As the core equipment for plastic molding and processing, injection molding machines are subject to various factors during long-term continuous operation, including material properties, process parameters, mechanical wear, and operating practices. As a result, various failures are inevitable. Minor issues can affect the appearance and quality of products, while severe ones can cause machine downtime, mold damage, and increased production costs. Effective prediction, analysis, and standardized maintenance of common failures are essential to ensuring stable operation, extending service life, and improving production efficiency of injection molding machines. This article analyzes frequently occurring failures in injection molding production and systematically summarizes key points for daily and periodic maintenance, providing practical guidance for injection molding production sites.
Common Faults and Cause Analysis in the Injection Molding Process
Product Quality-Related Defects
This type of fault is the most common; it manifests directly in the molded products and serves as the primary basis for on-site troubleshooting.
Flash and Overflow
Flashing typically occurs at the mold parting line, ejector pin holes, and slider interfaces. The primary causes include insufficient clamping force, wear on the mold mating surfaces or the presence of foreign matter, excessive injection pressure and speed, excessively high material temperature resulting in excessive plastic fluidity, and an injection volume exceeding the mold cavity capacity. Minor flashing can be remedied by adjusting process parameters, whereas severe flashing necessitates an inspection of the mold and the clamping mechanism.
Short Shots and Incomplete Filling
Defects such as incomplete edges or unfilled sharp corners in molded parts are typically caused by factors such as insufficient injection volume, excessively slow injection speed, excessive resistance in the runners, poor mold venting, low material temperature, or insufficient back pressure. Furthermore, wear on the screw non-return ring, which leads to melt backflow, can also result in a reduction in the actual injection volume.
Sink Marks, Depressions, and Bubbles
Sink marks are commonly observed in areas of a molded part where wall thickness is uneven; the primary causes are insufficient holding pressure, an excessively short holding time, or inconsistent cooling rates. Internal voids, conversely, typically result from moisture content in the raw material, material decomposition caused by excessive barrel temperatures, uneven mold cooling, or air entrapment resulting from an excessively high injection speed.
Silver Streaks, Flow Marks, and Color Variation
Silver streaks are primarily caused by moisture in the raw materials, excessive shear heating resulting from high screw speeds, or the entrapment of gas within the molten material. Flow marks are associated with gate design, injection speed, and mold temperature. Color variation issues typically stem from excessive temperature fluctuations, uneven mixing by the screw, poor dispersion of color masterbatch, or residual material left uncleaned within the barrel.
Equipment Action-Related Faults
Mold Opening/Closing Anomaly
Issues include stuttering during mold opening and closing, unstable speed, failure to reach the target position, and abnormal noises. Common causes include insufficient hydraulic system pressure, lack of lubrication in the guide rails, uneven loading on the tie bars, leaks in the mold-clamping cylinder, misaligned limit switches, or wear and binding in the mechanical toggle mechanism.
Ejection Mechanism Failure
Issues such as ejector pins failing to extend, failing to fully retract, or exerting uneven force are typically caused by faults in the ejector cylinder, a stuck solenoid valve, deformation of the ejector plate, or insufficient lubrication-or seizing-of the mold ejector pins.
Injection and Melting Abnormalities
Weak injection force, failure of the screw to rotate, and erratic melting speeds are typically associated with hydraulic pump malfunctions, damaged solenoid valves, excessive clearance between the screw and the barrel, back-pressure valve failure, or contamination of the hydraulic oil.
Temperature Control and System-Related Faults
Temperature Runaway
If the temperature of the barrel or nozzle fluctuates erratically or fails to reach the setpoint, the issue is typically caused by a damaged heating band, poor contact or aging of the thermocouple, a faulty temperature control module, or wiring contact issues.
Hydraulic System Failure
Excessive oil temperature, unstable pressure, oil leakage, and abnormal noises are primarily caused by hydraulic oil degradation or excessive impurities, a clogged cooler, oil pump wear, aging seals, or a clogged filter element.
Abnormal Noises and Vibrations
The occurrence of abnormal noise and vibration during equipment operation is typically attributed to issues such as loose mechanical components, damaged bearings, bent screws, misaligned couplings, or excessive hydraulic shock.
Injection Molding Machine: Daily Maintenance and Upkeep Standards
Maintenance of injection molding machines adheres to the principles of "prevention first, periodic inspection, and timely intervention." It is categorized into daily, weekly, and monthly maintenance, as well as annual overhauls, with each cycle emphasizing different priorities.
Routine Maintenance (Before and After Each Shift)
Before startup, verify that the power, water, and air supplies are functioning normally, and check that the hydraulic oil and grease levels fall within the standard ranges.
Clear impurities from the hopper, ensure that the raw materials are dry and free of moisture, and prevent foreign objects from entering the barrel and damaging the screw.
Check whether the mold is securely installed, and whether the safety door and emergency stop switch are responsive and functional.
During operation, monitor parameters such as temperature, pressure, and speed to ensure their stability, and watch out for any abnormal noises, oil leaks, or electrical leakage from the equipment.
Before the end of the shift, clean waste materials and oil stains from the equipment surfaces, shut down the heating and power systems, and complete the production records.
Weekly Maintenance
Apply lubricant to moving parts-such as mold opening and closing guide rails, ejector pins and guide pillars, and hinges-to ensure smooth lubrication.
Clean dust and scale from the cooler surface to ensure effective cooling and prevent the hydraulic oil and barrel temperatures from becoming excessively high.
Check whether the various wiring connections, heating bands, and thermocouples are loose or damaged.
Check the hydraulic system for leaks, tighten oil pipe joints, and clean oil stains from the surface of the oil tank.
Monthly Maintenance
Inspect the screw check ring, flange, and nozzle for wear, and replace them if necessary.
Verify the accuracy of all temperature control points, calibrate temperature sensors, and replace aging heating elements.
Inspect safety protection devices-including hydraulic safety valves, mechanical limit stops, and safety door interlocks-to ensure reliable operation.
Clean the hydraulic oil filter element, check the fluid for emulsification or deterioration, and filter it if necessary.
Annual Overhaul and In-depth Maintenance
Completely replace the hydraulic oil, and clean the oil tank, oil lines, and cooler.
Disassemble and inspect the screw, barrel, and bushings for wear; if wear exceeds tolerance limits, repair or replace the components.
Inspect and repair hydraulic pumps, cylinders, and solenoid valves; replace aged seals.
Calibrate clamping force, mold opening stroke, and injection precision; inspect the electrical control system and wiring.
Perform rust prevention, fastening, and commissioning on the complete machine to restore the equipment to its original factory performance.
The Critical Importance of Maintenance Management to Production
Adherence to standardized procedures for troubleshooting and maintenance can significantly reduce injection molding machine downtime, minimize wear and tear on molds and mechanical components, stabilize product acceptance rates, and reduce waste of raw materials and energy. Furthermore, sound maintenance practices can extend equipment service life by 3 to 5 years, thereby substantially lowering an enterprise's costs for equipment replacement and repair.
In actual production environments, operators must be thoroughly familiar with the equipment's structural design and operational principles. They should promptly address minor faults as they arise and proactively anticipate major malfunctions, while also cooperating with professional maintenance personnel for scheduled inspections. Only through such diligence can injection molding machines be sustained in a state of high efficiency, stability, and safety over the long term-thereby providing a reliable foundation for continuous, large-scale production.
