Specialized molding techniques PDF
Preview Specialized molding techniques
Edited by Svcic~j' (Jf,Dlu:'.IIC,~ l"ngmeer.1 Hans-Peter Heim H. Potente Plastics Design Library Copyright'2001,PlasticsDesignLibrary.Allrightsreserved. ISBN1-884207-91-X LibraryofCongressControlNumber:2001091835 PublishedintheUnitedStatesofAmerica,Norwich,NYbyPlasticsDesignLibraryadivisionof WilliamAndrewInc. Informationinthisdocumentissubjecttochangewithoutnoticeanddoesnotrepresentacommitment onthepartofPlasticsDesignLibrary.Nopartofthisdocumentmaybereproducedortransmittedin anyformorbyanymeans,electronicormechanical,includingphotocopying,recording,oranyinfor- mationretrievalandstoragesystem,foranypurposewithoutthewrittenpermissionofPlasticsDesign Library. Comments,criticismandsuggestionsareinvitedandshouldbeforwardedtoPlasticsDesignLibrary. PlasticsDesignLibraryanditslogoaretrademarksofWilliamAndrewInc. Please Note: Great care is taken in the compilation and production of this volume, but it should be madeclearthatnowarranties,expressorimplied,aregiveninconnectionwiththeaccuracyorcom- pletenessofthispublication,andnoresponsibilitycanbetakenforanyclaimsthatmayarise.Inanyin- dividual case of application, the respective user must check the correctness by consulting other relevantsourcesofinformation. Theuseofgeneraldescriptivenames,registerednames,trademarks,etc.inthispublicationdoesnot imply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevantprotec- tivelawsandregulationsandthereforefreeforgeneraluse. ManufacturedintheUnitedStatesofAmerica. PlasticsDesignLibrary,13EatonAvenue,Norwich,NY13815Tel:607/337-5080Fax:607/337-5090 One-shot Manufacturing: What is Possible with New Molding Technologies James F. Stevenson GenCorp Technology Center, Akron, OH 44305, USA INTRODUCTION New molding technologies1 together with a revolution in thinking about how to design and manufacture products2-3 have merged to open exciting new possibilities in polymer part manufacturing. The new technologies offer versatile, cost effective forms of materials and more unified, efficient production methods. Even greater benefits, especially from consoli- dation, exist for parts previously made of metal. These processes move forward and, in some cases, realize the goal of forming a complex product in a single manufacturing step, or one-shot manufacturing. Conventional manufacturing processes generally employ homogeneous materials and simple primary shaping processes to form components which were then assembled by vari- ous joining methods. These subassemblies then go through various secondary operations and ultimately are combined to form the finished product. This sequential process is labor intensive, time consuming, and costly; it requires large inventories and long changeover times and is prone to produce scrap. This paper presents and analyzes common features of ten of the new material-process technologies. These technologies, along with chapters on predicting orientation and warpage by T.A. Osswald and on lean molding by Colin Austin are presented in greater depth in a recent book Innovation in Polymer Processing: Molding.1 All of these process and material innovations are based on antecedent technology and well-known physical principles. The key to realizing these innovations was conceptual. NEW TECHNOLOGIES The innovative molding technologies are summarized in Tables A1-A10 which also include a listing of advantages/disadvantages, applications, and materials. These tables are self con- tained; readers are referred to them as independent sources of information and as descriptive material for the processes cited in Tables 1-3. 2 Special Molding Techniques PRODUCT ELEMENTS The framework given in Tables 1-3 facilitates classification and comparison of the new molding technologies. It serves both as a guide to applications and as a means of locating opportunities and gaps in the new technologies. Technologies at similar locations in the tables can be considered as potential alternatives for each other. In the tables materials are divided into polymers, consisting of rubber and plastics, and nonpolymers, primarily solids or gases. Solids, typically fibers, serve as reinforcements, whereas gases reduce density or increase stiffness for a given cross-sectional area by distrib- uting material to increase the moment of inertia. The term macroscopic refers to dimensions that are on the order of the thin part dimension, e.g., a 2-mm diameter void in a 3-mm rod. Microscopic means dimensions two orders of magnitude or more smaller (e.g., a 300-layer laminate) than the part thickness. Achieving one-shot manufacturing requires an optimal combination of MATERIALS, PROCESS, and GEOMETRY. MATERIALS In terms of MATERIALS, the innovative molding technologies are classified accord- ing to COMPOSITION, one or more POLYMERS with or without NONPOLYMERS, and SCALE, MICROSCOPIC or MACROSCOPIC, as shown in Table 1. Table 1. Materials: composition and scale Composition Scale Polymers combined with Other polymers Gas Solids Macroscopic Blow Molding Gas-Assisted Molding Laminate Molding (LPM) Coinjection Molding (MMP)* Liquid-Gas Molding Multimaterial Molding (MMP) (LPM) In-Mold Coating (MMP) Dual Blow Molding Molding (LPM)* Microscopic Lamellar Molding Microcellular Plastics Sheet Composites Controlled Density (LPM) Reactive Liquid Molding * Processes designated LPM (Low Pressure Molding) are described in Table A4; those designated MMP (Multimaterial Multi- process) are given in Table A10. One-shot Manufacturing 3 Two of the innovative processes shown in Table 