Compression molding is the process of molding in which a preheated polymer is placed into an open, heated mold cavity. The mold is then closed with a top plug and compressed in order to have the material contact all areas of the mold. This process is able to produce parts with a wide array of lengths, thicknesses, and complexities. The objects it produces are also high in strength, making it an attractive process for a number of different industries. Thermoset composites are the most common type of material used in compression molding.
Compression molding is an old and widely used molding process for thermosetting plastics. Its applications also include thermoplastic phonograph records, rubber tires, and various polymer matrix composite mold parts. The process, illustrated for a TS plastic; consists of (1) loading a precise amount of molding compound, called the charge, into the bottom half of a heated mold; (2) bringing the compressing mold halves together to compress the charge, forcing it to flow and conform to the shape of the cavity; (3) heating the charge by means of the hot mold to polymerize and cure the material into a solidified part; and (4) opening the compressing molds halves and removing the part from the cavity.
The initial charge of molding compound can be in any of several forms, including powders or pellets, liquid, or preform. The amount of polymer must be precisely controlled to obtain repeatable consistency in the molded product. It has become common practice to preheat the charge prior to 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 (borrowed from injection molding) is also used to meter the amount of the charge.
Pressing moulds (pressing tools) presses are oriented vertically and contain two platens to which the mold halves are fastened. The presses involve either of two types of actuation: (1) upstroke of the bottom platen or (2) downstroke of the top platen, the former being the more common machine configuration. They are generally powered by a hydraulic cylinder that can be designed to provide clamping capacities up to several hundred tons.
Molds for compression molding are generally simpler than their injection mold counterparts. There is no sprue and runner system in a compression mold and the process itself is generally limited to simpler part geometries due to the lower flow capabilities of the starting thermosetting materials. However, provision must be made for heating the mold, usually accomplished by electric resistance heating, steam, or hot oil circulation. Compression molds can be classified as hand molds, used for trial runs; semiautomatic, in which the press follows a programmed cycle but the operator manually loads and unloads the press; and automatic, which operate under a fully automatic press cycle (including automatic loading and unloading).
Materials for compression molding include phenolics, melamine, urea-formaldehyde, epoxies, urethanes, and elastomers. Typical TS plastic moldings include electric plugs, sockets, and housings; pot handles, and dinnerware plates. Advantages noted for compression molding in these applications include: molds that are simpler, less expensive, and require low maintenance; less scrap; and low residual stresses in the molded parts (thus favoring this process for flat thin parts such as phonograph records). A typical disadvantage is longer cycle times and therefore lower production rates than injection molding.
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