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Unformatted text preview: Special and Emerging Injection Molding Processes LIH-SHENG TURNG Department of Mechanical Engineering University of Wisconsin-Madison, ' 1 513 University Avenue Madison, WI53706-1572 New variations and emerging innovations of conventional injection molding have been continuously developed to further extend the applicability, capability, flexi— bility, productivity, and profitability of this versatile mass-production process. These special and emerging injection molding processes introduce additional design freedom, new application areas, unique geometrical features, enhanced part strength, and sustainable economic benefits, as well as improved material properties and part quality that cannot be" accomplished by the conventional injection molding process. This paper provides a general review of several special and emerging inj ec- tion molding processes, namely, co—injection molding, fusible core injection molding, gas-assisted injection molding, injection-compression molding, in-mold decoration / lamination, low—pressure injection molding, micro—injection molding, and microceL lular (MuCell) injection molding with emphases on current state-of—the-art technol- ogy, applicationsprocess physics, technical challenges, and applicable materials. " INTRODUCTION njection molding is one of the most versatile and im- portant operations for mass production of complex plastic parts. The injection—molded parts typically have excellent dimensional tolerance and require almost no finishing and assembly operations. In addition to thermoplastics and therrnosets, the process is also being extended to such materials as fibers, ceramics, and powdered metals, with polymers as binders. Among all the polymer-processing methods, injection molding accounts for 32% by weight of all the poly- ‘ meric material processed (1)..Nevertheless, new varia— tions and emerging innoVations of conventional injec- tion molding have been continuously developed to offer special features and benefits that cannot be accom- plished by the conventional injection molding process. This paper provides a general review of several spe- cial injection molding processes, namely, co-injection molding, fusible core injection molding, gas-assisted injection-molding, injection-compression molding, in- mold decoration/lamination, low—pressure injection \ molding, micro-injection molding, and microcellular I (MuCell) injection molding with emphases on current state-of-the-art technology, applications, process phys- ics, technical challenges, and applicable materials. References listed at the end of this paper provide de- tailed information for more in—depth studies. With this This paper is partially based on the book chapter “Special Injection Molding Processes" in the Injection Molding Handbook to be published by Hanser Pub- lishers. . information. readers will be able to evaluate the tech— nical merits and applicability of the relevant processes in. order to determine the most suitable production method. It is also hoped that a collective presentation of these special and emerging molding processes, which descend from the same origin and yet mature with diversified creativity, will spark innovative ideas that lead to further improvement or new inventions. CATEGORIZATION OF‘SPECIAL INJECTION MOLDING PROCESSES Technically speaking, it is very difficult to cover all special injection molding processes, not to mention those new processes that are being developed and field—tested. Furthermore, due to the diversifiedna~ ture of these special injection-molding processes, there is no unique method to categorize them. As a preliminary attempt._ Table 1 classifies the various processes based on the specific techniques employed by the process or the unique characteristics of the process. Figure 1 illustrates some of the characteris- tics of those special injection molding prOcesses for therrnoplastics. It should be noted that, for a special purpose or application, a new or viable special injec- tion molding process could employ multiple specific techniques listed in Table 1 (e.g., co-injection molding with microcellular plastics, gas-assisted push-pull injection molding, gas—assisted powder injection mold- ing, multi-component powder injection molding, etc.). In addition, more sophisticated “hybridized” technology such as in—line compounding—injection molding [2) / 160 JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging Iry'ection Molding Processes Table 1. Categorization of Special Injection Molding Processes. 1. Incorporation of additional material(s) or component(s) into the molded part . a. Adding or injecting additional plastics i. Co-injection molding ii. Mum-component injection molding (overmolding) iii. Lamellar (microlayer) injection molding b. Injection around (or within) metal components i. Insert/outsert molding ii. Fusible- core (lost core) injection molding c. Injecting gas into the polymer melt . i. Gas-assisted injection molding d. Injecting liquid or water into polymer melt ‘ i. Liquid gas-assisted injection molding ii. Water-assisted injection molding e. Injecting gas into the metal (or ceramic) . powder-polymer mixture i. Gas-assisted powder injection molding f. Incorporating reinforced fiber mats inside the cavity i. Resin transfer molding 1 ii. Structural reaction injection molding g. Incorporating film, foil, fabric, or laminate to be back—molded by polymer melt i. ln-mold decoration and in-mold lamination ii. Low-pressure injection molding 2. Melt formulation a. Mixing polymer melt with super-critical fluids i. Microcellular injection molding b. Mixing polymer melt with chemical or physical blowing agents i. Structural foam injection molding c. Mixing polymer melt with metal or ceramic powders i. Metal/ceramic powder injection molding d. Mixing pre-polymer (monomers or reactants) prior injection i. Reaction injection molding ii. Structural reaction injection molding iii. Resin transfer molding iv. Thermoset (reactive) injection molding 3. Melt manipulation a. Providing vibration and oscillation to the melt during processing i. Multi live-feed injection molding ' ii. Push-pull injection molding iii. Rheomolding iv. Vibration gas(-assisted) injection molding b. Providing vibration to the melt prior to processing to reduce melt viscosity i. Melt disentanglement c. Using serew speed and back pressure to control melt temperature -i. Low-pressure injection molding 4. Mold movement a. Applying compression with mold closing movement i. Injection-compression molding 5. Special part or geometry features a. Producing parts with miniature dimensions or relatively thin sections i. Micro-injection molding ii. Thin-wall molding has also been developed to reduce the overall cost and improve product quality. Detailed description of some of these processes can be found in the following sec- tions. CO-INJECTION (SANDWICH) MOLDING Process Description Co-injection molding (also called sandwich molding) comprises sequential and / or concurrent injection of a material and a dissimilar but compatible “core” material into a cavity (3—6]. This process produces parts that have a sandwich structure, with the core material embedded between the layers of the skin ma- terial (cf. 2). This innovative process offers the inherent flezdbility of using the optimal properties of ’ each material to reduce the material cost, injection pressure, clamping tonnage, and residual stresses, to modify the property of the molded part, and/ or to achieve particular engineering effects. For example, this process allows such applications as components with class “A” surfaces and low-cost recycled cores, steering wheels and grab handles featuring soft-touch skins over rigid cores, underhood components with chemically resistant skins, and electronic housings with electromagnetic shielding (EMI) skins or cores. Co-injection molding is diflerent from multi-compo— nent molding (or overmolding] that injects material. over a pre-molded plastic component placed in a larger cavity by way of rotary