Trexel has developed a new screw design that, used in conjunction with Trexel's MuCell microcellular foam process, is designed to minimise damage to long glass fibre-filled materials.
These are typically materials with a glass fibre length of 12mm or longer produced through a pultrusion process. The new screw design will also improve the property performance of standard chopped glass fibre filled materials. This design is now commercially available for all customers running glass filled materials and has been successfully tested on Ticona's Celstran polypropylene in industrial trials along with glass filled PA 6 and PA 66. However, Trexel officials confirm that the design can be used with any base resin.
Engel and Trexel have agreed to install MuCell technology for mould trials using long glass fibre filled material, including a 120mm screw based on Trexel's new design on an Engel DUO (2-platen) 1000 ton machine by early July, 2007- to be located at Engel's new technical centre and training facility in York, PA.
David Bernstein, President of Trexel said, "The MuCell process, when used with our proprietary screw design offers long glass fibre material users the potential to realize the tremendous benefits and mechanical property advantages of retained long glass fibres, the potential for weight savings of anywhere from 5% - 12%, as well as with a typical 10-20% cycle time reduction. More important, you get these benefits along with very little warpage, which has always been a big problem for long glass fibre materials."
Levi Kishbaugh, VP of Engineering for Trexel noted that "comparison testing of sample plaques of polypropylene 40% long glass fibre material moulded solid and then moulded using the MuCell microcellular foam process showed anywhere from a five-fold to six-fold reduction in warpage when molded using the MuCell process."
The new screw design uses a lower compression ratio, 2.0:1 as compared to 2.5:1, and incorporates significant changes to the centre restriction element and the wiping and mixing sections to reduce shear. Testing to date on these screws has been done using the MuCell microcellular foam process with up to 0.6% nitrogen in a 40% glass fibre filled PP without observing process variations.