Advances in injection molding of fiber-reinforced thermoplastics during 1991—Part II: Processing, design, and manufacture

SPECIAL REPORT

Advances in Injection Molding of Fiber-Rein forced Thermoplastics During 1991 -Part 11: Processing, Design, and Manufacture PAUL G. KELLEHER New Jersey Polymer Extension Center at Stevens lnstifute of Technology, Castle Point, Hoboken, New Jersey 07030

ABSTRACT This article is the second part of a survey of developments in the open literature during 1991 on subjects related to the injection molding of fiber-reinforced thermoplastics. Topics discussed include injection molding machines, screw design, process control systems, modeling, product design, mold design and manufacture, characterization techniques, and applications of composites. Although these technological advances are broadly applicable to the processing, design, and manufacture of thermoplastics generally, they are essential to the enhanced engineering, fabrication, and utilization of fiber-reinforced thermoplastics.

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0 1993 John Wiley & Sons, Inc.

I Introduction

his article is the second part of a review of the T open literature during 1991 and late 1990. Part I appeared in an earlier issue.] This part of the review covers the subjects of injection molding machines, screw design, process control systems, modeling, product design, mold design and manufacturing, product manufacture, characterization techniques, and applications of composites.

I Injection Molding Machines

Wilde9 reported on newly available injection molding machines. Specific machines and manufacturers are cited along with details of the various special features of each. Statistical data on machinery sales by both domestic and export manufacturers have been p~bl i shed .~ Brunner" considered the limiting values of technological parameters in relation to the plasticizing units of injection molding

Advances in Polymer Technology, Vol. 12, No. 3, 297-311 (1993) 0 1993 by John Wiley 8 Sons, Inc. CCC 0730-6679/93/030297-15

ADVANCES IN INJECTION MOLDING II

machines. Klein and Bichle? developed a fast cy- cling machine capable of molding parts with wall thicknesses of 1 to 1.5 mm in multicavity molds. The authors achieved increases in output, used less material, and obtained narrower weight and dimensional tolerances. Information was given on firms offering new choices in moderately sized (112 tons or less) machines.6 This development is a response to the demand for small parts, particularly technical components as well as short runs necessitated by just-in-time production. Mapleston’ discussed the current status of hybrid injection molding machines which combine hydraulic and servo-electric drives and mentioned specific machines and sup- pliers.

Several articles appeared dealing with large injection molding machines. Siegert and Fullers developed a new clamping unit for large machines. Details were provided on its advantages as well as the CAD method used for this development. Eck- ardt et al.9 stated that the development of large machines has been driven by the automotive and large container markets. Large units now have clamping forces of 15,000 kN and higher. The authors covered the choice and decision-making criteria and also features of the clamping unit, hydraulics, and control systems. Wilderlo described one machinery supplier’s expansion into the market for large machines and multicomponent equipment, including robotics and hot runner designs. Details on machine types and sizes were given. Another article” dealt with a supplier’s 2000 ton machine weighting 321,000 pounds.

Urbanek et a1.12 discussed the advantages of molding machines without tie-bars. These provide easy accessibility to the mold mounting area resulting in an improved facility for automation and better utility of large molds in relation to machine size. Operational experience has been positive with the machines now in use. Another articleI3 described a line of standard machines without conventional tie-bars.

PallerI4 studied adhesive wear between the screw and barrel. The author evaluated com- pounded plastics with regard to load carrying capacity; the behavior of the melt film and its effect on wear; and the selection of wear resistant materials for the screw and barrel. He also determined the fundamentals of the tribological system consisting of the screw flight and barrel.

Screw Design

Sepe15 designed an advanced screw which improved melt uniformity in injection molding. The study, based on long term production runs using both conventional and advanced screws, showed the latter’s effectiveness for both amorphous and semicrystalline thermoplastics. Amano and Ut- sugi16 designed 12 screws and used them to inves- tigate melt temperature uniformity and plasticating rate. Based on the data obtained, the authors designed a new screw with a wide barrier flight zone and a deep channel, which achieved better melt performance and doubled plasticating capacity.

I Process Control Systems

CONTROL SYSTEMS Rogers” discussed the two competing protocols

for the standardization of electronic communica- tions between computers and processing equipment. These are the U. s. Society of the Plastics Industry (SPI) protocol and the Switzerland based European Committee of Machine Manufacturers for the Rubber and Plastics Industry (Euromap) protocol, both developed separately. The SPI has 32 licensees. Euromap 17 has been ratified by nine nations and is reportedly the less complex of the two. Konzelmann18 covered the subject of operational data gathering (ODG) as applied to injection molding. This system exposes weak points by providing detailed information about the overall production cycle and obliges machine operators and production managers to follow up, record, and analyze the cause of every disturbance in the process. Quality control tests on the moldings can be put into the ODG system and linked to process data resulting in more effective statistical process control. KreisherI9 described a system which reportedly processes data 20 times faster than microprocessor based controls. It has been adapted to controlling the injection molding process resulting in a number of listed advantages. In a general dis- cussion of control technology, Wilde9O pointed out that tighter dimensional requirements and just-in-time manufacturing require microprocessor

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ADVANCES IN INJECTION MOLDING I1

controllers to achieve machine repeatability. Some of the developments mentioned were the use of piezoelectric sensors rather than strain gages, non- contacting ultrasonic path measuring systems, in- line rheometers, and closer control of clamping times in high speed operations.

