Compression molding powders/preforms

Preform A compressed tablet or biscuit of plastic composition used for efficiency in handling and accuracy in weighing materials, (v.) To make plastic molding powder into pellets or tablets. 

Metal powder processing techniques in which a preform is molded and sintered are used to process PTFE. Compression molding may also be used to fabricate PTFE parts. Its dispersions are applied by similar techniques to other coatings. Paste extrusion in which PTFE is blended with a hydrocarbon, prior to molding a preform, is used to continuously fabricate PTFE into tubes, tapes, and wire insulation. The hydrocarbon is vaporized before the parts are sintered. PVF is dispersed in a polar latent solvent such as dimethyl acetamide and is melt-extruded as a plastisol, followed by solvent removal by drying. 

Preform v. (1) To make plastic molding powders into pellets, tablets, or biscuits of known mass that facilitate accuracy in compression molding. (2) To prepare by hand cutting of reinforcing cloth or mat, or by blowing chopped fibers onto a contoured screen, the reinforcement for a fiber-reinforced molded object. The reinforcement, which has a shape close to that of the final molded object, is placed into or onto the mold along with the required amount of resin, then wet out and cured. 

Compression molding is a closed-mold filling process in that the resin is placed in the mold in the form of powder, pellets, preforms, bulk... 

A recent development of the dry-laid techniques can be ascribed to the preformed GMT produced by a fluidized bed process [3]. In this case, fiber rovings are cut and mixed with thermoplastic powder, conveyed and deposited by air stream onto a mold. The loose web formed on this mold is heated by hot gas stream prior to being transferred to a matched die compression mold. So, in this case, the web (i.e. the GMT preform) is consolidated and formed simultaneously in the compression molding... 

PTFE powder is put into a mold to make billets. Powder is compressed uniformly (carefully) at pressures of 2,000 to 5,000 psi (14 to 34 MPa). Tbis preform is removed from the mold and sintered by heating unconfined in an oven at temperatures 680 to 715°F (360 to 380°C) for times ranging from a few hours to several days depending on the size and shape of the billet. Billet sizes go from 2 to 1,600 lb (1 to 726 kg), among the largest thermoplastic moldings made of any plastics. Time with temperature variation during cure is closely controlled. 

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