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COMPRESSION AND TRANSFER MOLDING. Compression Molding. The process of molding a material in a confined shape by applying pressure and usually heat. Almost exclusively for thermoset materials Used to produce mainly electrical products. Compression Molding. Background.
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Compression Molding • The process of molding a material in a confined shape by applying pressure and usually heat. • Almost exclusively for thermoset materials • Used to produce mainly electrical products
Background • One of the oldest known molding processes. • Widely used to produce articles from thermosetting and thermoplastic materials. • The plastic material is placed in a mold cavity and formed by heat & pressure. • Most compression-molding equipment is typically sold by the press or platen rating. • A force of 2900psi is usually required for moldings up to 1inch (25 mm) thick. An added 725psi should be provided for each 1inch (25 mm) increase.
In terms of processing, the past begins with the manufacturing process known as “compression molding” The very first plastic products included items such as, buttons, brush and mirror handles that were compression molded. All were relatively small and had relatively simple geometries. Hand made compression mold for a hand held mirror (c.1870).
The items shown below were compression molded from a shellac based thermoplastic molding compound known as Florence compound. Its inventor, Alfred Critchlow, founded the Pro Corporation, Florence MA in 1847. Pro is said to be the very first plastics molding company. Daguerreotype case (c.1856). Hand held mirror frames (c.1866).
Over the years, compression molding (and transfer molding) press capacities (size) increase so that larger thermoplastic and thermoset plastic parts, such as this radio housing, could be molded. Trimming flash from Ekco radio cabinets (c.1934). Large scale compression molding press (c.1935).
Stages in Compression molding cycle The various stages of the compression molding cycle time can be represented as a function of the force required to close the mold at a constant rate. In the “plastication” stage (t<tf) the force increases rapidly as the polymer feed is compressed and heated. The second stage flow commences, once the yield stress of the elastomer is exceeded. tc is the point at which the mold fills and compression of the melt occurs. Ideally, to aid mold filling the majority of chemical reaction should take place after tc.
Transfer Molding • A process of forming articles by fusing a plastic material in a chamber then forcing the whole mass into a hot mold to solidify. • Used to make products such as electrical wall receptacles and circuit breakers
T R A N S F E R M O L D I N G
T R A N S F E R M O L D I N G
Injection Compression Process • CPI Process
MATERIALS • Thermoplastic • Thermoset • Composites
Variables • Amount of charge • Molding pressure • Closing speed • Mold temperature • Charge temperature • Cycle time
Process Window There is an optimum process window for the filling stage, which can be represented by a Moulding Area Diagram (MAD).
C O M P R E S S I O N M O L D I N G
Press calculations • Press capacity • Required force to press a part
Mold material • Mold closure types
Advantages and Disadvantages ADVANTAGES Little waste (no gates, sprues, or runners in many molds) Lower tooling cost than injection molding Good surface finish Less damage to fibers Process may be automated or hand-operated Material flow is short, less chance of disturbing inserts, causing product stress, and/or eroding molds. DISADVANTAGES High initial capital investment Labor intensive Secondary operations maybe required Long molding cycles may be needed.
