Control of injection pressure and injection speed in injection molding
First, the control of injection pressure:
1. The actual applied pressure should be higher than the full cavity pressure. During the injection process, the mold control pressure rises sharply and finally reaches a peak. This peak is the so-called injection pressure. The injection pressure is obviously higher than the full cavity pressure. High.
2, the role of holding pressure: after the cavity is filled with plastic until the gate is completely cooled for a period of time before closing, the plastic in the cavity still needs a relatively high pressure support, that is, pressure retention, its specific role is:
A: Replenish the amount of material near the gate position, and stop the plastic that has not been hardened in the cavity before the gate condensation to close, and backflow to the gate source under the residual pressure.
B: Prevent the shrinkage of the parts and reduce the vacuum bubbles.
C: Reducing the phenomenon of sticking or bending deformation of the mold due to excessive injection pressure of the part. Therefore, the holding pressure is usually 50%~60% of the injection pressure. If the holding pressure or time is too long, it may be The cold material in the gate and the runner is filled with cold spots near the gate, and the cycle is extended without any benefit.
3, the choice of injection pressure
A. According to the shape of the workpiece. Thickness selection. B. For different plastic raw materials.
In the case of production conditions and the quality standards of the parts, it is recommended to use the process conditions of temperature and low pressure.
4, the adjustment of back pressure
Back pressure represents the pressure that the plastic plasticizing process is subjected to. It is also called plasticizing pressure.
A. The mixing effect of the color is affected by the back pressure, the back pressure is increased, and the mixing effect is enhanced.
B. Back pressure helps to eliminate various gases in plastic parts and reduce the phenomenon of silver streaks and bubbles.
C. Appropriate back pressure can avoid local stagnation in the barrel, so the back pressure is often increased when cleaning the barrel.
Second, the control of injection speed
The effect of speed: low speed filling is the smooth flow rate, the workpiece size is relatively stable, the fluctuation is small, the internal stress of the workpiece is low, and the internal and external stresses are consistent. The disadvantage is that the workpiece is prone to delamination and poor melting point. Traces, water marks, etc., high-speed filling can use lower injection pressure, improve the gloss and smoothness of the product, eliminate seam line phenomenon and delamination, shrink shrinkage and uniform color.
The disadvantage is that it is easy to produce "free jet", that is, there is stagnation or eddy current. The temperature rise is too high, the color is yellow, the exhaust is poor, and sometimes the mold is difficult. The plastic with high viscosity may cause melt fracture and fog on the surface of the workpiece. The plaque also increases the tendency of the wings and thick parts to be cracked along the seam line caused by internal stress.
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