Prevent the injection machine from slipping
Textile Organza,Organza Fabric,Organza Silk,Printed Organza Fabric JIANGSU ZJA NEW MATERIAL CO., LTD , https://www.zjafiber.com
June 22, 2025
The issue of screw slippage in an injection molding machine can lead to material degradation, negatively impacting product quality. But what exactly causes this slippage, and how can it be resolved? This article aims to provide answers.
Screw slippage typically happens when the material accumulates at the feed throat and fails to reach the nozzle end of the injection unit. During the process where the screw retreats inside the barrel to convey material for the next injection, slippage occurs in the plasticizing zone. At this stage, the screw keeps rotating but halts its axial movement, leading to slippage. Such slippage often results in material degradation prior to injection, diminished product quality (such as incomplete filling), and a longer molding cycle.
Several factors contribute to screw slippage. Excessive back pressure, overheating or overcooling at the barrel's end, wear on the barrel or screw, a shallow feed section, improperly designed hoppers, clogged hoppers, moisture in the resin, excessive lubrication, overly fine materials, or irrational cutting of resins and recycled materials could all be culprits.
Process Settings
One major reason for screw slippage is subcooling at the barrel's end. The injection machine barrel is divided into three zones. At the feed zone, a melt film forms on the screw during heating and compression. Without this film, pellets won’t easily move forward.
The feed zone material needs to reach a critical temperature to form this essential melt film. However, due to the short residence time of the material in the feed zone, the required temperature is often not achieved. This is common with smaller injection machines. Short residence times can lead to incomplete melting and mixing of polymers, causing the screw to either stall or slip.
Two straightforward methods can help identify whether a screw is slipping. First, adding a small amount of material at the barrel’s end to measure melt temperature. If the residence time is too short, the melt temperature will be lower than the set barrel temperature. Second, inspecting the product: if patterns or light and dark streaks appear, it indicates uneven mixing in the barrel.
Once slippage occurs, one solution is to raise the barrel temperature at the feed zone until the screw resumes normal rotation and retraction. This might require exceeding the recommended set point.
High back pressure can also cause the screw to stall or slip. While increasing back pressure boosts the energy applied to the material, excessively high settings prevent the screw from overcoming the resistance and delivering material forward. At this point, the screw performs additional work on the material, raising melt temperatures, reducing product quality, and prolonging the molding cycle. Back pressure can be adjusted via the injection cylinder valve.
Hardware Issues
If the slippage originates from equipment rather than process settings, wear on the screw and barrel might be to blame. The resin melts in the transition zone and adheres to the barrel walls, similar to the feed zone. As the screw turns, it shears the melt and pushes it forward. If the screw and barrel are worn, efficient material transport becomes challenging. If wear is suspected, measure the gap between the screw and barrel. If tolerances aren't met, repair or replacement is necessary.
Screw design, particularly the compression ratio, is crucial for plastication. A short feed section, i.e., a low compression ratio, reduces throughput and causes slippage. Resin suppliers often recommend optimal compression ratios for their materials.
Improper functioning of the check valve can also cause slippage. During injection, the retaining ring should be positioned at the front end (open position) and securely fastened to the retaining ring frame. If the ring is at the rear (closed position) or somewhere in between, polymer melt struggles to pass through. Any issues with the retaining ring should be promptly addressed.
Moreover, the hopper design might contribute to slippage. Proper hopper design ensures uniform feeding, yet this aspect is often overlooked. A square hopper with a rapid compression section (narrowing abruptly at the bottom) works well for uniform raw materials but isn’t ideal for recycling materials. The wide particle size range of recycled materials affects feeding consistency, making it hard for the screw to transport the melt uniformly under consistent pressure, ultimately causing slippage. Using a circular hopper with a graded compression section (gradual taper at the bottom) helps handle materials with varying particle sizes.
Material Uniformity
As mentioned earlier, the size and shape of materials influence feeding uniformity—poor pellet shape can also lead to slippage, affecting output. Uniform-sized pellets pack tightly in the feed zone, facilitating easier melting and transportation.
Irregularly shaped pellets create excessive free volume (gaps between pellets), making transportation difficult and causing slippage. Raising the barrel temperature at the end can help start the melting process earlier, compacting the material more tightly.
For hygroscopic materials like nylon, high resin moisture levels can also cause slippage. Improper drying significantly reduces material viscosity and generates water vapor in the barrel, decreasing the screw’s conveying ability. To address this, use a moisture analyzer before processing to ensure material dryness meets supplier recommendations.
In summary, addressing screw slippage involves both process adjustments and hardware maintenance. Understanding the root cause and taking appropriate measures can mitigate this issue and improve product quality.