Multi-stage adjustment techniques to prevent gas marks when rapid filling is required
Rapid filling is an important means of improving injection molding production efficiency, especially for thin-walled plastic parts and materials with poor fluidity. However, rapid filling can easily cause turbulence in the melt within the mold cavity, entraining air to form air streaks, which appear as silver or black streaks on the surface of the plastic part, affecting its appearance and mechanical properties. Air streaks are caused by severe shearing, elevated temperatures, and gas stagnation during high-speed melt flow. For example, in the production of thin-walled PC lenses, air streaks caused by rapid filling can reduce light transmittance by 5%-10% and result in scrap rates exceeding 20%. The multi-stage machine adjustment technique is an effective way to resolve this contradiction by controlling the injection speed, pressure, and temperature in sections, ensuring filling speed while avoiding air streaks. This technique sets different process parameters according to the flow stage of the melt within the mold cavity to achieve smooth and efficient filling.
The core strategy for preventing gas lines is to set the multi-stage injection speed in stages. The injection process is divided into 3-5 stages, and the speed is adjusted according to the cavity structure and melt flow state to maintain the optimal flow state of the melt at different stages. In the initial stage (0-30% cavity filling), a low-speed injection (10-30mm/s) is used to ensure that the melt enters the cavity smoothly, avoids impacting the air near the gate, and reduces gas entrapment. In the middle stage (30%-80% filling), the speed is gradually increased (50-100mm/s), and high speed is used to quickly fill most of the cavity to improve production efficiency, but turbulence caused by a sudden increase in speed must be avoided. In the final stage (80%-100% filling), the speed is reduced again (20-50mm/s), and the end of the cavity and complex structures are slowly filled to expel residual gas and prevent gas lines caused by excessive compression. For example, when producing ABS thin-walled housings, using a three-level speed of “low speed 20mm/s → medium speed 80mm/s → low speed 30mm/s” can reduce the air mark defect rate from 15% to below 3%.
Coordinated adjustment of injection pressure and holding pressure parameters is a crucial component of speed control. Multi-stage pressure settings must be matched to the speed stages. A higher pressure (80-120 MPa) is used during the initial, low-speed stage to ensure the melt overcomes initial flow resistance and advances smoothly. The pressure is appropriately reduced (60-100 MPa) during the intermediate, high-speed stages to reduce shear heating of the melt. The pressure is then increased again (70-90 MPa) during the final stage to assist in gas discharge. The holding pressure and time should be adjusted based on the thickness of the part, generally ranging from 60% to 80% of the injection pressure. The holding time should be maintained until the gate solidifies to prevent gas marks caused by melt backflow. For example, for a 2mm thick PC part, a holding pressure of 70% of the injection pressure and a holding time of 8 seconds can effectively compensate for shrinkage, avoiding surface depressions and gas marks. Furthermore, controlling the back pressure at 5-10 MPa ensures uniform melt plasticization and reduces bubble formation.
Fine-tuning the barrel and mold temperatures can improve melt flow and reduce gas lines. The barrel temperature should be set according to the material’s characteristics. For materials prone to gas lines, such as PC and PMMA, the temperature should be appropriately increased (PC: 280-300°C, PMMA: 230-250°C) to reduce melt viscosity, flow resistance, and shear heating. However, excessively high temperatures can cause material degradation and gas generation, so they must be controlled within a reasonable range. Mold temperature also significantly affects gas lines. Increasing the mold temperature (PC: 80-100°C, ABS: 50-70°C) slows melt cooling, allowing more time for gas to escape. It also reduces friction between the melt and the mold surface, preventing gas lines caused by excessive shear. For example, increasing the PC mold temperature from 60°C to 90°C can reduce gas line incidence by 40%, but this requires extending the cooling time by 5-10 seconds. A balance must be struck between efficiency and quality.
Coordination of the exhaust system and dynamic optimization of process parameters are auxiliary measures to prevent gas marks. During rapid filling, the gas exhaust rate within the mold cavity must match the filling speed. Venting grooves, 0.02-0.05mm deep and 5-10mm wide, should be added at the point where the melt reaches the last part to ensure smooth gas discharge. For example, installing venting grooves at the corners and ends of ribs in a plastic part can reduce gas mark defects by over 60%. During production, the location and form of gas marks should be observed through mold trials, and multi-stage parameters should be dynamically adjusted. If gas marks appear near the gate, the initial speed should be reduced; if gas marks occur at the end of the cavity, the final hold time should be extended or the venting groove size should be increased. The injection molding machine’s process monitoring system can be used to record the pressure and speed curves at each stage in real time, analyze abnormal fluctuations, and optimize parameters accordingly. For example, a sudden pressure increase at a certain stage indicates excessive flow resistance, requiring a reduction in speed or an increase in barrel temperature.
The application of multi-stage machine tuning requires flexible adjustment based on the specific part and material characteristics. For thin-walled, complex parts, the number of stages can be increased (4-5 levels) to refine speed and pressure control. For thick-walled parts, the number of stages can be reduced (2-3 levels), focusing on controlling the holding pressure during the later stages of filling. Regarding materials, PE and PP with good flowability can adopt a simpler multi-stage setup; PC and POM with poor flowability require more refined parameter adjustments. For example, when machining 3mm-thick POM gears, a four-stage control scheme (low speed 15mm/s → medium speed 60mm/s → high speed 90mm/s → low speed 25mm/s) combined with a mold temperature of 80°C can completely eliminate gas lines. Through continuous testing and analysis, a multi-stage parameter database for different products has been established, enabling rapid response to gas line issues in production and maintaining a part qualification rate above 95% for rapid filling.
