Meaning
Numerical relationship comparing the surface area or depth of a thermoformed part to that of the initial flat sheet determines the severity of the forming process. A high thermoforming draw ratio indicates that the sheet must undergo extreme stretching, increasing the risk of localized thinning or rupture.
Thickness Control
Wall distribution of the formed part is heavily influenced by how far the sheet must stretch. When the thermoforming draw ratio is high, pre-stretching the heated sheet with a plug assist is required to distribute the material more evenly before vacuum is applied. This prevents the bottom corners of the part from becoming excessively thin and weak.
Sheet Selection
Initial sheet thickness must be chosen based on the depth of the mould. To achieve the required minimum wall thickness on a part with a high thermoforming draw ratio, a thicker starting sheet must be sourced. This increases raw material costs, highlighting the importance of optimizing the sheet thickness to balance part performance and material economy.
Using advanced simulations during the design phase helps engineers identify the minimum starting thickness that can be processed without compromising the part’s structural load capacity.
Process Defect
Mould design must account for the stretching limits of the selected polymer grade. Exceeding the allowable thermoforming draw ratio results in defects such as webbing, tearing or severe warping. Moulders must adjust the mould radii and draft angles to ease the transition of the stretched material and ensure smooth ejection.
