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PIM molding feedstock Q/A simplified, costs cut, too

Injection molding machine manufacturer Arburg (Lossburg, Germany) reports it can help processors using powder injection molding to quickly and easily conduct preventive quality assurance through feedstock testing, using the control on the press.

Injection molding machine manufacturer Arburg (Lossburg, Germany) reports it can help processors using powder injection molding to quickly and easily conduct preventive quality assurance through feedstock testing, using the control on the press.

In principle, powder injection molding (PIM), used for the production of metal (MIM) or ceramic (CIM) components, does not differ significantly from the injection molding of plastic parts, notes Hartmut Walcher, powder processing development manager at the manufacturer. Walcher also heads the multi-company Ceramic Injection Moulding (CIM) group formed in April 2008 by a number of machinery, mold, and materials suppliers, plus processors.

Arburg has devised a time- and cost-saving means for testing powder PIM feedstock quality.
The main difficulty in PIM is that an assessment of part quality is only possible following the necessary debinding and sintering stages—at which point quality problems in the production process can no longer be rectified.

A preliminary assessment of feedstock quality is therefore critical, adds Walcher. He says that, using the company’s Selogica machine control system, a processor can monitor the pressure at the changeover point of injection molding machines and collect meaningful, dependable values. These permit the reliable assessment of possible fluctuations in batch quality.

For PIM processors, the ability to judge batch quality during processing, and prior to completion of molding, can be a huge time savings. A period of several days is required for the subsequent processing of PIM parts, which poses the following problem for processors: Should they trust that the batch is OK, continue to produce green compacts (the phrase given unsintered PIM parts), or stop the machines until the results of the quality testing become available?
Not only is time wasted if feedstock quality is questioned, but material feedstocks, be they metal or ceramic compounds, can be priced in the hundreds of dollars per kilogram. Added to these costs are the machine-hour rate and the hourly rates for debinding and sintering.

Many influencing factors determine part quality

In order to ensure the flawless volume production of PIM parts, states Walcher, the injection molding machine must operate perfectly and the dimensional tolerances of the PIM cylinder, PIM screw, and non-return valve must be adhered to. Constant ambient conditions must be maintained to ensure production at balanced temperatures and unvarying viscosity of the feedstock, which is a central factor during processing. The viscosity depends on numerous factors, including the binder, the binder-agent content and particle size distribution, as well as the precision of feedstock preparation.

In the case of PIM feedstocks, incoming goods inspections are neither as easy to perform nor as widespread as with thermoplastics. Setting up a laboratory with a high-pressure capillary viscosimeter represents a high-five-figure investment, and the results cannot be measured under the actual pressure and speed conditions of the injection molding machine. Determination of the melt flow index would be less costly but the information gleaned is of little use, he adds.

One alternative he recommends is the processing of test pieces on a small, or even lab-scale, molding machine. The testing mold used forms test pieces, whereby the cavity is filled at various injection speeds and to less than 100%. The control system records the maximum injection pressure in relation to the respective speeds as a measured value. This allows information on the quality of the feedstock to be derived from the injection molding machine pressure values during injection. This information can then be applied to the setting parameters. Here, it is the flow characteristics of the material that are tested.

Control-system based solution saves time and costs

Walcher described testing completed on an Allrounder 170 S, the company’s micromolding machine with clamp forces of 14, 17, or 20 tons. During testing, the test pieces are injected as described above. Debinding and sintering of the parts is unnecessary; the green compacts can be regranulated and the material reused. Significant information on feedstock quality can be derived from the pressure recordings in relation to the respective injection speeds, he says, and both ceramic and metal feedstocks with different binders can be tested.

The recording of 20 to 30 cycles is generally sufficient in order to achieve meaningful reference curves. The purchase of a small injection molding machine with the Selogica control system is more cost-effective than that of a high-pressure capillary viscosimeter, he claims, as well as more accurate.

The correlation between feedstock quality—i.e. viscosity—and the pressure/speed curves of the control system is sensitive. Changes in material viscosity have an immediate impact on the characteristics of these curves. The pressure used during injection is a significant differentiation criterion for the viscosity of feedstocks and consequently for their processing quality. A quick feedstock test is possible on almost any mold, he adds, particularly as no internal pressure measurement is required for this purpose.

Sample calculations reveal major savings potential
Based on a molded part weight of eight grams and the use of a 316L feedstock with an average price of €22/kg, the following cost situation results for a MIM batch oven: If a four-cavity mold and a cycle time of 30 seconds are used, 8.25 production hours are required to fill a hypothetical sintering oven. The feedstock costs for a throughput time of 24 hours amount to around €696 for one sintering batch. Added to this are operating costs of around €1200 for the debinding and sintering stations and the operating costs for the injection molding machine, amounting to €123.75. If the quality of the feedstock cannot be tested in advance and the complete batch is not produced flawlessly as a result, total costs of €2019.75 are incurred, which have to be written off as reject part production.

Comparable results are obtained in the case of a CIM batch oven filled with parts made from a ZrO2 (zirconium oxide) feedstock (kilo price: €120) with the same part weight. Based on a four-cavity mold and a cycle time of 30 seconds, the feedstock costs for one sintering batch amount to some €1420, the operating costs for the debinding and sintering stations are €900 and those for the injection molding machine are €46.25. The total costs for a normal run therefore amount to €2366.25, which are incurred even if the batch produced is defective.

Using these figures it is clear that the purchase of an injection molding machine for the purpose of preliminary feedstock testing can be amortized within a very short time. If the test data is collected and compared over a prolonged period, acceptable batch-fluctuation ranges can be determined without the need for injection molding parameter changes. New injection parameters can then be devised for batches that lie outside this range. [email protected]
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