Mold venting and Shrinkage
Particularly with thick-walled parts of Desmopan® and Texin™, good venting of the mold cavities is essential. At the parting line, grooves of 0.02 to 0.04 mm in depth and 5 mm in width have proved suitable (Fig. 14). If the area in which the air is compressed is not in the area of the mold parting line, adequate venting can also be achieved through pins and inserts with the appropriate play.
Fig. 14: Schematic diagram of the venting grooves
It is only possible to fix the shrinkage data for mold design for thermoplastic polyurethanes within certain limits because the shape of the article, its wall thickness and the processing conditions all exert a significant influence on shrinkage. As a rough guide for mold design, a shrinkage of approx. 1 % can be assumed.
Minor differences in size are balanced out by the good elastic deformability.
Post-molding shrinkage is greater with soft Desmopan® and Texin™ grades and thin-wall articles than it is with hard grades and greater wall thicknesses. Figure 15 gives an idea of the extent of overall shrinkage.
Fig. 15: Comparison of the shrinkage ranges with Desmopan® and Texin™