COMPRESSION MOULDING
Process
There are a number of methods but the process is broadly similar. This is a process for the moulding of thermosets and differs from injection moulding in that the mould is heated to form the part rather than cooled to solidify it.
The most common method is Dough Moulding Compound (DMC) or Bulk Moulding Compound (BMC). Thermosetting resins are used in a partially cured state, either in the form of granules, putty-like masses or performs.
The preheated material is placed directly into a heated mould. The material is forced into all cavities when the mould is closed with a top force. Heat and pressure are maintained until the material is cured.
Another method is Sheet Moulding Compound (SMC). This process involves placing a reinforcing material, such as glass fibres, between layers of a thermoplastic and heating the materials so as to bond them together into a single sheet of material. This is then cut to size and reheated before being placed in a compression moulding press. This has a higher cost due to the handling and manufacturing costs in making the sheet. The material used in making the sheet is also heated three times – when making the initial thermoplastic sheet, then again when bonding with the reinforcement and finally, when making the part itself.
Materials
Phenolics (“Bakelite”), some polyesters, urea formaldehyde, melamine formaldehyde.
Products
Applications where mechanical strength, electrical and thermal insulation properties are required.
Electrical switches and sockets
Radio and appliance knobs
Electrical parts
Automotive exterior panels (especially commercial vehicles)
Advantages
Lower cost moulds
Low material wastage
Internal stress and warping are minimised
Excellent dimensional accuracy and stability
Thick and variable sections are readily achievable
Large parts are practical
Disadvantages
Least product consistency
Not suitable for fragile mould features
Mould design problems where there are uneven parting lines
High impact compounds make flash control and removal difficult
Thermosets cannot be remelted like thermoplastics
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