The following points highlight the four types of moulds used for compression moulding of plastics in industries. The types are: 1. Flash Type 2. Positive Type 3. Landed Positive Type 4. Semi-Positive Mould.
1. Flash Type:
This type of mould is widely used because it is simple to construct and holds the part thickness and density within close limits. As the mould closes, the excess material escapes over the land where it forms a very thin fin.
This fin hardens first, preventing the escapement of the mould charge. Flash type moulds may be loaded by volume since excess material is permitted to escape. Material lost through flashing is higher than for other types of compression moulds, but the original mould cost is relatively low.
2. Positive Type:
The characteristics of positive type mould are deep cavity and a plunger that compresses the compound at the bottom of the mould. As there is very little escapement of material, it is necessary to weight the charge accurately if the size of the part is to be controlled. Such moulds are used for high impact materials and parts requiring deep draw.
3. Landed Positive Type:
This type of mould is similar to the positive mould except that lands are incorporated in the design to stop the travel of the plunger at a predetermined point. The density may vary depending upon the charge. In this process, greater loading space is provided thus making possible the processing of bulky materials.
4. Semi-Positive Mould:
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This is a combination of flash type and landed mould. In addition to the flash ridges, land is incorporated to restrict the travel of the plunger while some of the pressure is taken up by the land, much pressure is being exerted on the part being moulded.
The amount of clearance between the flash ridges regulates the thickness of flash and controls to a large extent the density of the moulded part.
Oil and air hardening tool steel are used for phenolic, urea and melamine, and cast steel for polyester materials requiring low pressure. Flash parting lines on the dies of these materials are usually flame hardened and the cavities are hard chrome plated.
