MGB Thermo Molding Services & RepairMGB Thermo Molding Most thermoplastics have a high molecular weight. The polymer chains associate through intermolecular forces, which weaken rapidly with increased temperature, yielding a viscous liquid. Thus, thermoplastics may be reshaped by heating and are typically used to produce parts by various polymer processing techniques such as injection molding, compression molding, calendering, and extrusion. Thermoplastics differ from thermosetting polymers, which form Fische chemical bonds during the curing process. Thermosets do not melt, but decompose and do not reform upon cooling.
Stress Strain graph of thermoplastic material above its glass transition temperature, Tg, and below its melting point, Tm, the physical properties of a thermoplastic change drastically without an associated phase change. Some thermoplastics do not fully crystallize below the glass transition temperature Tg, retaining some or all of their amorphous strands. Amorphous and semi-amorphous plastics are used when high optical clarity is necessary, as light is scattered strongly by crystallites larger than its wavelength. Amorphous and semi-amorphous plastics are less resistant to chemical attack and environmental stress cracking because they lack a crystalline structure. Brittleness can be decreased with the addition of plasticizers, which increases the mobility of amorphous chain segments to effectively lower Tg. Modification of the polymer through copolymerization or through the addition of non-reactive side chains to monomers before polymerization can also lower Tg. Before these techniques were employed, plastic automobile parts would often crack when exposed to cold temperatures. |
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