With thermoforming, a plastic sheet is heated until it’s pliable, then stretched into or onto a mold to form a part. But the sheet gets thinner as it stretches deeper onto the mold, which is why prestretching of the hot plastic is sometimes useful.
Prestretching is the process of stretching the plastic once it’s at its forming temperature, but before molding to help ensure even thickness when formed, minimizing undesirable material thinning. There are two primary methods of prestretching:
- Snap-Back: In this process, a draw box is pushed onto the heated plastic, creating a seal. The hot plastic sheet is then pulled into the box with vacuum until it reaches the correct thickness. At that point, it is quickly “snapped” onto the surface of the mold.
- Plug-Assist: Plug assist is a process in which the mold is pushed into the heated sheet while a plug pushes the sheet into the mold, creating a seal around the edges that prevents contact between the hot plastic sheet and the cool mold as the sheet is stretched into the cavity.
When precision is required for your part, you may want a thermoformer whose machines have pre-stretching capabilities for optimal uniformity.
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