Working principle of injection molding machine screw

The working principle of injection molding machine screw is the core link in the injection molding process, which is directly related to the melting quality of plastic, injection pressure, injection speed and the molding effect of the final product. The following is a detailed analysis of the working principle of injection molding machine screw:

1. Plastic conveying and preheating
The injection molding machine screw conveys plastic from the hopper to the barrel through rotation. During the conveying process, the screw groove and screw ridge push and compact the plastic, and the external heating device of the barrel preheats the plastic to gradually soften it. The main purpose of this stage is to ensure that the plastic can enter the barrel smoothly and prepare for the subsequent melting process.

2. Plastic melting and stirring
As the screw continues to rotate and advance forward, strong friction and shearing are generated between the plastic and the bottom surface of the screw groove, the screw ridge pushing surface and the inner wall of the barrel. This friction and shearing not only softens the plastic further, but also generates a lot of heat. When this heat accumulates to a certain extent, the plastic begins to melt. At the same time, the screw groove and screw ridge stir the molten plastic to make it evenly mixed. The main purpose of this stage is to ensure that the plastic can be completely melted and reach a uniform molten state.

3. Injection and holding pressure
When the screw rotates to a certain position, the injection cylinder starts to work and pushes the screw forward. At this time, the molten plastic is injected into the mold cavity under the push of the screw. During the injection process, the screw applies sufficient pressure to the molten material to ensure that the molten material can fill the entire cavity. When the cavity is filled, the injection molding machine maintains a certain pressure (called holding pressure) for a period of time to compensate for the volume reduction caused by the cooling and shrinkage of the plastic. The main purpose of the holding pressure stage is to ensure that the plastic can be fully cooled and hardened in the cavity.

4. Screw retreat and pre-molding
After the injection and holding pressure are completed, the screw begins to retreat. During the retreat process, the screw groove and screw ridges of the screw stir and compact the plastic again to prepare for the next injection. At the same time, the external heating device of the barrel continues to heat the plastic to keep it in a molten state. When the screw retreats to the set position, the pre-molding stage ends. The main purpose of the pre-plasticization stage is to prepare enough molten material for the next injection and ensure the temperature and uniformity of the molten material.

5. Cyclic work
The above process constitutes a complete working cycle of the injection molding machine screw. In actual production, the injection molding machine screw will repeat this process continuously to achieve continuous production. By adjusting the screw speed, injection pressure, injection speed and other parameters, the production of plastic products of different types and specifications can be achieved.

After the injection is completed, whether the screw continues to move forward depends on the specific injection molding machine design and injection molding process requirements.

In general, after the injection is completed, the behavior of the screw may vary depending on the type of injection molding machine, the control system and the injection molding process. Here are some possible situations:

Pressure holding stage: After the injection is completed, in order to compensate for the reduced volume of the plastic cooling and shrinkage, the injection molding machine usually enters the pressure holding stage. In this stage, the screw may maintain a certain forward position in order to apply pressure to the molten material in the mold to ensure the compactness of the product. However, it should be noted that the forward speed of the screw may be adjusted according to the demand for holding pressure, rather than continuously moving forward at a high speed.
Screw retreat: In some cases, the screw may retreat after the injection is completed. This is usually to prepare for the next injection cycle, that is, the pre-molding process. During the pre-molding process, the screw rotates and retreats, melts the plastic and accumulates it at the front end of the barrel in preparation for the next injection.
Multi-stage injection and holding pressure: In advanced injection molding machines, in order to achieve more precise injection and holding pressure control, multi-stage injection and multi-stage holding pressure technology may be used. In this case, the behavior of the screw after the injection is completed may be more complicated, including advancing or retreating at different speeds at different stages.
The role of the control system: The control system of the injection molding machine plays a vital role in the whole process. It can adjust the screw's rotation speed, forward speed, and retreat position according to the preset process parameters and real-time feedback signals to ensure the stability of the injection molding process and the quality of the product.