Molded parts processing aids

As discussed in Chapter 10, a wide variety of additives is used in the polymer industry. Stabilizers, waxes, and processing aids reduce degradation of the polymer during processing and use. Dyes and pigments provide the many hues that we observe in synthetic fabrics and molded articles, such as household containers and toys. Functional additives, such as glass fibers, carbon black, and metakaolins can improve dimensional stability, modulus, conductivity, or electrical resistivity of the polymer. Fillers can reduce the cost of the final part by replacing expensive resins with inexpensive materials such as wood flour and calcium carbonate. The additives chosen will depend on the properties desired. 

As with other mold operation process mold release agents are usually required because the plastic melt can adhere to the surface of the mold cavity. Molds with very little or no draft usually require a release agent. Like other molding operations, a textured cavity can provide a textured product surface. Most texturing of cavities is by chemical etching so it is important to use the appropriate mold material to create a particular texture. An effective release is needed at the parting line to aid in demolding. 

The final product may take any one of a vast multitude of final forms. These include, but are not limited to, construction materials, toys, automotive parts, kitchen aids, industrial parts, sports and recreational equipment, containers of all sizes and description, and an unlimited variety of packaging products for foods, drugs, cosmetics, and other consumer products. The processes that are used to produce or mold or fabricate the final products are also varied and depend on the design and purpose of the product. Some of these processes include extrusion, compression molding, roto-molding, blow molding, sheeting, and films. 

Acrylics. Acetone is converted via the intermediate acetone cyanohydrin to the monomer methyl methacrylate (MMA) [80-62-6]. The MMA is polymerized to poly(methyl methacrylate) (PMMA) to make the familiar clear acrylic sheet. PMMA is also used in molding and extrusion powders. Hydrolysis of acetone cyanohydrin gives methacrylic acid (MAA), a monomer which goes direcdy into acrylic latexes, carboxylated styrene—butadiene polymers, or ethylene—MAA ionomers. As part of the methacrylic structure, acetone is found in the following major end use products acrylic sheet molding resins, impact modifiers and processing aids, acrylic film, ABS and polyester resin modifiers, surface coatings, acrylic lacquers, emulsion polymers, petroleum chemicals, and various copolymers (see Methacrylic acid and derivatives Methacrylic polymers). 

Processing aids for dosed-mold processes are sometimes the key additives that make a molded product both possible and profitable In injection molding particularly, melted resin must flow quickly from saew to mold, and parts must release from the mold quickly and cleanly. And these things must happen with compounds that may be highly filled, viscous when melted, or fast-shrinking when solidifying. 

Internal melt lubrication Metal stearates fatty amides, ester, acids, or alcohols polymeric agents Increased output Reduced melt viscosity and temperature Process energy savings and reduced torque Improved mold-filling Part-surface contamination possible (and reduced printability) Lower physical properties (or higher, depending on the processing aid)... 

Uses Resin for thin-film coatings, molded elec, parts, coatings, motor wire varnish, transformer impregnating improves die swell in rubber extrusions process aid plasticizer Features Thermosetting... 

Uses Inj. molding and extrusion resin for cosmetic pkg., fan blades, toys and games, business machines, interior refrigerator parts, medical parts, beverage tumblers, food containers, tableware, dinnerware, containers, automotive parts, cassettes PVC modifier and processing aid color cone, carrier in resinous/polymeric food-contact coatings plasticizer in food-contact rubber articles for repeated use... 

Processing aids to assist material flow during extrusion or injection molding. The internal and external lubricants are recognized. Internally lubricated resins use oils. Teflon , M0S2, or other materials to give the molded part a lower coefficient of friction. The external lubricant can be a sohd, such as sodium or zinc stearate, a fluoropolymer, or silicone resin or liquid. 

Kabanemi KK, Crochet Ml (1992) Thermoviscoelastic calculation of residual stresses and residual shapes of injection molded parts. Int Polym Process 7 60-70 Kaelble DH (1985) Computer-aided design of polymers and composites. Marcel Dekker, New York... 

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