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Thermoforming
History of Thermoforming
One source dates thermoforming back to the ancient Egyptians. They found that animal horns and tortoise shells could be heated and formed into a variety of vessels and shapes. Keratin, a component found in tortoise shells and in animal horns and hoofs, can be softened by immersing it in boiling oil or water, then forming into a sheet. The sheet was then manually draped over a form and held until cool. Natural cellulose, a primary element in tree bark, was shaped in a similar fashion by Native Americans.
Modern thermoforming began about in the 1940’s with major development in two important areas. Research in thermoplastic resin chemistry led to commercialization of sheet grades. Continuous forming was achieved with the invention of the screw extruder and the roll-fed sheet thermoformer. These breakthroughs allowed a wide variety of pre-war domestic products to be developed. And this in turn, prepared manufacturers for war product developments such as airplane canopies and war survey relief maps.
The packaging industry adopted thermoforming as a basic process in the late 1940’s to such an extent that the thermoformed package is considered as the most significant packaging development of the 1950’s. In the 1970’s demand for convenience food containers, oven-able portion servings, and more ductile disposable drink cups spurred development of foam, PS, CPET, and PP pressure forming processes.
Basic Process Sequence / Steps
1. FEEDING THE MATERIAL
Rolls of plastic material are mounted onto an unwind stand at the rear of the machine. The material is pulled through the machine by a mechanical or electrical mechanism.
2. HEATING
The next step in the process is to heat the material up to its forming temperature. This can be done in several ways. In some cases, a heating tunnel is used, whereby the material is pinched along the edges and transferred through a hot tunnel oven. In other cases the material is moved over a hot plate and forced against it by the surface of the mold. In either case, heating is the most important part of the process. Many materials today require temperatures to be controlled to within 2° or 3°F. This close temperature control ensures proper forming.
3. SEALING OR TRAPPING
After the correct forming temperature has been reached, the mold cavities are closed against the plastic in order to create a seal. Forming or shaping material can only be achieved by creating an air tight chamber. The sealing or trapping step is achieved by what we call a clamping frame or a knife around the outside of one or more cavities.
4. FORMING
Next, the heated material is made to take the shape of the mold. This is done by applying vacuum to one side of the material and air pressure to the other. The vacuum creates a void or absence of atmosphere on the one side allowing the air pressure to more freely or rapidly move the material into position. Once the heated material has taken the shape of the mold, it is allowed to remain against the mold surface for a short period of time in order to cool. This sets the new shape of the material.
5. TRIMMING AND STACKING
When the forming cycle is complete, the parts are now ready to be trimmed out of the web.
In the case of molds that have knives wrapped around each cavity, a secondary cylinder applies pressure and forces the knives through the material.
In the case of the molds that have a clamping frame around the cavities, the parts are forwarded to a separate cutting station, where they are cut from the web.
Parts then move to another station where they are either manually or automatically stacked.
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