**Post-blow**

Post-blow is similar to the internal blistering and always appears at the place of hot spots (see Figure 1-5d). The post-blow defect is caused by following two factors. One is that the cooling is not enough at the hotspots; the other is that too much gas enters the some certain bubbles due to the high SCF concentration and form some large size bubbles. When the pressure inside the bubbles is higher than the outside one, the post-blow will happen. So the method to eliminate this defect is to enhancing cooling at the hot spots and adjusting SCF concentration.

#### **Surface roughness**

In addition to the above serious defects, surface roughness is another problem that limits the application scope of microcellular foam injection molding process. During bubbles growing up, some small bubbles break up near the surface, and the gas is trapped on the mold surface when the polymer-SCF mixer begins to solidify. So the surface roughness of microcellular foam injection parts is higher than that of traditional injection parts.
