Plastic Product Material and Process Selection Handbook Part 11 pps

t,,o t,d 4~ "o Table 8.I Examples o f r'gid Flas:'c fcarr prcpe'ties Phenolic ASTM Properly Density, [b.gfLr (kg/m:'l Tensile slret~gth, p~i ~MPal (:~nprcssi , :-2"~ ~ APS MOLDED PART Figure 8.3 View of PS beads in a perforated mold cavity that are expanding when subjected to steam heat The heat cycle is followed by the cooling cycle Because EPS is an excellent thermal insulator, it takes a relatively long time to remove the heat before &molding If the heat was to remain, the product would distort Cooling is usually by water spray over the mold To facilitate removal, particularly for complex shapes mold release agents are used The final density is about 0.7 to 10 lb/ft (11 to 160 k g / m 3) or in normal molding the density of the product will closely approximate the bulk density of the unheated beads EPS molds have double walls; the inner wall is the actual shape to be formed It is perforated with vents to allow steam to penetrate the foam; the hot gases that develop leave the product through these vents Thus, the double walls allow for encasing the steam that is delivered to the mold and in turn flows throw the vents Before removal from the mold, products are stabilized by creating a vacuum and spraying water on the inner mold wall, causing diffusion of gasses from the many cells as well as a reduction in temperature EPS molding generates pressures of less than 30 psi (2 kPa) in most mold applications This low pressure allows the use of inexpensive molds such as aluminum To process the other expandable plastic foams (EPFs), such as PE, PP, and PMMA, the equipment for EPS can be used with only slight modifications Pentanc has been used as a gas-blowing agent to produce different foamed plastics or elastomers, particularly in EPS Pentane is used to produce certain rigid polyurethane insulation foams as an alternate to the past used CFC blowing agents As an cxample during P U R processing, it can be added separately to the mixture bypassing on the high pressure side of the mixing head, thereby bypassing explosive-proof mix chamber and polyol metering pump Because of pentane's flammability and chemical makeup, no problems exist when properly processed It is Foaming halogen-flee, non-polar, and accepted as non-toxic The flammability of foam products can be controlled through proper use of flameretardants It is a hydrocarbon in the methane series occurring in petroleum Expandable polyethylene (EPE) is a low-density, semi-rigid, closed-cell, weather-stable PE homopolymer that is easier to compress than EPS but less compliant than flexible PUR EPE foam follows a similar processing technique as that of EPS starting with the use PE beads Conventional EPS molding presses can process EPE with the addition of a modified filling device, provision for higher molding pressure, and postmold oven curing Their density range is 1.8 to 7.8 lb/ft (29 to 120 kg/m3) The most commonly used density is 1.8 lb/ft (29 kg/m3) Expanded polyethylene copolymcr (EPC) is a / wt% of polyethylene and polystyrene Combining the properties of both plastics widens the selection of resilient materials for packaging engineers EPC is a material that falls between EPS and EPE in performance, but exceeds both materials in toughness The tensile and puncture resistance of EPC is superior to all of the moldable resilient foams available It has good multiple-impact performance characteristics with better memory than EPS, but not as good as EPE The cushion performance of EPC parallels EPE but at higher levels even after repeated drops EPC is especially good for reusable material-handling trays and packaging applications that require a nonabrasive, solvent-resistant, impact absorbing material with a superior toughness that elongates, compresses, and flexes without material fatigue EPC is a low density, semi-rigid, closed-cell material that requires refrigerated storage below 40F (4.4C) in its raw granular form and has a shelf life of at least one month The material expansion and conveying of the sensitive pre-puff requires special handling and molding within a short period of time The molding process and equipment are similar to EPS, but with slower molding cycles Expandable styrene-acrylonitrile (ESAN) is a moldable, lightweight, semi-rigid, closed-cell, highly resilient plastic foam The raw materials can be stored without refrigeration and they have a shelf life of about months provided some simple precautions are observed as provided by the material supplier Processing is similar to EPS except that it has extended fabricating cycles due to the higher level of blowing agent Postmold oven curing is not necessary A low density of more than 40 times expansion can be reached during the preexpanding phase, as low as 1.0 lb/ft (16 k g / r n 3) on the first pass through the expander and 0.8 lb/ft (13 k g / m 3) on the second pass 360 Plastic Product Material and Process Selection Handbook Molding Foam molding operations arc those in which a liquid mixture of foam components is used It is poured into a mold cavity to form a cellular shaped product The molded product is later removed after