Causes of Screw Slipping in Injection Molding Machines

Regarding the Issue of Screw Slipping in Injection Molding Machines

People use high-speed injection molding machines mainly to shorten the injection molding cycle and increase production efficiency. However, during the injection molding process, screw slipping in injection molding machines is a major concern. This is particularly true when dealing with hygroscopic materials (such as nylon), as high resin moisture content can lead to screw slipping. Improper drying significantly reduces material viscosity and generates steam in the barrel, thereby reducing the conveying capacity of the screw.

To address this, a humidity detector can be used before processing to determine the moisture content of the dried material, ensuring that the material’s dryness level reaches the recommended value by the supplier.

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- Impact of Screw Slipping

Screw slipping in an injection molding machine can lead to material degradation, thereby affecting product quality.

When the screw in the barrel slips, the material tends to accumulate around the feeding port and cannot be conveyed properly to the end of the injection machine. As the screw rotates and retracts within the barrel to transport the material and prepare for the next injection, screw slipping occurs in the plasticizing section.

At this point, the rotation of the screw continues, but the axial movement of the screw stops, resulting in slipping. Screw slipping often leads to the degradation of the material before injection, resulting in reduced product quality (such as material shortages), and an extended molding cycle.

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Causes of Screw Slipping

There are various reasons for screw slipping, including the following factors:

  • Excessive back pressure
  • Overheating or undercooling at the end of the barrel
  • Wear and tear on the barrel or screw
  • Shallow threads in the feeding section
  • Unfair material hopper design and blockage
  • Moist resin
  • Excessive lubrication of the resin
  • Improper cutting of materials that are too fine or resin and recycled materials.
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Among them, the cold end of the barrel is one of the main reasons for screw slippage. The barrel of the injection molding machine is divided into three sections. And at the end, the feeding section, during the heating and compression process, a layer of molten film adheres to the screw. Without this thin film, the granules are not easily transported to the front end.

The material in the feeding section of the injection molding machine must be heated to a critical temperature to form the crucial molten film. However, the residence time of the material in the feeding section is usually short and cannot reach the required temperature. This condition tends to occur in small injection molding machines. Too short a residence time can lead to incomplete melting of the polymer and the inclusion process, resulting in screw slippage or stalling.

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