table, core-back, or robotic transfer. Instead, the co-injection molding process is characterized by its ability to completely encapsulate an inner core with an outer skin within one molding cycle without change of cavity geometry. The process mechanics rely on the sequential (namely, skin-core— skin. or A—B-A) or hybrid sequential—concurrent (i.e., A—AB—B-A) injection of two different materials through the same gate[s]. Figure 3 illustrates some of the co- injection molding devices and the resulting flow of skin and core materials inside the cavity. Process Physics The principle of the co-injec‘don molding process is relatively simple: two dissimilar polymer melts from their injection units are injected one after the other into a mold cavity. Due to the flow behavior of the polymer melts and the solidification of skin material; a frozen layer of polymer starts to grow from the colder mold walls. The polymer flowing in the center of the cavity remains molten. As the core material is injected, it flows within the frozen and relatively viscous skin lay- ers pushing the molten, running skin material at the hot core to the eXtremities of the cavity. Because of the fountain—flow effect at the advancing melt front (an outward trajectory of fluid particles from the cen- tral region to the bounding walls), the skin material at the melt front will show up at the region adjacent to the mold walls. This process continues until the cav- ity is nearly filled, with skin material appearing on the surface and the end of the part. Finally, a small JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No.‘ ’ 161 .moumuiogmfi LoSmmmmmoEQ ©5309: totem? mSmngm Btu EBMQW .~ .5 9.902 838: 839: 8902 commie. €afi§ ,, . 96.08032 flu... . .// 9662:9695 vmfimmoao©nsc= 05.062 50:83 vvmpm_mwo..m+o\s , 96.02 8:8: gémmogw w Iii... @5302 conom_c_ 55653 m2 5 N00 _oo:_5-.oaam + :9». 58208 , DEEDS. cozoflg 5.2.8922 , §_mEo._ OSQOE. Stow: 900 @533 96.02 toxaoxtmmg \ mcfioE6>O w. \‘ co__bc_E9_o_oE..c_ m cofiuoomo. u,_oE-c_ T « Lih—Sheng Tumg 966.2 c309; 9339930,. QEOEQEEE + :9: 5:203 0590.2 eczema. 5038. 96.0.2“ :98: =9).th I gamma, ngona + :wE 5520.8 0:662 8:85, 90:2 96.02 E08 65625 JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 162 Special and Emerging Injection Molding Processes Fig. 2. Cross section of a co—iry'ec- iion molded part showing the sand- wiched (laminated) structure and the distribution of the skin and core materials in the part. :53. m a, <, r 4 additional amount of skin material is injected again to core content ratio, injection speed, and melt and mold- “seal” the part and to purge the core material so that wall temperatures) (see, e.g., 8—1 1}. Figures 4 and 5 it will not appear on the part surface in the next shot. illustrate the effect of some influencing factors on the The spatial distribution of the skin and core materi- skin/ core material distribution in the cavity. As shown als depends on the materials used (primarily the vis— in Fig. 4, core material will penetrate into the hot core cosity ratio), the geometry in terms of part and mold as long as the skin material can be displaced. Accord- designs, and the processing conditions (e.g., skin and ingly, there will be little or no core penetration for Fig. 3. (Top left) Two-channel and - (top right) three-channel co-injec-' tion molding nozzles that allows sequential or concurrent, concen— tn'cflow of skin and core materials (4). (Bottom) Multi—gate co-irijection hot runner system with separate flow channels for skin and core materials, which join at each hot runner nozzle (7). Co-injeciion Nozzles JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 v 163 Lih-Sheng Tumg Skin Materiel Skin injection Core injection End of Injection / - / 2‘ /'x ///x Core Materiel MO“? /////'///t////Lsi$ ) A 1' —, Mold Wall Fig. 4. Cross section of a cavity showing the ejfect of mold design {e.g., gate location) on the skin/core material distribution in the cavity. Note that there is no core penetration into the lefi portion of the cavity,‘ as skin material in that region cannot be displaced. regions that get completely filled prior to injection of core material or areas that exhibit stagnant flow dur— ing core injection stage (e.g., weld lines behind an open- ing). In addition, experimental studies have found that the viscosity ratio of core vs. skin materials (more/115m) plays an important role in determining the thiclmess Frozen Layer Skin Injection Core injection Wiih mos/mm“ Core injection with nomad uniformity and length of core penetration (8—10). More specifically, the most uniform skin/ core thickness distribution can be achieved by injecting a core mate- rial that has slightlyhigher viscosity than that of the skin material. As illustrated in Fig. 5, a much more vis- cous core material results in thinner skin thickness Mold Wail /. Mold Wail Fig. 5. Cross section of a cavity showing the effect of viscosity ratio (mm/715M“) on the skin/core material thickness ratio and distribution in the cavity. 164 , JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging Injection Molding Processes deposited on the wall and a shorter core penetration (8]. On the other hand, if more/715k,“ is too low, a much thicker skin thickness will be deposited on the wall lead- ing to longer core penetration. If there is not enough skin material displaced to the melt front, the core ma- terial will eventually penetrate through the skin mate- rial leading to “core breakthrough." That is, the skin material is eventually depleted during the filling proc- ess and the core material shows up at the advancing melt front. Such “core surfacing" or “core breakthrough” is generally undesirable, and can be avoided by ad- justing the content ratio of skin and core materials. In general, about 40% of the total shot volume can be molded beneath the skin material that satisfies strin- gent appearance requirements (12). If (more/115kb,) con- tinues to decrease further, fingering effect, as commonly seen in gas—assisted injection molding, will occur (9). Fingering effect is a result of flow instability due to low viscosity of the core material and its penetration following the path of least resistance. As shown in Skin i Materioimw Material Core Materiel End Of injection Fig. 6, the pressure gradient along the core-penetrated domain is relatively small, due to the low viscosity of the core material. Given the equal pressure drop from the polymer entrance to the skin melt front, flow path that has slightly longer core penetration length will produce higher pressure gradient in the skin domain and thus higher skin velocity. As the skin material is moving (or being displaced) faster along a certain flow path (e.g., path A vs. path B in Fig. 6), it promotes a localized acceleration of core penetration resulting in an irregular core penetration pattern. Finally, other processing conditions, such as tem- perature control and injection speed affects the vis- cosity of the materials and the thickness of the skin frozen layer, Which, in turn, influence the spatial dis— tribution of the skin and core materials. More specifi- cally, a high core temperature was the most significant ' variable promoting a constant core thickness, while core content was the most significant factor influenc-‘ ing a breakthrough of the core (1 1). Pressure Flow-Length Pressure Flow Length 93 A a 39’ :3. Flow Length 9 a i 0. Flow Length Fig. 6. Cairy’ection molded parts showing finger-like" core penetration, a result of flow instability due to low viscosity of the core material and its penetration following the path of least resistance (see text). JOURNAL OF INJECTION MOLDING ‘I'ECHNOLOGY, SEPTEMBER 2001, Vol. 5, NO. 3 165 Lih-Sheng Turng Technical Challenges Co-injection molding offers a number of cost and quality advantages as well as design flexibilities and environmental friendliness through material cost re- duction and recycling, and modification of the part quality and property. However, the technical challenges lie in proper design of the part, mold, and process as well as the selection of materials to obtain the