Solutions for cracks on the surface of injection molded products

I. Terminology

Cracks on the surface of molded products are called cracks.



Figure 1 Crack diagram

II. Failure analysis and troubleshooting method

1. The residual stress is too high. In-mold design and production, the direct gate with minimum pressure loss and high injection pressure can be used. When designing side gates, a tab-type gate can be used to remove the broken part after molding.

In terms of process operation, reducing residual stress by lowering injection pressure is the easiest way to reduce residual stress, because injection pressure is proportional to residual stress. The barrel and mold temperature should be increased appropriately to reduce the temperature of the melt and mold, and the cooling time and speed of the mold embryo should be controlled so that the oriented molecules even have a longer recovery time.



Figure 2 Schematic diagram of residual stress

2. External force causes residual stress concentration. In general, this type of failure always occurs around the top bar. After this type of failure, the ejector should be carefully checked and adjusted, and the ejector should be set in the part with the greatest resistance to release, such as the tab. Reinforcements, etc. If the number of ejector bars set is not possible to expand due to the condition of pushing the roof area, the method of a small area with multiple ejector bars can be used. If the mold cavity release slope is not enough, the surface of the plastic parts will also appear scratched to form a crease pattern.

3. There is a difference between the coefficient of thermal expansion of the molding material and the metal insert. The metal insert should be preheated, especially when the cracks on the surface of the plastic parts occur at the beginning of the machine, most of them are caused by the low temperature of the insert.

In addition, in the choice of insert material, should try to use the coefficient of linear expansion close to the characteristics of the resin material. In the selection of molding raw materials, should also be used as far as possible, high molecular weight resin, if you must use low molecular weight molding raw materials, the thickness of the plastic around the insert should be designed thicker.

4. improper selection of raw materials or not pure. Practice shows that low-viscosity sparse resin is not easy to produce cracks. Therefore, in the production process, should be combined with the specific situation to select the appropriate molding raw materials. In the operation process, special attention should be paid not to mix polyethylene and polypropylene, and other resins together, which can easily produce cracks. In the molding process, the release agent is also a foreign substance for the melt, such as improper dosage will also cause cracks, should try to reduce its dosage.

5. Poor design of the structure of the plastic part. The sharp corners and notches in the structure of the plastic part are most likely to produce stress concentration, resulting in cracks and ruptures on the surface of the plastic part. Therefore, the outer and inner corners of the structure of the plastic part should be made of the largest possible radius arc. The test shows that the best excessive arc radius is the ratio of the radius of the arc and the wall thickness at the corner is 1:1.7.



Figure 3 Deformation caused by the unreasonable design of the product

6. Cracks on the mold are reflected on the surface of the plastic part. In the injection molding process, due to the repeated action of injection pressure on the mold, fatigue cracks will be generated in the cavity with sharp edges, especially in the vicinity of the cooling hole.

When the cracks on the surface of the mold cavity are reflected on the surface of the plastic part, the cracks on the surface of the plastic part always appear continuously in the same shape at the same part. When such cracks occur, the surface of the cavity corresponding to the crack should be checked immediately for the same crack. If the cracks are caused by the re-imaging effect, the mold should be repaired by mechanical processing.