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How to control screw speed during injection production

323 Published by Sabrina wang Mar 18,2020

How to control screw speed during injection production

It is also necessary to control the screw speed during the injection molding machine production process. The following points are methods to teach you how to operate.

 

injection production

 

1 Screw speed

1. The melting of plastic is generally due to the heat generated by the rotation of the screw, so the speed of the screw is too fast, it has the following effects:
  ①. Thermal decomposition of plastic.
  ②. The glass fiber (fiber-reinforced plastic) is shortened.
  ③. The screw or heating barrel wears faster.
2. The setting of the rotation speed can be measured by its circumferential speed (circumferen-tial screw speed): Circumferential speed = n (speed) * d (diameter) * π (circumferential ratio), usually, low-viscosity thermal stability is good The peripheral speed of the screw rod rotation of plastic can be set to about 1m / 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.

2 back pressure

1. When the screw rotates to feed, the pressure accumulated by the melt glue advanced to the front end of the screw is called back pressure. During injection molding,
  It can be adjusted by adjusting the oil return pressure of the injection hydraulic cylinder. The back pressure can have the following effects:
  ①. Melt the glue more evenly.
  ②. The toner and filler are more evenly dispersed.
  ③. Make the gas exit from the blanking port.
  ④. 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 will lengthen the feeding time, and it is easy to overheat the plastic due to the increase of the rotational shear force. Generally, 5 ~ 15kg / cm2 is suitable.

injection production

injection production

 

3 loose

1. After the feeding of the rod is completed, the screw can be properly withdrawn to reduce the melt pressure at the front end of the screw. This is called loosening, and its effect can prevent dripping at the nozzle.
2. Insufficient, easy to make the main channel (SPRUE) sticky mold; too much loosening, it can suck air, causing air marks on the molded product.

4 Cooldown correction

Gradually reduce the cooling time and confirm that the following conditions can be met: 1. The molded product is ejected, clipped out, trimmed, and the packaging will not be whitened, cracked or deformed. 2. The mold temperature can be balanced and stable. Simple algorithm for cooling time of products with wall thickness above 4mm:
Theoretical cooling time = S (1 + 2S) ……. Mould temperature is below 60 degrees.
Theoretical cooling time = 1.3S (1 + 2S) ……. Mould above 60 ° [S represents the maximum wall thickness of the molded product].

5 Correction of plasticization parameters

Determine if the back pressure needs to be adjusted;
Adjust the screw speed to make the measuring time slightly shorter than the cooling time;
To confirm whether the measurement time is stable, you can try to adjust the gradient of the heating coil temperature.
Check whether the nozzle is dripping, whether pig tails or sticky molds are present in the main stream, and whether there are air marks on the finished product. Adjust the temperature of the nozzle or the loosening distance appropriately.

6 Holding pressure and rate of fire

Generally speaking, under the condition of not affecting the appearance, the injection should be based on the principle of high speed, but it should be performed at a lower speed before switching through the gate and holding pressure;
The holding pressure should be gradually reduced to avoid the residual stress in the molded product being too high and the molded product to be easily deformed.

injection production