Why does the screw of injection molding machine slip

People use high-speed injection molding machines mostly to shorten the injection cycle and improve production efficiency. However, the slippage of the injection molding machine screw is a major problem during the production of injection molded products. Especially against hygroscopic materials (e.g. nylon), high moisture content of resin can cause slippage of the injection molding machine screw. Improper drying can significantly reduce the viscosity of the material and generate water vapor in the barrel, thus reducing the conveying capacity of the screw.

In this regard, a moisture meter can be used to determine the moisture content of the dried material prior to processing, so that the material is dried to the supplier's recommended value.

Injection molding machine screw slippage can cause material degradation, thus affecting the quality of the product. What are the causes of screw slippage? How to "catch" the slippery screw? When the barrel screw slips, the material may collect at the feed port and not be delivered properly to the end of the injector. Screw slip occurs in the plasticizing section when the screw is rotating and retreating in the barrel to deliver material and prepare for the next injection.



At this point, the screw rotation continues, but the axial movement of the screw stops, resulting in slippage. Screw slip often results in degradation of the material prior to injection, reduced product quality (e.g., missing material), and longer molding cycles.

The causes of screw slippage are many, probably related to high back pressure, overheating or overcooling of the barrel end, barrel or screw wear, shallow threads in the charging section, poor hopper planning and blocked hoppers, wet resin, over-lubrication of resin, too fine material or unfair cutting of resin and recycled material, etc.

The end of the barrel is too cold is one of the main causes of screw slippage. The barrel of the injection machine is divided into 3 sections, at the end, i.e. the filling section, the granule is heated and compressed in the process, a layer of molten film will be formed to stick to the screw. Without this film, the pellets are not easily transported to the front end.

The material in the charging section of the injection molding machine must be heated to a critical temperature to form that molten film at the end. However, usually, the residence time of the material in the charging section is too short to reach the required temperature. And this environment typically arises in small injection machines. Too short a dwell time can cause incomplete melting and intercalation processes of the polymer, which can lead to screw slippage or stall.