Voids

Voids are formed inside the wall of a part and can adversely affect the structural performance of the part. Except in transparent parts, voids can be invisible to the naked eye. Voids are formed as a result of shrinkage of the molten core after the wall has solidified on the cooler mold surface. The shrinkage of the molten core causes the layers of material to pull away, forming an opening in the part, or void

Causes:

1. Higher melt temperatures slow down the cooling rate of the polymer in the mold, causing higher shrinkage in the hotter core, leading to voids

2. Injecting the material at high speeds raising melt temperatures due to shear heating can also cause overheatingelt

3. Short packing times will not provide enough time to hold pressure on the core to prevent shrinkage

4. Gate diameters that are too small are source of shear heating increasing melt temperature

5. A gate that freezes-off too soon will prevent material from filling out the cavity completely, causing the molten core to shrink

6. Small runner diameters will cool faster and hinder cavity filling, creating the potential for voids to form in the part

7. A cold mold will only enhance the formation of the skin surface long before the core cools sufficiently. As a result, the core forms more slowly and shrinks more 

8. The workpiece with not good design

Solutions:

1. Lower melt temperatures can help reduce the formation of voids

2. Low packing pressures will allow voids to form since there is not enough resistance applied on the core to prevent shrinkag

3. Enhancing the packing time and holding pressure

4. Increasing the gate size

5. Measuring gate seal time can indicate whether the cavity is full before the gate closes

6. Larger-diameter runners, such as full round, trapezoidal, and modified trapezoidal are recommended

7. Increasing mold temperature slows down the cooling rate, and the formation of the skin prevents voids. Improvements can be made in mold cooling, such as the addition of bubblers, cooling pins, thermal pins, or steel inserts to drive out more heat from the part

8. Designing the part to flow from thick to a thin wall eliminates the formation of voids. Coring out thick sections allows for improved cooling of the part, reducing voids