How to adjust the process parameters of the injection molding machine to solve product defects?

To adjust the process parameters for defects in injection molding products, it is necessary to accurately optimize the core parameters such as temperature, pressure, speed, and time in combination with the defect type. The following are process adjustment strategies and operation points classified by typical defects:
I. Process parameter adjustment plan by defect type
1. Flash (burrs)
Pressure parameters:
Injection pressure: Reduce by 10%-20% (such as from 100MPa to 80-90MPa) to avoid excessive filling of the mold gap with molten material.
Clamping force: Calculate the minimum clamping force based on the projected area of ​​the product (formula: clamping force = projected area × cavity pressure, cavity pressure is 30-80MPa), and ensure that the clamping force is sufficient and uniform (check the tightness of the coring column nut).
Temperature parameters:
Barrel/mold temperature: Reduce by 5-10℃ (such as from 230℃ to 220-225℃) to speed up the solidification of the molten material and reduce the risk of overflow.
Speed ​​parameters:
Injection speed: reduce the proportion of high-speed section, adopt multi-stage injection (such as 60% speed in the front section and 40% speed in the back section) to avoid high-speed impact causing mold expansion.
2. Shrinkage (shrinkage depression)
Pressure and time:
Holding pressure: increase to 60%-80% of injection pressure (such as injection pressure 120MPa, holding pressure adjusted to 72-96MPa), extend holding time 5-10 seconds to ensure sufficient filling of thick-walled areas.
Injection volume: increase 5%-10% (adjusted by injection stroke) to avoid insufficient barrel melt causing missing filling.
Temperature parameters:
Mold temperature: increase 10-20℃ (especially cooling water circuit in thick-walled area), slow down the surface solidification speed, and promote internal melt flow and shrinkage compensation.
3. Lack of material (short shot, insufficient filling)
Temperature and speed:
Barrel temperature: Increase by 5-15℃ (such as from 200℃ to 205-215℃) to improve melt fluidity; mold temperature is increased by 10-20℃ simultaneously to reduce filling resistance.
Injection speed: Increase the proportion of high-speed section (such as from 50% to 70%-80%), shorten the filling time, and avoid premature cooling of the melt in the flow channel.
Pressure parameters:
Injection pressure: Increase by 10%-20% (such as from 80MPa to 90-100MPa) to enhance melt penetration; back pressure is appropriately increased (5-10MPa) to ensure the density of the melt.
4. Surface defects (silver streaks, flow marks, black spots)
Silver streaks (air streaks):
Raw material drying: Hygroscopic materials (such as PA, PC) are dried until the moisture content meets the standard (PA needs to be dried at 120℃ for 4-6 hours, dew point ≤-40℃), and a dehumidifier is installed in the hopper.
Injection speed: Low-speed pouring in the front section (30%-40% speed) to avoid high-speed air entrainment; the speed can be appropriately increased in the back section to ensure smooth filling.
Flow marks (cold material streaks):
Barrel/mold temperature: Increase 5-10℃ to reduce the viscosity of the melt; use multi-stage injection (low-speed over-gate, high-speed filling cavity) to prevent condensation of the melt at the gate.
Black spots/scorch marks:
Back pressure and melting time: Reduce back pressure (such as from 20MPa to 10-15MPa), shorten melting time, and avoid overheating of molten material in the barrel; use cleaning material to clean the barrel 3-5 times before stopping.
5. Deformation/warping
Cooling and pressure:
Cooling time: Extend by 10%-20% (such as from 15 seconds to 18-20 seconds), reduce cooling water temperature (5-10℃), and ensure uniform cooling of each area of ​​the product (check water flow ≥5L/min).
Pressure holding parameters: Reduce pressure holding pressure (to 50%-60% of injection pressure) and pressure holding time (reduced by 5 seconds) to avoid internal stress accumulation caused by excessive compaction.
Speed ​​parameters:
Injection speed: Use medium-speed injection (50%-60% speed) to reduce shear stress, especially for glass fiber reinforced materials (such as GF-PA), high speed should be avoided to cause uneven orientation.
6. Bubbles/voids
Raw materials and pressure:
Drying treatment: strictly control the proportion of recycled materials (≤30%) to avoid contamination by recycled materials; hygroscopic raw materials are dried to a dew point of ≤-40℃, and filters are installed in the hopper to filter impurities.
