Thermoplastic molten

Flash - In welding thermoplastics, molten plastic that seeps out of the joint area during the welding process. In molding, surplus plastic attached to the molding along the parting line. Flash must usually be removed before parts are considered to be finished. 

Injection Molding. In iajection mol ding a molten thermoplastic is iajected under high pressure iato a steel mold. After the plastic solidifies, the mold is opeaed and a part ia the shape of the mold cavity is removed. Geaeral descriptioas of the process and related equipment are given ia Refereaces 29—34. 

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

Examples of plasticizers include adsorbed water and ethylene glycol for vinyl binders, stearic acid and oleic acid for wax binders, glycerine and ethylene glycol for clay bodies, and molten oils and waxes for thermoplastic polymers used in injection mol ding. 

The most chemical-resistant plastic commercially available today is tetrafluoroethylene or TFE (Teflon). This thermoplastic is practically unaffected by all alkahes and acids except fluorine and chlorine gas at elevated temperatures and molten metals. It retains its properties up to 260°C (500°F). Chlorotrifluoroethylene or CTFE (Kel-F, Plaskon) also possesses excellent corrosion resistance to almost all acids and alkalies up to 180°C (350°F). A Teflon derivative has been developed from the copolymerization of tetrafluoroethylene and hexafluoropropylene. This resin, FEP, has similar properties to TFE except that it is not recommended for continuous exposures at temperatures above 200°C (400°F). Also, FEP can be extruded on conventional extrusion equipment, while TFE parts must be made by comphcated powder-metallurgy techniques. Another version is poly-vinylidene fluoride, or PVF2 (Kynar), which has excellent resistance to alkahes and acids to 150°C (300°F). It can be extruded. A more recent development is a copolymer of CTFE and ethylene (Halar). This material has excellent resistance to strong inorganic acids, bases, and salts up to 150°C. It also can be extruded. 

The melt-spinning process used to convert mesophase pitch into fiber form is similar to that employed for many thermoplastic polymers. Normally, an extruder melts the pitch and pumps it into the spin pack. Typically, the molten pitch is filtered before being extruded through a multi-holed spinnerette. The pitch is subjected to high extensional and shear stresses as it approaches and flows through the spinnerette capillaries. The associated torques tend to orient the liquid crystalline pitch in a regular transverse pattern. Upon emerging from the... 

Hot melts are 100% solid thermoplastic compounds. They are compounded and applied in the molten state at elevated temperatures. The resultant properties are obtained by cooling. Due to the quick cooling, bonds can be established in a very short time. 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

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). 

Injection moulding The injection moulder is a machine which first melts a thermoplastic and then injects that molten polymer into a mould. Such items as baskets, bowls, bins, telephones and electronic housings are produced by this technique. It can be used for lining valves. In this case the valve would be used as part of the mould. Very thick coatings are produced which give chemical resistance to the valve. At the same time, the metal valve housing will protect the valve from mechanical damage. 

The properties of molten thermoplastic are independent of the concentration of gas dissolved therein. This assertion is probably true, and very original. It clearly requires special discussion. 

The number-average molar mass of thermoplastic polyesters varies from about 20,000 for film and fiber applications to 40,000 for injection-molding or blowmolding resins. Relationships between intrinsic or molten viscosity and molar mass have been published for PET,131-136 PBT,135,137 and PEN.138... 

Fibers are made from thermoplastic polymers. The polymers are made into fiber form, normally by extrusion of molten polymer or a polymer solution through tiny holes. The resulting fiber is stretched to orient the molecules. This orientation of the molecules lines up the polymer molecules and produces the strength and other properties needed in a textile yam. 

The blanks of this consolidated material can be shaped in steel tools by heating under pressure to a semi-molten state. The matrix is generally polypropylene (in more than 95% of GMTs) or, more rarely, thermoplastic polyester. 

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