When operating high-speed small injection molding machines, the goal is typically to shorten the injection cycle and improve overall production efficiency. However, screw slippage during the production of injection molded products can be a major frustration. This problem is especially prominent when processing hygroscopic materials (like nylon), as high moisture content in the resin is a direct cause of screw slippage. When moisture is present in the material, the water vapor generated during processing reduces the screw’s effective rotational conveying force.
To prevent this, using a material dryer before processing to bring the plastic resin’s moisture level to the standard recommended by the supplier is a critical first step.
Consequences and Causes of Screw Slippage
Screw slippage in a small injection molding machine can cause premature material decomposition, which directly impacts the quality of the final product. When the screw slips, material accumulates at the feed port and cannot be properly conveyed to the front of the barrel. Specifically, screw slippage occurs during the plasticizing stage—at this point, the screw continues to rotate, but its axial retraction stops. This is the “slippage” phenomenon.
Screw slippage often leads to material degradation before injection, a decrease in product quality (such as short shots), and an extension of the molding cycle.
The causes of screw slippage are multifaceted and can be related to the following factors:
- Back pressure is set too high
- The rear zone of the barrel (the feed section) is too hot or too cold
- Wear on the barrel or screw
- The screw’s feed section threads are too shallow
- Improper hopper design or a blockage
- Moisture in the resin
- Excessive lubricant in the resin
- Material granules are too fine or inconsistent in size
- Improper mixing of virgin and regrind materials
Among these, an overly cool rear barrel zone is a primary cause of screw slippage. An injection machine’s barrel is generally divided into three sections. The rear feed section is responsible for melting the pellets during heating and compression to form a thin melt film that adheres to the screw. Without this film, the pellets cannot be smoothly conveyed to the front.
Often, the material’s residence time in the feed section is very short, especially on smaller injection molding machines. If the residence time is too brief, the material may not reach the critical temperature needed to form this crucial melt film, leading to an incomplete polymer melting and mixing process, which ultimately triggers screw slippage or stalling.
Solutions for Screw Slippage Provided by MINHUI
Method 1: Check the Melt Temperature
An effective troubleshooting method is to purge a small amount of material from the nozzle and check its melt temperature. Due to the short residence time, you might find that the actual melt temperature is lower than the barrel’s setpoint. This indicates that you need to appropriately increase the temperature setting for the feed zone.
Method 2: Inspect the Molded Products
Carefully examine your finished products. If you notice black specks, light-colored streaks, or marbled patterns on the product, it indicates that the material was not mixed thoroughly inside the barrel. This phenomenon confirms the presence of a screw slippage issue in the process, which requires you to make timely adjustments to your process parameters to resolve it.
