Vacuum forming

This method of forming plastic is used for the production in industry of cheap items such as plastic cups, chocolate box trays, thin type plant pots and trays. It is also the main way of forming plastic in school.
The process involves heating a plastic sheet until it is pliable and sucking it over a mould of the required shape. Any excess material is trimmed and in industry it is usually recycled.
As was previously mentioned only thermoplastics can be formed using this process. It is a heat process which does not require the need for a chemical change.

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Stages in Vacuum Forming

1. A mould is made and placed on a platten which can be raised.

2. A thermoplastic sheet is clamped around its edges above the mould.

3. A heater above the plastic heats the sheet until it is soft (or plastic).

4. The platten is raised to press the mould into the soft sheet.

5. A vacuum pump is switched on to fully suck the plastic onto the mould.

6. When cold, the plastic sheet is blown off the mould by making the vacuum pump blow instead of suck.

Sometimes when deep shapes are required the plastic is blown first to stretch it, sucked to make it take the form of the mould, then blown again to lift it from the mould (the second blowing is done when the plastic has set.

Stage One
The plastic sheet is heated by the heater above it (which is rather like a grill that is on an electric cooker). When the sheet is soft it can be blown to stretch it. This will give the plastic sheet a uniform thickness preventing thin areas occurring when moulding takes place.

Stage Two
When the sheet is soft the platen (table underneath the plastic sheet which has the mould on it) is raised and the mould is pushed into the plastic which is being sucked by a vacuum underneath to suck the plastic sheet tightly over the mould.

Stage Three
Finally after the vacuum has caused the sheet to take the shape of the mould, the sheet is lightly blown (as opposed to sucked) to gently release the plastic from the pattern. (Obviously at this stage the plastic is not as soft.

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