Urquhartzl reported on hydraulic and electronic advances in injection molding machine controls which have resulted in faster response, greater data storage and manipulation, and better repeatability. Examples were given along with effects on molded parts. Srinivasan et al." considered two classes of closed loop control applications. The first controls part weight and thickness. Procedures for this control algorithm design resulted in more effective control. The second class controls processing variables such as nozzle and cavity gate pres- sures during injection. The authors compared simulation and experimental results. HarryB performed experiments to determine which of the injection molding response variables best predicted product characteristics and concluded that cavity pressure demonstrated the strongest relationship to the molded properties. Wu et al.24 described an expert system for aiding the setup and adjustment of injection machine parameters consisting of a data acquisition system, programs for extracting and identifymg pressure patterns, and a cycle to cycle statistical process control program which permits the diagnosis of most molding problems.

Daca and K0glel.2~ provided a detailed comparison of analogue vs. digital temperature controllers in plastics processing, presenting the state of the art and development trends in this field. Billman and Bergma+ designed, implemented and provided details on an adaptive temperature control concept which met the requirements of reduced machine cycle times, robustness, simplified operation, and adaptation to process instrumentation.

Kommaye~?' discussed microprocessor controlled mixing and metering for the incorporation of colorants and other additives to plastics. The author pointed out the advantages and gave several examples of automatic material supply systems.

ON-LINE MEASUREMENTS Wang and Wangz8 developed a process control

system based on a hybrid scheme of on-line data acquisition and simulation using a simplified P-V- T thermodynamic model. Calculations for on-line

simulation, experimental verification, and model based process control were provided. Wiegmann and Oehmkez9 compared direct and indirect methods and measurement principles for obtaining P-V-T data for polymers. Direct methods were slightly more advantageous. Differences in methods or measurement principles produced measured values not exceeding 4%. Fakhreddine and ZolleP discussed the advantages and disadvan- tages of different P-V-T techniques which provide data related to the equation of state of the solid and melt as well as the transition behavior. Data were given for PMMA and PBT.

Anders et al.31 reported on the use of an automatic on-line method of measuring filling pressure during molding. This procedure was a means of directly evaluating the ability of a melt to flow during the injection molding process. It was more advantageous than other characteristic values for melt viscosity. In a subsequent article,3z these authors commented further on this method and pointed out that in combination with a proper data acquisition system, it is an effective tool for quality assurance. Bader et a1.% utilized piezoelectric length-sensor plugs in the cylinder head of the injection molding machine to measure the stresses and strains arising from melt pressure. Material flow characteristics can be determined with this system during production. Kamal et a1.34 measured heat transfer coefficients for convective heat transfer between the mold surface and the polymer melt using heat flux sensors at different posi- tions in the mold cavity. Amorphous and semicrystalline polymers were evaluated. The values obtained showed a negligible effect of melt temperature at the nozzle on heat transfer coefficients for both types of polymers.

Modeling

CRYSTALLIZATION Hsiao et al.35 used a crystallization kinetics

model to analyze the isothermal crystallization of semicrystalline matices in advanced polymer composites including PEEK, PEKK, and PPS reinforced with carbon fiber. The authors discussed the effect of fiber on total crystallization kinetics as well as the relative volume crystallinity of different crystallization stages. Cebe and Chung36 utilized a

ADVANCES IN POLYMER TECHNOLOGY 299


model to explain the behavior of PPS crystallized over a wide temperature range from the melt or from the rubbery-amorphous state. The model may also be applicable to PEEK. Pate1 and S p r ~ i e l l ~ ~ studied the Nakamura and Kamal models for predicting nonisothermal crystallization of nylon 6 from isothermal data. Hsiung and Cakmak= originated a model for simulating the crystallinity gradients developed during the injection molding of a slowly crystallizing polymer such as PPS. The study included calculations of temperature, velocity, and pressure distributions as well as effects of mold temperature, injection flow rate, and holding time. Friedl and M~Caffrey~~ advanced a model for predicting crystallinity levels in injection molded parts and obtained good correla- tion with experimental results. The method is applicable to many semicrystalline polymers.

RESIN F'LOW AND MOLD FILLING A text on the modeling of polymer processing40

covered recent advances in the field. Major em- phasis was give to simulation of kinematics and dynamics of flow processes in various polymer processing operations. Axtell and Haworth4* wrote an article on a practical verification of a computer simulation for injection molding applied to amorphous thermoplastic polyesters. White" wrote about the state of the art in computer-aided engineering, particularly the subject of molding simulation. H a u e showed that jetting can be predicted and corrected by using flow analysis software. Papathanasiou and KamaP reported on modeling the flow of viscous materials in complex cavities as applied to the filling stage during molding of semicrystalline thermoplastics through the use of boundary fitted curvilinear coordinates. Chiu et al.45 developed a nonlinear mathematical model for the mold filling process during injection molding. The comparison between experimental results and the model was a reasonable representa- tion of the mold filling dynamics for an ABS in- jected into a disk shaped mold. Chiang et al. published two studies on the filling and postfilling stages of the injection molding process. The first46 described the theoretical model utilized; the sec- ond47 compared the numerical simulation with experimental results obtained in instrumented test molds. Chiu et a1.& provided an analysis of the filling, packing, and cooling stages during injection molding in disk cavities. It was pointed 0ue9

that mold filling analysis is effective in reducing defects and lowering costs.

