First, the screw speed
1. The melting of plastic is generally due to the heat generated by the rotation of the screw. Therefore, if the screw speed is too fast, the following effects are affected:
Thermal decomposition of plastics;
Glass fiber (plus fiber plastic) is shortened;
The screw or heating cylinder wears up faster.
2, the setting of the speed can be measured by the size of the circumferential speed (circumferen-tial screw speed): circumferential speed n (speed) * d (diameter) * π (pi) usually, low viscosity thermal stability of the plastic, The circumferential speed of the rotation of the screw rod can be set to about 1 m/s, but the plastic with poor thermal stability should be as low as about 0.1.
3. In practical applications, we can reduce the screw speed as much as possible so that the rotary feed can be completed before the mold is opened.
Second, back pressure
1. When the screw is rotated and fed, the pressure accumulated by the melt that is pushed to the front end of the screw is called back pressure, and can be adjusted by adjusting the oil pressure of the injection hydraulic cylinder during injection molding. The back pressure can have the following effects. :
The melt melts more evenly.
The toner and filler are more evenly dispersed.
The gas is withdrawn from the discharge opening.
The measurement of the feed is accurate.
2, the level of back pressure is determined by the viscosity of the plastic and its thermal stability. Too high back pressure makes the feeding time prolonged, and the plastic shearing force is easy to make the plastic overheat. Generally, 5-15kg/cm2 is preferred.
Third, loose
1. After the rod is rotated and the feed is finished, the screw can be properly withdrawn, and the melt pressure at the front end of the screw can be lowered. This is called loosening, and the effect can prevent the dripping of the nozzle portion.
2, loose retraction, easy to make the mainstream road (SPRUE) sticking; and too much retreat, it can suck in the air, so that the molded article has gas marks.
Fourth, the modification of plasticization parameters
Confirm if the back pressure needs to be adjusted;
Adjust the screw speed so that the metering time is slightly shorter than the cooling time;
To confirm whether the measurement time is stable, try to adjust the gradient of the heating coil temperature; confirm whether the nozzle has dripping, whether the pig tail or the mold is generated in the main channel, whether the finished product has air marks or the like, and properly adjust the nozzle temperature or the retreat distance.
V. Multi-stage pressure keeping and multi-segment rate of activity
1. Generally speaking, in the case of not affecting the appearance, the injection should be based on the principle of high speed, but should be carried out at a lower speed before switching between the gates and the pressure holding;
2, the pressure should be gradually reduced to avoid excessive stress in the molded product, so that the molded article is easy to change