Compression molding is essentially a forging process, performed in a heated die that forms a premeasured quality of the polymer. The process is most widely used for the forming of the thermosetting plastics.
The basic procedure for compression molding, consists of placing a measured amount of powder or a compressed perform (called the charge), into the open mold cavity, closing the mold and then applying heat and pressure through a downward moving die (called the force, plug or core) to the material until it softens and is forced fill the mold cavity. In the closed mold, a chemical reaction or polymerization , that cross-links the polymer chains takes place and the material hardens into the required shape. Heat for polymerization or curing is supplied through the walls of the cavity by steam or electricity.
Molding pressures (hydraulic) may be a low as 0.35 kgf / mm2 (3.5 Mpa) for polyester and epoxy but most thermosets require 1.4 to 4.2 kgf/mm2 (13.8 to 41.4 Mpa), depending largely on the filler or plasticizer used. Molding temperatures are from 1100 to 2200*C depending on the plastic, filler, etc.
The complete cycle may take from 10 sec, for small parts under 2.5mm thick to 5 or 10 min for large , thicker parts. This curing time may be reduced as much as 50 % by pre-heating the charge.
Compression molding may be of the positive -type, semi-positive type , or the flash type. In the latter, some of the material is allowed to escape , usually long the moving - die perimeter , over a land or cut-off area, in the form of thin flash or fin which is finally trimmed off . A mold of this type gives closes tolerances and is usually cheapest to make.
Cold Molding :
Although compression molding is mostly done hot, some cold molding is done. A powder or fibers (often of refractory materials) are mixed with a binder and compacted in a cold die. These procedures are followed by curing in a separate oven. This method is not suitable where close tolerance and good surface finish is required.
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