Setting injection molding process parameters is crucial for ensuring product quality and improving production efficiency. This requires following a scientific process and focusing on key points to achieve precise and standardized parameters. These parameters must be determined based on raw material characteristics, mold structure, product requirements, and equipment performance. A stable and reliable parameter combination is ultimately determined through a process of “theoretical calculation – preliminary settings – trial mold adjustments – and curing optimization.” This process emphasizes the synergy between the three major parameters of temperature, pressure, and time to avoid product defects (such as sink marks, flash, and missing material) caused by parameter conflicts.
Preliminary preparation and determining basic parameters are the first steps in process setup. Comprehensive information collection and preliminary calculations are required. First, key raw material properties must be determined, such as melt temperature (e.g., 170-220°C for PP), melt flow rate (MI), density, and crystallization characteristics. This allows the temperature ranges for each barrel section to be determined (the feeding section is typically 50-80°C below the melt temperature, while the homogenization section is closer to the upper melt temperature limit). For example, the melt temperature of PA66 is 250-260°C, so the barrel feeding section temperature can be set at 200-220°C, and the homogenization section at 250-260°C. Next, the mold structure must be analyzed. Based on the number of cavities, runner length, and gate size, preliminary injection pressure values (50-80 bar for small, single-cavity parts, 100-150 bar for large, multi-cavity parts) and holding pressure (typically 50-80% of the injection pressure) must be determined. Furthermore, the cooling time should be calculated based on the part’s wall thickness (generally 1.5-3 times the wall thickness, e.g., 5-9 seconds for a 3mm wall-thick part). After the basic parameters are determined, they need to be recorded in the process card as a basis for trial mold adjustments.
Detailed temperature parameter settings require layered control, encompassing barrel, nozzle, and mold temperatures, to ensure a balance between melt fluidity and cooling efficiency. The barrel temperature should be set using a “gradient” principle, gradually increasing from the feeding section to the homogenization section (with a temperature difference of 10-20°C per section) to prevent premature melting of the raw material in the feeding section or degradation in the homogenization section. For example, barrel temperatures for PC are set as follows: 240-250°C in the feeding section, 260-270°C in the middle section, and 280-290°C in the homogenization section. The nozzle temperature should be 5-10°C higher than the homogenization section to prevent melt solidification and clogging the gate. However, for heat-sensitive plastics (such as PVC), the temperature should be lowered by 5-10°C to reduce the risk of degradation. The mold temperature should be adjusted based on the crystallinity of the raw material. Crystalline plastics (such as PE) require a higher mold temperature (40-60°C) to promote crystallization, while amorphous plastics (such as PS) can be set to 20-40°C. After the temperature is set, a stability test is required. After the machine is preheated for 30 minutes, an infrared thermometer is used to detect the actual temperature of each section. The deviation must be ≤±2℃, otherwise the temperature control system needs to be calibrated.
Matching pressure and speed parameters is central to process design. “Segmented control” is required to achieve a smooth transition between filling, holding, and demolding. Injection pressure is divided into a primary shot (filling the cavity 70%-80%) and a secondary shot (filling the cavity completely). The primary shot operates at a lower pressure (50%-70% of the set value) and a slower speed (30%-50% of the maximum speed) to prevent melt splashing and air entrapment. The secondary shot increases pressure to the set value and speeds up (60%-80% of the maximum speed) to ensure rapid filling. Holding pressure is determined based on the shrinkage characteristics of the part. For thick-walled parts (≥5mm), the holding pressure is 70%-80% of the injection pressure, with a holding time of 10-20 seconds. For thin-walled parts (≤3mm), the holding pressure can be reduced to 50%-60% of the injection pressure, with a holding time of 5-10 seconds. For example, for a PP lunch box (2mm wall thickness), the injection pressure was set to 80 bar, the primary injection speed was 40 mm/s, the secondary injection speed was 70 mm/s, the holding pressure was 50 bar, and the holding time was 6 seconds, effectively avoiding sink marks and flash. Pressure and speed adjustments must be made simultaneously, with each adjustment not exceeding 10% to prevent excessive parameter fluctuations.
Timing parameters must be set to cover the entire molding cycle, including injection time, hold time, cooling time, and demolding time, to ensure smooth transitions between each step. Injection time is calculated based on cavity volume and injection speed (time = volume ÷ flow rate) and is generally controlled between 2 and 10 seconds. Too long can lead to excessive melt shearing, while too short can result in insufficient filling. The hold time must be sufficient to compensate for shrinkage but not exceed the melt solidification time (this can be determined gravimetrically: the optimal value is when the part weight stops increasing with increasing hold time). Cooling time should ensure that the part is fully solidified and can be demolded without deformation. Crystalline plastics typically cool down 50% to 100% longer than amorphous plastics. Demolding time must be coordinated with the robot’s movements, generally between 1 and 3 seconds. Excessive times can affect production efficiency. For example, an ABS toy (wall thickness 1.5mm) achieves stable production with a 99.5% pass rate with an injection time of 3 seconds, a hold time of 5 seconds, a cooling time of 8 seconds, and a demolding time of 2 seconds, for a total cycle time of 18 seconds. After the time parameters are set, cycle optimization is required. Under the premise of ensuring quality, unnecessary time can be gradually shortened (such as reducing the cooling time by 0.5 seconds each time) to improve production efficiency.
Parameter verification, consolidation, and optimization are the final steps in process design. Trial mold data must be used to assess parameter rationality and create standardized documentation. During trial molds, 10-20 molds should be produced continuously to inspect the product’s dimensions, weight, appearance, and mechanical properties. Once all indicators meet the requirements, the parameters are finalized. If defects occur, targeted adjustments are made: for material shortages, increase injection pressure or speed; for flash, reduce pressure or mold temperature; for sink marks, extend the holding time or increase the holding pressure. Once parameters are finalized, they must be incorporated into standard operating procedures (SOPs), clearly defining the permissible fluctuation range for each parameter (e.g., ±3°C for temperature, ±5 bar for pressure). Parameters should be regularly reviewed and optimized, re-verifying their suitability every 1,000 molds or when changing raw material batches, and making minor adjustments as necessary. For example, through quarterly parameter optimization, one production line reduced its defect rate from 1.2% to 0.6%, while also reducing energy consumption by 8%.
