Tham khảo tài liệu 'composites manufacturing - materials, product, and process engineering part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | which isocyanate and polyol are generally mixed in a ratio of 1 1 in a reaction chamber and then rapidly injected into a closed mold containing short or long fiber reinforcements. RRIM and SRIM processes are low-cost and high-volume production methods. The automotive industry is a big market for these processes. Polyurethane is currently used for automotive applications such as bumper beams hoods body panels etc. Unfilled polyurethane is used for various applications including truck wheels seat and furniture cushions mattress foam etc. Polyurethane is also used for wear and impact resistance coatings. Polyurethane can be a thermosetting or thermoplastic resin depending on the functionality of the selected polyols. Thermoplastic-based polyurethane contains linear molecules whereas thermoset-based resin contains crosslinked molecules. Polyurethane is obtained by the reaction between polyisocyanate and a polyhydroxyl group. There are a variety of polyurethanes available by selecting various types of polyisocyanate and polyhydroxyl ingredients. Polyurethane offers excellent wear tear and chemical resistance good toughness and high resilience. Thermoplastic Resins Thermoplastic materials are in general ductile and tougher than thermoset materials and are used for a wide variety of nonstructural applications without fillers and reinforcements. Thermoplastics can be melted by heating and solidified by cooling which render them capable of repeated reshaping and reforming. Thermoplastic molecules do not cross-link and therefore they are flexible and reformable. Thermoplastics can be either amorphous or semicrystalline as shown in Figure . In amorphous thermoplastics molecules are randomly arranged whereas in the crystalline region of semi-crystalline plastics molecules are arranged in an orderly fashion. It is not possible to have 100 crystallinity in plastics because of the complex nature of the molecules. Some of the properties of themoplastics are given in Table
