Polystyrene injection molding

Fabrication. Acetal resins are most commonly fabricated by injection mol ding. A homogeneous melt is essential for optimum appearance and for performance of injection molded parts. A screw compression ratio of no less than 3 1 is advised and the size of the injection molded shot should be 50 to 75% of the rated capacity (based on polystyrene) of the barrel. 

There are five basic types of polystyrene foams produced in a wide range of densities and employed in a wide variety of apphcations (/) extmded polystyrene board (2) extmded polystyrene sheet (2) expanded bead mol ding (4) injection molded stmctural foam and (5) expanded polystyrene loose-fiU packaging. 

FIG. 41 Antistatic effect of alkanesulfonates in polystyrene. Surface resistance according to DIN 53 482. Test specimen 1-mm injection-molded sheets, stored for 1 day at 23°C and 50% relative humidity. 

Siloxane containing polyester, poly(alkylene oxide) and polystyrene type copolymers have been used to improve the heat resistance, lubricity and flow properties of epoxy resin powder coatings 43). Thermally stable polyester-polysiloxane segmented copolymers have been shown to improve the flow, antifriction properties and scratch resistance of acrylic based auto repair lacquers 408). Organohydroxy-terminated siloxanes are also effective internal mold release agents in polyurethane reaction injection molding processes 409). 

In 2002, the world production of polymers (not including synthetic libers and rubbers) was ca. 190 million metric tons. Of these, the combined production of poly(ethylene terephthalate), low- and high-density polyethyelene, polypropylene, poly(vinyl chloride), polystyrene, and polyurethane was 152.3 milhon metric tons [1]. These synthetic, petroleum-based polymers are used, inter alia, as engineering plastics, for packing, in the construction-, car-, truck- and food-industry. They are chemically very stable, and can be processed by injection molding, and by extrusion from the melt in a variety of forms. These attractive features, however, are associated with two main problems ... 

Processing and Uses. Polystyrene is an outstandingly good injection molding material. In this process the material is melted and forced through dies into a mold where it cools and solidifies. The process is very adaptable and is particularly suitable for the mass-production of small articles. Components for the electrical industry, objects for everyday use (buttons, combs, tube tops, fountain pens, etc.) and also articles for the military field (components for detonators in grenades, parts of gas-masks etc.) were manufactured in this way at that time. 

Processing and Application. Polystyrene and the other thermoplastics would not have been able to enter on their triumphal march if the manufacturers of processing equipment had not been able to provide efficient fabrication methods. A decisive factor was that between 1950 and 1960 the injection molding machine with a screw preplasticizer was developed and extrusion through slit dies to give sheeting or film followed by thermoforming was evolved. 

Processing of impact-resistant polystyrene to large injection moldings (weighing up to 10 kg). 

An ounce is a unit used to represent the capacity of an injection molding machine, showing the polystyrene equivalent weight of polymer for one shot . 

Spalding, M. A., Nelb, R.G., Patterson, J., Youngson, C., and Zawisza, M. J., Processing Ignition-Resistant Polystyrene Resins In Injection Molding Machines, SPE ANTEC Tech. Papers, 47, 476 (2001)... 

For higher throughput applications, injection-molded plastic microtiter plates have served as the formats of choice for automated assay development. Thermoplastics such as polystyrene, polycarbonate, and polypropylene are used for a variety of purposes including storage and assay plates, lids, pipette tips, and Eppendorf PCR tubes. Polystyrene plates are used for cell culture and ELISAs. Polycarbonate reagent bottles are popular, while polypropylene storage plates and PCR tubes are standards. 

Monsanto Hi-Flow 77 Polystyrene. The viscosity-average molecular weight found in this work was 284,000. The ratio Mw/Mx reported by the manufacturer is 2.67. The samples were prepared by injection molding in a 1-ounce Mini-Jector molding machine, followed by annealing for 24 hours at 75°C. 

Du Pont Lucite 130 Poly (methyl methacrylate). This material was injection-molded in the same way as the polystyrene. No molecular weight measurements were made. 

Foams have limited use for these purposes. Rigid cellular PVC is good as a thermal barrier but not for structural parts. Doors and frames of structural molded foam, eg, foamed high impact polystyrene, can be made by injection molding, with recesses for hinges, striker plates, and miter corners. Solid polystyrene and structural foam-molded polyurethane have been molded for door frames. 

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