Meaning
Geometric processing metrics calculate the relationship between the depth of a thermoformed part and the smallest dimension of the mold opening. A high deep draw ratio requires significant material stretching, which can lead to excessive thinning at the corners of the part. Understanding this ratio helps designers select the appropriate sheet thickness.
Wall Thickness
Sheet temperature must be controlled precisely to allow uniform stretching into deep cavities. If the deep draw ratio exceeds the limits of the polymer, the bottom corners of the thermoformed part become too thin. This creates a weak point that can fail under load or during subsequent assembly.
Tooling Design
Assist plugs are often used to push material into the mold before vacuum is applied to ensure even distribution. When the deep draw ratio is severe, these heated plugs help pre-stretch the sheet. This tooling modification allows the use of thinner starting sheets while maintaining the required minimum wall thickness in the finished part.
Choosing the correct plug geometry prevents the sheet from sticking to the plug and creating cosmetic defects known as chill marks.
Resin Selection
High melt strength polymers prevent the sheet from tearing during the forming process. Materials with low melt strength cannot tolerate a high deep draw ratio because they sag excessively or rupture when stretched. Polystyrene and polypropylene grades are specifically designed with high extensional viscosity to accommodate deep-draw thermoforming.
