What is the working principle of the injection molding screw? How does it affect the quality of injection molding?

The injection molding screw is one of the key components in the injection molding machine. Its main function is to heat and extrude the plastic raw materials so that they are evenly melted and injected into the mold. The working principle of the injection molding screw can be divided into the following stages:

Feeding stage: The screw brings plastic particles or powder from the hopper into the barrel and pushes the raw materials forward through the rotation of the screw.

Compression stage: In the middle of the screw, the screw groove gradually narrows and the raw materials are compressed. This process increases the density of the plastic and forces air out of the material.

Melting stage: The compressed plastic enters the heating zone of the barrel under the further push of the screw. Here, the plastic is heated to its melting point and completely melted under the shear force of the screw and the heat in the barrel.

Metering stage: The molten plastic accumulates at the front end of the screw, and the rotation and retreat of the screw push the molten plastic quantitatively to the nozzle, ready to be injected into the mold.

Injection stage: The screw stops rotating, moves forward, and injects the molten plastic into the mold through the nozzle. The speed and pressure of the screw determine the filling speed and density of the plastic in the mold.

Impact on injection molding quality
Uniformity: The efficiency of the screw in the process of melting and mixing plastic directly affects the uniformity of the melt. If the mixing is not sufficient, it may lead to uneven material distribution in the finished product, affecting the mechanical properties and appearance of the product.

Shear force and temperature control: The design and operating parameters of the screw (such as speed and compression ratio) affect the shear force and melting temperature of the plastic. If the shear force is too large or the temperature is not suitable, it may cause the plastic to degrade or produce bubbles, thus affecting the strength and surface quality of the finished product.

Compression ratio: The compression ratio of the screw is crucial to the melting and degassing effect of the material. The appropriate compression ratio can ensure that the plastic is fully compressed and degassed during the melting process, thereby improving the density and surface finish of the product.

Filling speed and pressure: The forward speed of the screw and the injection pressure determine the fluidity and filling effect of the plastic in the mold. Too fast speed or too high pressure will cause uneven plastic flow in the mold, resulting in flash or internal stress; while too slow speed or insufficient pressure may lead to insufficient filling or surface defects.