Powder injection molding yields new control technology.

Injection molding of metal or ceramic powder with a thermoplastic binder is the province of a small number of firms that have mastered the specialized processing know-how and expensive post-molding techniques. While definitely not for everyone, powder injection molding (PIM) has generated some interesting new control technology that could benefit other types of injection molding. That fact emerged during the annual PIM Symposium held recently in State College, Pa., by the Metal Powder Industry Federation of Princeton, N.J.

HIGH-SPEED INJECTION CONTROL

One area in which PIM processing shares concerns common to other precision molding applications is in controls technology. As explained by Dr. Karl Hens, program director of the PIM Consortium at Penn State University in University Park, PIM often requires fill rates as fast as 0.12 sec in order to counteract the high thermal conductivity of metal-powder compounds, which promotes fast cooling. Such fast injection in turn requires some novel control techniques. In Penn State's PIM laboratory, a specialized Engel machine with a RISC-based CC100 controller and robust sensing capabilities does the trick. For each shot, Hens says, this "advanced control with in-situ correction" uses both cavity-pressure - from piezo-electric sensors - and temperature measurements to determine the optimum transfer pressure at which to switch from speed to pressure control. (See PT, Aug. '94, p. 15 for an earlier report on this development program.)

Recently emerging from the lab, this control technology has been incorporated in a commercial system from Kistler Instrument Corp., Amherst, N.Y. Kistler's Automatic Quality Control System (AQCS) automatically determines the optimum transfer pressures needed to get a good packing-pressure profile. The system identifies the end of volumetric filling by a sharp upward bend in the cavity-pressure signal. AQCS monitors material viscosity during cavity filling and applies this data to the transfer-pressure calculations. Kistler maintains that mathematical methods alone won't yield the optimum transfer pressure in real time, so it also incorporates some "fuzzy-logic" decision making into its automatic calculations.

From shot to shot, AQCS will choose different transfer pressure points because of variations in feedstock viscosity, but the AQCS will hold a consistent maximum cavity pressure. Kistler product manager Karl Bauer says that AQCS works on the principle that "consistent maximum cavity pressure is equivalent to consistent part quality." In the case of thick-wall parts, the system shoots for a cavity-pressure integral, rather than a maximum cavity pressure. But otherwise the control works the same way.

At the symposium, Bauer announced that the AQCS cavity-pressure control system will soon be embodied in a computer chip that can be installed in a variety of OEM controllers - thanks to EuroMAP integration standards. The chip's official introduction will be at the upcoming K'95 show in Dusseldorf, Germany.

NEW PIM MATERIALS

BASF Corp. of Wyandotte, Mich., now offers ready-to-mold PIM feedstocks combining various metals and ceramics with an acetal resin binder. Besides eliminating the need for molders to compound their own feedstocks, BASF also provides engineering support for processors who know injection molding but lack expertise in debinding and sintering. "We provide help all along the process - from the molding to the sintering," says David Krueger, who works on PIM applications for BASF.

According to BASF's Dr. Jan Ebenhoch, the use of acetal has several advantages over the conventional wax/polyolefin binders: Acetal gives the molded parts enough strength to withstand automatic handling. As an engineering resin, it fosters tighter tolerances than the wax binders. And acetal debinding can be accomplished through a catalytic process that is easier to control and up to 10 times faster than thermal debinding, depending on part geometry and powder particle size. The acetal binder also complements BASF's developmental reactive sintering method, in which sintering in a hydrogen atmosphere yields low-carbon stainless steels.

PIM PROCESSING TIPS

A wear-resistant injection barrel and screw are the most obvious requirements for PIM, but speakers at the symposium pointed out several other technical considerations that should be considered by anyone exploring this field.

* Screws: Torsten Kruse, technical training engineer for Arburg Inc., Newington, Conn., notes that screws do not need a compression section because the highly filled PIM feedstocks don't compress. Arburg sells complete systems for PIM, adding the requisite pre-mold and post-molding equipment to its standard AllRounder machines.

* Gating: "End users want a tiny gate," notes Dr. Hens, but PIM parts are likely to require larger gates than normal plastic parts. The guideline, he says, should be to avoid separation of powder from polymer binder, which means keeping shear rates below 10,000 [sec.sup.-1].

* Hot runners: "You don't have to rush the filling as much when you use hot runners," says Hens, because hot runners get the feedstock to the mold hot enough to forestall premature solidification. Hens recommends only cylindrical hot-runner channels, which have a smaller surface area than trapezoidal channels of equal volume, resulting in less heat loss.

RELATED ARTICLE: PIM in Hot-Runner Parts

Metal-powder injection molding is starting to turn up in components of injection molding systems. At K'95 in Dusseldorf, Mold-Masters Ltd., Georgetown, Ont., will introduce powder-injection-molded hot-runner nozzles that will become part of the company's standard Dura series. Mold-Masters, which also offers other hot-runner systems for PIM, is molding these new components in-house on a machine from Engel Canada Ltd., Guelph, Ont. According to Mold-Masters R&D director Hakim Belhadjhamida, PIM allows the nozzles to be better optimized for high-wear applications.