In our machine adjustment, we usually use multi-stage injection. The first level injection control gate, the second level injection control main body, and the third level injection fill 95% of the product, and then start to maintain the pressure to produce the complete product. Among them, the injection speed controls the melt filling rate, the injection pressure is the guarantee of the filling rate, the injection position controls the melt flow position, and the pressure maintaining pressure is used to adjust the product weight, size, deformation, and shrinkage.
>> Initial determination of injection pressure during product startup and commissioning:
When we first started the machine for parameter adjustment, the injection pressure would be higher than the actual set value.
Because the injection pressure is too low, the injection mold (temperature) is very cold, and the oil stain on the surface of the mold cavity will inevitably cause great resistance. It is difficult to inject the melt into the mold cavity, and it may not be formed due to insufficient pressure (sticking the front mold, plugging the gate); When the injection pressure is too high, the product will have a large internal stress, which is easy to cause burrs and shorten the service life of the mold. It may also lead to the plugging position of the product, difficulty in demoulding, scratches on the product surface, and even the mold will be expanded in serious cases. Therefore, the injection pressure should be set according to the following points during startup and commissioning.
1. Product structure and shape.
2. Product size (melt flow length).
3. Product thickness.
4. Materials used.
5. Gate type of mold.
6. Screw temperature of injection molding machine.
7. Mold temperature (including mold preheating temperature).
>> Common defects caused by injection pressure in production
The injection pressure is mainly used for the filling and feeding of the melt in the mold cavity.
In injection molding filling, the injection pressure exists to overcome the filling resistance. When the melt is injected, it needs to overcome the resistance from the nozzle runner gate cavity to eject the product. When the injection pressure exceeds the flow resistance, the melt will flow. It is not as accurate as the injection speed and injection position. Generally, we debug the product with the speed as the reference. The increase of the injection pressure can maintain a higher temperature of the melt and reduce the resistance loss of the channel, The inner part of the product is tight and thick.
>> Stabilize process parameters after product commissioning
The factors that directly affect the injection pressure: the flow stroke of the solution, the viscosity of the material and the mold temperature.
In the ideal state, it is the most scientific that the injection pressure is equal to the pressure of the mold cavity, but the actual pressure of the mold cavity cannot be calculated. The more difficult the mold filling is, the greater the injection pressure is, and the farther the melt flow length is. The injection pressure decreases with increasing filling resistance. Therefore, multistage injection is introduced. The injection pressure of the front melt is low, the injection pressure of the middle melt is high, and the injection pressure of the end segment is low. The fast position is fast and the slow position is slow, and the process parameters need to be optimized after stable production.
>> Precautions for selection of injection pressure:
1. During parameter adjustment, when the mold temperature or storage temperature decreases, it is necessary to set a larger injection pressure.
2. For materials with good fluidity, lower injection pressure should be used; For glassy and high viscosity materials, it is better to use a larger injection pressure.
3. The thinner the product is, the longer the process is, and the more complex the shape is, the greater the injection pressure used is, which is conducive to filling and molding.
4. The scrap rate of the product is directly related to whether the injection pressure is set reasonably. The premise of stability is that the molding equipment is intact and free of hidden defects.
Post time: Dec-09-2022