Injection molding cold spots and solutions
Cold slug spot in injection molding is a common surface defect in plastic parts, appearing as dark spots or streaks, often located near the gate or at the transition point of the melt flow. This occurs when the cold front of the melt enters the mold cavity and fails to fully merge with the subsequent melt, resulting in a different appearance from the surrounding area after cooling. A PP toy wheel exhibited a cold slug spot with a diameter of 2-3 mm near the gate. Microscopic examination revealed that the crystallinity in the cold slug area was 5% higher than that of the surrounding area, resulting in differences in optical properties. Sources of cold slug include coolant at the front of the barrel, residual agglomerate in the runner, and a hardened layer formed by the melt cooling in the runner. Test data showed that the probability of cold slug spot increased by 40% when the melt temperature dropped 15°C below the process setpoint. Cold slug spot not only degrades appearance but can also weaken the strength of the plastic part. In one example, cold slug spot reduced the impact strength of an ABS electrical appliance housing by 12% due to cold slug spot, leading to cracking during a drop test.
Optimizing the design of runners and cold slug wells is a fundamental measure for eliminating cold slug spot. Through a rational structure, cold slugs are directed to the runners rather than the cavity. A cold slug well of sufficient capacity is designed at the end of the main runner, with a depth of 1.5-2 times the main runner diameter (e.g., a 10mm diameter main runner should have a cold slug well depth of 15-20mm). This inverted tapered structure prevents cold slug from being drawn into the cavity. Increasing the cold slug well volume reduced the cold slug spot defect rate in a PA66 gear mold from 10% to 2%. Circular cross-sections and polished runners (Ra 1.6μm) reduce melt flow resistance and prevent cold slug accumulation in dead corners. After optimizing the runners in a six-cavity connector mold, the distribution of cold slug spot was improved, and the defect rate variation between cavities was reduced from ±5% to ±1%. For micro-plastic parts (weighing less than 5g), a hot runner system can be used to eliminate runner cold slug. A micro-motor housing achieved a 99.8% appearance acceptance rate by using a hot runner to completely eliminate cold slug spot.
Adjustments to process parameters should focus on increasing melt and mold temperatures to minimize the generation and impact of cold slugs. Raising the barrel temperature by 5-10°C (without exceeding the material degradation temperature) maintains a high melt temperature before entering the mold cavity. For a PC lens, increasing the barrel temperature from 280°C to 285°C reduced the brightness difference of the cold slug spot from 30% to 5%. Increasing mold temperature can slow the cooling of the cold slug. For PP parts, increasing the mold temperature from 50°C to 70°C increased the fusion time between the cold slug and the subsequent melt by 0.5 seconds, blurring the boundaries of the cold slug spot and reducing its visibility by 80%. Optimizing injection speed is also crucial. A two-stage “slow-fast” injection process is employed: Initially (near the gate), the speed is reduced by 30% to ensure smooth cold slug entry into the mold cavity; the speed is restored to normal in the subsequent stages. This approach reduced the cold slug spot area by 60% for an ABS television housing, while also avoiding turbulence caused by high-speed injection.
Material pretreatment and barrel cleaning are significantly effective in preventing cold slug spot, especially for hygroscopic and easily degradable materials. Hygroscopic materials such as PA and PBT require thorough drying (moisture content < 0.05%). Wet materials hydrolyze in the barrel, causing a decrease in viscosity and forming an unstable melt front. After thorough drying, the cold slug spot rate for a PA6 part dropped from 15% to 3%. Regular barrel cleaning (every 8 hours of production) removes residual cold and degradable material from the inner wall. A PC production line experienced persistent cold slug spot due to carbon deposits in the barrel; cleaning immediately eliminated the defect. For materials sensitive to residence time, such as PVC, the melt residence time in the barrel should be controlled to no more than 10 minutes. By shortening the cycle time for a PVC pipe fitting, the incidence of cold slug spot was reduced from 8% to 1%. Furthermore, by selecting a material grade with good flowability, a PP part that switched to a higher MFR grade (from 10g/10min to 20g/10min) achieved a 70% improvement in cold slug spot.
Optimizing the gate form and size can effectively prevent cold material from entering the mold cavity, and the effect can be further improved by combining it with a venting design. Using a fan-shaped gate instead of a point gate increases the contact area between the melt and the mold cavity, dispersing the cold material before entering the cavity. The cold material spot on a cosmetic bottle cap disappeared after the gate form was changed. The gate size must match the wall thickness of the plastic part, usually 0.8-1.2 times the wall thickness. For a 2mm thick ABS plastic part, increasing the gate thickness from 1.5mm to 1.8mm reduced the cold material pass rate by 40%. An auxiliary venting groove (depth 0.01mm) is set near the gate to allow gas in the cold material to be discharged in a timely manner, preventing bubbles from overlapping with cold material spots. In one case, after the venting was optimized, the composite defect rate dropped from 9% to 1%. For cold spots that cannot be solved by mold and process adjustments, post-processing methods can be used. For example, cold spots on PC lenses can be removed by polishing (removal amount 0.02mm), but it must be ensured that the dimensional accuracy is not affected. After polishing, the appearance qualification rate of a batch of lenses increased from 82% to 95%, but the cost increased by about 5%. Therefore, post-processing is only used as an emergency measure.
