlo discussed a systematic molding procedure coupled with ASTM testing methods for providing valid property and processing comparisons among resins. Data were presented showing excellent agreement on 5 different resins in 12 ASTM tests.

Cooper"' wrote about process analysis, a technique of process management that reportedly provided sig- nificant improvements in manufacturing capability and productivity. Data derived from continuous recording of machine and process variables can be used in devel- oping control capability parameters. Efner et a1.112 performed a critical analysis of the cycle time, ease of flow, and part dimensional stability of five glass fiber- reinforced thermoplastics. Lawrence et al. 'I3 discussed thermal and skin-core effects in the processing of thermoplastic composites. Farrell et a1.'14 carried out an energy audit on an injection molding process and found that it takes nearly eight times as much as an "ideal" process, most of the energy being used for cooling requirements. Davis and Hudson"s described the nature and duration of variations in cavity conditions and part weight following a process interruption

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or pressure adjustment. Data analysis for trends, oscil- lation, and process stabilization were presented. Burke and Malloy116 published an article on the forces required to strip moldings of both amorphous and semicrystalline polymers from the mold core. Ejection forces were affected by the quality of the mold surface, molding conditions, tool geometry, and material processed. Galic and MausiI7 developed an injection mold design and process in which the cavity remains closed to airborne contaminants, ejecting the part by means of a movable core, thus maximizing mold and part cleanliness. The method is suggested for molding optical and medical applications. Zippmannli8 dis-. cussed injection-molded parts made of thermoplastics and thermosets with long fiber reinforcement.

Yokoi et al.Ii9 described a method for visualizing flow patterns along the cavity thickness consisting of magnetizing a mixture of resin and magnetic particles as it was injected into the cavity at the gate. The parts were sectioned to observe the magnetic patterns. Three-dimensional measurements of flow patterns have been achieved. HobsonIzn reported that constant material viscosity can be achieved at very high shear rates, resulting in increased mechanical strength owing to a reduction in orientation stress in the part. Briedis and TeterisIz1 predicted the non-Newtonian viscosity of a melt of a polymer composite with a hybrid filler. WieczorekIz2 indicated that automated material feed contributed to improvements in the flexibil- ity of the injection molding process. Hertlein and Fritztz3 made on-line measurements of the degree of dispersion of pigment concentrates during the compounding process using a dynamic rheometer in con- junction with filtration and pressure.

Fiber Breakage

Kottyan and RosenthalIz4 studied the effects of injection molding equipment and process variables on fiber attrition and performance of a long glass fiber- reinforced amorphous thermoplastic. The check ring was one of the critical variables. Screw compression ratio had less effect. Detailed fiber length analysis and property evaluations were given. Wu et al.IZ5 compared the behavior of short and long fiber-reinforced thermoplastics. They identified key variables controlling fiber breakage as the gate dimensions and injection speed. Thieltges and Michaeli126 determined the effects of processing on fiber length. The fiber fracture mechanisms observed were caused by mechanical breakdown, shearing of protruding fiber ends, shear stressing, and fiber-fiber interaction.

Shrinkage and Warpage

S t i t ~ I ~ ~ reported the agreement between predicted amounts of shrinkage and warpage in injection-molded neat and glass fiber-reinforced PBT. The author commented that new developments make possible the computation of fiber alignment so that distortions due to fiber orientation can be analyzed with more cer- tainty. Bea11128 discussed this subject and stressed the need for uniform wall thickness. Pierick and NolleriZ9 studied the effects of processing conditions on shrinkage. Shrinkage was directly related to cavity pressure and flow direction at any particular point in the mold. Cavity pressure was influenced by hold pressure and melt temperature. Delbarre et al.I3" investigated the effects of holding pressure and melt and mold temperatures on the shrinkage and warpage of parts molded from fiber-reinforced and talc-filled PP homopoly- mers. Analysis of microtomed layers revealed a com- plex distribution of internal stresses owing to hetero- geneous shrinkage. Davidoff et al.131 used the finite element method for stress analysis, flow simulation, and mold cooling analysis. Knowledge of the part thermal history makes possible warpage and residual stress calculations.

