Meaning
Sheet forming of thermoplastic materials involves shaping a heated sheet over a mold while the material cools. In non-isothermal thermoforming, the polymer sheet is formed under a rapid thermal gradient as it contacts the colder mold surface, affecting material distribution. This process governs the final wall thickness variation and the level of frozen-in stresses in the formed plastic part.
It requires precise control over sheet transfer speed and mold temperature to avoid premature freezing or thinning of the polymer sheet.
Sheet Forming
Heating a polymer sheet above its glass transition temperature allows it to be stretched over a mold cavity. The deformation behavior is highly dependent on the localized temperature of the sheet. Uniform heating prevents localized thinning during forming.
Thickness Variation
Contact with the cool mold surface rapidly increases the polymer viscosity, stopping further material flow in that region. Non-isothermal conditions cause the thickest section to form where the sheet first touches the mold. This variation must be compensated for during part design.
Warpage Reduction
Uneven cooling after forming induces thermal stresses that cause the final part to twist or warp. Controlling the mold temperature profile minimizes these internal stresses. This adjustment ensures dimensional accuracy of the thermoformed part.
