It is widely used molding process for thermosetting plastics. The process consists of
Loading a precise amount of molding compound, called the charge, into the bottom half of a heated mold
Bringing the mold halves together to compress the charge, forcing it to flow and conform to the shape of the cavity
Heating the charge by means of the hot mold to polymerize and cure the material into a solidified part
Opening the mold halves and removing the part from the cavity.
Compression molding for thermoplastics are
Charge is loaded
Charge is compressed and cured
Opening the mold halves and removing the part from the cavity.
The initial charge of molding compound may be in any of many forms including powders, liquid.
The number of polymer should be precisely controlled to get repeatable consistency in the molded product.
It has become common practice to preheat the charge before its placement into the mold.
This softens the polymer and shortens the production cycle time. Preheating methods include infrared heaters, convection heating in an oven and use of a heated rotating screw in a barrel.
The latter technique is also used to meter the amount of the charge.
Compression molding presses are oriented vertically and contain two platens to that the mold halves are fastened.
The presses involve either of two types of actuation
Upstroke of the bottom platen
Down-stroke of the top platen the former being the more common machine configuration.
They are typically powered by a hydraulic cylinder which will designed to provide clamping capacities up to many hundred tons.
Molds for compression molding are typically easier than their injection mold counterparts.
There’s no sprue and runner system during a compression mold and therefore the method itself is mostly limited to simpler part geometries because of the lower flow capabilities of the starting thermosetting materials.
However, provision should be made for heating the mold sometimes accomplished by electric resistance heating, steam, or hot oil circulation.
Compression molds may be classified as hand molds, used for trial runs semiautomatic, in which the press follows a programmed cycle however the operator manually loads and unloads the press and automatic which operate under a fully automatic press cycle.
Materials for compression molding include phenolics, melamine, epoxies, urethanes, and elastomers.
Typical TS plastic moldings include electric plugs, sockets, housings, pot handles and dinnerware plates.
ADVANTAGES :
Simple method
Less expensive
Need low maintenance
Less scrap
Low residual stresses in the molded parts .
DISADVANTAGES :
Longer cycle times and therefore lower production rates than injection molding.
APPLICATIONS :
Thermoplastic phonograph records
Rubber tires
Various polymer matrix composite parts.
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