Compression moulding is a conventional manufacturing process which uses heat and pressure to mould plastic resins and thermosets into a desired shape using a cavity tool.
What is Compression moulding?
In Compression moulding, the material is placed directly inside a heated cavity and pressed into the desired shape using heat and high pressure. The material could be either powder, pre-shaped volume or liquid resin. Sometimes, manufacturers add filler materials to enhance the part and improve the process.
The process is primarily used with thermosetting plastics, although thermoplastics and elastomers are also used.
Compression moulding is well suited for creating parts with intricate features and high strength requirements. It is commonly used in automotive, aerospace, electrical and construction industries.
Types of Compression moulding
- Flash type – Flash type compression moulding is for shallow or flat components such as control panel dashboards and ashtrays.
- Positive type – This type of compression moulding is used for high-density parts.
- Semi-positive type – Combination of the above two types.
How does Compression Moulding work?
The compression moulding process typically follows the following steps.
- Material preparation – Raw material, often in the form of pellets, granules or sheets, is pre-heated to material-specific temperature to soften and make it malleable.
- Mould preparation – The mould is heated to the desired temperature to suit the material used. The mould cavity with the desired shape is lubricated to avoid the material sticking.
- Material loading – Pre-heated material, generally called “Charge”, placed inside the open mould cavity. This controlled material amount ensures that the mould cavity is filled and there is no excess.
- Mould closing – As shown in the figure, a simple mould consists of two halves, an upper and a lower, which close together like an injection moulding tool. Then, the pressure is applied to force the material to flow into the mould cavity. The combination of heat and pressure allows the material to flow and fill the mould cavity.
- Curing and setting – The pressure is maintained until the material is cured or set to take the mould cavity shape. The curing time varies depending on the material. Curing time also includes cooling time.
- Mould opening and part ejection – Once the material is cured, the mould is opened, and part ejected.
Advantages & disadvantages of compression moulding
Advantages
- Lower tooling cost – Compared to Injection moulding, the moulds used in moulds used in compression moulding are simple and less expensive.
- Cost-effective – With its low tooling cost, the process can be cheaper than injection moulding and plastic extrusion.
- Suitable for large parts – Compression moulding is well-suited for producing large parts like car bumpers. Depending on the design, compression moulding can be vastly cheaper for substantial parts compared to Injection moulding.
Disadvantages
- Slow cycle time – Compression cycle time is longer than manufacturing processes like injection moulding. Hence, the lower production rate makes it less suitable for high-volume production.
- Material waste – Depending on the part design and the type of mould used, compression moulding can produce more material waste
- Limited automation possibility – It is challenging to implement automation.
DFM guidelines for Compression moulding
- Material selection – Choose a material suitable for process, generally thermosetting plastics like Silicone, Phenolics and melamine.
- Surface finish – Choose a suitable surface finish. Avoid specifying a highly polished surface finish as it adds cost.
- Avoid undercuts – Avoid or minimise undercuts as the cost of adding a manual side action is high and labour-intensive. If required, a mould tool can be manufactured to open sideways so the moulded part can be removed from the side.
- Avoid sharp edges and corners – Use fillets and radii to reduce stress concentration points whenever possible. Radii also helps material flow better, creating uniform parts.
- Uniform wall thickness – Design parts with consistent wall thickness to promote even material flow and minimise warping. Add gradual transitions to avoid sudden changes if different wall thicknesses are required for full functionality.
- Draft angles – Always incorporate draft angles on vertical surfaces to help ease the ejection of the part from the mould.
- Ejector pin location – If needed, allow a flat surface so the manufacturer can place an ejector pin.
As a product designer, you must work closely with the manufacturer from an early stage to improve the design for material flow and part geometry.