desir- able skin/ core material distribution and adhesion. Im— . proper part and mold design and material combina— tion will result in non-uniform skin/ core distribution within the cavity, as shown in Figs. 4, 5, and 6. Recall that the skin thickness and extent of 'core penetration depends on the visCosity ratio of the materials and the ’ selection of process conditions. As a result, the devel- ’ opment for a co-injection mold and’process set-up take longer time than that with the conventional in- jection molding process. For complex part geomenies, computer simulation developed specifically for co-in- jection (see, e.g., 6, 13—16] provides useful insight into 1 how the skin and core materials interact as they fill the cavity as well as the resulting material distribu- tion within the part. In addition, useful application guidelines can be found in (17). Applicable Materials Co—injection molding can be employed for a wide vari- . ety of materials. Although most of the materials used are thermoplastic, there are some promising develop- ments with thermosetting materials co-injected with thermoplastic materials as well as with co-injected metal powders (15). Since two materials are used in co—injection molding processing. the flow behavior and :> injection Molding ' Fusible Core 05 An lnseri _ Die Casting Liquid Metal <3 the compatibility of material properties are very im- portant. In considering the material selection, the most important properties are viscosity difference and the adhesion between the sldn and core materials. Since the materials are laminated together in the part, an effective adhesion and similar molding shrinkage are desirable for optimum performance. Reference (19)’pro- vides basic reference guidelines on a wide range of material combinations. FUSIBLE (LOST, SOLUBLE) CORE INJECTION MOLDING Process Description The fusible (lost, soluble) core injection molding proc- ess produces single-piece components that feature complex, Smooth internal geometry and a high dimen— sional stability, which cannot be obtained through the conventional injection molding process [20, 21); This process can be classified as an insert molding proc— ess, as the plastic is injected around a temporary core of lowtmelting-point material, such as tin—bismuth alloy, wax, or a thermoplastic. Different approaches, such as fusible core, soluble core, and salt core tech- niques, have been developed. All these techniques em- ploy the same principle, namely, the production of injection’molding with a lost core that gives the in- ' ‘ temal contour of the molded part. Among these lost core processes, fusible core techniqueicf. Fig. 7) is' the most energy intensive method. Nevertheless, this drawback is offset by the low core losses, smoother in- ternal surface requiring low finishing cost, faster heat dissipation by using a stronger and highly conductive metal core. After molding, the core will be physically Temperature Profile Final Plos’ric Pon‘ with Hollow Sections Fig. 7. Schematic illustration of the fusible core injection molding process. 166 JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging I ry‘ection Molding Processes melted or chemically dissolved, leaving its outer geom— etry as the internal shape of the plastic part. This process reduces the number of components required to make a final assembly or substitutes plastic for metal castings to boost performance (e.g., corrosion resist- ance] while saving weight, machining, and assembly cost. The fusible core injection molding has so far been used primarily for automotive air intake manifold. The main disadvantage of fusible core injection molding is high tip-front investment cost, which requires a sig- nificant capital investment for a very large injection molding machine, casting machine and equipment, a “melt-out” station, and robotic handling devices for they typically heavy plastic molding with core. As a result, it is facing competition from less-capital-in- tensive methods such as standard injection molding, Where part halves are welded (twin-shell welding) or mechanically fastened (22). Process Physics To minimize the core loss due to oxidation and en- ergy consumption, a- number of eutectic alloys have been found suitable for fusible core injection molding. These eutectic alloys exhibit stable dimensional stabil- ity (with low expansion or shrinkage], a sharp melting point and a rapid phase transition. It is necessary that the melting temperature of the fusible alloy core be lower than that of the over-molding plastic so that it Can be melted out in the melting bath. Nevertheless, due to the high thermal difl‘usivity as well as its mas— sive thermal inertia and latent heat of the alloy, the surface temperature of the core remains below its melt- ing point during molding, as illustrated in Fig. 7. This phenomenon is akin to the relatively small temperature rise at the mold-wall in conventional injection molding where the high thermal conductivity of the metal mold rapidly removes the heat from the molten plastic. GAS-ASSISTED INJECTION MOLDING Process Description The gas-assisted injection molding process consists of a partial or nearly full injection of polymer melt into the mold cavity followed by injection of inner gas (typi- cally nitrogen) into the core of the polymer melt through the nozzle, sprue, runner, or directly into the cavity. The compressed gas takes the path of the least resist- ance flowing toward the melt front where the pressure is lowest. During the gas—injection stage, gas pene- trates the hot core of the thickest sections where the material remains molten and fluid, displacing it to fill the extremities of the cavity. As a result, the gas pene— ~ trates and hollows out a network of pre—designed, thick—sectioned gas channels, displacing molten poly- mer at the hot core to fill and pack out the entire cavity. Ideally, the essentially inviscid gas transmits the gas pressure effectively as it penetrates to the extremities of the part, thereby, requiring only a relatively low gas pressure to produce sufficient packing and a fairly uniform pressure distribution throughout the cavity. Consequently, this process is capable of producing lightweight, rigid parts that are free of sink marks and have less tendency to warp. ” In the so-called “gas-pressure control” process, the compressed gas is injected with a regulated gas pres— sure profile (constant, ramp, or step). In the “gas-vol~ ume control" process, gas is initially metered into a compression cylinder at preset volume and pressure; then it is injected under pressure generated from re- ducing the gas volume by movement of the plunger. Conventional injection molding machine with precise shot volume control can be adapted for gas-assisted injection molding with add-on conversion equipment, a gas source, and a control device for gas injection. However, gas-assisted injection molding requires a dif- ferent approach to product, tool, and process design due to the need for control of additional gas injection and the layout and sizing of gas channels to guide'the gas penetration in a desirable fashion. A comprehen- sive review of the technology and applications can be found in references (23—27) and the cited references. As an illustration, Fig. 8 shows three typical types of gas-assisted injection molding applications,‘ namely, (a) tube— and rod-like parts for saving material, reduc- ing the cycle time, (b) large, sheet-like, structural parts for reducing part warpage and clamp tonnage as well as to enhance rigidity and surface quality, and (c) com- plex parts consisting of both thin and thick sections, ‘ where the process is used primarily for decreasing manufacturing cost by consolidating several assem- bled parts into one single design. Process Physics The gas-assisted injection molding process begins with a resin-injection stage, same as the conventional injection molding process, followed by the gas injec— tion. There is an optional delay time between the end of resin injection and the beginning of gas injection to allow the polymer at the thin section to cool so