1, lamellar molding and microcellular plastics, serve primarily as the means of generating a unique material on the microscopic scale and secondarily for shaping the material. Other processes listed in the table combine polymers and nonpolymers on a microscopic or macroscopic scale to from products where specific properties of the multiple materials meet local functional needs. This localized opti- mization ultimately enhances overall product performance. PROCESSING PROCESSING is the bridge between the unshaped raw MATERIALS and the GEOMETRY (MACROSCOPIC STRUCTURE and SIZE & SHAPE) of the product as shown in Table 2. Table 2. Processing: materials and geometry Geometry Materials Macroscopic structure Size & shape Laminate Segmented Large Hollow Polymers In-Mold Coat- Multimaterial Sheet Composites Dual Molding ing Molding In-Mold Coating (LPM) Coinjection (MMP), (MMP) Blow Molding Blow Molding Blow Molding Injection-Compres- Dual Molding sion (MMP) (LPM) Poly- Solid Laminate Molding (LPM) Laminate Molding mers and Reactive Liquid Molding (LPM) non- Sheet Composites polymers Reactive Liquid Molding Gas Blow Molding Blow Molding Blow Molding Gas-Assisted Molding Controlled Density Gas-Assisted Mold- Liquid Gas Molding (LPM) Molding (LPM) ing Fusible Core Liquid Gas Mold- ing (LPM) Fusible Core Many processes under MACROSCOPIC STRUCTURE in Table 2 allow the combina- tion of a polymer with other polymer(s) or nonpolymers on a macroscopic scale to give 4 Special Molding Techniques LAMINATES which have uniform properties over the surface of the part but variable proper- ties through the thickness. SEGMENTED PARTS which exhibit a variation of properties along the surface but are uniform over a given cross-section. Combinations of laminated and segmented parts are possible, for example gas-assisted parts with hollow sections only in certain regions of the part. In terms of part SIZE & SHAPE, innovative technologies are generally needed when the part is LARGE, especially when a cosmetic surface is required, or when the part has a COMPLEX, often hollow shape, particularly when it is load bearing. GEOMETRY Part GEOMETRY can be considered in terms of FUNCTION where the part is located on a scale ranging from ENCLOSURE (containers or panels, often with cosmetic surfaces) to LOAD BEARING, and COMPLEXITY which allows for geometric complexity ranging from SYMMETRIC (planar or axisymmetric) to fully THREE DIMENSIONAL. Table 3. Geometry: function and complexity One-shot Manufacturing 5 Table 3 suggests that molded parts and the associated innovative processes to make them generally range from more or less symmetric enclosures in the upper left to three dimensional load bearing parts in the lower right. REFERENCES 1 Stevenson, J.F. (Ed.) Innovation in Polymer Processing: Molding, Hanser, Munich, Hanser-Gardner, Cincinnati (1996). 2 Womack, J.P., Jones, D.T., and Roos, D., The Machine that Changed the World, Macmillan, New York (1990). 3 Gooch, J., George, M., and Montgomery, D., America Can Compete, Institute of Business Technology, Dallas (1987). 6 Special Molding Techniques APPENDIX Table A1 and A2. Gas-assisted injection molding: process [1.1] and simulation [2.1] PROCESS: Nitrogen gas under high pressure is injected through the nozzle or mold wall into plastic partially filling a mold. The gas flows preferentially through local thick sections with hot interiors and pushes the plas- tic ahead to fill the mold. SIMULATION: Commercial software now available to predict gas flow paths, polymer thickness, clamp force, and contraction during cooling for various geometries and process variables including gas pressure, injection time, and prefilled polymer volume. Simulations and experiment generally show • increasing gas pressure decreases fill time, gas penetration distance, and (by conservation of mass) poly- mer wall thickness, • melt temperature has a variable effect on gas penetration length, • increasing delay time before the start of gas injection increases wall thickness and gas penetration length, • increasing gas injection time increases gas penetration distance, • decreasing the prefilled polymer volume fraction increases the penetration length until a critical level when gas blows through. • increasing gas pressure level and time decreases shrinkage. Simulations are generally able to predict undesirable air traps and gas penetration into thin sections. Simula- tion of a freezer bottom part converted to gas-assisted molding showed a 70% reduction in packing pressure, feasibility of using a less expensive material, and reduced warpage due to lower, more uniform pressure and higher part stiffness. Eight design guidelines are given based on both experiment and computer simulation [2.1]. Advantages/Disadvantages Applications Materials PROCESS: Part weight and cooling time can be reduced up to 50%. Handles, Panels ABS, PA, PE, Sink marks are eliminated. Warpage is reduced. Clamp force and with Ribs, Appli- PP, PS, PPO, injection pressure are lower. Part stiffness is increased because of the ance/machine PC, PBTP, higher moment of inertia. Licensing is necessary. Housings (TV ben- PC/PBTP, SIMULATION: Simulation helps identify optimal process conditions zels) SAN, TPE, including runner layout and size, and location and timing of gas Automotive Parts TPU introduction. Software is available but new. (Clutch Pedals, Mirror Housings) One-shot Manufacturing 7 Table A3. Fusible core injection molding [3.1] Complex hollow parts are formed by injection molding plastic around a fusible alloy core which is subse- quently