The Thermoforming Process

Douglas Fabrication & Machine - The Thermoforming ProcessVacuum forming is accomplished by taking a flat piece of plastic, heating it until it softens, then using a vacuum to pull it onto a contoured surface where it is held until it cools and hardens. Tooling costs for this process are the lowest of any plastic molding process. View thermoforming illustration.

Thermoforming compares favorably with other methods such as injection molding because of the low pressures required for production. Prototypes and short runs can use tooling of wood, plaster, epoxy, aluminum or similar materials. Machined or cast aluminum is used in actual production.

When using vacuum forming there are only two pressures we have to be concerned with: 1) the atmospheric pressure exerted against the mold when holding the sheet in place, and 2) the force necessary to lift or lower the mold into the hot sheet before the vacuum is applied.

Before forming, the heated sheet is virtually stress free. When properly formed, the sheet is almost completely stretched at the forming temperature before it is cooled against the mold. This, of course, sets up a very minimum of internal stress in the finished part. In addition, the starting resin in many cases can be of a lower melt flow, which usually has better physical properties than resins with a high melt flow. Injection molding requires raw materials with high melt flow characteristics because of the small openings into the mold and the narrow area between mold walls. On the other hand, sheet extrusion uses much larger orifices to form sheets. Consequently, extrusion can use the better resin so the thermoformer can take advantage of the resins with superior performance.



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