The post-processing of polymer powder sintered parts mainly involves two techniques: resin impregnation and wax impregnation. When the prototype part is primarily used for lost-wax casting, a wax impregnation treatment is necessary. On the other hand, when the prototype part is used to improve mechanical strength, a resin impregnation treatment is required.

Polymer powder-based sintered parts typically exhibit poor mechanical properties. Therefore, resin enhancement is often needed for sintered parts used as prototypes. Among various resin coatings, epoxy resin is preferred due to its low shrinkage rate, which helps maintain the dimensional accuracy of the sintered prototype parts. This, in turn, expands the potential applications of polymer powder sintered parts.

1. Influence of Shrinkage Accuracy

Sintered parts inherently have a significant number of voids between the particles, in addition to the fused adhesion between particles. When epoxy resin is applied through capillary action, it fills these voids, as shown in Figure 4-14. However, since epoxy resin experiences volume shrinkage during curing, the sintered part undergoes a volumetric shrinkage process. Moreover, the resin layer that forms on the surface of the sintered part may expand, creating a combination of expansion and contraction, either positive or negative.

For thicker parts of the sintered piece, the resin coating has minimal effect on the volume change. However, for thinner sections, the thickness of the resin coating and its curing shrinkage significantly affect the shrinkage rate. In the case of sintered parts with holes in small-sized regions, after resin impregnation, the expansion of the cured resin is minimal, but the curing shrinkage is more pronounced.

SEM images of cross-sections clearly illustrate the differences. The untreated sintered part shows numerous voids between particles, apart from the fused adhesion. In contrast, the resin-treated sintered part shows that the resin has filled most of the voids, leading to substantial improvements in both strength and impact resistance.

2. Impact on Mechanical Properties

Plastic hardness, which refers to a material’s resistance to indentation by harder objects, quantitatively reflects its soft or hard properties under specific conditions. The surface hardness of sintered materials is moderate after sintering, but after resin impregnation, the Shore hardness of the sintered part improves from 51HD to approximately 73HD, marking a significant enhancement in hardness.

Impact resistance is highly sensitive to the macro and microstructural differences in composite materials. After resin treatment, the impact resistance of the sintered part improves, with a 1.9 to 2.7 times better performance compared to untreated parts. This means the treated parts can withstand high-speed impact tests during actual assembly without fracturing.

The untreated sintered parts have a maximum compressive strength of 26.894 MPa, while sintered parts treated with different epoxy resin systems exhibit compressive strengths ranging from 47.207 MPa to 67.137 MPa. This treatment increases the compressive strength by 1.8 to 2.5 times compared to the untreated parts. Additionally, after resin treatment, the tensile load-bearing capacity of the sintered parts increases exponentially.

In conclusion, resin treatment significantly enhances the mechanical properties of polymer powder sintered parts. The voids between particles are filled, leading to increased impact resistance, compressive strength, and overall durability of the parts. This process provides a crucial improvement for polymer powder sintered parts, making them more suitable for demanding applications requiring higher strength and resistance to impact.