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Polymers are perhaps the most commonly formed material in the D&T workshop due to the ease with which they can be turned into a liquid (or softened) and back to a solid at relatively low temperatures. The following methods rely on the polymer being heated up until it becomes plastic i.e. flexible enough to form but it never becomes a liquid.
Line Bending
Line Bending
A process which involves heating up a small area of plastic sheet over the top of a hot wire, so it can be folded or bent like paper. Usually this is done around a 'former' to help guide the shaping and allow the plastic to cool in position.
Vacuum Forming
Vacuum Forming
Vacuum Forming is a process which involves heating up a sheet of plastic then raising a 'former' or 'pattern' up through the plastic while removing the air below to create a vacuum. This then causes atmospheric pressure outside the vacuum to ‘force’ the plastic down onto the former.
Vacuum Forming is commonly used to produce hollow ‘shell like’ items such as yoghurt pots and other food containers but they can be used to produce larger items such as baths and hot tubs. They are also useful for making moulds for pouring in plaster and concrete.
How Vacuum Forming works
Vacuum Forming moulds should have the following characteristics:
The sides should slope outwards with no ‘under hang’ so the product can be removed from the mould. This is called a draft angle.
There should be no sharp edges (radius edge) to avoid thinning of the plastic.
There should be air holes to avoid trapped air especially in the corners.
There should be sufficient space between peaks (and peaks should not be too high) toavoid webbing.
Pressure forming
Pressure forming
Pressure forming is a relatively new process but is very closely related to vacuum forming. However, where vacuum forming relies solely on the vacuum to ‘pull’ the softened polymer around the mould, pressure forming uses an additional former to help in the process and allows for a greater level of detail such as lettering and sharp edges to be achieved.
Blow Moulding
Blow Moulding
Blow Moulding is a manufacturing process by which hollow plastic parts are formed. The blow moulding process begins with melting down the plastic and forming it into a 'parison' or in the case of injection (and injection stretch blow) moulding, a 'preform'. The 'pariso'n is a tube-like piece of plastic with a hole in one end through which compressed air can pass.
The 'parison' is then clamped into a mould and air is blown into it. The air pressure then pushes the plastic out to match the mould. Once the plastic has cooled and hardened the mould opens up and the part is ejected.
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