Multilayer Extrusion Die Geometry for Thermoformed Polypropylene Foam Sheets and Biaxially Oriented Foam Films, 1993 Polymers, Laminations & Coatings Conference Proceedings

Jaakko Raukola, Antti Savolainen

Because of the environmental demands of the CFC-free foamed products and also in striving to replace PS with more environmental friendly and with higher temperature resistant polyolefine, a development for a new foam technology for polyolefins was started. The idea was to develop a new two staged film blowing line, which could be used for both thermoformed PP foam sheets and for biaxially oriented PP foam films.

To reach the accepted level in quality of foamed films and sheets it is necessary to destroy the stripes born in the product by homogenizing the structure. This can be founded by effective mixing in the die. For this purpose a rotor - stator die, which homogenizes the melt, was developed. After the die exit, thermoformable foam tube is first rapidly cooled down and it can be reheated for the orientation.

The results were encouraging foam structure was homogenous (bubble diameter less than 100 pm), no stripes appeared, smooth solid skins (multilayer structure) and low density of the product.