Moulding solid thermoplastics

Kel-F Trade name for a polymeric chlorotri-fluoroethene, often copolymerized. May be a liquid or solid. Inert to chemical attack and a thermoplastic (Teflon cannot be moulded). 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

Foamed thermoplastic articles have a cellular core with a relatively dense (solid) skin. The foam effect is achieved by the dispersion of inert gas throughout the molten resin directly before moulding. Introduction of the gas is usually carried out either by pre-blending the resin with a chemical blowing agent which releases gas when heated or by direct injection of the gas (usually nitrogen). 

Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

The iQ automotive battery incorporates a micro-chip which monitors and controls the electrochemical process of the lead-acid battery, as well as its physical environmental and operating conditions. The thermal insulation of the battery also plays an important role. Neopolen P, a thermoplastic particle foam from BASF which can be processed completely without blowing agents, was chosen for this application. The foamed battery container calls for a solid frame that can be mounted onto the car body, and this frame is configured as a solid PP injection-moulded article, and fused to the container. 

The route from raw material to end-product proceeds in one of more steps, one of which at least takes place in the fluid condition. Most of the raw materials are available in the solid state granules or powders of thermoplasts, moulding powders or moulding masses of thermosets, bales of rubbers. 

Thennoplastics are heat softening materials which can be repeatedly heated, made mobile and then reset to a solid state by cooling. Under conditions of fabrication these materials can be moulded (shaped in a mould) by temperature and pressure. Examples of thermoplastics are more numerous than thermosets, e.g. polyethylene, polyvinylchloride, polystyrene, polypropylene, nylon, polyester, polyvinylidene chloride, polycarbonate. Thermoplastics may be further divided into homopolymers which involve one type of monomer, e.g. ethylene polymerised to polyethylene, and copolymers, terpolymers, etc., which involve two or more monomers of different chemical substances. Polymerisation producing thermoplastics and thermoset materials usually follows two basic chemical mechanisms, i.e. condensation and addition polymerisation. 

Many cellulose esters have been developed. They include acetates, acetopropionates, acetobutyrates and nitrates. As the crystallinity of cellulose is suppressed by such substitutions, the esters are thermoplastic and can be manufactured by usual methods, e.g. extrusion or moulding. Properties of these compounds depend on the type of substituent and on the degree of substitution. Cellulose nitrates were the first modified polymers. Depending on the degree of substitution, different applications have been developed Highly nitrated cellulose is a well-known explosive, whereas a little less substituted cellulose is used as solid fuel for rockets and the least substituted cellulose is a thermoplastic called celluloid. Celluloid was used to make films for early movies and moulded objects, e.g. dolls and table tennis balls. However, as celluloid is highly flammable, other esters have almost completely replaced cellulose nitrates for manufacturing everyday objects. 

A silicone rubber can be bonded to a solid substrate if it is clamped into or onto it. A very good example is the baby soother. The silicone nipple is injection moulded by a common LR process and then post cured. After that the nipples are clamped into a plastic construction or even overmoulded by thermoplastic. 

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