Weissmanso reviewed the use of mold filling simulations over several years, covering various reasons why simulations are performed, and pointed out areas where improvements are needed. RoeS1 discussed a validation study to determine the accuracy of a commercial mold filling software package in predicting the pressure profiles during injection molding of ABS using a large plaque mold over a range of processing conditions. Favorable agreement was found between predicted and experimental pressure profiles. Sajan52 examined mold filling analysis to determine the accuracy and economic advantages and disadvan- tages. Two interpretations furnished similar con- clusions. The experiment showed that mold filling analysis can warn of unforeseen injection molding conditions if implemented in the design phase. Friedl and Sherbelis= described a set of consistent procedures required to venfy properly an injection molding simulation. Lee et al.% used an iterative boundary pressure reflection method for the simulation of injection mold filling. Lee55 described an improved flow analysis network method for irreg- ular geometries.

Hieber et a1.% simulated the flow during filling and postfilling of a center gated disk. Comparisons were made with experimental pressure traces in the cavity as well as with the radial dependence of measured gapwise shrinkage. Strong and weak aspects of the simulation were discussed. Nguyen and KamalS7 performed a two dimensional analysis of the packing stage. In addition to the velocity and pressure distributions, the model predicts the stresses in the planar direction, which could be used to calculate residual stresses. Palit and Chen% compared Computer-Aided Engineering (CAE) software with experimental results relative to pressure vs. time history during filling and packing of the mold cavity. The outcome was influenced by mesh size, tightness of convergence criteria, and type of viscosity and PVT models.

MOLD COOLING Chen et al.59 developed simulations of the pack-

ing and cooling processes which take into account the effects of the channel layout and coolant flow rate. Analyses of shrinkage and warpage were done on the basis of the temperature and density profiles obtained. Chen et a1.60 presented a detailed article on hybrid methods for injection mold

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cooling system analysis. Temperature profiles of the polymer melt were calculated by coupled and decoupled methods. Examples illustrated the opti- mization of mold cooling operations by applying the three hybrid methods. Dietzel et a1.61 published a procedure for calculating the cooling and sealing times for an injection molding tool.

FIBER-REINFORCED MATERIALS Ha et a1.62 discussed the modeling of the vis-

coelastic behavior of fiber-reinforced thermoplastic matrix composites at elevated temperatures. ChoW63 used a model to predict stress-strain relationships in polymer composites. Fried1 and Brouwel.61 utilized a numerical simulation of the development of fiber orientation in injection molding to predict mechanical properties. Wagner et al.65 investigated thin polymeric films, in which single fibers were accurately positioned, as model composite materials. This approach allowed the control of experimental parameters and the possi- bility of verifying theoretical models in the range of low fiber content. Sun and ChertM advanced a micromechanical model for the plastic behavior of fibrous composites. Eduljee et al.67 outlined a micro- mechanics theory for predicting the elastic and thermoelastic properties of dispersed, short fiber systems. The theory was applied to the prediction of macroscopic thermal residual stresses in a PEEK-glass fiber system in an effort to determine the influence of crystallinity on residual stress development. Matsuoka et al.@ developed injection molding analysis programs related to mold cooling, polymer filling-packing-cooling, fiber orientation, material properties, and stress analysis. The programs were integrated to predict warpage of molded parts and showed good agreement with experimental results.

I Product Design Including Computer-Aided Design (CAD) and Computer-Aided Engineering (CAE)

Rao69 has published a text on design formulas for plastics engineers. Included are formulas for part design as well as those pertaining to rheology, thermodynamics, and heat transfer. The objective is to provide access to the formulas on which

mathematical models are based in order to improve existing or new computer programs. Aus- t in70 reported results of a recent survey of injection molders' experience with CAE. Those firms which have used CAE most successfully used a concurrent process in which the product designer, mold builder, and molder participated together in material selection and part and mold design to take advantage of CAE technology. Analysis of various parts were provided as examples. Leonard7* considered the increasing use of CAD/CAM technology and pointed out that its use promotes faster introduction of products, reduces development costs, and improves product quality. Specific firms using this computer technology are referenced. Prototype software has been developedn in which the interactive concurrent engineering software (ICE) allows high resolution graphics output from a CAD program to be simultaneously viewed and manipulated from several locations. Martino" reported on the development of software which permits electronic linkage of diverse functions such as product design, construction of models, production of prototypes, flow analysis, mold design, and machining of tool steel. Turner and Dean7* discussed criteria in determining mechanical properties for design. The authors dealt with quality of data, different standpoints of data generators and data utilizers, the avoidance of a proliferation of data, and possible styles of data storage, particularly as related to modulus and impact resistance. Chivers et al.75 devised a method for predicting stiffness properties for use in the design of engineering thermoplastics. Bamkin and Piear~ey~~ wrote about the nature of material selection within the design process. The authors concentrated on a component as a working part of a larger assembly, and modeled it by using object-oriented software techniques. Papers presented at the twelfth POLY- MODEL Conference" have been published. Top- ics comprise general surface modeling, lubrication and wear, geometric surface modeling, and statistical modeling. Crippa and G e n n a r ~ ~ ~ reported on the design of injection molded parts considering fiber reinforcement orientation. Tobinn commented on ways of avoiding product failures by design.

