What Are the Common Problems and Solutions in Plastic Injection Molding Machines?

Plastic injection molding machine, as core equipment of modern manufacturing industry, is widely used in automotive, electronics, medical and other industries. These machines efficiently produce structurally complex products by heating and melting thermoplastic or thermoset plastics, which are then injected into mold cavities under high pressure. However, in the process of use, equipment will often encounter problems such as improper process parameter setting, mold design defect, mechanical failure, and so on, affecting production efficiency and product quality. In this paper, the common problems of plastic injection molding machines are summarized systematically, and the solution is provided with typical cases, which can provide technical reference for the industry.

1.Incomplete Filling (Short Shot)
Phenomenon: The product is not fully filled with mold cavity, indicating a shortage of some materials or overall undersized dimensions.
Reason:

• Process Parameters: Insufficient injection pressure (less than 70 MPa), slow injection speed (less than 1 s/shot) and low barrel temperature (less than 230°C).
• Mold Design: Excessively narrow runners, the gate section is small, the poor venting, causes the cavity air to remain.
• Material Characteristics: Poor resin flowability (e.g., high-viscosity PC material), excessive moisture content of raw materials (insufficient drying).

Solutions:

• Process Adjustment: Increasing injection pressure gradually to 100–150 MPa, increasing injection speed to 3-5 sec/sec, and adjusting barrel temperature to 250–300°C, depending on material properties.
• Mold Optimization: Enlarge runner diameters (≥ 8 mm recommended), increase the number of gate or steering fan gates, and increase the exhaust groove at the end of the cavity (depth 0.02 -0.05 mm).
• Material Handling: Use special grade raw materials (such as high-flow PC-110) and follow a strict drying process (vacuum drying ≥ 4 ≥ 4 hours – 115°C).

Case study: In the production of car interiors, the small gate diameter (2 mm), there is a short shot of the edge. By enlarging the gate diameter to 4mm and adjusting injection pressure to 120 MPa, the defect rate decreased from 15% to less than 2%.
2. Bubbles and voids in products
Phenomenon: Bubbles or holes appear inside or on the surface of the product, affecting mechanical properties and aesthetics.
Reason:

• Material Factors: High moisture content in raw materials (e.g. PA6 not drying to less than0.2% humidity), incomplete removal of volatiles.
• Process Parameters: Insufficient injection pressure (<80 MPa), short retention time (<2 ss), excessive barrel temperature (>300°C, resulting in degradation of the material).
• Mold Design: Improper gate placement results in uneven filling of melt clogged venting systems.

Solutions:

• Material Handling: Predrying of raw materials using molecular sieve dryers (dew point ≤-40°C) with a return pressure of 5–10 MPa to remove volatiles.
• Process Optimization: increase injection pressure to 100–130 MPa, extend retention time to 5–8 s, and reduce barrel temperature to 260-280°C.
• Mold Improvement: Exhaust grooves (width 3–5 mm) at the dividing line are added, and gate placement of thick-walled section of the product is optimized.

Case study: During the production of electronic connectors, internal bubbles are created due to excessive moisture content the raw material (0.5%). The bubble issues was resolved by increasing the 120ºC drying step (4 hours) and adjusting injection pressure to 120MPa.
3. Flash on product (Burrs)
Phenomenon: Too many flakes appear on product sorting lines or core surfaces, requiring secondary repair.
Reason:

• Equipment Factors: Insufficient clamping force (< pressure required for product), increased clearance due to wear of clamping mechanisms.
• Process Parameters: High injection pressure (>150 MPa) and high barrel temperature (>300°C, reducing material viscosity).
• Mold Defects: Uneven parting surfaces (gap >0.05 mm), severely worn guide posts/bushings.

Solutions:

• Equipment maintenance: calibrate clamping force to 1.2 times the required pressure (for example, set the 600 force of 500 tons of product to 600 tons) and replace worn clamping components.
• Process Adjustment: reduce injection pressure to 100–120 MPa and control barrel temperature at 250-270°C.

Mold Repair: The flatness of the parting surface flatness is checked using CMM and repaired by grinding or plating (surface roughness ≤ 0.8 μm).
Case study: Flashbulbs appear in home appliance housing production due insufficient clamping force (450t machines are used in 500t products). By upgrading to a 600t machine and optimizing injection pressure to 110 MPa, the flash rate dropped from 8% to below 0.5%.

1. Screw failed to Rotate or retract
Phenomenon: During injection, screws become stuck or fail to fully plasticize and contract.
Reason:

• Barrel Issues: High temperature at the back of the barrel (>300°C) causes material carbonization and blockage of screws.
• Mechanical Failures: Too much roughness on the screw surface (Ra >1.6 μm), bearing wear or drive gears damage.
• Electrical control: Overload protection is triggered in screw drive motor or misaligned limit switches.

Solutions:

• Cleaning and maintenance: Remove screws after closing, remove carbonized material with a copper brushes, and repolish screw surface (Ra ≤0.8 micron).
• Temperature control: Reduce the temperature at the back of the barrel to 260-280°C and increase the coolant flow to 15 L/min.
• Electrical inspection: Check the motor's three-phase power supply (voltage fluctuation less than5%) and recalibrate the position of the limit switch (error <0.1 mm).

