Visible defects on the surface of injection molded parts include dark spots, gloss differences, and surface wrinkling. Usually these defects appear near the gate or behind sharp corners. We can find out the causes 

and solutions of these defects from the mold and production process. For example， the Spots on product surface

In fact, dark spots often appear near the gate at the beginning of the injection cycle. Experiments show that this phenomenon is due to the injection speed

One way to solve this problem is to reduce the injection speed. In order to reduce the melt flow rate at the gate, the injection can be divided into several steps, and gradually increase the injection speed

in order to obtain a uniform melt flow rate in the whole stage

Low melt temperature is another reason for dark spots. This phenomenon can be reduced by increasing barrel temperature and screw pressure. In addition, the mold temperature is too low will produce surface defects

so increasing the mold temperature is another feasible way to overcome the surface defects

Mold design defects can also produce dark spots near the injection gate. The sharp corner of the injection can be avoided by changing the radius. In the design, we should pay attention to the position and diameter of the gate to see if the gate design is appropriate.