Lee and GreineP wrote about methods such as laser digitizing and computed X-ray Tomography which can be incorporated into standard CAD systems to create 3-D descriptors of objects. WildeP discussed the development of a software program for the design of one-way, two-way, annular, and



torsional snap-fit applications. The program can accommodate various plastics and can analyze more than 100 options without the need for finite element analysis. The program reportedly simpli- fies selection of geometries, cross sections, beam lengths, force deflection, and material choice. SarabiS2 dealt with the problem that high performance plastics are often used to the limits of their load bearing capacity. Thus, material behavior under every condition of stress must be known. The author covered rapid static loading, static fixed loading, and impact loading and commented on the need for materials testing to determine the characteristic properties relevant to design. Beaum gave an example of problems encountered with a plastic microscope slide designed with nonuni- form wall thickness and the steps involved in pro- ducing acceptable parts.

RosatoM presented an overview of the many design features that influence part performance, particularly detractors or constraints which are re- sponsible for failures. By taking these constraints into account, unwanted stresses are avoided and reliable parts are produced. LeonardE5 gave a number of examples taking into account the solid waste problem. Some of the designs incorporate easy dis- assembly to facilitate economic recycling. Parts consolidation, source reduction, and recycled plastics are advocated. Friedmans6 promoted the concept of designing for recycling as well as the use of recycled plastics. HubeP’ considered the qualita- tive aspects of design, especially aesthetics such as style, function, meaning, value, form, and visual organization. Curass examined the design process for melt processable plastics materials and empha- sized that knowledge of part design, materials, and fabrication are necessary in each of the three design process stages, viz. preliminary, engineering, and manufacturing. Sierodzinski and Weiss- mannS9 wrote about the criteria for part design and material selection, contrasting the bases of specifi- cation sheet data vs. those data obtained on actual molded parts. The authors demonstrated the effects of molding conditions, material flow characteristics, part design, and gating through the use of a number of case studies. Jarosgo investigated the stresses and deflections that are a result of the anisotropic properties of a nonlinear part. Data were obtained on samples molded under a matrix of process conditions and a computer model was prepared. Linear analysis was compared with finite element analysis of the molded parts.

Mold Design and Manufacturing

Thienel and Schmidtg1 discussed the mechanical design of injection molds. The authors compared different programs for mold design, studied the deformation of an injection mold, and evaluated and compared calculated and measured results. They concluded that the use of programs for di- mensioning molds, linked with the experience gained with related molds, afforded the best success in achieving the desired quality and long ser- vice life. Nollerg2 dealt with designing for tight tolerances with glass fiber reinforced plastics. The author covered material properties such as anisotropic shrinkage and processing variables, as well as design details of core pins, slides, side actions, and mold gating. Kisch et aLg3 described a tooling project in its various stages, viz., prototyping using 3-D based product designs, flow simulation, CAD mold design, mold start-up, and debugging. Examples were used to show that this approach reduces time to market.

Urquhartg4 discussed integration of the mold with the injection molding machine, covering factors such as mold design, mold cooling, ejection, alignment, access for maintenance, balanced filling of cavities, and compatibility with the particular machine. In a detailed report on the status of the mold making industry, Hofmanng5 pointed out that the mold designer could be more efficient if the customer provided complete drawings and the desired tolerances and specifications for mechanical, thermal, and chemical loads on the part rather than sketches and incomplete calculations. Mackay% reported on several types of specialized mold designs including multimaterial molds, molds with unscrewing mechanisms, specialized ejection methods, and in-mold handling de- vices. Tarahomig7 analyzed the use of collapsible cores, their capabilities, limitations, and types available.

Nedess and Kosing8 considered the benefits of mold standardization. Time and money could be saved owing to: reusability of data once generated, access to existing know-how in the form of CAD data, use of mold construction data already com- piled, and utilization of a uniform mold clamping system. Groleau% wrote that most chronic problems with injection molds result from improper tryouts and run-in procedures. The author covered

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specific requirements for a comprehensive mold tryout to insure that molds are capable of produc- ing good parts. Falkelm pointed out that tunnel gating has the advantage that sprue and molded part are cleanly separated immediately on demold- ing so that no finishing is required. This is difficult to accomplish with plastics having low elongation at break. The author described a design suitable for glass-reinforced thermoplastics. Zou and Pandeli- dislol optimized runner design and demonstrated their methodology by means of a complex indus- trial mold. Hubrich and Nachtsheimlo2 advocated the use of three-plate stack molds as an alternative to high speed machines. Advantages and disad- vantages were given. Variations in design, hot runner systems, and ejector arrangements were covered. Such molds reportedly result in lower costs per part owing to increased output. Ro- zema103 proposed a four-face stack mold and reported that output was quadruple that of a conventional single-face mold and double that of a two-face stack mold.

