Cracking of injection molded products includes hairline cracks, microcracks, whitening, cracking on the surface of the workpiece, as well as damage due to component adhesion, flow path adhesion or traumatic damage, splitting and cracking during cracking time. There are mainly the following reasons:
- Processing:
(1) If the processing pressure is too high, the speed is too fast, the more filling, the injection and heat preservation time are too long, the internal stress will be too large and crack.
(2) Adjust the mold opening speed and pressure to prevent demoulding cracking caused by rapid stretching of the workpiece.
(3) Properly increase the mold temperature to make the parts easy to demould, and appropriately lower the material temperature to prevent decomposition.
(4) Prevent cracking caused by weld deterioration due to plasticity reduction caused by plastic degradation.
(5) When using release agents appropriately, attention should be paid to eliminating aerosols and other substances attached to the surface of the mold.
(6) Eliminating internal stress by performing annealing heat treatment immediately after molding can reduce the residual stress of the workpiece, thereby reducing the occurrence of cracks.
- Mold aspect:
(1) The balance should be balanced. For example, if the number and cross-sectional area of ejector pins are sufficient, the draft angle should be sufficient, and the cavity surface should be smooth enough to prevent cracking due to residual stress concentration caused by external forces.
(2) The structure of the workpiece should not be too thin, and the transition part should be arc-shaped as much as possible to avoid stress concentration caused by sharp corners and chamfers.
(3) Minimize the use of metal inserts to prevent internal stress from increasing due to shrinkage differences between the insert and the part.
(4) Appropriate stripped air intake channels should be provided for deep bottom components to prevent vacuum negative pressure.
(5) The main channel is large enough to release the gate material during future curing, so it is easy to demould.
(6) The main channel bushing and nozzle joint should prevent the cooling material from being dragged and causing the workpiece to stick to the fixed mold.
3.Materials:
(1) The content of recycled materials is too high, resulting in low strength of parts.
(2) The humidity is too high, causing some plastics to react with water vapor, reducing their strength and causing cracking.
(3) The material itself is not suitable for the environment or quality being processed, and contamination will cause cracking.
- Machinery:
The plasticizing capacity of the injection molding machine should be appropriate. If the plasticization is too small, it will not mix completely and become brittle. When it gets too big, it degrades.
Polishing of injection molds has two purposes when used. One is to increase the smoothness of the mold, which can make the surface of the product produced by the mold smooth, beautiful and beautiful. The other is to make the mold easy to demould, so that the plastic will not be stuck. The mold is on and cannot be taken off.
Things to note when polishing injection molds are as follows:
(1) When starting to process a new mold cavity, you should first check the surface of the workpiece and clean the surface with kerosene so that dirt will not stick to the oilstone surface and cause the cutting function to be lost.
(2) When grinding coarse grains, proceed in the order of difficult first and then easy, especially for some difficult corners. Deeper bottoms should be grinded first,
(3) Some workpieces may have multiple pieces assembled together for polishing. The rough lines or spark lines of individual workpieces must be ground separately first, and then all workpieces must be polished together until they are smooth.
(4) For workpieces with large flat surfaces or side surfaces, use a whetstone to grind away the rough lines and then use a straight steel sheet for light transmission inspection to check whether there are any uneven or undercut defects. If there are undercuts, it will cause The part is difficult to demould or the part is strained.
(5) In order to prevent the mold workpiece from being undercut or having some fitting surfaces that need to be protected, you can use a saw blade to paste it or stick it to the edge with sandpaper, so that you can get the ideal protection effect.
(6) When grinding the flat surface of the mold, pull it back and forth, and keep the handle of the dragging oilstone as flat as possible, not exceeding 25°; because the slope is too large, the force rushes from top to bottom, which can easily lead to many rough lines on the workpiece.
(7) If the flat surface of the workpiece is polished with copper or bamboo sheets pressed against sandpaper, the sandpaper should not be larger than the area of the tool, otherwise it will grind into places that should not be grinded.
(8) The shape of the grinding tool should be close to the surface shape of the mold, so as to ensure that the workpiece is not grinded and deformed.
When designing the gating system of an injection mold, first realize the gate position. The gate position selection is directly related to the product molding quality and the smooth progress of the injection process. The gate position selection should follow the following principles:
① The gate position should be chosen on the parting surface as much as possible to facilitate the cleaning of the gate during mold processing and use;
②The distance between the gate position and each part of the cavity should be as consistent as possible, and the tooling process should be kept short;
③The position of the gate should ensure that when the plastic flows into the cavity, it should be in a wide and thick-walled part of the cavity to facilitate the smooth flow of plastic;
④The gate should be located at a thicker section of the plastic part;
⑤ Prevent the plastic from flowing directly into the cavity wall, core or insert when flowing down the cavity, so that the plastic can flow into all parts of the cavity as quickly as possible and avoid deformation of the core or insert;
⑥ Try to avoid welding marks on the product, or make welding marks occur on unimportant parts of the product;
⑦The position of the gate and its plastic inflow direction should ensure that when the plastic flows into the cavity, it can flow evenly along the direction parallel to the cavity and facilitate the discharge of gas in the cavity;
⑧The gate should be set in an easy-to-clean location on the injection molded product while minimizing any impact on the appearance of the product.
