THE VACUUM FORMING PROCESS
Thermoforming is one of the oldest and most common methods of processing plastic materials. Thermoformed plastic products are all around us and play a major part in our daily lives. lt is a very versatile process used to manufacture a wide range of products from simple packaging trays to high impact aircraft cockpit covers. lt is also used extensively to make design prototypes of products to be produced by other processes.The process, however, is basically the same in each case. in its simplest form thermoforming is the heating of a plastic sheet which is then draped over a mould whilst a vacuum is applied
THE VACUUM FORMING PROCESS
In its simplest form the process consists essentially of inserting a thermoplastic sheet in a cold state into the forming clamp area, heating it to the desired temperature either with just a surface heater or with twin heaters and then raising a mould from below. The trapped air is evacuated with the assistance of a vacuum system and once cooled a reverse air supply is activated to release the plastic part from the mould. The process is shown in the following diagram form.
PRE-STRETCH (BUBBLE)Once the plastic has reached it's forming temperature or ‘plastic’ state it can be prestretched to ensure even wall thickness when the vacuum is applied. Pre-stretch is an invaluable feature when forming deep draw parts with minimum draft angles and high mould surface detail. The method of controlling the bubble height should be such that consistent results are obtainable. Vacuum, air pressure and optional aids such as a plug assist are then used to assist in moulding the heated, stretched plastic.
VACUUM
Once the material is suitably pre-stretched a vacuum can be applied to assist in forming the sheet. A dry vane vacuum pump is used to draw the air trapped between the sheet and the mould. The vacuum pump should be capable of maintaining a differential pressure of approx 27" mercury. With larger machines a vacuum reservoir is used in conjunction with a high volume capacity vacuum pump. This enables a two stage vacuum to be applied ensuring rapid moulding of the heated sheet ( before the sheet temperature drops below its ideal forming temperature).