MATERIALS FOR MOLDS Tamey'" commented on material choices for in-

jection molds. The author presented information on the metallurgical basis for development of mold steel properties and compared conventional with recently introduced steels. Differences in wear resistance, toughness, and corrosion properties were given. Beng t s s~n '~~ discussed the development of two grades of stainless steel for mold making, particularly for the molding of electronic parts, large optical lenses, and medical applications. Gersonlo6 gave examples of the growing variety of mold steels and presented a rational basis for alloy selection. Rajendran'O' reported on the development of carbide bearing composite alloys for improving the wear resistance of molds. Some possible wear mechanisms were proposed. Bryce108 reported the use of aircraft aluminum as a replacement material for tool steel in production molds for plastic parts. This approach was taken to reduce the overall product development cycle. Wend1 and WupperlW studied the effect of microstructure of the steel grades used for tooling on their wear properties. The authors provided information on chemical composition, heat treatment, hardness, carbide content, and particle size, and their relationships to wear resistance, corrosion, and machineability .

OTHER FACTORS Thienel and Sass'lO provided a comprehensive

survey of steel surface treatment methods such as thermal, thermochemical, electrochemical, and gas phase deposition procedures. The authors treated these methods in detail, and gave the important process characteristics, achievable surface properties, potential applications, and limitations of each. Ritter et al."' discussed the use of an indentation technique for evaluating the protection afforded by polymeric coatings to a hard, brittle substrate such as glass. Ralston112 investigated the effects of injection molding polymer bonded magnets on gate insert wear. The thermoplastic-rare earth magnetic material was characterized rheologically and practical processing conditions were determined using computer flow analysis.

PROTOTYPING Ogale et al.l13 described the processing of dis-

continuous reinforced composites by 3-D photo- lithography. The study indicated the feasibility of composite production using polyacrylates. The method was reportedly suitable for applications in rapid prototyping without the use of molds or dies. Fidorra114 studied stereolithography for rapid prototyping and examined spray-on zinc alloy, sol- der alloys, ceramics, and rubber-epoxy for cores and cavities. The author described tool construction methods, approximate costs and mold life- times. It was reported115 that modification of the stereolithography process and computer program development improved accuracy, ease of use, and increased throughput.

Hicks1I6 evaluated the performance of prototype mold materials including aluminum filled epoxy, polyester casting compound, filled polyester, and polyester photopolymer by measuring the number of parts produced and the wear resistance. A glass fiber reinforced PBT compound was used as the test polymer. The aluminum-filled epoxy was rated best in performance. A case was reported117 in which prototyping time was markedly reduced by two firms working concurrently with compati- ble CAD/CAM software. One firm directed the design activity and nontooling decisions, while the other provided mold design, mold manufacture, and part production. By using finite element analysis, a firm was able to shorten the time to production by 50%. The prototyping step was eliminated



and a production tool was built directly with ap- parently successful results.118

WELD LINES Fisa and Rahmani119 studied weld line strength

in injection molded glass fiber reinforced polypro- pylene (PP). In the weld line zone the fibers were oriented almost perfectly in a plane parallel to the weld line formed by flow around a circular insert. Variables such as distance of flow, insert diameter, or lateral expansion had no significant effect on weld line strength. A simple model based on the assumption of complete debonding of the fiber- matrix interface when failure occurred could be used to predict the strength loss in the weld line. The same authors'" also designed a mold in which observations of the fiber orientation in and away from the weld line better simulated actual molded components and gave a more reliable estimate of weld line strength in multiphase polymer systems. Vaxman et a1.lz1 investigated weld line behavior in neat and fiber-reinforced PP and Noryl. The strength ratio of double-gated to single-gated spec- imens decreased with increasing fiber concentra- tion. Quantitative determination of the glass fiber orientation distribution within the weld line region was done by analyzing photomicrographs of pol- ished sections.

Lalandel" studied the weld line strength and structure of long fiber reinforced PP, evaluating structure in and away from the weld line zone and measuring mechanical properties, which were then related to the observed fiber orientation. Weld line strength was significantly less than ex- pected. Dharia and W o l k o w i c ~ ~ ~ ~ reported that the weakness of the weld line in short fiber reinforced PP was related to the mean fiber length, length distribution, and melt flow rate of the composite and not only to fiber orientation. Increasing the melt flow rate increased the retention of strength. Rahmani and Fisa'" examined the effects of planar, fibrous, and spherical filler shapes and geo- metrical mold parameters on the weld line performance and structure of PP. Variables studied were insert diameter, cavity thickness, and distance from the insert.