Case study: In the production of medical devices, the screw became stuck due to the high temperature in the barrel (320°C). Normal operations were resumed by lowering temperatures to 270 degrees Celsius and increasing the flow of the cooling system.
2. Nozzles and Sprue Bush Leakage
Phenomenon: Melt Molten material leaks from the nozzle-sprue bush interface during injection.
Reason:

• Alignment Errors: Axial misalignment between nozzle and sprue bush (>0.1 mm).
• Design Flaws: The convex spherical radius on the sprue bush is too small (less than0.5 mm of nozzle radius) or worn nozzle convex surfaces is worn.
• Temperature loss: nozzle temperature is too high (>280°C), enhancing material flowability.

Solutions:

• Mechanical correction: nozzle-sprue bush coaxiality is checked using dial indicators and mold positioning ring is adjusted for correction (error ≤0.05 mm).
• Component Replacement: replace nozzle bushing with standard convex spherical radii (bushing 12 mm and nozzle 11.5 mm) and restore nozzle convex surfaces (surface roughness Ra ≤0.4 μm).
• Temperature control: Reduce nozzle temperature to 250 -270 ℃ and improve temperature sensor accuracy (±1°C).

Case study: During automotive lampshade production, leakage may occur due to nozzle-sprue bush misalignment (0.2 mm). The leakage leakage issues solved by adjusting the mold positioning ring and replacing the sprue bush.

1. Mold Jamming and damage
Phenomenon: Die cannot be separated when opening, leading to forced operation and core breakage.
Reason:

• Thermal Expansion/Contraction: mold temperature is not uniform (difference >20°C), causing local thermal stress.
• Insufficient Lubrication: Lack of regular lubrication of Guide posts/bushings (dry friction coefficient >0.15).
• Contaminant Intrusion: Residual debris (particle size greater than0.5 mm) in cavity obstructing movement.

Solutions:

• Temperature control: Balancing mold temperature (difference ≤ 5 ℃) with mold temperature controller, increasing cooling channel cross section (diameter ≥10 mm).
• Lubrication Maintenance: Spray molybdenum disulfide lubricant is sprayed on guide posts/bushings every 8 hours (friction coefficient ≤0.05).
• Clean management: Install magnetic separators to filter metal debris in cooling water and regular cleaning of cavities (using compressed air).

Case study: During the production of 3C product housing production, mold jamming occurred due to uneven temperatures (120°C, below local temperature). 80°C elsewhere). By upgrading the die temperature controller for precise control (±2°C), the life of the die is extended to more than 500,000 cycles.

2. Poor Mold Venting
Phenomenon: Products with burn marks, silver streaks or incomplete fillings.
Reason:

• Design Flaws: Inadequate exhaust slot dimensions (width <3 mm, depth <0.02 mm).
• Processing error: Too much surface roughness (Ra >1.6 μm) for type separation results in sealing.
• Material Blockage: Low-viscosity materials (such as PP) penetrates the vent and solidifies.

Solutions:

• Exhaust Slot Optimization: Exhaust groove added at end of cavity (width 5 mm, depth 0.03 mm) and vacuum venting systems (vacuum ≤-90 kPa).
• Surface treatment: Superfinish parting surfaces (Ra ≤0.4 micron) coated with polytetrafluoroethylene (PTFE) coatings to reduce friction.
• Material management: Increase the return pressure of low viscosity low-viscosity materials – 15 MPa) to prevent infiltration and clean the vents every 24 hours.

Case study: In optical lens production, due to poor venting, there are silver streaks. The product yield increased from 70% to 95% by increasing vacuum vent and optimizing slot dimensions.

1. Equipment Maintenance Strategies
Daily Inspection: Check hydraulic oil levels (error less than5%), cooling water flow (≥10 L/min) and electrical connections (looseness rate less than1%).
Periodic Maintenance: hydraulic oil filters are replaced every 500 hours (filtration精度 process ≤ 10 microns) and screws/ drums are inspected every 2000 hours (gap ≤0.1 mm).
Spare Parts management: Stock critical wear (such as seals, heating coils) to ensure inventory turnover rate ≥90%.
2. Intelligent Upgrades
Condition Monitoring: Installation of Install vibration sensors (frequency range 10–1000 Hz) and temperature sensors (accuracy ±0.5°C) for real-time monitoring of screw and barrel conditions.
Data analysis: Process parameters (injection pressure, retention time) were collected through MES systems and machine learning algorithms were used to predict failure (accuracy >85%).
Remote maintenance: expert remote guidance using 5G + AR technology reduces fault response time to <30 minutes.
Case study: through the implementation of an intelligent maintenance system, a home appliance enterprise, overall equipment effectiveness increased by 15%, maintenance costs decreased by 2 million yuan a year.
Conclusion:
The stable operation of plastic injection molding machines depends on accurate control of process parameters, reasonable mold design and scientific equipment maintenance. By systematically analyzing common problems (e.g., incomplete filling, flash flooding, mold jamming, etc.) and implementing targeted solutions (such as process optimization, mechanical calibration, intelligent upgrading, etc.), production efficiency and product quality can be greatly improved. In the future, with the advance of Industry 4.0 and Intelligent Manufacturing, equipment failure prediction and automatic maintenance will become industry trends, creating higher value for enterprises.