S e ~ e " ~ utilized shrinkage vs. time plots to predict whether the molding process required adjustment. The method reportedly could be used to quantify the effects of process changes and material selection on the final part dimensions. J a r ~ s ' ~ ~ prepared glass-reinforced PBT specimens using a specially designed mold under varying process conditions. Shrinkage values, measured at different locations, were compared with published linear flow shrinkage data. W a l ~ h I ~ ~ published an article on linear and nonlinear buckling analysis of an injection-molded PP crate. Liou et al.I3' reported a study of sink marks in injection-molded parts. Potsch and M i ~ h a e l i ' ~ ~ discussed the prediction of linear shrinkage and warpage of thermoplastic injection moldings.

Chiang et a1.13' developed an integrated filling, post- filling, and mold cooling analysis system that was in- terfaced with a nonlinear stress analysis program to predict shrinkage, warpage, and sink marks in injection-molded parts. Ross et al.13* described melt viscosity monitoring as a measure of melt quality in injection molding. Brouwers and P ~ p p e I ~ ~ discussed the mea- surement of the internal pressure in the mold while processing semicrystalline polymers. The authors commented on the proper placement of sensors and compared their measurements with those derived from computer simulations Yu et al.14n wrote about thermal contact resistance in injection molding. Dontula et

31 0 VOL. 11, NO. 4


al.141 studied the effects of screw speed, back pressure, shot size, and polymer viscosity on melt temperature during plastication. Measured temperatures were in qualitative agreement with a first-order model of the process.

foam molding, and gas counterpressure techniques. The authors described machine and mold requirements, advantages over injection molding, and areas of application.

SUBSEQUENT REPORT ALTERNATIVE MOLDING METHODS

Gas Injection Molding

Shah142 published an article on current practices in gas injection molding and provided information on process and apparatus details, as well as advantages and limitations of the process. Anders and S a ~ e r ' ~ ~ discussed the effects of process parameters on the quality of the molded part and also the influence of the polymer selected on wall thickness and surface quality. Shah and H l a ~ a t y I ~ ~ used this process for molding a thermoplastic composite window guidance channel that required dimensional stability, a high strength-to- weight ratio, no warpage, and low wear. The authors discussedpart and tool design and optimization of gate design and process parameters.

Other Processing Methods

Burkle et al.i45 described the Alpha 1 plant, which can carry out two-component injection molding, combined compression and injection molding, gas injection molding, structural foam molding, and in-mold coating. This makes possible such applications as complete moldings for automobiles, e.g., front ends, hoods, doors, and structural components, as well as other large items for the building industry. The chief feature of the plant is that different forming and shap- ing techniques can be carried out in combination. With this equipment, control of flow and orientation of fi- brous reinforcements is possible. Braunlich and Jo- h a n n ~ o n l ~ ~ evaluated three methods for producing thick-walled plastic moldings-gas injection molding, two-component injection molding, and encapsulation of an insert-and gave reasons for choosing the latter method. W i g o t ~ k y I ~ ~ discussed innovations in a number of processing methods such as injection-compression molding, extruder screw modification, vacuum compression molding, blowmolding, and low-pressure injection molding, and cited specific examples in which these processes are used. Another articleI4* covered the performance advantages of alternatives to injection molding. Eyerer and B ~ r k l e l ~ ~ reported on alternative processing methods including injection stamping for flat moldings, flow casting, structural

Part I1 of this review is in preparation and will include the following subjects: injection molding machines and screw design; process control systems; modeling; computer-aided product design; mold design and manufacture; product manufacture; characterization techniques; and applications.

ACKNOWLEDGMENT

This work was carried out under a contract of Stevens Institute of Technology with the Great Lakes Composites Corporation and was supported by the U.S. Navy Manufacturing Technology program. The Polymer Processing Institute/New Jersey Polymer Extension Center is a subcontractor in this program.

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Advances in injection molding of fiber-reinforced thermoplastics during 1991—Part I: Materials and processing