A blown film producer of custom and engineered blends sees loss of film properties and surface appearance from a small amount of wear. A particular process makes foamed MW PE and wear of the screw mixer leads to temperature differences, poor mixing and surging at the die interface. As these are high performance liners, consistency of output and process stability are extremely important for high quality products.
This customer extrudes foamed PE sheet which is used to seal consumer products. Sheet thickness ranges from 4 to 40 mils and the material is used in applications such as food bottling or pill bottles. Wear is a critical factor as too much heat input due to screw wear causes poor control of foamed product sizing. In a foaming operation, process stability is very important. A reduction in output as a result of wear is not an issue compared to the poor quality product that results from an increase in melt temperature.
This is a 4-1/2”, 24:1 screw with a barrier flight and double mixers. The screw has a weld inlay of Colmonoy 56 on flight lands and hard chrome plating. At .010” of OD wear, process stability is affected and at .020” wear a screw is removed and rebuilt. The average screw life is about 12 to 14 months. The foamed PE also contains 4-5% titanium dioxide as a whitening agent.