Krasne~ky'~~ used mold filling software to predict weld line appearance and recommended computer simulation of molding trials as a time saving procedure for the start-up of new molds. Lauten- bach et a1.lz6 used computer software to success-

fully predict the number and location of weld lines on two production molded parts. This can reportedly be done before the mold is built so that weld lines can be placed advantageously. Yokoi et al.'27 observed weld line formation and disappearance using a glass inserted visual mold capable of being operated under normal molding conditions. Ex- periments showed that the weld line disappeared at a certain meeting angle of two flow fronts particular to the molding material and regardless of most molding conditions.

Product Manufacture

COMPUTER-INTEGRATED MANUFACTURE ( C W Michaeli et al. published three articles on vari-

ous aspects of the manufacturing process. The first128 was an extensive analysis of a complex production planning and production control system which handles data management of the following functions: production program planning; management of quantities, e.g., purchase orders and stock; schedule and capacity planning; order execution release; and order execution monitoring. The authors discussed the requirements of the system to be met by the molded object and gave detailed cost estimates for such a system. The second article'29 dealt with a production planning system which can do the following tasks: handle a large number of quotations or orders; manage all products including injection molds and moldings; plastic materials; standard requisites and components; control of work and price lists; and monitor the progress of an order. Information on the details of the system were provided. In the third report,*30 the authors described a program for mold making and injection molding. The program took into account the vaned special requirements and problem areas in injection molding plants as well as the numerous details involved in mold design, manufacture, and maintenance.

Results of a market survey on commercial production data acquisition systems were presented by Michaeli et al.I3l A detailed comparison of the systems was made based on the following characteristics: acquisition of data on job status; machine parameters; manufacturing factors; quality control; time factors; material aspects; and mold status. Each of these categories was further subdivided

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into appropriate subcategories. The survey provided a comprehensive assessment of the commercially available production data acquisition systems. K~rnrnayer l~~ described an interactive central computer system in which was stored information on all existing molding machines, molds, materials, uncompleted jobs, and jobs com- pleted within a certain time. By providing an in- stantaneous display of the current situation, the program assisted in solving production problems. Herwe'% considered the question of whether the use of flexible production units is an economically feasible proposition. The author analyzed a plant which manufactured precision sealing rings and provided costs for injection molding machines, auxiliary equipment, robotics, and mold changers. The use of robotics increased production and reduced costs. Automatic mold changers were eco- nomical for small batch sizes. Kang et a1.IM discussed the measurement of processing variables in the manufacture of thermoplastic composites.

Holliday135 commented on the continuing evo- lution of CIM and pointed out that experience with these systems indicated that they can now be used and relied upon like standard machine controls and auxiliary equipment. Engelmann et al.136 described a CIM cooperative for injection molding. It was supported by the resources of educational in- stitutions, vendors and local industry to provide a facility for the use of local molders and the instruc- tion of college students.

KNOWLEDGE-BASED SYSTEMS Rogers137 commented on the current status of

artificial intelligence (AI) systems. A1 process controls and knowledge-based expert systems for control of the molding operation are in the develop- mental stage. Some programs are available based on information gathered from 50 molders over a period of 20 years. These are valuable for training new machine operators, since they were developed from the expertise of experienced molders. The author provided names of specific firms and programs. Bernhardt and Berta~chi '~~ developed a software program which identifies the cause of a molding problem based on the operator's answers to multiple choice questions, and suggests correc- tive action. The user can create data bases for specific molds and machines. An expert system for supplementing operator skills in dealing with injection molding problems was described by T.-C. Jan and K. T. O 'B~ien . '~~

Paulson140 presented information on a software package which allowed expert systems to be con- structed by the expert without the need for pro- gramming expertise. This reportedly permits rapid development of modules which can be integrated into the CIM software. Wang et al.14' described a three layer approach to the solving of potential processing problems, mold filling, mold cooling, and part performance based on several expert system modules. Hunkar and Mastine'" wrote about the development of neural shell programs for training operators to recognize and solve processing difficulties.

AUTOMATION K r ~ t h ' ~ ~ discussed robot linked injection mold-

ing and assembly operations. Robots are capable of performing the following tasks: feeding inserts into the machine; part removal; placing the molding in a cooling station; sprue removal; pressing an insert into place; placing the molding in a welding machine; functional and quality checks by a testing machine; placing good parts on a conveyor belt and disposing of rejects. Factors on the choice of a robot were examined. The various tasks done by a robot were described in detail. Knapp and Schmitz'44 stated that the most important problem in an assembly process is handling. They dealt with grippers for automated assembly, describing mechanical and vacuum types as well as varieties developed for flexible assembly.

M a l l ~ n ' ~ ~ reported that robots are used by only 10 to 20% of U.S. injection molders compared with 70% in Western Europe and 90% in Japan. The author commented on the processing advantages and economic benefits of robots and described the various types available. The use of robots reportedly promotes increased production, reduced la- bor costs, and consistent part quality. Costs of the robots described were in the $5000-$10,000 range. An article'& described robots having 1-5 axes with drive options including air, hydraulic, AC electric, and servo motors. Accuracies up to 0.004 inch were claimed with speeds as high as 13 feet per second and payloads of 55-550 pounds. Wagner147 wrote about robots used commercially in several segments of the industry including injection molding, assembly, measuring, and testing and gave details of their operation. W i t t m a n ~ ~ ' ~ ~ discussed safety aspects in the operation of robots. The author listed safety regulations for West Germany, Great Britain, Austria, Switzerland, and the U.S.



