Process principles and characteristics
The core principle of the injection molding process is to utilize the thermoplastic properties of plastics. After heating and melting solid plastic particles, the molten plastic is injected into the mold cavity at high pressure through the screw of the injection molding machine. After cooling and solidification, the plastic part is produced in the same shape as the cavity. The entire process can be divided into five stages: plasticization, injection, pressure holding, cooling, and demolding. The parameters of each stage are interrelated and influence each other, and together determine the quality of the plastic part. For example, the plasticization stage requires heating the plastic to a molten state. The temperature must be above the melting point of the plastic but below the decomposition temperature. For example, the plasticization temperature of polypropylene is usually between 200-230°C. This ensures that the plastic is completely melted while preventing molecular chain breakage caused by high temperature.
The injection molding process is characterized by its efficiency and flexibility. Its efficiency is reflected in its high degree of automation, enabling fully automated production from raw material delivery to mold release. Its short cycle time per mold makes it suitable for high-volume production. For example, the production cycle for small plastic gears can be controlled within 10-15 seconds, with 4-6 pieces produced per minute, and daily output reaching tens of thousands of pieces. Its flexibility lies in its ability to mold a wide variety of complex plastic parts, from microelectronic components to large automotive parts. Furthermore, the injection molding process allows for rapid switching between product types by simply changing molds, meeting the demands of high-variety, small-batch production.
The injection process parameters, primarily injection pressure, injection speed, and injection volume, significantly impact the filling quality of a plastic part. Injection pressure should be determined based on the plastic’s fluidity and the complexity of the part. Plastics with poor fluidity, such as polycarbonate, require higher injection pressures, typically between 80-120 MPa; whereas polyethylene, with its better fluidity, can use pressures between 50-80 MPa. The injection speed should be selected according to the principle of “fast filling, slow holding.” Fast filling reduces plastic cooling in the runners, ensuring complete cavity filling; slow holding prevents flashing. For example, for parts with thin ribs, a stepped injection speed is required: initially filling the main and branch runners at a high speed (80-100 mm/s), then switching to a lower speed (30-50 mm/s) after entering the ribs to prevent underfill or burning in the ribs.
The characteristic of the holding phase is to maintain a constant pressure to compensate for the volume loss in the mold cavity due to plastic cooling and shrinkage, thereby ensuring the dimensional accuracy and density of the plastic part. The holding pressure is typically 60%-80% of the injection pressure, and the holding time is determined by the wall thickness of the plastic part, generally 50%-70% of the cooling time. For example, for a part with a wall thickness of 5mm, the cooling time is approximately 25 seconds, and the holding time can be set to 15 seconds. This continuous pressure replenishment keeps the shrinkage of the plastic part within 0.5%. If the holding pressure is insufficient or the holding time is too short, the plastic part is prone to defects such as sink marks and dents. Excessive holding pressure increases the internal stress of the plastic part, causing warping after demolding.
The cooling stage is characterized by the rapid removal of heat from the molded part through the mold’s cooling system, allowing the molten plastic to solidify and take shape. Cooling time accounts for 50%-70% of the entire molding cycle and is a critical step influencing production efficiency. The cooling rate must match the crystallization characteristics of the plastic. For crystalline plastics such as polyethylene and nylon, the cooling rate must be controlled to achieve the appropriate degree of crystallinity. For non-crystalline plastics such as polyvinyl chloride and polystyrene, rapid cooling is required to improve production efficiency. For example, controlling the cooling rate of polypropylene parts at 5-10°C/s can achieve a crystallinity of 60%-70%, ensuring the rigidity and heat resistance of the parts. Polycarbonate parts, on the other hand, require a cooling rate of 15-20°C/s to avoid surface fogging caused by slow cooling. Furthermore, the uniformity of the cooling system is crucial to ensuring the quality of plastic parts. A cooling rate deviation of more than 10% in any part can result in dimensional instability or cosmetic defects in the parts.
