Plastic mold manufacturers introduce the factors that cause mold cracks

Update:07 Jan 2021


   Cracking often occurs in the plastic mold molding process, which is a common defect in plastic products. The main reason for occurrence is due to stress and deformation. There are mainly residual stress, external stress, and stress deformation caused by the external environment. Let the plastic mold manufacturer specifically talk about the causes of the cracking phenomenon and the corresponding solutions:

   1. Cracking caused by residual stress:

  The residual stress is mainly composed of the following three conditions, namely, overfilling, ejection from the mold, and metal inserts. As a crack that occurs under the condition of overfilling, the solution can be mainly based on the following aspects:

  1. Because the pressure loss of the sprue is the smallest, if the crack occurs mainly near the sprue, you can consider switching to a multi-point distribution point gate, side gate and shank gate method;
  2. On the premise of ensuring that the resin does not differentiate or deteriorate, appropriately increasing the resin temperature can reduce the melt viscosity and improve the fluidity. At the same time, the injection pressure can also be reduced to reduce the stress;
  3. Under normal conditions, stress is likely to occur when the mold temperature is low, so the temperature should be increased appropriately, but when the injection speed is high, even if the mold temperature is lower, the occurrence of stress can be reduced;
  4. If the injection and holding time are too long, stress will occur. It is better to shorten it appropriately or perform Th times of holding pressure switching;
  5. Non-crystalline resins, such as AS resin, ABS resin, PMMA resin, etc., are more prone to residual stress than crystalline resins such as polyethylene, polyoxymethylene, etc., so be careful;

  A. When the ejector is ejected, the ejection force is too large due to the small ejection angle, the mold rubber and the roughness of the punch, and the ejection force is too large, causing stress, and sometimes even whitening or splitting occurs around the ejector rod. Only by carefully observing the location where the crack occurred, the cause can be determined;

  B. When the injection molding is embedded in the metal parts, stress is likely to occur, and it is easy to crack after a period of time, which is very harmful:

   This is mainly because the thermal expansion coefficients of metal and resin are very different and stress occurs, and with the passage of time, the stress exceeds the strength of the gradually degraded resin material and cracks occur. In order to prevent the resulting cracking, as a rule of thumb, the wall thickness is 7" and the outer diameter of the embedded metal is.

  C, general-purpose polystyrene is basically not suitable for adding inserts, and inserts have minimal impact on nylon. Because glass fiber reinforced resin material has a small thermal expansion coefficient, it is more suitable for inserts;

  D. Preheating the metal insert before forming also has a good effect;