Thermoforming is a process of heating a material until it is pliable, stretching it over a mold, and cooling it to hold the desired shape. It is mainly used for plastic sheets and films and can be done through various methods, including linear vacuum forming, matched die forming, and pressure bubble cap assisted vacuum forming. Other methods include trapped sheet contact pressure thermoforming, vacuum snap-back processing, billow forming, and matched die forming.
Thermoforming is a manufacturing process that involves heating a material until it is pliable, stretching it over a mold, and then cooling it so it holds the desired shape. A variety of materials, including bone and shells, have been worked in this way for centuries, although thermoforming is now mainly used to shape plastic sheets or films. Examples of thermoformed products include blister packs for pills and single portions of food, plastic trays and even plastic pallets.
The most common method of thermoforming is linear vacuum forming. This process involves heating a sheet of plastic to the sag point and then lowering it into an encased mold. The air is removed by vacuum, sealing the sheet against the walls of the mould. The plastic is cooled and then cut and removed if necessary. The equipment required for linear vacuum forming is generally much cheaper than the equipment required for pressure or mechanical machining.
A more expensive thermoforming method used for parts that require fine detail is matched die forming. In this process, a sheet of plastic is stretched between two matched heated molds. The molds are pressed together and air bubbles are removed by the vacuum through the holes in the moulds.
When plastic sheets need to be molded into deep shapes that require a uniform thickness, pressure bubble cap assisted vacuum forming is a preferred method. In this thermoforming process, controlled air pressure is exerted against the heated sheet to create a bubble. Once the bubble reaches the correct height, a male plug-assist is lowered, forcing the bubble into the mold.
Trapped sheet contact pressure thermoforming uses both vacuum and air pressure to force heated plastic into a mold. This process requires a lower temperature, is faster and has a finer finish. Vacuum snap-back processing is a slower and slightly more complicated process that can be used when uniform thickness is more critical. In this method, the heated plastic sheet is placed on a vacuum box which sucks it into a concave shape. The vacuum box then releases the plastic and snaps back into a male plug mold.
Billow and Matched Die are two thermoforming methods that do not use a vacuum. In wave forming, heated plastic is attached to a wave, air blowing chamber, and the air pressure presses the plastic against a mold. Mate forming is used to create parts with two halves that mate. The heated plastic sheet is clamped together between two molds and no vacuum or air pressure is required.
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