Feature article: PIM International, Vol.4 No. 2 June 2010, pages 31-39, 5625 words

Author: Professor Randall M. German, San Diego State University, USA

Associate Dean of Engineering, College of Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1326, USA 

 

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The initial demonstrations on powder injection moulding date from the 1930’s while metal variants have been in production since 1975. The core process is well known, supported by a wide variety of powder, feedstock, and equipment firms.

Although there is much focus on the core technology, based on high-pressure moulding and high-temperature sintering, many variants have emerged with shifts in binders, shaping, or other processing steps.

In this article Prof. Randall German introduces several variants to illustrate how core principles, similar powders and binders, and other parallel aspects spill over to a wide range of alternative approaches that share many of the same concepts used in PIM.

Introduction 

Within the core PIM process we recognise many possible binders, powder types, moulding conditions, and even a wide range of debinding and sintering approaches. Still, in spite of such variation there seems to have emerged a fairly similar set of approaches, even though moulding might be at low pressures (few atmospheres), intermediate pressures, or high pressures.

However, in spite of the apparent diversity in the PIM industry, there are many other technologies out there that share some of the same platform, yet innovate around the same concepts for a variety of applications. Accordingly, this article is organised on the technical variants to the powder injection moulding process. Several variations in powder-binder combinations are reported.

Many examples of different additive and reshaping routes are described, with a few comments on subtractive efforts that use green machining of PIM feedstock blocks to form green bodies. Several additive efforts are well known in the form of rapid prototyping techniques. Although binders might be tailored to the specific needs of the forming process, for most cases the debinding and sintering steps remain similar to that seen in PIM. The range of invention shows the possible new materials, products, features, and property ranges possible from the core PIM technology...........

Further sections of this article include:

- The central idea
- Variations in powders and binders
- Variants on moulding

   Additive processes
   Ink jet printing
   Laser sintering
   Nozzle extrusion
   Laminated objects
   Dip coating
   Sprinkling
   Electrorophoritic Deposition

   Reshaping processes
   Extrusion
   Tape casting
   Slip casting
   Slurry casting in soft tooling
   Centrifugal compaction
   Gel casting
   Putty and clay materials
   Die compaction

   Subtractive processes
   Green machining

- Variants on sintering

- Summary

Figures and Tables:

Fig. 1 Simple process flow chart for powder injection moulding. Within each step there is a large possible variation. Many shaping approaches use similar feedstock, but avoid high pressure moulding

Fig. 2 This key chain includes an injection moulded wood component, showing evidence that PIM works even for organic particles

Fig. 3 A scanning electron micrograph of a green body built using stereo-lithography, showing the laminated layers resulting from the progressive build process

Fig. 4 Porous torpedo noses formed by stereo-lithography with selective laser sintering

Fig. 5 An example turbine component generated by stereo-lithography techniques after infiltration

Fig. 6 A tool set for molding a camera body. This tooling was generated using rapid prototyping approaches adapted from PIM technologies

Fig. 7 This structure shows the electrophoritic deposition grid over a copper base used to generate a heat dissipation design out of silicon carbide with copper electrodeposited into the pores (photograph courtesy of Eugene Olevsky)

Fig. 8 Translucent alumina tube formed by extrusion of a powder-binder paste which is sintered to full density

Fig. 9 Tape casting relies on powder and binder to form a constant thickness sheet, but unlike PIM where the rheology is adjusted with heat, tape casting relies on solvent to allow shaping

Fig. 10 Metallographic cross section through a diamond bonded to a stainless steel substrate. In this case the braze layer was formed using tape casting and the diamonds were glued to the braze layer prior to firing. The stainless structure is at the bottom and the cross-sectioned diamond is evident as the angular object

Fig. 11 This 2.5 kg bronze statue (150 mm high) was formed in rubber tooling using a slurry casting process with a feedstock designed for 82 vol. % solid loading. Shown here is the final product after debinding and sintering with a dark patina

Fig. 12 A hard tool composite system designed for slurry casting, consisting of large cemented carbide (WC-10Co) grains in a matrix of 17-4 PH stainless steel with bronze infiltration. Wear tests determined tooling fabricated with this composition exhibited properties similar to tool steels (courtesy of Anthony Griffo)

Fig. 13 Superalloy gel cast turbine component, with an outside diameter of approximately 200 mm (component courtesy of Mark Janney)

Fig. 14 Tungsten putty deformed to illustrate the pliable character of this 50 vol. % solids loading (sample courtesy of Travis Puzz)

Fig. 15 Agglomerated stainless steel powder ready for die compaction. The small particles provide good sintering and the large agglomerates enable automated feed during uniaxial die compaction (photograph courtesy of Louis Campbell)

Fig. 16 Metallographic cross-section in a die compacted stainless steel formed by die compaction of an agglomerated MIM powder, the black spots are residual porosity (photograph courtesy of Louis Campbell)

Fig. 17 A stainless steel green body imaged using scanning electron microscopy after green machining, showing only slight damage around the drilled hole

Fig. 18 Bronze block, consisting of powder and binder, and a final object carved from the block. The block is easily carved using hand tools, and then is debound and fired in a single firing cycle. A few chips have been lifted out of the block using the carving tool                                                                                                                                   

Table 1. Outline of the PIM process for 316L stainless steels