Influence of wax-based binder formulations on rheological properties of feedstocks used in low-pressure metal injection

Abstract

Low-pressure metal injection molding (LPIM) is a powerful technology used to produce small complex net shape metallic parts from a large variety of metals in a cost-effective way. The advantages of LPIM process include high production volumes, high buy-to-fly ratio, wide variety of materials (both for ceramic powders and for metallic powders), excellent mechanical properties, good surface finishes, and good geometric and dimensional tolerances of the produced part. However, efforts to optimize the mechanical properties of LPIM metallic materials typically involve debinding and sintering using feedstocks whose optimal moldability has not yet been demonstrated. Therefore, achieving the full potential of the LPIM process is still limited by the poor understanding of the fundamental mechanisms underlying the moldability of LPIM feedstocks. So far, there is no database that engineers can use to design an appropriate feedstock and the link between the feedstock formulation and its impact on moldability is not yet clear. For this reason, the main objective of this work was to study the influence of wax-based binder systems on the rheological properties of feedstocks to better understand and predict the moldability properties of powder-binder mixtures used in low pressure powder injection molding process. To achieve this objective, a rheological characterization campaign was carried out using seven groups of different feedstock formulations consisting in more than 195 different mixtures. Each feedstock family was tailored to discretize the influence of single-binder and multiple-binder formulations on viscosity. In this study, three different 17-4PH stainless steel powder lots were used to prepare different feedstock formulations containing 60 vol. % of powder, five different waxes (one paraffin wax (PW), one beeswax (BW), one carnauba wax (CW), and two microcrystalline waxes (MW1 and MW2), two surfactants agents (stearic acid (SA) and oleic acid (OA)) and two thickening agents (ethylene vinyl acetate (EVA) and low-density polyethylene (LDPE)). Two different powder sizes (12 and 3 μm) produced by water-atomization and one powder produced by gas atomization (12 μm) were selected to quantify the influence of powder size and powder shape on rheological behavior. Differential scanning calorimetry analysis were performed to assess the melting point of all feedstocks, while rheological tests were performed at 90°C for all feedstocks (i.e,, above melting point of feedstocks) to study the influence of binder constituents on the feedstocks viscosity. It was demonstrated that stearic acid (SA) and oleic acid (OA) produce the same surfactant comparison other conventional thickening agent such as low-density polyethylene (LDPE) can also be used as surfactant agent but at the price of a too significant increase in feedstocks viscosity. Since no significant interaction between SA and EVA was seen, it was proposed that a proportion as low as 0.5 vol.% of SA could be added into EVA-waxes mixtures to produce surfactant effect into that kind of feedstocks. Finally, it was demonstrated that a combination of two surfactant agents (stearic acid and oleic acid) into EVA-waxes based feedstocks produces no significant effect on viscosity compared to similar feedstocks using a single surfactant agent. This systematic binder mapping performed in this study can be now used as the first complete database to better understand the impact of each ingredient and interactions between binders on feedstocks viscosity in order to tailor the LPIM feedstocks properties in future. effect, and since it is more difficult to work with oleic acid, it was concluded that there is no real advantage to use this surfactant agent in the future. It was also proved that a quantity as small as 0.2 vol. % of stearic acid was enough to produce the surfactant effect. Likewise, ethylene–vinyl acetate (EVA) was confirmed as a good binder constituent to increase the viscosity and possibly tailor the viscosity of feedstocks in order to decrease the segregation. In comparison other conventional thickening agent such as low-density polyethylene (LDPE) can also be used as surfactant agent but at the price of a too significant increase in feedstocks viscosity. Since no significant interaction between SA and EVA was seen, it was proposed that a proportion as low as 0.5 vol.% of SA could be added into EVA-waxes mixtures to produce surfactant effect into that kind of feedstocks. Finally, it was demonstrated that a combination of two surfactant agents (stearic acid and oleic acid) into EVA-waxes based feedstocks produces no significant effect on viscosity compared to similar feedstocks using a single surfactant agent. This systematic binder mapping performed in this study can be now used as the first complete database to better understand the impact of each ingredient and interactions between binders on feedstocks viscosity in order to tailor the LPIM feedstocks properties in future.

Date	19 Sept 2018
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Vincent Demers (Supervisor) & Nicole R. Demarquette (Co-supervisor)