Specific safety precautions for the different types of injection molding operations as well as the pack- aging of molded articles were provided.

POSTMOLDING OPERATIONS Herkt-Maetzky'" reported on the laser marking

of plastics listing the advantages and applications. The author also commented on interactions between the laser light and the plastic, how color change reactions occur, development of a molding material for use with a specific laser, and the types of lasers which provide optimum results. K i l ~ ' ~ gave a brief review of laser marking technology and described the utilization of carbon dioxide lasers. Ways of overcoming the inherent insensitiv- ity of some plastics were covered.

Taylor151 described the construction of an ultrasonic welder for use with short glass fiber reinforced thermoplastics and discussed optimum welding parameters, process tolerances and repro- ducibility as well as the influence of fibers on the joining mechanism. Lambing et al.152 gave detailed information on the design, construction, and testing of an automated resistance welder for thermoplastic composites. Process conditions and data were compared with traditional resistance welding processes. Weld quality was determined by ultrasonic evaluation, mechanical tests, and micros- copy. Potente et al.153 experimented with computer-aided heated tool welding of thermoplastics having different flow characteristics. The authors describe a method for rapidly calculating the heat- ing parameters.

Majthan et a1.IM described the equipment and method used for ultrasonic staking of a plastic heater support and metal conductor strips in the manufacture of a coffee maker. The authors cite the advantages of ultrasonic staking vs. mechanical fastening or adhesive bonding. Smiley et al.155 disclosed a process for dual polymer bonding of thermoplastic composites and discussed resin compatibility, the effect of process conditions, and applications.

Dennis'56 reported on the machining of fiber- reinforced plastics with diamond tools which have high abrasion resistance. Electroplated diamond tools as well as polycrystalline diamond tipped tools were used to obtain high surface quality. The author gave details of operation and conditions of use. ZanderI5' investigated the use of a robot- guided water jet for precise cutting of fiber composite moldings. This device was reported to be

fast, accurate, pollution-free, and avoided the wear associated with conventional cutting tools.

Tobin et a1.'% presented mechanical and analytical data to show that ion implantation increased surface properties such as density, microhardness, resistance to wear and scratching as well as resistance to chemicals and etchants.

QUALITY ASSURANCE (QA) B ~ u r d o n ' ~ ~ wrote about the planning and opti-

mization of quality requirements in injection molded parts. The author considered the factors and dependencies in statistical process control. The relationships between process parameters and molded part properties as well as those involving the machine-process parameters and quality characteristics were discussed. Wortberg and Han- ninglm described in detail the concept of continuous process control in which each part is judged immediately on manufacture as to whether it meets specified requirements, thus assuring 100% on-line quality testing. The authors pointed out that the financial risks of product liability laws can be minimized by effective, documentable quality assurance. Ackermann16* reported that shipping of unsatisfactory moldings can be avoided by preven- tive QA through machine control systems in which centralized acquisition, evaluation, and documen- tation of the various machine parameters are used to control quality. Schmidt162 gave a comprehensive description of the development of a computer- aided statistical process control (SPC) system for a mid-sized company. Continuous use of the system increased quality consciousness and qualifications of production and QA personnel. Schwabl@ elabo- rated on the steps involved in a computer-aided QA program for injection molding and discussed processing problems, machine control, and software capabilities.

Wilder'" stated that improvements in QA are attainable by adding SPC, proportional hydraulics, quick mold changing equipment, and robotics to existing injection molding machines. Details on sources and types of retrofit equipment were given. Mikulac165 advocated the development of a "Continuous Improvement" environment coupled with SPC to improve QA. McDermott16 presented an approach to the training of line operators in the use of SPC and team problem solving techniques. Case studies were provided. HunteP7 gave examples of cost savings in transportation and distribu-

306 VOL. 12, NO. 3

ADVANCES IN INJECTION MOLDING If

tion functions resulting from a quality improvement program. Weis and Volgmanl@ described the use of infrared spectroscopy for measuring formu- lation components quantitatively as a method of on-line QA. Malberg and We~sl ing '~~ wrote about the utilization of combined instrumental methods for chemical analysis in compounding operations. Froitzheim and Michaelil'O dealt with the subject of computer controlled QA in the injection molding shop. Buchman and Isayev'" reported on QA studies of various thermoplastic composites.

Characterization Techniques

A great variety of methods have been reported for characterizing and testing composites. A collec- tion of 38 articles dealing with the fatigue and fracture of composite materials has been reviewed.In Fatigue crack propagation in short fiber reinforced thermoplastics has been characterized by Chow and L u . ' ~ ~ Failure processes in the fiber-polymer interphase were investigated by l3gg0tt.l~~ Kolle et al.175 reported on a micro/macro approach to fracture in composites. A volume on the nondestructive testing of fiber reinforced plastics composites has been p~b1ished.I~~ Agarwal and Broutman edited a text on analysis and performance of fiber composites. I n Acoustic emission analysis178 has been used by Bohse et al. to show damage in short fiber reinforced thermoplastics and to assess the different damaging processes viz. debonding, slid- ing, or fiber pullout. Shelby et a1.Im conducted a feasibility study on the utilization of vibration spectrum analysis for the nondestructive evaluation of polymeric composites. The method was sensitive to fiber breakage, delamination, and matrix cracking. Crane and GillespielW characterized the vibration damping loss factor of glass and graphite fiber composites.

