The depression on the surface of molded parts is called shrinkage cavity, which is caused by the shrinkage of plastic molding, so most of them are difficult to eliminate. In the process of injection molding, the molten plastic is injected into the cool mold cavity. Because of the poor thermal conductivity of the plastic, the cooling is very complicated.
Especially, the cooling rate of the thick wall is slower than that of the thin wall part, so the shrinkage cavity is easy to appear in the thick wall part. In addition, the higher the mold temperature is, the slower the condensation is, so the local temperature difference of the mold is formed. If the heat conduction difference of the mold itself is added, the parts with high mold temperature and poor conduction will have shrinkage cavity.
Therefore, it is necessary to consider the forming parts and die structure which are difficult to produce shrinkage cavity in the design. For example, the ribs and protrusions are thinned and rounded; or the ribs are designed to be non solid; and the surface is designed to cover up the defect.
1. Insufficient compression
When the runner material (composed of main runner, runner and gate) is thicker than the wall thickness (volume is too small) of the molded part, the injection pressure can not fully act on the molten material in the mold cavity, resulting in the increase of shrinkage and the appearance of large shrinkage cavity.
Especially when the gate is too small, even if the holding time is sufficient, the gate has solidified, so that the pressure can not be transmitted to the molten material in the mold cavity. In particular, it is easier to produce this phenomenon when the solid melting point is not consistent. There are also easy to burr molds, which are not tightly closed and can not add enough molding pressure, and are prone to shrinkage.
The screw type injection machine is provided with a check ring to prevent the molten material from flowing back along the screw, but it is easier to produce shrinkage cavity than the plunger type injection machine. In this regard, it can be said that the plunger type injection machine is better than the screw type injection machine. As mentioned above, the shrinkage cavity can be prevented by adding enough pressure at the end of the mold before solidification.
Therefore, it is very effective to increase the main gate, gate and gate, especially the gate diameter. In addition, it is important to increase the injection pressure, or add enough holding pressure. The lack of molten material also contributes to shrinkage. The plastic has good fluidity. If the pressure is increased, the burr will also cause shrinkage cavity. If necessary, reduce the material temperature or use the plastic with poor fluidity to prevent shrinkage.
It is also easy to have shrinkage cavity at the end of the flowing molten material which is far away from the gate, which is caused by the pressure loss caused by the resistance of the flow path from the molten material to the end. Therefore, it is effective to open the gate near the shrinkage cavity or increase the thickness of the part. Therefore, it is more effective to increase the number of point gates or change the gate location according to the situation.
2. Improper adjustment of injection volume
At the end of injection, there must be a proper amount of molten plastic between the screw head and the nozzle (about 5mm according to the size of the machine) to cushion it.
If the buffer amount is zero and the injection volume is adjusted to the end, the screw will be pushed to the bottom at the same time. In this way, the screw can not move forward when maintaining pressure, so the plastic shrinkage will become shrinkage cavity.
The way to solve this problem is to reserve a certain amount of buffer so that the cuttings can still advance several millimeters or even more than ten millimeters at the end of injection. If the buffer amount is zero (that is, when the screw is pushed to the bottom at the end of injection), the service life of the injection machine itself will be shortened.
3. Shrinkage appears on the working face of the workpiece
Some molding parts even if the internal shrinkage, sometimes there is no obstruction. In this case, as described at the beginning, the side with high temperature is easy to produce shrinkage cavity, while the surface with low temperature is difficult to produce shrinkage cavity.
Therefore, it is also effective to cool the surface that is not allowed to shrink, or on the contrary, to form the opposite surface with shrinkage cavity at high temperature.
4. Uneven cooling
When the wall thickness of the molded part is extremely uneven, the cooling rate of the thick wall part is slower than that of the thin wall part, so the shrinkage cavity occurs in the thick wall part.
In theory, it is difficult to eliminate the shrinkage cavity caused by uneven wall thickness. Therefore, the wall thickness should be uniform when designing parts. In other words, the key point is to reduce the variation of wall thickness.
For example, when designing the boss, if there are requirements for the external diameter size, the process hole for eliminating shrinkage hole should be set in the center; when the strength of the boss is required, the boss itself should not be thickened, but the way to increase the strength by using the reinforcing rib should be adopted.
The shrinkage cavity in the gentle concave is less obvious than that in the sharp depression. Therefore, the parts with no precision should be taken out of the mold when the outer layer has solidified and the central part is still soft and can be ejected, and then slowly cooled in air or warm water, so as to make the shrinkage cavity not obvious and not affect the use.
5. Excessive shrinkage
When the coefficient of thermal expansion of molding plastics is large, of course, shrinkage is easy to occur.
For example, PE shrinkage is 0.02 ~ 0.05, PP shrinkage is 0.01 ~ 0.02, PS shrinkage is 0.002 ~ 0.006, even if there is a little stiffener, there will be dents
Therefore, low temperature molding of this plastic is not easy to appear shrinkage. If the injection pressure is increased, more plastic can be injected into the cavity, so the higher the pressure, the smaller the shrinkage cavity.
However, when the temperature drops below the minimum required temperature of the plastic, even if the injection pressure is increased, it is difficult to prevent shrinkage of the resultant plastic.
