High-speed injection molding machines are utilized to minimize injection cycles and boost production efficiency. However, a common challenge encountered during production is the slipping of the injection screw. This issue is especially pronounced when working with hygroscopic materials, such as nylon, where high moisture content can lead to screw slippage. Inadequate drying of these materials can significantly lower their viscosity, generating water vapor in the barrel and reducing screw conveyance efficiency.
To mitigate this problem, a moisture analyzer can be employed prior to processing to assess the moisture content of the dried material, ensuring it aligns with the supplier’s recommended drying levels.
When the injection screw slips, it can cause material degradation and diminish product quality. Specifically, material may accumulate at the feed port, obstructing proper conveyance to the end of the injection machine. Slipping often occurs in the plasticizing section when the screw rotates backward within the barrel to transport the material in preparation for the next injection cycle.
During this process, the screw continues to rotate, but its axial movement ceases, leading to slippage. This phenomenon can result in degraded material prior to injection, ultimately impacting product quality—such as leading to short shots—and prolonging the molding cycle.
Several factors can contribute to screw slippage, including excessive back pressure, extreme temperatures at the barrel’s end, wear on the barrel or screw, shallow threads in the feed section, poor hopper design or blockages, moisture content in the resin, excessive resin lubrication, inadequate particle size, or improper cutting of the resin and recycled materials.
The barrel of an injection molding machine consists of three sections, with the feeding section being critical. Without this film, the pellets cannot be efficiently conveyed to the front end.
To ensure proper conveyance, the material in the feeding section must be heated to the critical temperature necessary for forming this essential molten film. However, the residence time of the material in the feeding section is often short, especially in smaller injection machines, which may prevent it from reaching the required temperature. This insufficient residence time can lead to incomplete melting and incorporation of the polymer, resulting in screw slippage or stalling.