Euro PM2021: Advances in MIM feedstocks
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Exploring the subtleties of the formulation and preparation of MIM feedstocks A technical session, comprising three papers, at the Euro PM2021 Virtual Congress, organised by the European Powder Metallurgy Association (EPMA) and held from October 18–22, 2021, assessed various aspects of the formulation and preparation of feedstocks for Metal Injection Moulding. Dr David Whittaker reports on three papers that addressed solvent debinding efficiency, strategies for enhancing the debinding of polyacetal (POM) feedstocks, and, lastly, an analytical method for enhancing the understanding of the homogeneity of MIM feedstocks and green parts.
Effect of backbone selection on the solvent debinding of Metal Injection Moulding feedstocks The first paper featured in this review came from Christian Kukla, Santiago Cano, Stephan Schuschnigg, Clemens Holzer and Joamin Gonzalez-Gutierrez (Montanuniversitaet Leoben, Austria). It considered the effect of backbone selection in MIM feedstocks on the efficiency of solvent debinding [1]. At least two components are necessary in MIM feedstock binders: the backbone, to provide strength and shape retention during the removal of the other binder components, and the main binder component to provide flowability during the injection moulding process. Organic solvents or water can be used to remove a portion of the binder, followed by thermal debinding to eliminate the remaining binder components. During solvent debinding, interconnected pore channels are left, allowing the
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decomposition products in thermal debinding to leave the part. If the binder components are not adequately selected, defects – such as cracking, distortions or slumping – can be encountered during solvent debinding. The binder component to be removed in thermal debinding is the backbone. The backbone must be low cost, chemically resistant to solvents used in solvent debinding and easily removable by thermal degradation. These requirements are fulfilled by polyolefins, the most commonly used being high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP). For high performance, the binder should show good adhesion to the powder. In many cases, an additive
achieves this. Stearic acid (SA) is commonly used for this purpose in PIM. Another possibility is the use of grafted polyolefins; grafting introduces suitable polar sites onto the chains of the polymers, thus improving the adhesion between polymers and particles with different polarities. The use of grafted polymers as PIM feedstocks has not, to date, been extensively studied. Therefore, the reported work looked at the behaviour of different backbones and grafted polyolefins, comparing the debinding performance, morphology and thermal degradation of feedstocks with grafted backbones. The major fraction of the binder in the studied feedstocks was composed of two waxes, added to
Composition (wt.%) Fe
C
Si
Mn
P
S
Cr
Ni
Mo
Cu
Balance
0.017
0.55
1.28
0.019
0.006
16.3
10.2
2.05
0.1
Table 1 Chemical composition of gas atomised 316L powders, as provided by the suppler [1]
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