Release time：2023-05-06
Plastic shell of power supplyDuring injection molding, cracks formed by surface cracking are called cracks. We solve this problem through fault analysis and troubleshooting.
1. Residual stress is too high
In terms of mold design and manufacturing, the direct gate with small pressure loss and high injection pressure can be used, and the forward gate can be changed into multiple needle point gates or side gates to reduce the gate diameter. When designing the side gate, the convex gate that can remove the broken part after forming can be used.
From the perspective of process operation, reducing the injection pressure is a relatively simple method to reduce the residual stress, because the injection pressure is in direct proportion to the residual stress. The temperature of the barrel and the mold should be appropriately increased, the temperature of the melt and the mold should be reduced, and the cooling time and speed of the parison in the mold should be controlled, so that the oriented molecules have a longer recovery time.
2. Residual stress concentration caused by external force
Generally, this kind of fault always occurs around the ejector rod. In case of such failure, the ejector shall be carefully checked and calibrated, and the ejector rod shall be set at the position with large demoulding resistance such as boss and stiffener. If the number of ejector pins cannot be expanded due to the limitation of pushing area, the method of small area and multiple ejector pins can be adopted. If the mold cavity demoulding slope is not enough, the surface of the plastic shell of the power supply will also be scratched to form wrinkles.
3. There are differences in thermal expansion coefficients between raw materials and metal inserts
Metal inserts should be preheated, especially when cracks appear on the surface of the plastic shell of the power supply at the initial stage of startup, most of which are caused by the low temperature of the inserts.
In addition, in the selection of insert materials, try to use materials with linear expansion coefficient close to resin characteristics. When selecting raw materials for molding, high molecular weight resins should be used as much as possible. If low molecular weight molding raw materials must be used, the thickness of plastic around the insert should be designed to be thicker.
4. Improper or impure selection of raw materials
Practice shows that low viscosity loose resin is not easy to crack. Therefore, in the production process, appropriate molding raw materials should be selected according to the specific situation. During operation, pay special attention not to mix polyethylene, polypropylene and other resins together, which is easy to produce cracks. During the molding process, the release agent is also a foreign matter for the molten material. If the amount is improper, it will also cause cracks, so the amount should be reduced as much as possible.
5. The structural design of the plastic shell of the power supply is poor
The sharp corners and cracks in the structure of the plastic shell of the power supply are easy to produce stress concentration, resulting in cracks and cracks on the surface of the plastic shell of the power supply. Therefore, the outer and inner corners of the plastic shell shape structure of the power supply should be made into an arc with the largest radius as far as possible. The experiment shows that the best transition arc radius is that the ratio of arc radius to wall thickness at the corner is 1:1.7.
6. The cracks on the mold are reflected on the surface of the plastic shell of the power supply
In the process of injection molding, due to the repeated action of injection pressure on the mold, fatigue cracks will occur at the edges with acute angles in the cavity, especially near the cooling hole.
When the cracks on the surface of the mold cavity are reflected on the surface of the plastic housing of the power supply, the cracks on the surface of the plastic housing of the power supply always appear continuously in the same shape and position. When such cracks appear, immediately check whether the cavity surface corresponding to the cracks has the same cracks. If the crack is caused by reflection, the mold should be repaired by machining.
