Injection Molding Screw Slippage and Solutions
Injection screw slippage is a common malfunction during the injection molding process. This occurs when the molten plastic is not effectively conveyed forward as the screw rotates and propels it forward. Instead, it slips relative to the barrel, leading to unstable melt flow and, in turn, impacting product quality and production efficiency. This phenomenon typically manifests as normal screw speed but insufficient shot volume and large fluctuations in injection pressure, potentially leading to defects such as material shortages and dimensional instability. Screw slippage not only reduces production efficiency but also increases raw material loss and equipment energy consumption. Long-term occurrence can also lead to increased wear of the screw and barrel, shortening the equipment’s service life. Therefore, timely diagnosis and effective solutions are essential.
Raw material properties are a major cause of injection molding screw slippage. First, uneven particle size or excessive fines in the plastic raw material can affect raw material flowability and filling efficiency in the hopper. This can easily form “bridging” during screw rotation, preventing the material from continuously and evenly entering the screw channel, leading to slippage. For example, some recycled plastics, due to their varying particle sizes, are prone to accumulation and blockage during conveying, preventing the screw from effectively pushing the melt. Second, high raw material humidity can also contribute to slippage. Water vaporizes in the barrel due to heat, forming bubbles. These bubbles occupy space in the screw channel, reducing the density of the melt and diminishing the thrust imparted by the screw, causing slippage. Furthermore, excessive lubricant added to the raw material or poor compatibility with the substrate can create an overly lubricated interface between the screw and barrel, reducing friction and preventing efficient melt conveyance.
Improper setting of molding process parameters is the main factor causing injection molding screw slippage. Excessively high screw speeds will cause the raw material to shear too quickly within the barrel, generating a large amount of heat. This causes the molten material to melt prematurely and adhere to the inner wall of the barrel, forming a “molten film.” Unmelted raw material particles will slide on the molten film under the action of the screw’s rotation and cannot be effectively pushed. At the same time, excessively high screw speeds will also subject the molten material within the screw channel to excessive centrifugal force, causing it to gather toward the outside of the screw channel, reducing the friction between the molten material and the screw and exacerbating the slippage phenomenon. Furthermore, improper back pressure settings can also affect the conveying of the molten material. Too low a back pressure will result in insufficient molten material density, containing more bubbles and prone to slippage. Excessively high back pressure will increase the resistance to screw rotation, causing the molten material to be overly compressed within the screw channel, which in turn affects conveying efficiency.
Wear and improper structure of the screw and barrel are also major causes of slippage. The screw and barrel are key components of injection molding machines, and the roughness of their working surfaces and the clearance between them are crucial to the transport of the molten material. After long-term use, the screw fins and the inner wall of the barrel become smooth due to wear, reducing the friction between the molten material and the components, making the molten material more likely to slip. Furthermore, wear increases the clearance between the screw and barrel, causing the molten material to flow back through the gap and reduce conveying efficiency. Furthermore, an improperly designed screw compression ratio can also affect the transport of the molten material. Too low a compression ratio will result in insufficient molten material density, while too high a compression ratio will result in excessive resistance in the compression section, making it prone to slippage and blockage.
The solution to injection molding screw slippage requires comprehensive consideration from multiple aspects, including raw material processing, process parameter adjustment, and equipment maintenance. First, the quality of raw materials must be strictly controlled, raw materials with uniform particle size must be selected, the fine powder content must be reduced, and the raw materials must be fully dried to reduce the moisture content and avoid the influence of water vapor on the molten material conveying. For raw materials that are prone to bridging, a stirring device or a vibration device can be installed in the hopper to ensure that the raw materials can smoothly enter the screw. Secondly, the molding process parameters must be optimized, the screw speed must be appropriately reduced, the shear heating and centrifugal force of the molten material must be reduced, and the molten material can evenly fill the screw groove; the back pressure must be reasonably adjusted. Generally speaking, appropriately increasing the back pressure can increase the density of the molten material, reduce bubbles, and improve the conveying effect. However, the appropriate back pressure value must be determined based on the material properties to avoid being too high or too low.
In terms of equipment maintenance and structural improvements, regularly inspect the screw and barrel for wear. Repair or replace any severe wear promptly to ensure proper clearance and surface roughness. For components with less wear, surface treatments such as chrome plating can be used to improve wear resistance. Furthermore, select a screw with an appropriate compression ratio based on the characteristics of the plastic raw material. For example, for raw materials with high melt viscosity, a screw with a lower compression ratio should be used to reduce conveying resistance. Furthermore, during production, the barrel and screw should be cleaned regularly to prevent residual melt from carbonizing and affecting proper conveying, thereby effectively preventing screw slippage.