that the incoming gas can only core out the designated thick sections which serve as the gas channels. Alter— natively, the gas injection can take place before the end of the resin injection to avoid hesitation marks at the melt front location during the gas delay time. How- ever, it can be done only when the gas—injection point is different from the polymer entrance. Otherwise, simul- taneous injection of polymer and gas at the same en- trance will result in an undesirable, corrugated polymer skin thickness. The gas-injection time is the duration over which gas pressure is imposed at the gas en- trancets). Since gas is injected to help fill and pack out the mold, the gas-injection stage actually consists of both gas-injection-filling and post-filling phases. The timer for gas injection and the gas-injection time are two important processing parameters that strongly in- fluence the gas penetration. Although the total gas—injection time takes a signifi- cant portion of the entire cycle, the actual elapsed JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 167 Lih—Sheng Turng (b) time from the introduction of the gas to the instant when the Whole cavity is filled is very short. Neverthe- less, this relatively short gas-injection—filling stage is crucial to the success of the molded part, because the various molding prOblems associated with this proc- ess, such as air trap, gas blow—through, short shot, gas permeation into thin section, uneven gas penetra- tion, etc., typically take place in this stage. The gas penetration created by displacing the polymer melt during the gas—injection—filling stage is often referred to as the primary gas penetration. Typically, this is the stage when the majority of gas penetration is de— termined (assuming a short shot of resin is injected), which impacts the quality of the final products. After the cavity is filled, a certain level of gas pressure is maintained to pack out the molded part. During this post-filling stage (or the holding stage), the so—called secondary gas penetration continues to compensate for the material shrinkage, primarily along the gas channels or thick sections. When the molded part cools down and becomes rigid enough inside the mold, the gas pressure is released prior to the ejection of the part. Figure 9 illustrates schematically the mechanism‘of how the polymer melt and gas interact in the cavity. It is well knoWn that in the conventional injection mold- ing process, the pressure required to advance the poly- mer increases with the amount‘of polymer injected (or, equivalently, the flow distance). figure 9a depicts schematically the rise in pressure during filling of the 8. (a) Grab handles (background) and automotive wiper arm (foreground) produced with gas-assisted co-iry‘ection molding. Black resin was injected, then white fiber-reinforced material, followed by gas iry'ection. (Photo courtesy of Mila— cron Inc.), (b) quarter-scale automotive hood panel (Part cour- tesy of GE CR&D), and (c) office chair leg (Partcourtesy of ITRI). mold cavity'without gas injection. Noted that the gap- wise averaged melt velocity is proportional to the mag— nitude of the pressure gradient and the melt flu- idity. Therefore, as the flow length of the polymer melt increases. the inlet pressure has to increase to main- tain a certain pressure gradient if the flow rate is to be kept constant, On the other hand, the. pressure requirementwith gas-assisted injection molding is the same as the con— ventional process during the resin-injection stage. Upon introduction of gas into the cavity, the gas starts to displace the viscous polymer melt, pushing it to fill the extremities of the cavity. Because the gas is essen- tially inviscid, it can eifectively transmit the gas pres- sure, without a significant pressure drop, to the ad— vancing gas/melt interface (cf. Fig. 9b). Therefore, as the gas advances toward the melt front, the pressure required to keep the melt ahead of the gas moving at the same velocity decreases, since the effective flow length decreases. Consequently, the gas pressure re— quired to fill the mold cavity can be lower than the required entrance melt pressure for the conventional injection molding process. Further, the resulting pres— sure distribution is more uniform in a gas-injected part, which induces less residual stresses as the poly- mer cools down during the post-filling stage. Accord- ingly, the gas-assisted injection molded part can be produced with a lower gas pressure requirement (which normally leads to lower clamping tonnage) and has less 168 q JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging Injection Molding Processes Pressure Pressure injection ........... - - Pressure Increases Lower and More Uniform Pressure Distribution s Polymer Melt Flow Length Decreases (b) Gas-Assisted Injection Molding Process 9. (a) The evolution of pressure distribution for the conventional iry‘ection molding process. (b) The evolution of pressure distri- button for the gas-assisted injection molding process. Note the lower but more uniform pressure distribution in case of gas-assisted. iry’ection molding. tendency to warp. Because the part is internally pres—- surized with gas during the packing stage. all shrink- age is taken up on the inside of the part. Thus, it helps to eliminate the sink marks on the surface of the part. Again, due to the negligible viscosity of gas, the finger- ing effect described in Fig. 6 is also common with the ' gas—assisted injection molding process. Gas fingering ‘ typically manifests itself as undesirable gas penne— ation into the thin sections from the gas channels. Varaiation of Gas-Assisted Injection Molding Owing to the versatile and promising capabilities of this process, some alternative gas—assisted injection molding processes have been developed and become commercially available. For example, instead of using the compressed nitrogen, the “liquid gas—assist proc— ess" injects a proprietary liquid into the melt stream. This liquid is converted to a gas in a compressed state with the heat of the polymer melt (28). After the filling and pacldng of the cavity, the gas is absorbed as the part cools down, thereby, eliminating the need to vent the gas pressure. In the so-called “water-assisted in— jection molding" process, water, which does not evap- orate during displacement of the melt, is injected after resin injection (29). Compared with “conventional” gas— assisted injection molding, water facilitates superior cooling effect and thus shorter cooling time as well as thinner residual wall thickness and larger component diameters. The “partial frame process”.injects com- presses gas into the strategically selected thick sec- tions to form small voids of l to 2' mm (0.04 to 0.08 inch) in diameter. Such a process can be employed to reduce sink marks and residual stresses (30) over the thick sections. Finally, the “external gas molding proc— ess" is based on injecting gas in localized, sealed loca- tions (typically on the ejector side) between the plastic material and the mold wall (31, 32). The gas pressure maintains contact of the cooling plastic part with the opposite mold wall while providing'a uniform gas pressure on the part surface supplementing or substi- tuting for conventional holding pressure. This kind of process is suitable'for parts with one visible surface where demands on surface finish are high and con— ventional use of gas channels is not feasible. Applicable Materials Most of the gas—assisted injection molding appli- _ cations employ thermoplastic materials. HoWever, gas— assisted injection molding can be extended to ther- mosetting polyurethane (33) and powder injection molding (34). Note that the material selection should be based on requirements of application performance, ‘ such as stiffness, chemical resistance, and material strength at the operating temperature. JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 ' 169 Lih-Sheng Tqu Technical Challenges Because gas-assisted injection molding involves dynamic interaction of two dramatically dissimilar materials flowing