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23 Injection Molding Defects And How To Prevent Them

2022-05-25 16:33:57 JTC Model Technologies Co., Ltd., 0

With the rapid development of the plastics industry, plastic products are becoming more and more popular and are now widely used in various fields. In recent years, due to the increasing demand for injection products in household appliances, communications, video, medical and other industries, the development and improvement of injection molding technology has been promoted.


In the current engineering plastics industry, 80% of the products are injection molded. In the process of injection molding of plastic products, due to the influence of various factors such as molding materials, molding molds, auxiliary equipment, and molding environment, various problems will occur in the internal and external appearance of injection molding products.


So what are the common defects of injection molded parts?


One. Lack of glue and unsaturated mold for injection parts: Refers to the incomplete product caused by the material flow not completely filling the mold cavity during the molding process, also known as incomplete, lack of material, etc. Occurs mainly away from slugs or thin-walled surfaces.


Cause Analysis:

1. Improper matching with the injection molding machine, resulting in insufficient plasticizing capacity or injection volume of the injection molding machine.

2. The nozzle, runner and gate are too small, the process is too long, and the plastic filling resistance is too large.

3. Poor runner design and large plastic flow resistance

4. Insufficient melting of plastic, poor fluidity, resulting in large injection pressure loss.

5. Insufficient or improper reservation of the cold material well in the runner, the cold material head enters the cavity and hinders the normal flow of the plastic, increasing the

6. The material temperature and mold temperature are too low, the plastic is difficult to flow under the current pressure, the injection speed is too slow, and the pressure holding or holding pressure is too low.

7. When the mold exhaust is poor, the air cannot be excluded.



1. Change the injection molding machine, and the finished product containing the sprue should not exceed 80% of the injection volume of the machine.

2. Increase the size of the injection port of the nozzle.

3. Modify the runner to meet the actual requirements.

4. Properly adjust the back pressure and screw speed, so that the plastic is mixed evenly.

5. Increase the storage space of the cold material well.

6. Properly increase the mold temperature according to actual needs.

7. Improve exhaust and regularly remove mold scale.


Two. Surface shrinkage, shrinkage cavity (vacuum bubble)

The phenomenon of depression on the surface of the molded part, which is a shrinkage of volume, usually found in the thick part of the meat, the ribs or the protruding back. Since the thick part of the molded product cools slowly and the outer part cools quickly, the molten plastic in the thick part is stretched outward, and the void in the center is actually the thick part of the vacuum bubble that is not easy to cool and occurs on the surface. When the plastic melt contains air, moisture and volatile gases, the air, moisture and volatile gases enter the product during the injection molding process and the remaining voids are called bubbles.


Cause Analysis:

1. The holding time of the holding pressure is insufficient, the plastic melt is insufficiently fed, and it is also easy to cause back flow.

2. The injection speed is too slow, and the plastic melt is insufficiently fed.

3. Insufficient injection volume.

4. The material temperature and mold temperature are too high, the cooling is slow, and the plastic shrinks completely, resulting in shrinkage and subsidence.

5. The size of the runner and gate is too small, the pressure loss increases, and the gate solidifies too early and the feeding is poor.

6. The local meat is too thick.

7. When the residual amount of CUSHION VOLUME of the injection molding machine is insufficient or the check valve does not move smoothly, the wall thickness of the product will also shrink, and the surface of the product will have waves.



1. Increase the holding pressure and prolong the holding time.

2. Increase the filling speed and achieve full compression before the plastic is cooled and solidified.

3. The transfer of plastic injection to pressure is too fast.

4. Make the meat thickness change smoother and improve the cooling efficiency of this part.

5. The colloidal particles are fully dried to remove moisture in advance.

6. The temperature of the barrel should not be set too high, which can effectively prevent the generation of plastic decomposition gas.

7. Use a small screw or machine to prevent the screw from being over-sheared.

8. Increase the back pressure so that the gas can be discharged from the barrel.

9. Appropriately reduce the filling speed, and the gas has sufficient time to discharge.


Three. Warping

The deformation can be divided into two phenomena: warping and twisting. The deformation of the parallel side is called warping, and the deformation of the diagonal direction is called twisting.


Cause Analysis:

1. Uneven thickness of meat and uneven cooling. The cooling speed of the plastic is different. The place where the cooling is fast shrinks less, and the place where the cooling is slow shrinks more, resulting in deformation.

2. The material temperature is high, the shrinkage is large, and the deformation is large.

3. Differences in molecular alignment; inner curvature of the sidewall.

4. The deformation caused by the internal stress when the product is demolded is caused by the product being ejected from the mold before it is fully cooled and solidified.

5. Generally, in order to prevent the deformation of the product, the product can be shaped with a fixture after being ejected to correct the deformation or prevent further deformation, but the product will recover if it encounters high temperature again during use, so special attention should be paid to this point. .


In actual production, there are still many quality defects of injection molded parts. The above list is just three of the most common ones. All defects are closely related to plastic materials, molding processes and molds. Only by finding out the reasons for the defects can they be found. come up with countermeasures to resolve defects.