Processing
1) Excessive processing pressure, too fast speed, more filling, injection, and too long pressure holding time will cause excessive internal stress and cracking.
2) Adjust the mold opening speed and pressure to prevent demolding and cracking caused by rapid forced metal drawing of parts.
3) Increase the mold temperature appropriately to make the parts easy to demould, and lower the material temperature appropriately to prevent decomposition.
4) Prevent cracking due to lower mechanical strength due to welding marks and plastic degradation.
5) Use release agent appropriately and pay attention to frequently eliminate aerosol and other substances attached to the mold surface.
6) The residual stress of the part can be eliminated by annealing heat treatment immediately after molding to reduce the generation of cracks.
Mold aspect
1) Ejection must be balanced. For example, the number of ejector pins and cross-sectional area must be sufficient, the draft slope must be sufficient, and the cavity surface must be smooth enough to prevent cracking due to the concentration of residual stress in ejection caused by external forces.
2) The structure of the parts should not be too thin, and arc transitions should be used as much as possible in the transition parts to avoid stress concentration caused by sharp corners and chamfers.
3) Use as few metal inserts as possible to prevent internal stress from increasing due to different shrinkage rates between the insert and the finished product.
4) Appropriate demoulding air inlets should be provided for deep-bottomed parts to prevent the formation of vacuum negative pressure.
5) The sprue is large enough to allow the gate material to be demoulded before it has time to solidify, making it easy to demould.
6) The connection between the main flow bushing and the nozzle should prevent the cold hard material from being dragged and causing the part to stick to the fixed mold.
Materials
1) The content of recycled materials is too high, resulting in low strength of the parts.
2) Excessive humidity causes chemical reactions between some plastics and water vapor, reducing their strength and causing ejection and cracking.
3) The material itself is not suitable for the environment in which it is being processed or is of poor quality. Contamination will cause cracking.
Different injection molds have different requirements. To sum up, the interpretation requirements of plastic mildew on mold steel are mainly imaged in: hardness, polishability, etching version, welding performance, wear opposition, toughness, dimensional steadiness, corrosion opposition, etc.
According to the causes of bubbles, the solutions include the following aspects:
1) When the wall thickness of the product is large, the cooling rate of the outer surface is faster than that of the central part. Therefore, as the cooling proceeds, the resin in the central part shrinks and expands toward the surface, causing the central part to be underfilled. This situation is called a vacuum bubble. The main solutions are:
- a) According to the wall thickness, determine the reasonable gate and runner size. Generally, the gate height should be 50% to 60% of the wall thickness of the product.
- b) Until the gate is sealed, a certain amount of filling injection material remains.
- c) The injection time should be slightly longer than the gate sealing time.
- d) Reduce the injection speed and increase the injection pressure,
- e) Use materials with high melt viscosity grades.
2) The main solutions to bubbles caused by the generation of volatile gases are:
- a) Fully carry out pre-drying.
- b) Reduce the resin temperature to avoid the generation of decomposition gas.
3) Bubbles caused by poor mobility can be solved by increasing the temperature of the resin and mold and increasing the injection speed.
In order to achieve higher precision and reduce errors in injection molding processing, not only the mold processing accuracy is required to meet the requirements, but also a reasonable reserve amount must be set during the processing process to maximize the improvement as much as possible on the premise of ensuring the smooth completion of the injection molding processing. Precision.
The precision needsfor mold processing in injection molding include the manufacturing precisionof mold sizes, verticality, coaxiality, parallelism, etc. The manufacturing accuracy of general injection molding mold size should be within the range of 0.005-0.02mm; the verticality needshould be within the range of 0.01-0.02mm; the coaxiality requirement should be within the range of 0.01-0.03mm; the upper and lower leave surfaces of the movable and fixed molds , the equality need of the lower two planes is within the range of 0.01-0.03mm.
At the same time, the gap between the parting surfaces after mold closing should be smaller than the overflow value of the molded plastic; the parallelism of the remaining template mating surfaces is required to be in the range of 0.01-0.02mm; the matching accuracy of the fixed part of the injection molding mold is average. Choose a range of 0.01-0.02mm. If there is no requirement for insertion or it has little impact on size, a gap fit of 0.01-0.02mm on both sides is recommended.
The fitting accuracy of the sliding part of the injection molding mold is generally selected from three types: H7/e6, H7/f7, and H7/g6. It should be noted that if there are inserts on the mirror surface that are attached to the steps, the fit must not be too tight, otherwise When the insert is knocked backward from the front, the tool used for knocking can easily damage the mirror surface. As long as the size of the product is not affected, a gap fit of 0.01~0.02mm on both sides is recommended.
During injection molding processing, those workpieces that need to be heat treated are required to add a grinding allowance of 0.25mm to each side of the outline dimensions before heat treatment; while for parts of the mold core and inserts that require CNC rough machining, a allowance should be reserved on one side. 0.2mm, you must know that this reasonable reserve will also affect the accuracy of the workpiece.