setting or curing As reviewed in the case of expandable polystyrene beads the preexpanded or virgin beads are poured into a mold and heated to form the desired object In this case, liquids arc not used, although the freeflowing beads might be considered a fluid A number of flexible foam components arc molded, rather than fabricated from slabstock For a production quantity of intricately shaped products, molding results in savings up to 15%, compared to slabstock with secondary operations (Chapter 18) Rigid foam can also be molded, although such applications are fewer in number In a typical low pressure molding operation a mold is preheated and coated with a mold release agent A preheated amount of liquid mix is poured into the mold, the mold is closed and the ingredients foamed to the mold cavity configuration After curing and cooling, the part is stripped from the mold Scrap rate could be from zero to up to wt% depending on mold design Mold temperatures for commodity plastics are usually in the range of 150 to 160F (66 to 71C) The best results are usually obtained when the mold release agent is spray-applied to the clean, warm mold just before each pour Care must bc taken, however, to remove all solvents from the mold release compound before the foam components are poured into the mold cavity Curing of the molded TS polyurethane foam product is usually carried out in two stages, a precure of 15 to 20 minutes of about 270F (132C), permitting removal of the part from the mold, and a final cure of 60 minutes at the same temperature Mold release of polyurethane foams can bc difficult since uncured polyurethane has well-known adhesive properties Two basic types of mold release agent are used The first requires the hot molding to bc stripped from the mold Mixtures of paraffin and microcrystallinc waxes arc used for this technique, in which hot wax releases the part from the mold The molds must be heated and coated with wax before each filling There is a tendency, however, for the paraffin wax to be slowly oxidized by the repeated heating For this reason a release agent containing an antioxidant should be used The breakdown products formed have no release properties, and it is important to use a thin layer of wax each time a molding procedure is carried out A release agent, such as polyethylene waxes, is used if the mold must be stripped away when cold In this case the foam comes away from the Foaming 361 release agent Mold release problems can be reduced by adding small amounts of dibutyl tin dilauratc, which promotes curing at the surface of the mold and thus improves mold release Slabstock molding requires very little curing A bank of infrared heaters suspended above the conveyor is often sufficient to facilitate curing In conventional molding, however, the exotherm generated is not sufficient to cure the foam, and external heat must be applied Microwave curing permits a reduction in curing time from 20 to minutes Plastic molds are used with a gel coat of epoxy resin containing iron powder The plastic molds must be cooled to an even temperature before refilling There is evidence to suggest that foam cured with microwaves have properties slightly superior to foam cured by conventional heating, especially in compression set Dielectric heating has also been developed for use in curing Injection Molding The conventional and slightly modified injection molding machines (IMMs) are used to produce different types of foams (Chapter 4) Low pressure or short-shot conventional foam IM (injection molding) processing methods arc the most commonly used because they arc easy, simple, and best suited to economical production, particularly of large, complex, 3-D products A controlled melt mixture (plastic and blowing agent) is injected into a mold cavity from the IM plasticator (Chapter 3) creating a low cavity pressure usually 1.4 to 3.5 MPa (200 to 500 psi) As shown in Figure 8.4 two steps occur using a reciprocating IMM First the plastic melt with blowing agent (nitrogen, carbon dioxide, or hydrocarbon gas) is directed into an accumulator The next step has the accumulator very quickly deliver the hot plastic mix into the mold cavity Also used are two-stage IMMs (Chapter 4) ~ [I I ccumu,a,or Accumulator ! -~ l~I Filling accumulator the _~;-7/f //llillilj t'~'~valve i I P'i'a;icaTor- ] Filling mold the Figure 8.4 Schematicof foam reciprocatinginjection molding machinefor low pressure 362 Plastic Product Material and Process Selection Handbook Along with about 0.5wt% of CBA, this mixture can be injected directly from the barrel of a conventional injection molding machine (with limited modifications) or via an accumulator (two-stage IMM) The mixture only partially fills the mold (short shot), and the gas bubbles, having been at higher pressure, expand immediately and fill the cavity As the cells collapse against the mold surface, a relatively solid skin of melt is formed over the rigid foam core Skin thickness is controlled by the amount of melt injected, mold temperature, type and amount of blowing agent, temperature and pressure of the melt, and capabilities of the molding machine, particularly its speed of injection There