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How to prevent the screw of plastic injection molding machine from slipping?

2025/06/09 By Topstar

plastic injection molding machine(1)

Screw slippage in plastic injection molding machine poses a serious threat to process stability and part quality. Slippage occurs when the screw’s flights lose their grip on the molten polymer in the barrel, resulting in inconsistent shot sizes, poor melt homogeneity, and fluctuating injection pressures. First, identify the symptoms, which typically manifest as a sudden drop in injection pressure, varying shot weights, and streaking or underfilled parts. Next, determine the root cause, which can be a worn drive coupling, reduced screw-barrel clearance, a failed check ring, or improper back pressure setting. By deciding how each component affects screw pull, injection molding machine operators can implement targeted strategies to maintain strong melt delivery and adhere to high-quality production standards.

Inspecting and Maintaining Drive Couplings for Plastic Injection Molding Machine

The drive coupling between the servo or hydraulic motor and the screw shaft plays a critical role in the torque transfer of these horizontal injection molding machines. Suppose the coupling’s elastomeric elements degrade, crack, or shift under load. In that case, the motor cannot fully transfer its rotational force to the screw, which causes it to slip when processing high-viscosity-filled resins or large shot sizes.

First, disconnect the power to the machine, remove the coupling cover, and inspect the flexible cross shaft or bushing for signs of fatigue. Next, manually rotate the motor and screw to detect axial or radial play. Axial play exceeding 0.05 mm usually requires immediate replacement. When installing a new coupling, use a feeler gauge or laser alignment tool to verify angular alignment according to the corresponding tolerance. Finally, ensure that the torque of the coupling bolts is within specifications and reapply the corresponding anti-corrosion protective coating.

Maintain the integrity of the screw and barrel of the plastic injection molding machine.

Wear between the screw threads and the barrel bore reduces the mechanical grip required to convey the polymer forward, especially when high back pressures are high or abrasive fillers are used. Whether on a large injection molding machine or a standard plastic molding machine, the continued wear of glass fiber, talcum powder or pigment additives will accelerate wear and push the screw-barrel clearance beyond acceptable limits. Therefore, you can disassemble the screw every 1,000 to 2,000 production hours. Clean the adhered polymer with a solvent, and measure the thread height with a micrometer. If the measurements are outside the wear tolerance, you have two options: refurbish the screw and barrel, or replace them entirely. For demanding injection molding, consider upgrading to a hardened alloy barrel or ceramic-coated screw, technologies that can significantly reduce long-term wear.

screw design of all-electric injection molding machine 1

Ensure the performance of check rings and check valves.

Check rings and check valves to prevent melt backflow during the high-pressure injection stage. When these parts wear or the return spring fatigues, the melt may bypass the advancing threads, causing “screw slippage” even if the motor and coupling are functioning correctly. When maintaining the check device on the plastic injection molding machine, you can disassemble the check ring housing, clean all components, and inspect the ring surface and spring cavity. Check for uneven contact, corrosion pitting, or spring breakage. Alternatively, replace the damaged ring or spring with the specified part, ensuring that the outer diameter of the replacement ring matches the micro-tolerance of the barrel.

After reassembly, perform a quick check by manually rotating the ring forward and backward: the ring should slide backward but lock firmly in the forward direction under moderate torque. By maintaining the integrity of the check ring, you can prevent the melt column of the plastic molding machine from backflowing, thus eliminating a common cause of screw slippage.

Optimizing Back Pressure and Screw Speed

Back pressure and screw speed directly affect the melt viscosity and mechanical grip of the injection molding machine. Too little back pressure during plasticization will result in a thin melt column that may “slip” through the screw flights; conversely, too much back pressure will overload the drive components and limit production. On plastic injection molding machines, the ideal back pressure is usually between 5% and 10% of the peak injection pressure, adjusted in increments of 5 to 10 bar.

Similarly, moderate screw speed strikes a balance between reducing residence time and maintaining traction. When the system runs at high speed, it introduces shear heating and reduces purchase power, while low speed increases the cycle time. When using a plastic injection molding machine for engineering polymers or high-viscosity compounds, it may be necessary to fine-tune further and combine incremental speed adjustments with back pressure adjustments to achieve a stable torque curve and uniform melt temperature.

injection molding machine 25-1

Adjust with servo drive control system

Traditional hydraulic systems are robust but lack the dynamic feedback needed to respond to slip events. In contrast, servo-driven injection units provide precise torque control, instantaneous response to changing melt loads, and other intelligent monitoring capabilities. Using servo-driven or all-electric plastic injection molding machines, built-in slip detection functions can be implemented.

Additionally, in Topstar’s injection molding machines, proportional back pressure control, screw speed, and barrel temperature can be adjusted in real-time to maintain optimal melt viscosity and traction. This level of intelligence can not only prevent slippage but also shorten startup time and reduce scrap. As these large injection molding machines handle multi-material or high-fill applications, intelligent drive devices ensure that the screw does not “overtake” the melt.

Preventing screw slippage through multiple dimensions

Screw slippage can compromise the core function of plastic injection molding machines. By maintaining the integrity of the drive coupling, the condition of the screw barrel, the performance of the check ring, and the optimized back pressure and speed, as well as the use of servo drives, slippage can be effectively prevented in actual production. Additionally, proactive maintenance programs with real-time monitoring can also help ensure the early detection of wear or misalignment, which in turn helps keep your plastic injection molding machine running smoothly.

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