Pay attention to several process conditions that affect injection molding quality
In injection molding, the setting and control of process conditions directly determine the quality of plastic parts. Even minor parameter fluctuations can lead to defects such as material shortages, flash, and warpage. Numerous process conditions influence injection molding quality, with injection pressure, melt temperature, mold temperature, holding time, and cooling time being key factors requiring particular attention. These conditions are interrelated and mutually influential. Only by optimally adjusting them can we consistently produce plastic parts that meet quality requirements, while also improving production efficiency and reducing costs.
Injection pressure is the key force driving the melt into the mold cavity. Its level should be determined based on the part’s structure, material flowability, and mold resistance. Insufficient injection pressure prevents the melt from overcoming flow resistance and fully filling the cavity. This can lead to problems such as material shortages and noticeable weld marks, especially in thin-walled and complex structures. Excessive injection pressure not only increases internal stress in the part, causing warping and cracking after demolding, but can also cause mold deformation and flash. For example, when molding precision electronic parts with 1mm wall thickness, if the injection pressure is below 80MPa, the melt will have difficulty filling the corners of the cavity. If it exceeds 120MPa, flash will easily form on the edges of the part and internal stress will increase significantly. Typically, injection pressure is set in stages: initially using a lower pressure to prevent melt impact on the cavity and vortexes, and then gradually increasing the pressure later to ensure complete cavity filling. This staged pressure control effectively balances filling efficiency and part quality.
Melt temperature directly affects the fluidity and plasticizing effect of plastics, and different materials have significantly different sensitivities to melt temperature. For crystalline plastics such as polyethylene and polypropylene, the melt temperature needs to be controlled at 50-80°C above their melting point to ensure good melt fluidity. However, for non-crystalline plastics such as polycarbonate and polyoxymethylene, excessively high melt temperatures can lead to material degradation, resulting in defects such as bubbles and discoloration. Uneven melt temperature can also cause inconsistent shrinkage of plastic parts. For example, when the melt temperature fluctuates by ±5°C, the dimensional deviation of polyamide plastic parts may exceed 0.1mm. In actual production, precise control of the melt temperature is required through segmented heating of the barrel. The temperature in the area near the feed port is lower to prevent premature melting of the raw materials, while the temperature in the area near the nozzle is highest to ensure sufficient plasticization of the melt. At the same time, the screw speed is adjusted to avoid localized excessive temperatures due to shear overheating.
Mold temperature has a significant impact on the cooling rate, crystallinity, and internal stress distribution of plastic parts. When the mold temperature is too low, the melt cools and solidifies rapidly on the surface of the cavity, forming a harder surface layer. When the internal melt shrinks, internal stress is easily generated, and the melt fluidity decreases, making filling difficult. If the mold temperature is too high, the cooling time will be prolonged, production efficiency will be reduced, and it may also cause deformation of the plastic part during demolding. For crystalline plastics, the mold temperature determines the morphology and distribution of the crystals. For example, polypropylene crystallizes quickly at a mold temperature of 50°C, but the crystal particles are coarse, and the toughness of the plastic part is poor. At a mold temperature of 80°C, the crystallization is uniform, and the strength and toughness of the plastic part are better. The uniformity of the mold temperature is also important. If the temperature difference between different areas of the cavity exceeds 5°C, the plastic part will warp due to inconsistent cooling rates. Therefore, the cooling water channel needs to be reasonably designed to ensure that the temperature of each part of the mold is balanced. If necessary, a temperature controller should be used to achieve precise temperature control.
The settings for holding and cooling times are directly related to the dimensional accuracy and stability of plastic parts. Insufficient holding time prevents the melt from adequately replenishing itself during cooling and shrinkage, leading to defects such as shrinkage cavities and sink marks. Excessive holding time increases internal stress and prolongs the molding cycle. Generally, the holding time should last until the gate solidifies. For a part with a 3mm wall thickness, the holding time is typically set at 5-8 seconds, while a part with a 10mm wall thickness requires 15-20 seconds. The cooling time must ensure that the part solidifies to sufficient strength to avoid deformation during demolding. Insufficiently cooled parts are prone to warping and may also cause subsequent dimensional changes due to incomplete internal solidification. Overcooling increases energy consumption and production time. The cooling time is typically 1.5-2 times the solidification time of the thickest part of the part. For example, for a polycarbonate part with a 5mm wall thickness, the cooling time should be controlled within 20-30 seconds to ensure the part’s shape is stable and improve production efficiency.
Stable process conditions are essential for ensuring consistent quality throughout mass production. Even with optimal parameter settings, if they aren’t maintained consistently, part quality can fluctuate. Equipment issues such as hydraulic system leaks, heater coil aging, and sensor degradation can all contribute to process fluctuations. For example, a hydraulic system pressure fluctuation of ±2 MPa can result in part weight deviations exceeding 1%. Therefore, equipment maintenance is essential during production, with regular calibration of pressure and temperature sensors to ensure accuracy within ±1%. Furthermore, a closed-loop control system monitors process parameters in real time and automatically adjusts them when deviations from set values occur. For example, if the melt temperature fluctuates by more than ±3°C, the system automatically adjusts the heating power to maintain parameter stability. Furthermore, operators must strictly adhere to process procedures to avoid uncontrolled parameters caused by manual adjustments. Through the coordinated efforts of people, machines, and processes, stable control of injection molding quality is achieved.
