In the two-layer molding technology, foamed polyurethane (PU) is used to reduce the weight of the footwear and one of three other materials is used as the wear layer:
• wear layer of densified PU with high abrasion resistance;
• wear layer of TPU (thermoplastic polyurethane);
• wear layer of nitrile rubber.
The process of forming the wear layer of the sole consists of several stages:
1. When using the PU/PU technology, a denser and harder polyurethane is injected into the lower part of the mold. When using the PU/TPU technology, granules of thermoplastic polyurethane (TPU), melting in an extruder, fill the lower part of the mold, precisely reproducing the tread pattern.
When using the PU/Nitrile technology, a vulcanized and pre-treated nitrile rubber insert is placed into the mold.
2. The upper blank, pulled over the last, is lowered into the mold without being tightly pressed against the formed lower wear layer. Then PU components are injected into the mold which, by reacting chemically and then expanding, fill the space between the upper blank and the wear layer (PU, nitrile rubber, or TPU) already inside the mold, thereby completing the formation of the sole.