within typically complex cavities, the product, tool, and process designs are quite compli- cated. Previous experience with the conventional in- jection molding process is no longer sufficient to deal with this process, especially with injecting gas and controlling its penetration pattem. Therefore, there will be a learning curve before one can fully benefit from this process. More specifically, the design process has to be carried out with the process physics in mind in order to avoid problems associated with the dynamic nature of this process such as air trapping, gas per— meation intothin sections, uneven gas penetration, gas blow-through, surface defects, short shot, and surface blisters. It has also been reported that the reproducibility of the polymer sln'n thickness is nor- mally poor for polymers witha strong shear—thinning viscosity (3'5). Interested readers can refer to references [24—27, 36) for useful design guidelines. In addition, computer simulation for gas—assisted injection mold— ing (see. e.g., 37—42) has proven to be very useful in helping engineers gain process insights and make rational design decisions. INJECTION-COMPRESSION MOLDING .. Process Description Injection-compression molding (ICM) is an exten- sion of conventional injection molding by incorporat— ing a mold compression action to compact the poly- mer material for producing parts with dimensional ' stability and surface accuracy. In this process, the mold cavity has an enlarged cross section initially, which allows polymer melt to proceed readily to the Fig. 10. Typical ity‘ection—compres- ' sion molding process sequence: (a) injection stage, (b) compression stage. extremities of the cavity under relatively low pressure and stress. After a pre—set amount of polymer melt is fed into the open cavity, a mold compression action is engaged and continues to the end of the molding process. The mold closing movement forces the melt to fill and pack out the entire cavity. This mold com- pression action results in a more uniform pressure ' distribution across the cavity leading to more homoge- nous physical properties and less shrinkage, warpage, and molded-in stresses than are possible with c0n- ventional injection molding. Figure 10 illustrates both the initial injection stage and the subsequent compres- ‘ sion stage. The compression can also take place when the polymer is being injected. The injection—compression molding is suitable for the production of high-precision, high volume prod- ucts, such as compact disks and CD-audio/ROMs, or products that require excellent surface replication and minimum molded-in residual stresses, such as optical lenses (43. 44). Lately, there is a renewed interest in injection-compression molding for molding of thin- walled parts and for in-mold lamination. Varaiation of Injection-Compression Molding ‘ Based on the process variation. injection-COmpres- sion molding can be classified into the following three categories (43): 0 Two-stage sequential injection-compression mold- ing—the simplest type of these three techniques, which consists of separate injection stage and compression stage; 0 Simultaneous injection-compression molding— which activates mold compression while resin is being injected to avoid such surface defects as ‘ the "hesitation" or “witness” mark and to facilitate continuous flow of the polymer melt; \ “““‘ 170 , JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging Injection Molding Processes - Selective injection-compression molding—which allows melt pressure to push the un-pressurized movable core back first before it is activated to compress the melt. Process Physics The primary advantage of injection-compression molding is the ability to produce dimensionally stable,- relatively stress—free parts, at a low pressure, clamp tonnage (typically 20% to 50% lower), and reduced cycle time. Recall that in conventional injection mold— ing, a high injection and packing pressure has to be applied at the machine nozzle to produce sufficient pressure level at the extremities of the cavity to drive the flow and pack out the material. For thin-walled applications such as compact disks, there is typically a significant pressure variation across the part due to high flow resistance. Such a high—pressure variation results in non-uniform packing and volumetric shrink- age within the part, leading to molded-in residual stresses which, in turn, cause part warpage. With in-- jection-compression molding, the packing pressure is now applied in the thickness direction. As a result, in- jection-compression molding permits a lower and yet much more uniform pack/hold pressure distribution that can effectively pack out the mold and minimize molded-in residual stresses and part warpage. Technical Challenges A typical injection-molding machine with precise shot-volume control can be adapted for injection-com- pression molding. However, an additional control mod- ule is required for the mold compression stage. As far as processing is concerned, proper timing and shot volume/weight control are crucial to the quality of the mold parts. Further, the compression delay time (from end of injection to beginning of compression) should be kept as low as possible to avoid hesitation marks. In terms of mold design, a vertical flash face design is necessary to prevent uncontrolled leakage of the melt into the mold partition surface (cf. Fig. 1 0). As an engi: neering tool, computer programs have been developed recently to simulate the melt front advancements and the evolution and distribution of pressure, tempera- ture, flow velocity during the injection filling, com- pression and post-filling stages of the process (see, e.g., 45-47). The molds for this process are relatively expensive and subject to high wear during the com- pression stage. In addition, a needle valve nozzle (or a mechanical stop for the hot runner system) must be used to seal the cavity mechanically and to ensure a precise metering of the melt into the mold. Applicable Materials For thin-wall applications, difficult-to-flow materi- als, such as polycarbonate (PC) and polyetherirnide, have been molded to 0.5 mm. On the other hand, high melt-flow-index PCs are the most suitable materials for compact disks. In addition, most of the lenses are produced with PC or polymethylmethacrylate (PMMA) due to their excellent optical properties. Other mate— rials used in injection—compression molding include: acrylic, polyethylene (PS), polyphenylene ether (PPEJ/ polyamide (PA) blends, polypropylene (PP), as well as thermoplastic rubber and other therrnosetting materi- als (43); ' IN-MOLD DECORATION AND IN-MOLD LAMINATION Process Description In the in-mold decoration process, a pre—decorated carrier laminated onto film stock from the roll is pulled through the mold and positioned precisely be- tween the mold halves. The film stock may be deco- rated by printing methods (such as screen printing, hot stamping, and ink jet printing) prior to molding (48). During the molding stage, the polymer melt contacts - the film and fuses with it so that the decoration can be lifted off from the carrier film upon de-molding and strongly attach to the surface of the molding. In the other in-mold decoration technique called “paintless film molding" (PFM) or laminate painting process, a three—layer coextrusion film with pigment incorporated into layers of clear-coat cap layers and core layer is first thermoformed into the shape of the finished part and then inserted into the Cavity and overmolded with therrnoplastics to produce a final part (49). With in- , mold decoration, it is possible to obtain a high quality, extremely smooth paint finish on thermoplasfic mold- ings ready for assembly without subsequent spray . painting or finishing. In addition to injection mold- ing, in—mold decoration can be used with a variety of other processes such as structural foam injection molding, injection-compression molding, compression molding, blow molding, thermoforming, resin transfer molding, rotational molding (43), and rnicrocellular in- jection molding. Figure 11 shows