Fiber pull-out tests have been reported by several authors. Piggott and Dails1 measured the forces required to pull-out single glass or Kevlar fibers from a low density polyethylene. Netravali and Sachse182 compared methods for carrying out single-fiber composite tests in which acoustic emission data were used to monitor the progression of fiber fracture. Figueroa et a1.IS reported on the mi- cromechanics of single filament composites. Di- BenedettoIM measured the thermomechanical sta-

bility of interphases by the embedded single-fiber test. Pitkethly and Doblel= characterized the fiber- matrix interface of carbon fiber reinforced composites using a single-fiber pull-out test. Miller et a1.'% discussed the measurement and mechanical aspects of the microbond pull-out technique for obtaining fiber-resin interfacial shear strength. Wu and Clayp001'~~ utilized an analytical approach to the microbond test method for characterizing the fiber-matrix interface. Chen and Young188 described a microbonding testing system as a method of quantifymg adhesion in composites. H~ueh '*~ commented on some applications of fiber pull-out analysis. Mital and Chamislgo devised a three-dimensional finite element computational simulation for a fiber push-out test. Waterbury and Dr~al '~l reported on the determination of fiber strengths by in situ fiber strength testing.

Maffezzoli et al.la studied the crystallization behavior of PPS-based composites using dynamic mechanical analysis and differential scanning calo- rimetry. Scobbo and Nakajima193 discussed the dynamic mechanical analysis of thermoplastic resins and composites. S a ~ a n o v ' ~ ~ examined thermal analysis as a method for characterizing polymeric composites. Ga l i~ t i s '~~ reported on interfacial studies of model composites by laser Raman Spectros- copy. Favre et a1.l" evaluated stress transfer by shear in carbon fiber model composites using a computer simulation of the fragmentation test. Gil- bert et al.197 explored a liquid droplet measurement technique for assessing the interlaminar shear strength of a fiber-reinforced composite. Pavan and M e r ~ a n t e ' ~ ~ provided preliminary results of a micromechanical analysis of yielding in particulate modified glassy polymers under multiaxial stresses.

Applications of Composites

Two texts concerned with applications of composites were published, one by Seymour,lW while the other was a compilation of the proceedings of the Engineering Society of Detroit and ASMs 1991 Conference.200 Michaeli et a1.201 dealt with problems encountered by the plastics industry in applications for automotive engineering. One of the major obstacles is the requirement that plastics in automobiles must be recycled when the car is



scrapped. Several organizations are engaged in design for recycling and recycling operations, including reinforced plastics. Suggestions were made to achieve increased recycling of these materials. Rutgers and VerlaanZo2 reviewed the use of polyamides for engineering applications. The authors estimated that 330,000 tons of nylons 6 and 66 were consumed in Western Europe. This figure includes glass fiber reinforced and mineral-filled grades as well as blends. Details were provided for applications in automotive, electrical, electronic, and sports industries. In an extensive article, Wi- gotskyZo3 discussed the use of composites in aircraft and aerospace applications. Thermosets are now used but thermoplastic composites are under consideration because of their advantages in toughness and elimination of autoclave curing. BaumannZM reported on the utilization of glass fiber reinforced nylon 6 or nylon 66 for an automotive clutch pedal. These plastics permit the elimination of welding, corrosion treatment, and machining and allow assembly with a minimum of individual components. Hansmann and Bartc- zakm5 described the development of plastic clutch and brake pedals molded of glass fiber reinforced nylon 6. Both short and long glass fibers were evaluated. LynnzM wrote about polymer composite characterization for automotive structural applications.

Eyerer et aLZo7 considered the assessment of products and processes to take into account the technical, economic, and environmentally relevant requirements in a balanced and integral way. De- tailed analyses were given for a car fender, a win- dow, and a milk pouch. The authors pointed out that future problems will be solved in many cases with composite materials. Goldbach and KochZm described the steps involved in the development of an injection molded plastic-steel composite car door. DimmockZw reported on the employment of composites in automotive suspensions. Brown et al. discussed the characterization of short fiber reinforced thermoplastics for fracture fixation de- vices.210 Mapleston211 presented information on the molding of hollow parts of complicated geome- try by the lost core process using primarily glass fiber reinforced nylons. Moldings such as air in- take manifolds, thermostat housings, water pumps, impellers, water taps, and refrigerator compressors are made by this process and provide improved air flow, vibration damping, light weight, and lower finished part cost than metal fabrications.

I Acknowledgment

This work was camed 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 Jer- sey Polymer Extension Center is a subcontractor in this program.

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ADVANCES IN POLYMER TECHNOLOGY 31 1

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Advances in injection molding of fiber-reinforced thermoplastics during 1991—Part II: Processing, design, and manufacture