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The working principle of screw injection molding machine

- Jan 03, 2019 -

The first stage: injection

injection molding screw 1

1. The screw moves forward under the driving of the cylinder

2. There is a component called the "reverse ring" at the front of the screw. The outer ring of the "Check Ring" has the same inner diameter as the "cylinder", and the inner diameter is larger than the diameter of the corresponding part of the screw. Therefore, the "stop ring" can move freely in a certain area.

screw tip sets

When the screw is forward, the "reverse ring" is subjected to a backward thrust and starts to move backward due to the reaction force generated by the plastic in the front section.

3. The screw shoulder is the limit of the backward movement of the "reverse ring". When the "reverse ring" comes into contact with the screw shoulder, it starts to move forward along with the screw. At this time, the inner hole of the "reverse ring" and the screw shoulder are combined into one, and the entire screw head becomes a device similar to "piston".


4. The entire screw continues to move forward. Since there is no backlash, the material in the barrel is ejected from the nozzle under the action of pressure and is injected into the cavity. (This process is the same as other injection principles)

The second stage: pressure maintaining, molding stage

5. When the whole mold cavity is filled, the injection molding machine receives the pressure increase feedback, then stops the pressurization. At this time, the pressure will remain for a period of time, which is called “holding pressure”. The “holding pressure” is mainly used to compensate the mold. Within the cavity, the volume is reduced due to shrinkage of the plastic cooling.


When the plastic is completely cooled, the plastic in the cavity is hardened and shaped, and the plastic molding process ends.

Third stage prefabrication

6. When the plastic in the mouth of the cavity is hardened, the motor in the "injection system" starts to rotate, and the material in the "cartridge" moves forward under the "screw". (The process works like a meat grinder)


7. The material that moves forward, at this point, produces a forward thrust on the “reverse ring”. The "reverse ring" is then separated from the screw shoulder and begins to move forward.

8. The "stop ring" of the forward movement stops until it comes into contact with the "screw head". Unlike the screw shoulder, the design of the "screw head" is somewhat special. Generally, semi-circular grooves are formed in four directions, so that the inner hole of the "reverse ring" forms a passage with the grooves.


The material pushed forward by the screw passes through this passage and continues to move forward.

 9. The raw material is continuously moving forward under the action of the screw rotation. Since the product in the cavity has not been taken out, the raw material cannot continue to enter the cavity. Therefore, the continuous forward material gathers at the front end of the "cartridge", which inevitably brings about an increase in the front end pressure, thereby bringing a backward driving force to the screw.


When this driving force is greater than the minimum pressure set in the hydraulic system (referred to as "back pressure"), the screw begins to move backwards. Until the retreat to the previously set position (the front end is prepared with enough raw material for injection), the motor stops the rotation of the screw, the material is no longer forward, and the "pre-plastic" is completed.


“Pre-plasticizing” is the process of preparing the raw materials for injection. This process is carried out while the product is being cooled, and the complete cooling of the product after completion of the “pre-plasticizing” is also completed.

The fourth stage

10. Once the product has cooled down, remove it from the mold cavity. The front of the barrel is also prepared with enough material to wait for the start of the next injection.

These are the working processes of the screw injection unit. We usually refer to the whole process as the “injection process cycle” and the time taken for the entire process cycle is called “Cycle Time”.

“Cycle time” has a very important meaning for production, especially indicating the level of production efficiency (when the cycle is short, the number of production per unit time is large), so the company will try to shorten the cycle in actual production. Time" (mostly in the case of quality). There are many different ways to shorten the cycle time. Here, I will not describe it for the time being, and I will leave it for later decomposition.

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