Basic reasons: from the perspective of plastic, the reasons for the change of size are as follows:
The change of pressure distribution in the U-shaped cavity will cause the size change;
U for crystalline plastics, the change of crystallinity due to the change of cooling rate will cause the change of size;
The change of post-treatment, ambient temperature and humidity of u product will also cause dimensional change.
Next, the change of the pressure distribution in the cavity and the change of the size are discussed. The causes of defects will be different with different size changes.
1、 Defect: the overall product size is too small.
Cause analysis: the plastic pressure in the cavity is too low.
Solution: increase the feeding pressure to reach the cavity pressure required by qualified products.
2、 Defect: the overall product size is too large
Cause analysis: the plastic pressure in the cavity is too high.
Solution: reduce the feeding pressure to achieve the cavity pressure required by qualified products.
3、 Defects: the size near the gate of the product is too small, and the size at other positions is normal
Cause analysis: the pressure near the gate is too low. It is mainly because the gate is not frozen and the plastic returns.
Solution: increase the pressure holding time, or adjust the process according to other reasons (such as plastic or mold temperature rise caused by gate not frozen).
4、 Defects: the size near the gate of the product is too large, and the size at other positions is normal
Cause analysis: the pressure near the gate is too high.
Solution: if the three-stage forming process is adopted, the feeding rate can be changed. Check the effective viscosity (injection pressure integral in filling stage), measure the plastic temperature and adjust it. If the two-stage forming process is adopted and the filling rate, feeding rate and pressure are changed, it can be solved.
5、 Defect phenomenon: the size of the product fluctuates back and forth, unstable.
Cause analysis: This is caused by the pressure fluctuation of the cavity. Observe the fluctuation of cavity pressure curve. The first mock exam is to see if the pressure distribution of the die cavity varies, so does the procuratorial trend change with time. The first mock exam is the first mock exam. If there is a change in temperature, or the change of material batch, if the die is different, it will indicate that the change of the feeding process will lead to the change of gate freezing. Some gates are frozen, some gates are not frozen, and the pressure changes due to the leakage of the reverse ring.
Solution: do gate freezing test and dynamic check ring leakage test, determine the cause, and solve the problem.