PIM feedstocks are sensitive to shear, and if solid powders separate and get into the screw’s transition area too soon, they will cause premature wear and possibly breakage. Around the world, PIM molders and suppliers have gone far beyond just focusing on wear-resistant materials of construction for screws and barrels. Many are developing means of improving injection unit service life by perfecting screw designs for PIM.
In the United States, Dave Larson, president of Westland Corp., a supplier of screws and barrels, recommends, like most, the use of low-compression screws for PIM. Westland has the experience. It supplied CPM 9V and 10V screws and barrels to Penn State University for its original Engel machine in its P/M lab, for instance.
If proper attention is not paid to a PIM screw’s compression ratio, Larson says that powders can separate and bind things up. Also, depending on the shot size, Larson says that, in addition to a proper compression ratio, a reverse heating profile along the barrel should be used to soften the feedstock before it hits the transition zone. Powdered metal, he reminds us, cannot be compressed.
Westland’s PIM screws have a longer compression section than a GP plastics screw to gradually squeeze the feedstock. The Westland screw has eight turns in the feed section, seven in the transition section, and five in the meter section on a standard 20:1 L/D. Compression ratio ranges from 1:2 to 1:5. The company designs screws with a compression ratio matching a particular feedstock’s powder/binder letdown.
In Germany, Uwe Haupt, Arburg’s PIM development engineer, contends that even the term “low compression” fails to tell the whole story. They should be called “no compression” screws, he jokes. Arburg has engineered a new screw design for processing feedstocks with a short zero-compression back end, a large zero-compression midsection, and a short metering zone on the front.
Haupt adds that Arburg also is working on optimizing static and dynamic mixers for feedstock injection units. Also, he says Arburg uses tungsten-carbide-cobalt and high-chromium carbide bimetallics for wear resistance, because CPM 9V and CPM 10V materials are hard to come by in Germany.
Dunstan H. Peiris of PIM molder Ceramet Technologies in Singapore also has been focusing his attention on injection units. In addition to designing his own low-compression screw and nonreturn valves, Peiris has developed an easily replaceable screwtip assembly. A pin bears the brunt of the on-the-job stress. It buckles under pressure, not the tip, so that replacements are much less time-consuming.
Arburg GmbH & Co.
Phone: +49 (74) 446-33-3887
Fax: +49 (74) 446-33-3389
E-mail: [email protected]