removed by melting. The fusible (or lost) core typically is cast form a bismuth-tin alloy with a eutec- tic melting point of 138oC. The molten metal fills a split steel mold from the bottom and then cools for 2 min to produce a heavy core with a mirrorlike surface. The still-hot core is positioned by a robot in a steel mold and plastic is injected. Flow channels are designed to balance forces around the core during filling to prevent core movement. For thermoplastics the injection temperature, e.g. 290oC for polyamide, can be well above the melting point of the core since the relatively high thermal diffusivity of the metal maintains a low interface temperature. After demolding, cores are melted out in a large bath or by induction heating or by injecting heat transfer fluid inside hollow cores. Advantages/Disadvantages Applications Materials Plastic parts made by fusible core technology have a weight Air intake manifolds, PA and cost advantage over metal parts. Fusible core molding tennis racquets, pump Poly(etherarylke- eliminates the need for mechanically complex molds or join- parts tone) ing separately molded parts. Interior surfaces of fusible core parts are smooth which increases gas flow. Disadvantages are loss or oxidation of expensive core metal and need for robots to handle heavy cores. Table A4. Low pressure molding [4.1] Low pressure molding, as developed by Siebolt Hettinga, enables a number of other molding technologies.In Low Pressure Molding (LPM) the mold cavity is filled at low speed through large gates with a controlled pressure profile in the shape of a broad inverted U. LPM has no packing stage and no cushion. The melt tem- perature profile is controlled by adjusting screw speed and flow resistance during plastication. LPM works better with low viscosity semicrystalline materials and is not suitable for thin-wall parts. Slow injection, lower melt and higher mold temperatures reduce residual stress to allow demolding at a higher temperature to maintain or reduce cycle times. For larger parts, low clamp force can be achieved using multiple valve gates with programmed opening [4.2]. Lower clamp force allows use of self clamping molds and multistation injectors. Laminate Molding involves molding plastic at low pressure directly behind textile, film, or metal. In Liquid Gas Injection Molding, a volatile liquid is injected at low pressure into the melt and then vaporizes to form hollow channels in the part. The liquid condenses and is absorbed in the part. Dual Molding, similar to Bayer’s Multishell Molding, forms an integrated hollow part by overmolding at low pressure an assembly formed from separately molded parts. In Controlled Density Molding the mold is partially opened once a skin has formed to give a low density interior. 8 Special Molding Techniques Table A4. Low pressure molding [4.1] Advantages/Disadvantages Applications Materials Substantial capital costs savings result from the Low Pressure Molding: Interior Thermoplas- use of presses with a lower camp force or self Vehicle Panels, Bumper Fascia. tics, especially clamping molds. Laminate Molding saves on Laminate Molding: Fabric/Plastic polyolefins, assembly and adhesive costs in fabric/plastic lam- Seats, Vehicle Trim Panels. Liq- thermosets inates. uid-Gas Assist Molding: Large Chairs, Chair Bases. Dual and Shell Molding: Manifolds, Pump Bodies, Valves, and Fittings. Low Density Molding: Fittings, Elec- tronic Enclosures, Table Tops. Table A5. Advanced blow molding [5.1] The advanced blow molding technologies described below have greatly extended the versatility and facili- tated product design. Deep-Draw Double-Wall Molding employs a mold with four hinged slides and an advancing core which close in a programmed manner around a partially inflated parison to shape a deep draw part. Press Blow Molding is used to form panels between shallow male and female mold halves which press together certain sections and inflate other sections to form hollow stiffening ribs.Three Dimensional (3D) Blow Molding forms serpentine three-dimensional parts without excessive scrap by manipulating the parison and positioning it in a convoluted mold cavity. Positioning the parison can be accomplished by (1) translating in two directions the mold which is titled at an angle, (2) movement of the parison by robotic arms in a mold with multiple sections which close sequentially, and (3) guiding the parison through the mold by sucking air along the length of the mold. Multimaterial Blow Molding employs multiple materials sequentially along the part length, in layers over the part thickness, or on opposite sides of the parison. New material developments include molding of 0.3-in fiber reinforced materials and foam layers [5.2]. Computer simulation of blow molding has been developed by A.C. Technology, Ithaca, NY in cooperation with G.E. Advantages/Disadvantages Applications Materials Deep Draw Technology increases draw (depth-to-length) ratio Insulated containers with PE, PS, PP, from 0,3 to 0.7 and allows forming of parts with undercuts, ribs, foam, Planters, Conduits, POA, and noncircular crosssections. Multimaterial applications allow Air Ducts, Bumpers, ABS, PPE soft surfaces on structural parts, flexible conduits with rigid con- Equipment Panels, Instru- elastomers nectors, or parts with opposite sides of different properties. 3D ment Panels, Portable Toi- Blow Molding consolidates complex parts and enhances func- lets, Golf Cases, Arm tion. Rests, Gasoline Filler Tubes, Gas Tanks.
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