the preparation of an in—mold decorative film for a cellular phone housing. Instead of using a thin film/foil as does in in-mold decoration, the in-mold lamination process employs a multi-layered textile laminate positioned in the part- ing plane to be overmolded by the polymer melt on the inner side. The decorative laminate can be placed in the mold as a cut sheet, pulled from the roll with nee- dle gripper, or, by means of a clamping frame method. The clamping frame method allows, by means of a . thermoforming operation, defined pre-deformation of the decorative laminate during the mold—closing oper— ation. In-mold lamination is also known as “laminate insert molding" (50) or “fabric molding” [51) for manu- facturing automotive instrument panels and interior panels, respectively. Technical Challenges Comrhon problems with in-mold decoration and in- mold lamination include longer cycle time (due to the insulating effect of the decorative layer), part warpage resulting from unbalanced cooling, air entrapment, as JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 171 Lih-Sheng Turng Fig. 11. An in-mold decoration application for a cellular phone housing. Plastic sheet is screen printed and ‘laminated‘ with a PC-ABS substrate material (top Lefi). Laminated film is themwfomed to required geometry (top right). Therrnoformed film is die-cut (bottom lefi). Formed and cut film (bottom right). (Photos courtesy of Flambeau Micro and John McGavigan, Ltd.) well as damage, creasing, folding, over-stretching, shade changes, ironing effect, thermal damage, and weld—line marks of the decorative layer during mold— ing. To avoid undesirable displacement of the decora- tive laminate, washout,” or “gate wash,” the initial injection speed and the overall injection pressure should be low. Thus, 10w~pressure injection molding, injection— compression molding, compression molding, and cas- cade injection molding with sequential valve-gate open- ing and closing are suitable candidates for in-mold decoration and lamination. Note that the cascade in- jection molding process also eliminates the common problem associated with the weld lines, which’are more evident with the in-mold decoration process. Since the 172 ' I 1 JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging Injection Molding Processes decorative laminate typically sits on the moving platen, the part must be ejected from the sprue side to avoid leaving ejector pin mark on the decorated side. For in-mold decoration or in—mold lamination, mold-wall temperature control is very important, as the decora- tive laminate is sensitive to the temperature. Normally, the melt temperature would exceed the maximum _ temperature that the decorative laminate can with- stand. That is the reason why an additional backing layer is needed, which provides the insulation against the melt while reducing the possibility of melt break- through and folding. Meanwhile, becauSe of the insu- lating effect of the decorative laminate, the mold—wall temperature on the decorated side should be set lower than the other side to promote balanced cooling and avoid part warpage. To ensure the quality of the molded parts with decorative laminate, special considerations I on designs of part and mold, selection of coating and base (carrier) materials, setup of process conditions have to be taken into account. General mold design and processing consideration and guidelines can be found in (52—56). Applicable Materials As far as the base material is concerned, the proc- esses are feasible with virtually all thermoplastics (56). The largest volume resin used in in-mold lamina— tion is PP, mostly for automotive applications. A wide range of other materials including ABS, ABS/PC blends, PS, modified polyphenylene oxide (PPO), poly- esters, polybuiylene terephthalates (PBT), PA 6,‘ PA 66, and, polyethylene (PE), havegalso been used suc— cessfully. Since the recyclability of the laminated com- posite is becoming an important issue, the use of a single polyolefin-based system for the decorative, inter— mediate, and base layers seems to be a viable and cost-effective approach [57). Moreover, clue to the pres- ence of a decorative outer layer, use of regrind as the ' base material is quite appropriate. injection Pressure. Suitable for in-moid Lamination LOW-PRESSURE INJECTION MOLDING Process Description Low-pressure injection molding is essentially an op- timized extension of conventional injection molding. This process differs from conventional (high-pressure) injection molding in that it integrates a series of uSe- ful practices stich as properly programmed injection speed and pressure, a ramps-up-and-down screw RPM and back pressure, generous gate size, a novel type of film- gate, and/or sequential valve gates, (cf. Fig. 12) to keep the injection pressure (and, consequentially, the clamp force) at the lower limits (28, 58]. The major benefits of low-pressure injection molding include sig- nificant reduction of the clamp force tonnage require- ment, less expensive molds and presses, and lower molded-in stress. It also facilitates reduction of manu— facturing cost by incorporatingdecorative film and / or textile into the molded components. I . Ptocess Physics Low-pressure injection molding employs a properly programmed screw rotation speed and the plasticat— ing back pressure profiles to control the melt temper— ature profile of theshot volume (cf. Fig. 12). Note that ' the effective L/D ratio of the reciprocating screw de- creases (by a length of one to five diameters) as the screw retracts in preparation for the shot volume. To compensate for the decreasing plastication length as the screw rotates and moves backward, an increase of back pressure at the screw tips is used to enhance the mixing and shearing. In addition, the injection pres- sure profile is set in such a way that it delivers a slow and controlled injection speed during filling. As illus- ' trated in Fig. 12, the injection rate starts slowly to as- sure an even flow into the mold. Once the melt enters the cavity, the rate is set so that the melt front travels at the same speed throughout the injection stage. Low- pressure injection molding does not employ distinct Bock Pressure ‘\x‘\\‘\\\‘\“\\“\\\‘\‘\\\‘\\‘\‘\‘ L\‘\\\\‘\‘ n \‘“““““‘‘\\\\\\‘\““‘\\“‘\““\\\“\a Constant Meii Front Speed Generous Gote Size Ram Speed Fig. 12. Typical profiles for screw RPM speed, back pressure, injection rate, and injection pressure for low-pressure injection molding. JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 I -173 Lih-Sheng Turng high-pressure packing and holding stages as does with conventional injection molding. The injection pressure profile is generally in the shape of an inverted U, which is aimed at reducing the rapid buildup in the clamp force at the end of the cycle commonly seen in con- ventional injection molding (58). Since the material solidifies almost immediately as it comes into contact with the cold mold wall, the ve- locity at the melt front determines the flow—induced stress and the degree of molecular and fiber orienta- tion in regions adjacent to the part surface. Variable orientation within the part, as a result of changing ve- locity at the melt front during filling, leads to differen- tial shrinkage and, thus, part warpage. Therefore, it is » _ desirable to maintain a constant velocity at the melt . front to generate uniform molecular and fiber orienta- tion. Note that if the injection pressure required to fill a cavity is plotted against the fill time, a U-shape curve typically results, with the minimum value of the re- quired injection pressure occurring at an intermediate fill time. (59). The curve is U—shaped because, on the one hand, a short fill time involves a high melt velocity and thus requires a higher injection pressure to fill the cavity. On the other hand, the injected polymer cools more with a‘prolonged fill time. This leads to a higher melt viscosity and thus requires a higher injection pressure to fill the mold. The optimal fill time corre- . spending to lower injection pressure depends on