Effects of acrylic acid-grafted HDPE in HDPE-based binder on properties after injection and debinding in metal injection molding

The objective of the present work is to investigate the effects of acrylic-acid grafted high density polyethylene (AAHDPE) content in a HDPE-based binder (at 0 vol%, 5 vol%, 10 vol% and 25 vol%) on physical, mechanical, and morphological characteristics of parts after injection as well as on physical properties and appearance of parts after debinding. Calculated theoretical densities of the parts after injection slightly decreased with an increase in the AAHDPE content, while the densities measured by Archimedes' principle and mass/volume measurement exhibited the opposite tendency. The cross-sectional fractured surfaces of hexane-treated injected parts indicated that the interaction between the binder and the metal powder was significantly increased when the AAHDPE content was greater than 10 vol%. The fractured cross-sectional surfaces of hexane-treated injected parts further revealed the phase separation between HDPE and paraffin wax (PW). The flexural strength and flexural strain at break increased with increasing AAHDPE content in binder before reaching a plateau region, when the AAHDPE contents were 5 vol% and 10 vol%, respectively. After thermal debinding, the distortion of debinded parts as well as volume expansion of materials was observed. The distortion angle analysis found that the distortion angles tended to decrease remarkably with increasing AAHDPE content. The results of the present study suggest that AAHDPE is a key component in the HDPE-based binder, i.e. acting as a compatibilizer for binder/metal powder. Consequently, mechanical properties of the parts after injection could be enhanced and in addition the distortion of the debinded parts could be reduced.