Everything you need to know about shrinkage holes on the surface of injection molded parts

Shrinkage holes are small, circular voids that may form on the surface of the molded part. They are caused by the difference in thermal expansion between the viscous plastic and the air. In this blog post, we will discuss the causes of shrinkage holes, how to prevent them, and how to fix them when they occur.

The depression on the surface of forming parts is called a shrinkage hole, which is caused by the shrinkage of plastic forming, so most of them are difficult to eliminate.

In the process of plastic injection molding, the molten plastic is injected into the cool mold cavity, and because of the poor thermal conductivity of the plastic, the cooling is very complicated.

Especially, the thick-walled part cools more slowly than the thin-walled part, so the thick-walled part is prone to shrinkage cavity.

In addition, the part of the mold with a slightly higher temperature condenses slowly, so that the local temperature difference of the mold is formed.

If the difference in the heat conduction of the mold is added, the part of the mold with a high temperature and poor conduction will appear shrinkage holes.

Therefore, the design should consider the use of difficult shrinkage cavity-forming parts and mold structure. For example, the tendons and protruding parts are tapered and rounded, or design the tendons are non-solid. The surface was patterned to conceal the defect.

Insufficient compression

In the mold where the runner material (composed of the main runner, runner, and gate) is thicker than the wall of the molded part (the volume is too small), the injection pressure can not fully act on the molten material in the mold cavity, so that the shrinkage occurs increases, resulting in a large shrinkage hole.

Especially if the gate is too small, even if the pressure holding time is sufficient, the gate has solidified, so that the pressure cannot be transferred to the molten material in the mold cavity.

Especially for plastic with an inconsistent solid melting point, this phenomenon is more likely to occur, and the mold is easy to burr, because of lax closure and insufficient forming pressure, and it is also easy to appear shrinkage holes.

The screw injection machine is provided with a reverse ring to prevent the melting material from backflow along the screw, but it is easier to produce a shrinkage hole than the plunger injection machine.

At this point, it can be said that the plunger injection machine is better than the screw injection machine. As mentioned above, when the molten material and even the molten material at the end of the cavity have not solidified, the addition of sufficient holding pressure can prevent shrinkage caused by insufficient compression.

Therefore, it is very effective to increase the main runner, runner, and gate, especially the gate diameter. It is also important to increase the injection pressure or to add enough to hold the pressure. The lack of molten material also promotes the shrinkage cavity.

Plastic fluidity is good. If the pressure is increased, it will also cause shrinkage due to burr. If necessary, it can prevent shrinkage by reducing the melt temperature of the cylinder or using plastics with poor fluidity.

Shrinkage holes also easy to appear at the end of flowing molten material far away from the gate, which is caused by pressure loss caused by the flow path resistance of molten material flowing to the end.

Therefore, it is also effective to open the gate near the shrinkage holes easily or increase the thickness of this part. Therefore, it is more effective to increase the number of point gates or change the gate location as appropriate.

Therefore, the mold design should consider the use of difficult shrinkage cavity-forming parts and mold structure. For example, the tendons and protruding parts are tapered and rounded; Or design the tendons are non-solid; The surface was patterned to conceal the defect.

Improper adjustment of injection volume

At the end of the screw injection molding machine, there must be an appropriate amount of molten plastic between the screw head and the nozzle (according to the size of the machine is about 5MM), and it is used to buffer.

If the buffer amount is zero, and the injection amount is adjusted to the end, the screw is also top to the end, so that the screw can not move forward when the pressure is maintained, and the pressure can not be maintained, and the plastic shrinkage will become a shrinkage hole.

The solution is to leave a buffer so that the spike can still advance by a few millimeters or even tens of millimeters at the end of the injection. Zero buffer (that is, the end of the injection screw tip to the bottom) will shorten the life of the plastic injection molding machine itself, which must be noted.

Shrinkage holes appear on the workpiece working surface

Some formed parts, even if there are shrinkage holes inside, sometimes do not hinder. As described in the beginning, shrinkage holes are easy to appear on the high-temperature side of the mold, while shrinkage holes are difficult to appear on the low-temperature side.

Therefore, the surface that does not allow the shrinkage hole should be fully cooled, or on the contrary will allow the shrinkage hole (that is, does not allow the relative surface of the shrinkage hole) high temperature molding is also very effective.

Uneven cooling

When the wall thickness of the injection molding parts is not uniform, the thick part of the wall cools more slowly than the thin part, so the thick part of the wall produces shrinkage holes.

It is theoretically difficult to eliminate shrinkage caused by uneven wall thickness, so uniform wall thickness should be made when designing parts. That is, the focus is on reducing the variation in wall thickness.

For example, when designing the boss, if there are requirements on the outside diameter size, the injection molding process hole should be set in the center to eliminate shrinkage hole;

When the strength is required, the boss itself should not be thickened, but the reinforcement should be used to increase strength.

The shrinkage hole under the gentle concave is less obvious than the shrinkage hole under the sharp concave, so the parts that do not require precision should be solidified in the outer layer, and the central part is still soft and can be pushed out of the mold, and then slowly cooled in the air or warm water, so that the shrinkage hole is not obvious and does not affect the use.

Excessive shrinkage

When the thermal expansion coefficient of injection molding plastic itself is large, shrinkage holes are easy to appear.

For example, PE shrinkage rate 0.02~0.05, PP shrinkage rate 0.01~0.02, PS shrinkage rate 0.002~0.006, even if there is a little reinforcement, it will produce dents

Therefore, low-temperature molding of this plastic is not easy to appear shrinkage holes. If the injection pressure is increased, more plastic can be injected into the mold cavity, so the higher the pressure, the shrinkage cavity will decrease correspondingly.

However, if the temperature drops below the minimum temperature required by the plastic, it is difficult to prevent shrinkage of the plastic, even if the injection pressure is increased.

For example, in polypropylene, high-density polyethylene, polyformaldehyde, etc., the density of its crystalline solid and molten state is significantly different, so it is difficult to prevent shrinkage holes.

At this time, if the substitution of a non-crystalline copolymer is allowed, the shrinkage cavity can be reduced. In addition, if filled with inorganic fillers, such as glass fiber, asbestos can also make the shrinkage cavity smaller.

The surface cures too slowly

The shrinkage or gloss of the part of the wall thickness is because the surface layer does not form a solid curing layer.

When the center shrinks, the surface will be pulled inward into the shrinkage hole, and even the heat from the inside out will melt the surface layer again and the gloss will appear.

Conversely, where the wall is thick, if the surface layer is strong enough, the contraction of the center will form a vacuum bubble. Can reduce the high mold temperature, reduce the material temperature, and reduce the speed of the molten material through the wall thickness area.

So that the curing layer is thicker (but easy to appears vacuum bubble); Adjust the wall thickness, such as the reinforcement thinning, thickness slowly change; Use low shrinkage plastics; Add foaming agent to plastic.