the material used, as well as the cavity geometry and mold design. ' Low-pressure injection molding optimizes mold filing of large parts and highly viscous materials by sequen- tially opening and closing ‘valve gates at strategically selected locations. The timing of gate opening and. clos- ing is controlled by volume injected instead of using standard time-based controls. The advantage of tim- ing-by-volume approach is that the switching process of the hot runners are always connected to the current stage of filling, regardless, of the speed changes. Con- ceivably, the use of, multiple gates reduces the flow length, thereby reducing the injection pressure require- ment (59). In addition, the sequential opening and closing of the gates eliminate the formation of weld lines. If necessary, the melt injection speed profile can be adjusted from gate to gate. For situations where packing pressure has to be used, all the valve gates can be opened again at the end of the injection stage. Technical Challenges Low-pressure injection molding requires exact shot size with no cushion to be injected into the mold in a carefully controlled, even fashion. Setting the correct injection volume is important, as a very minimum of packing is still necessary for filling the cavity safely. Because the gate does not freeze off after the mold cavity‘fills completely, some compensation for thermal shrinkage is possible. Conceivably, voids or sink marks will occur at thicker sections, ribs, etc. due to the fact that the polymer volumetric shrinkage is not completely compensated for. However, for molding with approxi- mately constant wall thickness, low—pressure injection molding remains suitable (56). Finally, it is claimed that the low-pressure injection molding generally al- lows material to be molded at lower temperature [58). This results in comparable cycle time, even if the cav- ity filling is slower. ' ' Applicable Materials Low-pressure injection molding is well suited to processing a broad range of materials such as ther- moplastics, thermosets, polymer alloys and blends, filled materials, recycled thermoplastics, and even rubber. In particular, PP is used extensively with the low-pressure injection molding due to the lower cost and improved physical and mechanical properties. There are a broad range of compatible textiles and films that can be molded with PP using in-mold lami- nation. For fiber-filled materials, the generous gate I size reduces the possibility of fiber breakage. MICRO-INJECTION MOLDING Micro-injection molding (also called micromolding) produces parts that have overall dimension, func- tional features, or tolerance requirements that are ex- pressed in terms of millimeters or even micro—meters (cf. E'g. 13). Among the various micro-machining and Fig. 13. Various micnrg'ears. (Photos courtesy of Battenfeld of America.) 174 ‘ JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging Iry'ection Molding Processes micro-fabrication processes (see, e.g., 60-61), injec- tion molding possesses the advantages of having a wealth of experiences available in conventional plas- tics technology, standardized process sequences, and high level of automation and short cycle times (62). Due to the miniature characteristics of the molded parts, however, it requires a special molding machine and auxiliary equipment to perform tasks such as shot volume control, evacuation of mold (vacuum), in- jection, ejection, inspection, separation, handling, de- position, orientation, and packaging of molded parts (63). Special techniques are also being used to make the mold inserts and cavities. ' Although there is no clear way to define micro-in- , jection molding, applications of this process can be - broadly categorized into three types of products or com- ponents (64): - Micro-injeciion molded parts (micromolding) that weigh a few mg to a fraction of a gram and possi— bly have dimensions on the micrometer (pm) scale (e.g., micro-gearwheels, micro operating pins), 0 Injection molded parts of conventional size but exhibit micro-structured regions or functional fea— tures (e.g., compact disc with data pits, optical lenses with micro surface features, and wafer for making micro-gearwheels with the Plastic Wafer Technology (65)), , - Micro—precision parts that can have any dimen- sions but have tolerances in the urn scale (e.g., connectors for optical fiber technology). Technical Challenges Due to its unique characteristics, micro-injection molding requires special attention to the machine equipment, processing, and tooling. With diminish- ing part volume and shot size, conventional injection molding machine is no longer an econOmically Viable solution. Micro-injection molding applications require machines equipped with special metering and injec- tion ram (pistons) or screw designs to accurately meter the shot volume and to eliminate problems associated with material degradation. Given the size and Weight of the micro-injection molded parts, ejection is performed by suction pads, electrostatic charging, or blowing out. Large runner and sprues are used to have a shot size large enough to reliably control the process and switch-over. Traditional quality control methOds, such as measuring the part weight, are becoming imprac- tical and are being replaced by the video inspection system. Control of mold-wall temperature (sometimes near or above the melting temperature at the expense of cycle time) is important to avoid premature solidi- fication of ultra thin sections. In particular, the “vario— therm” process employs two oil circuits at different temperatures to heat up and cool the mold at filling and cooling stages, respectively (66). Alternatively, the induction heating technique and electric cartridge heaters have been used to generate a peak mold tem- perature prior to injection (67, 68). Furthermore, mold evacuation may be needed if the wall thickness of the micro-injection molded part is down to 5 pm, the same order as the dimension of vent for air to escape. Other machine requirements include shutoff nozzle to avoid drooling from the nozzle due to high melt tempera— ture, precise alignment and gentle mold movement to avoid deformation of delicate parts, and local clean room enclosure or laminar flow boxes to avoid con- tamination of molded parts. Traditional methods of tooling, such as various ma- fchining methods and electrical discharge machining (EDM), have quickly reached their limitations with decreasing dimensions of mold inserts and cavities. Existing technologies in the field of microelectronics, such as the LIGA technique, have been employed to fabricate mold inserts and cavities for micro-injection molding. LIGA is an acronym derived from the Ger- man wOrds for deep X—ray lithography, electrofonning (electroplating), and injection molding replication (69, 70). Other processes include micro-cutting, ultra-pre- cision machining, laser machining, and micro—EDM technologies (62). Applicable Materials Almost any material suitable for macroscopic mold— ing can be micro-molded. Materials reported for mi- cro—injection molding include polyacetal' (POM), PC, PMMA, PA, liquid crystalline polymers (LCP), poly— etherimide, and silicone rubber. Reaction injection molding has also been applied using material on the basis of acrylate, amides, and silicones (60). MICROCELLULAR MOLDING Process Description Microcellular molding (also commercially known as _ MuCell process) blends “supercn'tical” gas (usually ni- trogen or carbon dioxide) with polymer melt in the machine barrel to create a single-phase polymer—gas solution. During the molding process, the gas forms highly uniform micro-scale cells (bubbles) of 0.1 to 30 microns in diameter. The internal pressure arising from «the foaming eliminates the need of packing pressure while improving the dimensional stability of the molded parts. Basically, microcellular injection molding proc- ess involves the following four distinctive steps (of. Fig. 14): - Gas dissolution—supercritical nitrogen (or car- bon dioxide) is injected into the machine barrel to form a polymer-gas