is low pressure with coinjcction This technique involves the usual separate injection of two compatible plastics that are coinjectcd using two injection plasticators (Chapter 4) A solid plastic is injected from one plasticator to form a solid, smooth skin against the surfaces of the mold cavity Simultaneously a second material, a measured short shot containing a blowing agent, is injected to form the foamed core This approach can also take a relatively full core shot and have the mold open [as in injection-compression molding (Chapter 4) ] after the skin solidifies, having the melted core expand with mold-opening action There is low pressure with surface finish in low-pressure surface-finish (LPSF) molding, not using coinjection or injection-compression molding (Chapter 4), the volume of the molding cavity is always larger than the volume of the plastic in the unfoamed state The low pressure allows microbubbles to nucleate and grow Foam expansion occurs during filling, and growing bubbles arc carried to the mold surface, creating unacceptable surface irregularities and imperfections called splay or swirl pattern The irregularities can be seen and felt; the surface roughness can be as much as 1000 pin (25 }am) Products needing smooth, finished surfaces require secondary operations, usually sanding, filling, and painting There are techniques to improve surface appearance during fabrication The principal process variables are melt and mold temperatures, injection rate, the nature or type of blowing agent, and its concentration Cyclic heating and cooling of the mold surface and direct injection of blowing agent into the melt as it is being injected into the mold are two of the methods used The gas counterprcssure IM method uses a scaled mold pressurized to 2.8 to 3.5 MPa (400 to 500 psi) with an inert gas, sufficient pressure to suppress foaming as the plastic mix enters the mold cavity After the measured shot is injected, the mold pressure is released, allowing the instantaneous foaming to form the core between the already formed Foaming solid skins [Figure 8.5(a)] The mold action is similar to injectioncompression molding Another technique is gas injection molding, used to develop similar foamed structures Once the plastic at the mold surface has solidified, the gas pressure is released to permit the remaining melt mix to foam, creating the product's core Figure 8.5 (a) Schematic of gas counterpressure foam injection molding (Cashiers Structural Foam patent) (b) Example of an IMM modified nozzle that handles simultaneously the melt and gas (c) Microcellular foaming system directing the melt-gas through its shutoff nozzle into the mold cavity 364 Plastic Product Material and Process Selection Handbook There arc different gas operating patented foam IMM systems Examplcs arc shown in Figures 8.5(b) and 8.5(c) The Figure 8.5(b) is a Hoover/ Carbide patent Figure 8.5(c) is the patented Demag Ergotech process for molding microcellular foam products 247 Liquid CO2 enters the melt downstream of a conventional or two-stage IMM plasticator (Chapter 3) There is Trexel Inc., Woburn, MA a licensor of the MuCell microcellular foam IMM or extruder process It injects C O o r N into the IMM plasticator There exists a licensing agreement between Trexel and Demag Ergotech The high pressure molding system that uses an expandable mold is a takeoff of conventional IM (Chapter 4) It starts by injecting the heated melt mix (with blowing agent) into the mold, creating a cavity pressure higher than the blowing-agent gas pressure (usually much higher) This action is to ensure no loss in gas pressure during injection Pressure for certain machines could be 5,000 to 20,000 psi (34.5 to 138 MPa) With the mold being entirely filled the melt next to the cavity wall forms a solid skin as it starts solidifying against the mold surfaces As soon as the skin surface hardens to a desired thickness, a second step occurs where the cavity mold opens reducing the pressure allowing the remaining melt to foam between the sldns The opening occurs whereby the male plug retracts but remains within the female cavity as in injection molding (Chapter 4) The mold can be modified to meet certain different shapes The molding can be made either by withdrawing cores or by special press motions that partially open the mold halves (such as the compression molds used in coining to provide 2-D action; 3-D mold actions are also used) The degree of foam density, wall thickness, and surface finish depends on the foam mixture (constituents and amounts) The machine controls the time cycle and the mold action required Structural-web molding is a low pressure foam molding method It is the phrase usually used to identify the gap between structural foam (SF) molding and injection molding Its surface does not have the usual SF characteristic swirl pattern It can produce very large, lightweight parts with smooth surfaces like conventional injection molded parts Reaction injection molding (RIM) includes fabricating rigid, flexible microcellular, and rigid microcellular polyurethane foams The process embodies high-pressure-impingement mixing of the liquid components before