Different material shrinkage will produce problems, such as product sink, deformation warpage, size abnormality, batch front, etc. Therefore, mastering the skill of using good pressure-holding can solve most of the lousy injection molding.
The material is in high temperature and high pressure at high speed from liquid to solid. The material is composed of molecules in the injection stage in a stretching state. When the pressure drops, molecules will shrink because the product temperature distribution is different. That is, the molecular shrinkage in each place of the product is different, so the molecules will produce different tensile contraction forces, that is, internal stress.
The critical thing to understand here is why material shrinkage produces so much bad? The final product is a qualified product, which results from a combination of material shrinkage in different stages. The post-cooling stage is 2 or 12 hours after the product is out of the mold. After this time, the product basically will not change. Different materials will have different stability times; forming stage is the forming stage of the product in the mold. We correspond to the forming stage in the mold. More than 80% of the problems can be solved, where the role of pressure-holding is the most important.
How to set the holding pressure to achieve the result we want? The product, material, and machine will be slightly different, but the direction is generally the same. According to my own experience, write out for reference only.
1. The cooling of the product in the mold is different: the cooling at the near gate is slow, and the cooling at the far gate is fast. That is, the temperature distribution of the product is from the near gate to the far end, and the temperature is gradually reduced. When the far part starts to cure and set, the material at the gate is still in the flow stage, so they are gradually changed simultaneously.
We can use this characteristic to set multiple holding pressures when we set the holding pressure. The first holding pressure is mainly at the distal end. Why? Because the material near the gate is still flowable, the pressure will only make the material flow forward.
As the cooling time increases, the product gradually cures towards the gate, and the location where the holding pressure can be transmitted changes. The multi-stage pressure-holding setting can be set to correspond to different positions according to the cooling of the product, and the last to cool is the water gate. So the water gate position shrinkage to increase the holding pressure time and the distal shrinkage to increase the holding pressure is the most useful. There is also an intuitive method to see whether the screw in the holding pressure forward, no forward, indicating that the pressure is not enough, need to add pressure, there is forward, indicating that the time is not enough, add time.
2. Material instability of the product: mainly lack of glue, the setting of holding pressure is the first section of pressure is large, the time can be shorter. Mainly with pressure-holding to compensate for the short shot caused by hazardous material when injecting. The general injection switching pressure-holding is switched by position, which cannot sense the change of material. The way to check the pressure-holding setting is to adjust the switching position so that the product appears short of glue, and then adjust to increase the pressure-holding so that the product is not short of glue that is the pressure to be used.
3. For products of varying sizes: we have to confirm the gate closing time, and the pressure-holding should end when the gate is closed. If the holding pressure ends before the gate is closed, the material of the gate will flow backward and affect the dimensions near the gate. How to confirm the gate closure time? The main thing is to set a reasonable holding pressure (usually the holding pressure during production) and gradually increase the holding time until the weight of the product does not increase anymore is the gate closing time.
4. The method of adjusting the pressure-holding when shrinkage and batching conflict: the material in the mold is cooled on the surface first and cooled inside. That is, when the injection is filled without batching, switch the pressure-holding, use multiple pressure-holding, the first pressure-holding is small, reduce the pressure on the batching, no batching will be produced. The pressure and time of the section to ensure that there is no batching. The shrinkage adjustment is handed over to the second section of holding pressure. Due to the transition of the first section of holding pressure, the batch front has been cooled and cured, and the second section of holding pressure can be increased to make up for the shrinkage. The pressure and time of the first and second sections must be adjusted to complement each other.
5. Pressure-holding protection for the machine: When setting pressure-holding, if the pressure of the last section is significant, at the end of pressure-holding, the pressure is suddenly unloaded without buffer transition, which has a significant impact on the injection motor. When setting multi-stage holding pressure, if the pressure of the last stage is slight, the machine can release the pressure to buffer the transition and protect the motor.
The adjustment of holding pressure varies greatly, and the ultimate goal is to produce stable products. Mastering some small skills, the work of adjusting the machine will still play a half-hearted role.