solution for processing. ,- Nucleation—a large number of nucleation sites (orders of magnitude higher than conventional structural foaming proCesses) are formed by rapid and substantial pressure drop when material is V being pushed into the cavity through the nozzle. - Cell growth—cell growth and cell coalescence take place during mold filling and post—filling stages and is controlled by the processing conditions such as melt pressure and temperature. JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, NO. 3 175 Lih-Sheng Turng . ‘4!!! RR Rapid Pressure Drop in Nozzle to Promote Ceii- Nucleation Polymer-Gas Solution Supercrii'rcoi \\\ \‘\“\\\\\\‘\\\ ;\\‘\\\\\\\\\\\‘ ‘\\“\“‘VL‘ * \\“\\\\\\‘\\\\\\n\\\\w L\\\w‘\“\\\\\\\\\u‘ N, or co2 Higher ,B'ock Pressure Special Screw with Large L/D Ratio Fig. 14. Schematic of microcellular injection molding with characteristic microstmctures. Molded part courtesy of Kaysun Corp. I Shaping—the shaping of the part takes place in- side the mold via solidification. ' Process Physics The original rationale of producing microcellular plastics (MCPs) is to reduce the amount of plastic used by creating enough voids smaller than the pre-exist- ing flaw in polymers so that the amount of plastic used could be reduced Without compromising the me- chanical properties (71). While realizing a part weight reduction of 5%—95% by replacing plastics with gas, the microcells also serve as crack arrestors by blunting crack tips, thereby greatly enhancing part toughness.- For example, when properly prepared, microcellular polystyrene (PS) exhibits higher impact strength than that of its unfoamed counterpart (72, 73). In addition, the fatigue life of .microcellular polycarbonate (PC) with a relative foam density of 0.97 exceeds that of the solid polymer (74). Since the gas fills the interstitial sites between polymer molecules, it effectively reduces the viscosity (75, 76) and glass transition tempera- ture of polymer melt (77—79). Therefore, the part can be injection molded with lower temperatures and pres- sures, leading to significant reduction of clamp ton- nage requirement and cycle time. Table 2 lists the comparison of some of the properties between MCPs and its solid unfoamed counterpart. Two common gases used in the microcellular mold— ing process are nitrogen and carbon dioxide. The solu- bility of gas increases with increasing pressure and Table 2. Property Comparisons of Microcellular Plastics With Unfoamed Solid Plastics. Solid Microcellular Plastics Properties Plastics . (MCPs) Specific density ratio (p/psélid) 0.05—0.95 100 35 (PS)*‘ Glass transition temperature Tg (°C) 145 65 (PC/002)"2 90.7 50.8 (PMMA/COZV3 . . . ~ 020*4 ViSCOSIty ratio (n/nsond) ~ 001410016 Tensile strength ratio (Tfl'sond) Elongation ratio at peak (E/Esond) impact strength ratio (l/lsond) Fatigue life ratio (F/Fsofid) _.._.._l_. oo'o'o ~ 1.7 (HDPE/iPP/COZYG ~ 1.1 (HDPE/iPP/002)*5 ~ 5.0"7 ~ 4.0 (PC/cozy?! ———-——————————-————_————_—__———.—__ *1. P8 with 10% co2 (Flef. (77)) *2. Cog/PC at 9 MPa (Ref. (78)) '3. COz/PMMA at 15.2 MPa (Ref. (79)) '4. PS with 4% (:02 (Ref. (75)) '6. Ref, (72) '7. Ref. (72. 73) '5. Under suitable conditions (Ref. (75)) '8. At a specific density of 0.97 (Ref. (74)) . 176 V JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 Special and Emerging Injection Molding Processes decreasing temperature. While the solubility of carbon dioxide is higher, nitrogen tends to provide finer cell structure and gives better» surface finish (71). To en— hance the mechanical blending and shearing of the gas with the polymer within the barrel, a specially designed screw with longer L/D ratio is used. The in- troduction of supercritical gas into polymer melt is provided by a free-standing pumping system, which brings the gas to the supercritical fluid, SCF, state for improved solubility. At this state, gas behaves like a fluid so that it can be metered precisely into polymer melt and yet it has high diffusion rate of a gas for even distribution and dissolving in the melt. Prior to injection, the melt is kept under a higher screw back pressure (12—20 MPa) to maintain the gas in solution. In addition, the process requires a certain injection speed to keep the gas in solution before gas foaming starts in the mold (80). Packing is not de--» sirable as it impedes the foaming of dissolved gas. Instead, the internal pressure of growing cells takes the place of the holding pressure. 7 During injection molding, the formation of micro cells is triggered by thermodynamic instability via sudden change of melt pressure as the polymer-gas solution is injected into the cavity through the nozzle (cf. Fig. 14). The size of the cells is generally inversely propor- tional to the cell density, both of which are determined by cell nucleation and growth and the amount of gas dissolved in the polymer. In principle, the microcellular nucleation can be achieved by a rapid heating of the polymer-gas solution. However, the nucleation by a rapid pressure drop would be more appropriate for the materials, which easily degrade at high temperature. In general, the larger the amount of gas dissolved in a polymer and the greater pressure drop rate through Fig. 15. Glass transition tempera- ture decreases with increasing gas content in the MCP for an amorphous PET. The solid curve is based on data reported in (71, 81). JOURNAL OF INJECTION MOLDING TECHNOLOGY, SEPTEMBER 2001, Vol. 5, No. 3 100 Gloss Transition Temp. (T9) (deg C) 00 O O O I}. O M O 0.00 the nozzle, the greater the cell density and the smaller of the cell size. During molding, the increased melt strength resulting from cooling prevents the cell coa— lescence and preserves the high nucleated cell density of microcellular foams. As shown in Fig. 14, the typi— cal cell diameter is in the order of 10—80 microns (as opposed to 250 microns or more with the conventional structural foam molding process). Typically, larger cells can be found at the center of the part while a thin, non— porous polymer skin layer is typical near part surface as the gas could diffuse out of the surfaces prior to foaming (83). One of the major benefits of microcellular injection molding is the cycle time reduciiOn. Figure 15 shows that the glass transition temperature of an amorphous PET decreases with dissolved gas content (e.g., from 70°C to 15°C with 8% gas) (71, 81). This allows in— jection of such a material at a temperature below the normal processing temperature of neat plastic. Fur- ther, the endothermic reaction of cell nucleation and growth accelerates cooling of the material. As the gas diffuses out of the MCP, the material recovers its glass transition temperature and vitrifies quickly (cf. Fig. 15). Therefore. it requires much less cooling as com- pared with conventional injection molding or struc— tural foam injection molding. Technical Challenges The major challenges in processing MCPs lie in (1) continuous generation of MCPs with proper gas con- tent at an acceptable rate for mass production and (2) control of the state of thermodynamic instability (via temperature and pressure variation) to create fine and uniform microcells throughout the part. In addition, Extension of ' Processing Temperature/ Initial T9 0.02 0.04 0.06 0.08 0.10 Gas Concentration (kg/kg) 177. Lih-Sheng Tumg Fig. 16. The swirling pattern on surfaces of microcellular iry‘ection molded parts. ' ' the process requires changes in the machinery com— ponents, licenses (82), and processing know-how to fully utilize the process benefits. Due to the presence of micro cells, it may have limited applications with . parts that require clarity. In addition, swirling pattern on part surfaces clue to cell formation (cf. Fig. 16) could potentially affect the cosmetic appearance and the cellular rnicrostructure may adversely change the mechanical properties of the molded part. Finally, parts molded by the microcellular molding process need to be stabilized for the gas inside the cells to diffuse out and its pressure equalizes with the atmospheric pres— sure. 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