Method for quickly setting process parameters during injection molding trial
Injection molding trial molds are a critical step in verifying mold design rationality and determining optimal process parameters. Quickly and accurately setting process parameters can significantly shorten the trial mold cycle (from the traditional 8-12 hours to 4-6 hours) and reduce trial mold costs. Rapid setup methods are based on the raw material characteristics and mold structure. Through a process of “initial setup based on empirical values – staged debugging – defect-driven optimization,” the parameter range can be efficiently determined while ensuring trial mold results. This process requires a combination of the trial molder’s experience and scientific testing methods to avoid wasted time caused by blind adjustments.
Initial parameter setting based on the raw material and mold is the foundation of rapid mold trial and should be completed one hour before the trial. Baseline values are primarily determined based on the raw material technical manual and basic mold parameters. Regarding temperature parameters, the barrel section temperature is set at the midpoint of the raw material’s melting temperature range (e.g., ABS melts at 220-250°C, initially set at 235°C). The nozzle temperature is 5°C higher than that of the homogenization section. The mold temperature is initially set based on the raw material type (40-60°C for crystalline molds and 20-40°C for amorphous molds). Regarding pressure and speed parameters, based on the mold cavity volume and gate size, the injection pressure is initially set at 80-120 bar (lower values for small molds and higher values for large molds). The injection speed is set to 50% of the maximum speed, the holding pressure is 60% of the injection pressure, and the holding time is one-third of the cooling time. For example, for a thin-walled PP product (mold cavity volume 50 cm³, gate diameter 1 mm), the initial parameters are: barrel temperature 200°C, mold temperature 50°C, injection pressure 90 bar, injection speed 50 mm/s, holding pressure 55 bar, holding time 5 seconds, and cooling time 15 seconds. After setting these initial parameters, they should be recorded on the mold trial sheet as a reference for subsequent adjustments.
Rapid parameter tuning during the filling phase focuses on the core objective of “filling the cavity.” Optimal injection pressure and speed are determined through testing three to five molds. Initially, perform low-pressure, slow filling (injection pressure 50 bar, speed 30 mm/s). Observe the melt flow path and filling conditions, assessing proper mold venting and the presence of significant resistance points (such as narrow gaps or complex structures). If the mold stalls at 30% to 50% fill, gradually increase the injection pressure (by 10 to 20 bar) until the melt resumes flowing. If flash appears, reduce the pressure by 5 to 10 bar. For example, during a mold trial of an automotive parts mold, low-pressure filling only reached 40%. The pressure was increased to 120 bar, then to 80%, and then to 140 bar for complete filling. Filling speed adjustment should be coordinated with pressure. If pressure is too high (over 150 bar), increase the speed (by 10 mm/s) to reduce melt viscosity and minimize pressure requirements. During the filling phase, the lowest pressure and speed that “just fills the cavity” need to be recorded as the starting point for subsequent optimization. This process usually takes no more than 1 hour.
Quickly determine the holding and cooling parameters through weighing and visual inspection within two to three molds. The holding pressure is tested starting at 50% of the injection pressure and increasing by 10% per mold until the part weight no longer increases (fluctuation ≤ 0.5%). This pressure is considered the optimal holding pressure. The holding time should start at half the cooling time and be gradually reduced (by 1-2 seconds at a time) until slight sink marks appear on the part. An increase of 1-2 seconds is considered a reasonable value. The cooling time can be determined through demolding tests: After the holding period ends, the cooling time is reduced by 0.5-1 second per mold until the part exhibits deformation upon demolding. An increase of 2-3 seconds is considered the optimal cooling time. For example, for a thick-walled PC part, the initial cooling time was 30 seconds. After testing, deformation occurred when the cooling time was shortened to 22 seconds. The final cooling time was determined to be 25 seconds, and the holding time to be 10 seconds. During the debugging of the holding and cooling parameters, changes in part weight and appearance must be recorded simultaneously to ensure the shortest possible cycle time while maintaining quality. This process takes approximately 40 minutes.
Rapid optimization of temperature parameters requires determining the optimal range through comparative testing of both the product’s appearance and internal quality. Barrel temperature adjustment can be made by observing the product’s gloss: if the temperature is too low, the product’s surface will appear dull, requiring a 5-10°C increase; if the temperature is too high, silver streaks or black specks will appear, requiring a 5-10°C decrease. The impact of mold temperature is reflected in product warpage and crystallinity: if crystalline plastic products exhibit severe warpage, the mold temperature should be increased by 5-10°C; if weld marks appear on the surface of amorphous plastics, the mold temperature should be raised by 10-15°C. For example, an ABS product exhibited poor surface gloss during mold trials. Increasing the barrel temperature from 230°C to 240°C improved the gloss, but slight silver streaks appeared. Lowering the barrel temperature to 235°C achieved the desired gloss. Temperature adjustments should only change one variable (barrel or mold temperature) per mold to avoid parameter conflicts. This process typically requires three to five mold runs and takes approximately one hour.
Defect-driven rapid parameter correction targets typical issues encountered during mold trials (such as flash, sink marks, and bubbles), using a one-to-one adjustment method to resolve them within 1-2 hours. Flash is typically caused by excessive pressure or mold temperature. Prioritize reducing the injection pressure by 5-10 bar. If this is ineffective, lower the mold temperature by 5°C. Sink marks require increasing the holding pressure or extending the holding time (by 10% each time). Bubbles are often caused by gas in the melt or excessive temperature. Exhaust can be eliminated by increasing the back pressure (5-10 bar) or lowering the barrel temperature by 5-10°C. For example, bubbles appeared in a POM mold trial. Increasing the back pressure to 10 bar reduced the bubbles, and lowering the barrel temperature by 5°C completely eliminated them. Defect correction should follow the order of “pressure first, then temperature; injection first, then holding pressure,” adjusting one parameter at a time to avoid unintended changes. Before the mold trial is concluded, five molds should be produced continuously using the optimal parameters. Dimensions, weight, and appearance consistency should be tested. If deviations are within the allowable range (±0.1mm for dimensions and ±1% for weight), the curing parameters are considered, and the mold trial is complete. Using this method, most molds can be tried out and stable parameters determined within 6 hours.
