Multi-stage machine adjustment method for serious overflow and whitening of injection molded parts
When injection molded parts exhibit severe flash and whiteout, single parameter adjustments often fail to completely resolve the issue. A multi-stage tuning approach is required, systematically optimizing multiple steps from mold clamping, injection, holding pressure, to ejection, to systematically eliminate the defects. Stage 1: Optimize clamping parameters to address the root cause of flash. Flash is primarily caused by cavity pressure exceeding the clamping force, resulting in separation at the parting surface. Therefore, first verify that the clamping force is sufficient. Gradually increase the clamping force (by 5-10% each time) until the flash is reduced. At the same time, maintain a moderate clamping speed (30-50 mm/s) to avoid impact and mold misalignment caused by overly fast clamping. If flash persists after increasing the clamping force, check mold parallelism. If the parallelism error is excessive, adjust the tie rod tension to ensure uniform force distribution. For example, if flash is concentrated on one side of the mold, the tie rod on that side may be too loose and should be tightened appropriately to balance the clamping force.
Level 2: Adjust injection parameters to reduce melt impact. High pressure and speed during the injection phase are key factors contributing to flash and whiteout. Reduce injection pressure and speed to minimize melt impact on the cavity and reduce cavity pressure. A staged injection process can be employed, dividing the injection process into two or three stages. In the first stage, maintain a lower speed (20-30 mm/s) and pressure to ensure smooth melt entry into the cavity and avoid impact on the parting surface. In the second stage, increase the speed (40-50 mm/s) when the cavity is 70-80% filled to accelerate filling. In the third stage, reduce the speed again ( 10-20 mm/s) as the cavity nears fullness to minimize flash. Simultaneously, lower the barrel temperature (by 5-10°C each time) to increase melt viscosity, reduce fluidity, and thus reduce cavity pressure. For example, for ABS parts, reduce the barrel temperature from 230°C to 210-220°C to observe whether flash improves.
Level 3: Optimize holding parameters and reduce residual stress. Holding pressure that is too high or for too long will cause the cavity pressure to continue to rise, resulting in overflow, while increasing the stress in the product and exacerbating whitening. It is necessary to reduce the holding pressure (usually 50-70% of the injection pressure) and shorten the holding time so that no sink marks appear on the product. Segmented holding can be used, with a slightly higher initial holding pressure, maintained for 1-2 seconds, and then the pressure is reduced to avoid continuous high pressure. For example, reduce the holding pressure from 80MPa to 60MPa and shorten the holding time from 5 seconds to 3 seconds. If slight sink marks appear, it can be adjusted back by 0.5-1 second. At the same time, reduce the melt temperature during the holding stage, and by accelerating the cooling rate, reduce the holding pressure demand and further reduce the cavity pressure.
Level 4: Adjust the ejection parameters to reduce the phenomenon of white ejection. White ejection is caused by excessive or uneven ejection force, and the ejection speed, pressure and sequence need to be optimized. Reduce the ejection speed (10-20mm/s) to reduce the impact force during ejection; reduce the ejection pressure to just enough to eject the product, and avoid local stress concentration caused by excessive pressure. For complex products, adopt segmented ejection, first eject a small distance (1-2mm) at low pressure and low speed to loosen the product, and then increase the speed to eject and reduce ejection resistance. Check the ejection mechanism to ensure that the ejector pins are evenly distributed and in good contact with the product. If it is found that a certain ejector pin is improperly positioned and causes white ejection, adjust its position or increase the number of ejector pins. For example, when white ejection occurs at the edge of the product, auxiliary ejector pins can be added in that area to disperse the ejection force.
Level 5: Combine raw material and mold adjustments to completely solve the problem. If defects still exist after the above four levels of adjustment, further processing is required from the raw material and mold aspects. Check whether the raw materials are mixed with impurities or degraded, replace with new materials and fully dry them; for plastics with excessive fluidity, add an appropriate amount of filler to reduce fluidity. In terms of molds, clean the parting surface and ejector pin holes, remove burrs and impurities; check the exhaust grooves to ensure that they are unobstructed, and deepen or widen the exhaust grooves if necessary (but they must be smaller than the plastic overflow value); for molds that are severely worn, repair the parting surface or replace vulnerable parts such as ejectors. For example, when scratches on the surface of the mold cavity cause white tops, the cavity surface needs to be polished to reduce frictional resistance. Through multi-level machine adjustment and comprehensive treatment, serious overflow and white top defects can be gradually eliminated and product quality can be restored.
