Feedstock Test Method for Powder Injection Moulding

In order to assess feedstock batches quickly and reliably prior to the volume production of PIM parts, the pressure at the change-over point of injection moulding machines provides meaningful, dependable values. These permit the reliable assessment of possible fluctuations in batch quality. This data can be collected with ease using the Selogica machine control system of the Allrounder machines.

In principle, powder injection moulding (PIM), used for the production of metal (MIM) or ceramic (CIM) components, does not differ significantly from the injection moulding of plastic parts. With these applications, the greatest possible consistency and highest quality of production must be ensured, based on predefined setting parameters. The main difficulty here is that an assessment of part quality is only possible following the necessary debinding and sintering stages. Quality problems in the production process can no longer be rectified after completion of these processing steps. A preliminary assessment of feedstock quality is therefore of decisive importance.

Costly assessment of moulded part quality
A period of several days is always required for the time-consuming subsequent processing of PIM parts, which poses the following problem for feedstock processors: Should they trust that the batch is OK, continue to produce green compacts, or stop the machines until the results of the quality testing become available?

Both alternatives are problematic from an economical standpoint. Of course, production must run as smoothly as possible from the outset. Moreover, feedstocks, be they metal or ceramic compounds, are comparatively expensive. Depending on the material - copper, titanium and precious metals, for example, are processed as well as steel alloys in metal injection moulding (MIM) - three-digit material prices per kilo are no exception. Added to these costs are the machine-hour rate and the hourly rates for debinding and sintering. Accordingly, the production of reject parts is always extremely problematic due to the significant costs incurred.

Many influencing factors determine part quality
In order to ensure the flawless volume production of PIM parts, the injection moulding machine must operate perfectly and the dimensional tolerances of the PIM cylinder, PIM screw and non-return valve must be adhered to. Furthermore, 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.

Various testing methods in use
Owing to the many influencing factors, it is critically important that batch fluctuations be detected prior to the actual start of production so that processors can respond quickly by implementing suitable measures or rejecting a particular batch. In the case of 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-digit figure investment. This option is only viable to a limited extent as the results cannot be measured under the actual pressure and speed conditions prevailing in the injection moulding machine. In comparison, determination of the mould flow index would make more sense due to the more modest investment required, but this option must be ruled out due to its poor informative value.

An interesting alternative is the production of test pieces at the laboratory scale. For this purpose, the use of a small standard hydraulic injection moulding machine such as the Allrounder 170 S machine suffices. On an Allrounder producing under balanced temperature conditions, the maximum injection pressure at the change-over point to holding pressure can be precisely determined via the Selogica machine control system. The testing mould used produces test pieces, whereby the cavity is filled at various injection speeds and to less than 100 per cent in each instance. 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 moulding machine pressure values during injection. This information can then be applied to the setting parameters. Here, it is the flow characteristics of the material which are tested.

Control-system based solution saves time and costs
The relevant tests were performed on an Allrounder 170 S. During testing, the test pieces merely have to be injected as described above. Debinding and sintering of the parts is dispensed with completely, the green compacts can immediately be regranulated and the material reused during subsequent volume production. Significant information on feedstock quality can be derived from the pressure recordings in relation to the respective injection speeds. Both ceramic and metal feedstocks with different binders and varying preparation requirements 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 moulding machine with the Selogica control system is far more cost-effective than that of a high-pressure capillary viscosimeter. Moreover, a viscosimeter measures the viscosity of the feedstock at different pressures and speeds than those prevailing in the injection moulding machine.

In other words, the injection moulding machine should only be used here as a measuring instrument in the quality assurance laboratory. Although the recording of the pressure curves described here is possible using any Allrounder from production, the necessary cleaning and conversion of these machines is considerably more costly and running production is interrupted.

Test procedure and significance of the results
The correlation between feedstock quality - i.e. viscosity - and the pressure/speed curves of the control system is extremely sensitive. Changes in material viscosity have an immediate impact on the characteristics of these curves. The result is simple and clear: The pressure requirement during injection is a significant differentiation criterion for the viscosity of feedstocks and consequently for their processing quality. A quick feedstock test is essentially possible on any mould, particularly as no internal pressure measurement is required for this purpose. The most effective method, however, is the use of a small injection moulding machine operating independently of the production process. The reject rate can be effectively reduced and manufacturing costs lowered significantly. The differences between different material batches can be determined easily and reliably via the new, control-based Arburg methodology.

Sample calculations reveal major savings potential
Based on a moulded part weight of eight grams and the use of a 316L feedstock with an average kilo price of 22 euros, the following cost situation results for a MIM batch oven: If a four cavity mould 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 euros for one sintering batch. Added to this are operating costs of around 1,200 euros for the debinding and sintering stations and the operating costs for the injection moulding machine, amounting to 123.75 euros. 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 2,019.75 euros are incurred, which have to be written off as reject part production.

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