Pressure holding parameters: increase the pressure holding pressure (70%-80% of the injection pressure) and the pressure holding time (extend 5-10 seconds), compact the melt and expel the gas.
Speed ​​parameters:
Injection speed: low speed in the front section (30%-40%) to prevent air entrainment, and the speed can be appropriately increased in the back section to ensure a smooth filling process.
II. General principles for adjusting core process parameters
1. Temperature control
Barrel temperature: set according to the raw material melt index (MI), the temperature difference in each section is ≤10℃, and the rear end temperature can be reduced by 5-10℃ when the screw shearing heat is obvious (such as PVC).
Mold temperature: The mold temperature of crystalline materials (such as PE and PP) needs to be ≥50℃ to promote crystallization; the mold temperature of non-crystalline materials (such as PS and ABS) is 30-50℃, and thin-walled products can be increased to 60-80℃ to improve filling.
2. Pressure control
Injection pressure: Initially set to 60%-70% of the recommended value of the material, and gradually increase to just fill the cavity (avoid excessive flash).
Holding pressure: Usually 50%-80% of the injection pressure, and the holding time is based on the solidification of the gate (the "injection-holding-cooling" time test method can be used: gradually extend the holding time to the shortest time when shrinkage is no longer improved).
3. Speed ​​and time
Multi-stage injection: segmented control (such as low-speed over-gate → high-speed filling → low-speed holding pressure) to avoid high-speed impact on the mold or gate freezing. The typical segment ratio is: 30% (gate) → 70% (cavity) → 40% (holding pressure).
Cooling time: The standard is that the product is ejected without deformation. It can be optimized by combining "mold temperature + cooling water temperature" (for example, when the mold temperature is 60℃ and the water temperature is 20℃, the cooling time = product wall thickness × 3-5 seconds/mm).
4. Injection volume and back pressure
Injection volume: Keep a 5%-10% buffer (the thickness of the melt at the front end of the screw is ≥5mm) to avoid screw idling or insufficient melt.
Back pressure: The back pressure of general materials (such as ABS) is 5-10MPa, and that of engineering materials (such as PC, PA) is 10-15MPa. Too high back pressure can easily lead to melt degradation, and too low back pressure can lead to insufficient melt density.
III. Adjustment steps and verification methods
Defect location: Determine the defect type (such as dimensional deviation, surface problem, structural defect) through first-piece inspection, three-coordinate measurement or mold flow analysis.
Parameter fine-tuning: Adjust a single parameter each time (such as temperature ±5℃, pressure ±10%), and record the defect changes before and after the adjustment to avoid difficulties in tracing problems caused by simultaneous adjustment of multiple parameters.
Process window verification: Based on the optimal parameters, the test is conducted with a 5% fluctuation range to confirm whether the defects are reproduced and determine the stable parameter range (such as material temperature 220±5℃, holding pressure 80±10MPa).
Standardized records: Establish a "Process Parameter Record Table" containing the raw material model, mold number, specific values ​​of temperature/pressure/time/speed, and corresponding defect solutions for reference in subsequent production.
IV. Adjustment points for special materials
Glass fiber reinforced materials (such as GF-ABS): Reduce the injection speed (40%-50%) to reduce the glass fiber orientation, increase the mold temperature to 60-80℃ to avoid surface floating fibers; the holding pressure is slightly lower (50%-60% of the injection pressure) to prevent stress concentration.
Transparent materials (such as PMMA, PC): The barrel temperature is strictly controlled (fluctuation ≤2℃), the mold temperature is 60-90℃, the injection speed is medium speed (50%-60%), and the holding time is extended until the gate is completely solidified to avoid silver streaks and weld marks.
Heat-sensitive materials (such as PVC, POM): The barrel temperature is 10-15℃ lower than the melting point, the back pressure is ≤5MPa, and the PE cleaning material is used to empty the machine when it is shut down to prevent the molten material from being retained and decomposed to produce black spots.
Through the logic of "single parameter fine-tuning → effect verification → interval solidification" to adjust the process parameters, combined with the mold state (such as exhaust, cooling) and raw material characteristics, the defects of injection molding products can be systematically solved. For complex defects (such as unbalanced multi-gates and filling of thin-walled products), it is recommended to cooperate with mold flow analysis for pre-judgment, and then iterate and optimize through on-site mold trials to finally form a standardized process plan.