Nature of use and limitations
Vacuum forming offers several processing advantages over other forming processes. Low forming pressures are used thus enabling comparatively low cost tooling.
Since the process uses low pressures, the moulds can be made of inexpensive materials and mould fabrication time can be reasonably short. Prototype and low quantity requirements of large parts, as well as medium size runs therefore become economical.
More sophisticated machines and moulds are used for continuous automated production of high volume items like yoghurt pots, disposable cups and sandwich packs.
Unlike other thermoplastic forming processes, where powder or resin are the starting point, vacuum forming uses extruded plastic sheet. With vacuum forming a secondary process may be required to trim the formed sheet to arrive at the finished part. The trimmed waste can then be re-ground and recycled.
The Stages Involved in Vacuum Forming
1. First, a former is made from a material such as a soft wood. The edges or sides are shaped at an angle so that when the plastic is formed over it, the former can be removed easily.
2. The former is placed in a vacuum former.
3. A sheet of plastic (for example, compressed polystyrene) is clamped in position above the mould.
4. The heater is then turned on and the plastic slowly becomes soft and pliable as it heats up. The plastic can be seen to 'warp' and 'distort' as the surface expands.
5. After a few minutes the plastic is ready for ‘forming’ as it becomes very flexible.
6. The heater is turned off and the mould is moved upwards by lifting the lever until it locks in position.
7. The 'vacuum' is turned on and this pumps out all the air beneath the plastic sheet. Atmospheric pressure above the plastic sheet pushes it down on the mould. At this stage the shape of the mould can be clearly seen through the plastic sheet. When the plastic has cooled sufficiently the vacuum pump is switched off.
8. The plastic sheet is removed from the vacuum former. The sheet has the shape of the former pressed into its surface.
9. The excess plastic is trimmed so that only the plastic bowl remains - the completed item. An enlarged view of the final dish is seen opposite.
Vacuum forming allows us to make unusual shapes (like a stormtrooper helmet for example) with ease. Have you got any interesting ideas? Share them with Pozible.Tech today!