Injection Molding: Now Mold Microcellular Foams with a Standard Screw & Barrel

Trexel Inc. has just announced a new way to make its MuCell microcellular foam process simpler and less expensive to implement without the need for the previously required special screw and barrel. This “breakthrough’ development was unveiled in March at Plastics Technology’s Molding 2019 Conference in Indianapolis by Leo Devellian, Trexel’s business development manager. Besides reducing costs, it allows MuCell foaming to be applied in a range of applications utilizing standard or specialized screw designs.

Until now, injection machines for the MuCell physical foaming process required a barrel with a gas injection port and a special mixing screw with a center check ring just before the gas-entry port, in addition to the standard check ring at the end of the screw. Now, a standard screw and barrel can be used, with just one standard check ring. The usual screw tip and check ring are replaced with Trexel’s new MuCell Screw Tip Dosing Module (TDM), which bolts onto the existing barrel in place of the end cap. It contains an optimized wiping/mixing screw section (much shorter than on previous MuCell screws) and has a standard pressure-transducer port, into which fits Trexel’s new high-pressure MuCell SCF gas injector. A standard check ring is fitted on the end of the screw just ahead of the TDM. In addition to a retrofit package for existing injection machines, a version of the TDM will also be available on new MuCell-equipped machines.

While the TDM does add approximately 3.5D to the standard screw length, it reduces costs of MuCell implementation, is easily removable, and provides improved output without sacrificing foaming performance, according to Devellian. One reason is that the screw plasticating length is increased from 15.5D with the previous MuCell screw to the full 20D with the TDM. Second, eliminating the center check valve reportedly provides gentler treatment of long-glass reinforcements. Devellian says a machine fitted with the TDM can be used for non-foam molding with no process impact. Although not yet available from all machine builders, TDM will become the MuCell standard in the future, Devellian notes.