Depressions on the surface of molded parts are called shrinkage holes, which are caused by shrinkage of plastic during molding, so most of them are difficult to eliminate. In the process of injection molding, molten plastic is injected into a cool mold cavity, cooling is very complicated due to poor thermal conductivity of plastic.
In particular, thick-walled part cools more slowly than thin-walled part, so thick-walled part is prone to shrinkage. In addition, part with a slightly higher mold temperature condenses slowly, thus forming a local temperature difference of mold. If difference in heat conduction of mold itself is added, shrinkage holes will appear in the part with high mold temperature and poor conduction.

 

Therefore, design should consider use of molded parts and mold structures that are difficult to shrink cavities. For example, ribs and protruding parts are thinned and rounded; or ribs are designed to be non-solid; surface is designed as a pattern to cover up this defect.

In mold with runner material (composed of main runner, runner and gate) thicker than wall of molded part (volume is too small), injection pressure cannot fully act on molten material in mold cavity, which increases shrinkage. larger, resulting in larger shrinkage cavities.
Especially if gate is too small, even if pressure holding time is sufficient, gate has solidified, so that pressure cannot be transmitted to melt in mold cavity. This phenomenon is more likely to occur especially for rigid plastics with inconsistent solid melting point. There are also molds that are prone to burrs. Due to lax closure, sufficient molding pressure cannot be applied, and shrinkage cavities are also prone to occur.
Screw type injection machine is provided with a check ring to prevent melt from flowing back along screw, but it is easier to produce shrinkage holes than plunger type injection machine. At this point, it can be said that plunger type injection machine is better than screw type injection machine. As mentioned above, when molten material and even molten material at the end of cavity has not solidified, sufficient holding pressure can be applied to prevent shrinkage cavity caused by insufficient compression.
Therefore, it is very effective to increase main runner, runner, gate, especially gate diameter. It is also important to increase injection pressure, or add enough holding pressure. Lack of melt also contributes to shrinkage. Plastic has good fluidity. If pressure is increased, burr will also cause shrinkage. If necessary, reduce temperature of barrel or use a plastic with poor fluidity to prevent shrinkage.
Shrinkage holes are also prone to appear at the end of flowing melt that is far away from gate, which is caused by pressure loss caused by flow resistance of melt flowing to the end. Thickness of this portion is also effective. Therefore, it is more effective to increase number of point gates or change gate location according to situation.

When injection of screw injection molding machine is finished, an appropriate amount of molten plastic must be left between screw head and nozzle (about 5MM according to size of machine) to buffer it.
If this buffer amount is zero, and injection amount is adjusted to the end, screw will also push to the bottom at the same time, so that screw cannot move forward during pressure maintaining, so pressure cannot be maintained, and plastic shrinkage will be manifested as shrinkage holes.
Solution is to leave a certain amount of buffer, so that screw can still advance several millimeters or even ten millimeters at the end of injection. When buffer amount is zero (that is, when screw is pushed to bottom at the end of injection), life of injection machine itself will be shortened, and attention must be paid to it.

Even if there are shrinkage cavities inside some molded parts, there are sometimes no obstacles. In this case, as mentioned at the beginning, side with high temperature of mold is prone to shrinkage, while side with low temperature is difficult to have shrinkage.
Therefore, surface that does not allow shrinkage holes should be sufficiently cooled, or conversely, high temperature forming that allows shrinkage holes (that is, opposite surface that does not allow shrinkage holes) is also effective.

When wall thickness of molded part is extremely uneven, thick-walled part cools more slowly than thin-walled part, so thick-walled part produces shrinkage holes.
It is also theoretically difficult to eliminate shrinkage holes caused by uneven wall thickness, so wall thickness should be uniform when designing parts. That said, focus is on narrowing wall thickness changes.
For example, when designing a boss, if there is a requirement for outer diameter size, a process hole to eliminate shrinkage holes should be set in the center; when strength of boss is required, boss itself should not be thickened, but method of using reinforcing ribs to increase strength should be adopted. .
Shrinkage cavity that is gently concave is less conspicuous than shrinkage cavity that sinks sharply. Therefore, for parts that do not require precision, mold should be released when outer layer is solidified and center part is still soft and can be pushed out. Slowly cool in warm water, so that shrinkage cavity is not obvious and does not affect use.

When thermal expansion coefficient of molded plastic itself is large, shrinkage cavities are of course prone to occur.
For example, shrinkage rate of PE is 0.02~0.05, shrinkage rate of PP is 0.01~0.02, and shrinkage rate of PS is 0.002~0.006. Even if there is a slight reinforcement, dents will occur.
Therefore, low temperature molding of this plastic is less prone to shrinkage cavities. If injection pressure is increased, more plastic can be injected into cavity, so the higher pressure, the smaller shrinkage cavity.
However, when temperature falls below minimum temperature required for plastic, even if injection pressure is increased, it is difficult to prevent shrinkage of resultant plastic.
For example, polypropylene, high-density polyethylene, polyoxymethylene, etc., density of crystalline solid and molten state is significantly different, so it is difficult to prevent shrinkage. At this time, if non-crystalline copolymer is allowed to be replaced, shrinkage cavity can be reduced. In addition, if filled with inorganic fillers, such as glass fiber, asbestos, etc., shrinkage cavity can also be reduced.

Shrinkage holes or luster appear in the part of wall thickness because surface layer does not form a solid solidified layer. When center part shrinks, surface will be pulled inward to form shrinkage holes, even heat diffused from inside to outside will melt surface layer again and appear glossy.
Conversely, if surface layer is strong enough at wall thickness, shrinkage of center will form a vacuum bubble. It can reduce mold temperature, reduce material temperature, reduce speed of melt passing through wall thickness area, and make solidified layer thicker (but prone to vacuum bubbles); adjust wall thickness, such as thinning ribs, and gradually change thickness; use low shrinkage plastics; add foaming agent in plastic.

 

Figure A Appends flow edges where dents are created

 

Figure B Widening of the pass edge of the resin