they are injected into the mold RIM has advantages over the standard low-pressure mechanical-mixing systems in that larger parts are possible, mold cycles are shorter, there is no need for solventcleaning cycles, surface finishes are improved, and rapid injection into the mold is possible (Chapter 12) Foaming 365 Liquid injection molding (LIM) is a variation of the RIM process The major difference is in the manner in which the liquid components are mixed In the LIM process the entire shot is mixed in a chamber before injection into the mold, rather than being continuously mixed and injected, as in the RIM process (Chapter 16) Structural Foam Different plastics are used such as PSs and PVCs to produce building trim and moldings, picture frames, etc The most important structural foam molding processes have been reviewed They are the low and high pressure injection molding processes Structural foams with solid skins and cellular cores are extruded in the form of profiles, pipes, tubes, sheet, etc using conventional extruders that include handling the blowing agents As reviewed with IMMs the blowing agents can be mixed with the plastic as it enters the hopper, enters the screw plasticator melt, or use a mixing device to mix the melt with the blowing agent Foam Reservoir Molding Foamed reservoir molding is also lmown as elastic reservoir molding It has had limited use This process creates a sandwich of plasticimpregnated, open-celled, flexible plastic foam between the face layers such as fibrous reinforcements When this plastic composite is placed in a heated mold and squeezed, the foam is compressed, forcing the plastic and air outward and into the reinforcement The elastic foam exerts sufficient pressure to force the plastic-impregnated reinforcement into contact with the mold surface and simultaneously removing entrapped air Polyurethane Process When processing PURs different processing techniques arc used The specific processes include free-rise, liquid (pour-in-place), froth, and spray foaming techniques When injected, there is the common injection molding process and others When the PUR liquid ingredients are mixed, gases are produced which cause the mass to expand as it stiffens and hardens The reaction is complete in a few minutes Figure 8.6 shows different process systems (top/liquid, c e n t e r / f r o t h / bottom/spray) where in each case the blowing agent can be added to either or both components A and B In a sandwich structure the liquid mix is pourcd between the cover sheets and foams between them, bonding directly to the sheets without 366 Plastic Product Material and Process Selection Handbook QJ C.J O D_ C~ E E c~ O cc~ cQJ C9 ct~ cc~ cc.J ca-J C) N C~ Foaming an adhesive As foamed-in-place materials expand, they can exert appreciable pressure, so the sandwich sheets have to be held in a rigid frame to prevent bulging until the reaction is complete To overcome this pressure, the liquid mix may be allowed to form a froth of almost its ultimate volume prior to pouring The result is little or no pressure and rigid foam Flexible PUR foam, such as that used in upholstery, is made by continuous deposition on a belt before being cut into blocks or sheets of desired shape and size Other foam materials may be handled in somewhat similar ways, and may be pre-foamed or foamed-in-place Water is the component-forming blowing gas in the formulation for PUR soft, flexible foam; it forms carbon dioxide with isocyanates The evolution of heat and the change in temperature caused by this gasgenerating reaction mean that other blowing agents (such as dichloromcthanc) have to be used, along with coolants, in order to produce low-density foam of less than 20 k g / m (Figure 8.7) 80L= 60 > l.,- m z 40 l,,IJ 20 t t 10 THICKNESS (cm) Figure 8~7 Example of flexible foam density profile A rigid, foamed crosslinked PUR, usually with closed cells, is formed by the reaction of a diisocyanatc and often methane diisocyanate (MDI) or polymeric MDI with polyester or more usually with a polycther polyol Foaming may result from the water, which reacts with isocyanatc groups to form carbon dioxide but is usually the result of using other 367 368 Plastic Product Material and Process Selection Handbook blowing agents, sometimes in combination with water They are more rigid than flexible foams because they contain more crosslinks This is accomplished by the use of polyols, usually polyoxypropylene glycols of low molecular weight, which are highly branched by mixing of higher functionality comonomers (such as sorbitol or pentacrythritol) ... with plastic PROCESS _ = _ _ _ _ Based on production output, the most important processes are extrusion (using PUR, PS, PE, PVC, CA, etc.), I 352 Plastic Product Material and Process Selection Handbook. .. as a replacement for molded-paper-pulp board in meat and produce trays and egg cartons 346 Plastic Product Material and Process Selection Handbook Popular is roof-deck PS foam insulation where... emerges from the die lips the stretching and orienting take place 354 Plastic Product Material and Process Selection Handbook Applications for these foamed products arc many An example is meat trays