How to avoid PIM defects

When the causes of part defects are known, molders understand how to avoid them, says Arburg’s Uwe Haupt. Unfortunately, this is often easier said than done in PIM.

Haupt admits that because of the interdisciplinary nature of the PIM process itself—with its feedstock compounding, pelletizing, molding, setter staging, debinding, and sintering—there may be any number of sources of problems like weldlines, cracks, or jetting.

Haupt and his colleague, Hartmut Walcher, have worked closely with their customers to identify some of the major causes of part quality problems in the molding process (see chart of molded part defects and their potential causes).

For example, lot-to-lot changes in powder surfaces and powder agglomeration during compounding can cause any number of problems, including part porosity. Through working with a variety of customers from around the world, Haupt and Walcher have found that variations of less than .1 percent by weight can influence flow.

To this end, Haupt champions the use of more advanced compounding systems than the commonly used twin-blade batch blenders. The use of shear roller compactors, twin-screw extruders, or continuous kneading extruders is recommended to improve feedstock homogeneity. Mixer effectiveness can be monitored by analyzing pressures.

When it comes to the phase of the PIM process nearest and dearest to his heart—injection molding —Haupt just shakes his head. “There must be more than 150 parameters that have to be controlled in injection molding,” he says. “If temperatures are out of control, the feedstock viscosity can almost double, the binder can degrade, and the material can block the screw.”