The eight conditions of injection molding process mainly include: injection rate, injection volume, injection position of screw, injection time, cooling time, screw speed, anti delay amount and residual material amount. The following describes the performance of these eight processes.


1. Injection rate: the injection rate refers to the rate at which the screw advances to fill the plastic melt into the mold cavity. It is generally expressed by the injection mass per unit flower (g/s) or the screw advance rate (m/s). Both it and the injection pressure are important conditions in the injection conditions. Different die speeds may lead to different effects.


During low-speed injection, the material flow rate is slow, and the melt gradually flows from the gate to the far end of the cavity. Before the material flow, it becomes spherical. The melt entering the cavity first cools down and the flow rate slows down. The part close to the cavity specific wall cools into a high elastic thin shell, while the part far away from the cavity wall is still a viscous heat flow. Continue to extend the spherical flow end. After the cavity is completely filled, the thickness of the cooling shell increases and hardens. Due to the long time for the melt to enter the mold cavity, the viscosity and flow resistance of this slow die increase due to cooling, so it is necessary to use a high injection pressure die.

2. Injection volume: the injection volume refers to the total mass of the product and the main flow and shunt channels. If its value is less than the large injection volume of the injection molding machine, it can be molded in theory. However, in general, the injection volume shall be less than 85% of the rated injection volume of the injection machine. However, if the actual injection volume is too small, the plastic will be thermally decomposed due to the long residence time in the barrel. To avoid this phenomenon, the actual injection volume should be more than 30% of the rated injection volume of the injection molding machine. Therefore, the general injection volume should be set between 30%-85% of the rated injection volume of the injection molding machine.


Injection molding


3. Injection position of screw: the injection position is one of the more important parameters in the injection molding process. The injection position is generally determined according to the total weight of plastic parts and coagulant materials, i.e. nozzle materials. Sometimes, the injection position of the backlog section needs to be reasonably set according to the type of plastic used, mold structure, product quality, etc.


4. Injection time: injection time is the time to apply pressure to the screw, including the time required for plastic flow, mold filling and pressure maintaining. Therefore, injection time, injection speed and injection pressure are important injection molding conditions. As for finding the correct injection time, two methods can be used: appearance setting method and weight setting method.

Although the injection time is very short and has little effect on the molding cycle, the adjustment of injection time plays a great role in the pressure control of gate, runner and cavity. Reasonable injection time is helpful to realize the ideal filling of melt, and it is of great significance to improve the surface quality of products and reduce the dimensional tolerance.

5. Cooling time: the cooling process is basically started by injection rather than after the completion of injection, and the length of cooling time is set based on ensuring that the plastic parts can be taken out after mold opening. Generally, the cooling time accounts for 70%-80% of the cycle time.


6. Screw speed: screw speed affects the conveying and plasticizing process and shear effect of injection molding boredom in the screw, and is an important parameter affecting plasticizing capacity, plasticizing quality, molding cycle and other factors. With the increase of screw speed and plasticizing ability, the uniformity of melt temperature of melt temperature machine is improved, and the plasticizing effect is decreased. The screw speed is generally 50-120r/min.

For heat sensitive plastics such as PVC and POM, low screw speed can be used to prevent material decomposition. For plastics with high melt viscosity, low screw speed can also be used

7. Anti roll back: that is, the screw looseness. After the screw is measured in place, it reverses a certain distance in a straight line to increase the space of the melt in the metering chamber, reduce the internal pressure, and prevent the melt from flowing out of the metering chamber. This backward action is called anti roll back, and the backward distance is called anti roll back or anti roll back stroke. Another purpose of anti casting is to reduce the pressure and internal stress of the nozzle channel system when the nozzle is not returned for pre molding, and it is easy to extract the main channel during mold opening. The setting of the anti stretching amount depends on the viscosity of the plastic and the condition of the product, which is generally 2-3mm. Too large anti stretching amount will make the melt in the metering chamber mixed with bubbles, which will seriously affect the quality of the product. The anti stretching amount may not be set for the materials with high viscosity.

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8. Residual material amount: after the screw injection, it is not expected that all the melt at the screw head will be ejected, but some will be retained to form a residual material amount. In this way, on the one hand, it can prevent the mechanical collision accident caused by the contact between the screw head and the nozzle. On the other hand, it can control the repetition accuracy of the injection volume through the amount of residual materials, so as to achieve the purpose of stabilizing the quality of injection molded products (if the amount of residual materials is too small, the purpose of buffering cannot be achieved, and excessive accumulation of residual materials will be caused). Generally, the amount of residual materials is 5-10mm.