Solutions to the problem of gate air marks in PC injection molded parts
PC materials, due to their excellent mechanical properties, heat resistance, and transparency, are widely used in electronics, automotive, medical devices, and other fields. However, during the PC injection molding process, gate air marks are a common quality defect. These marks appear as cloud-like or silvery lines near the gate, which not only affects the product’s appearance but can also reduce its mechanical properties. The occurrence of gate air marks is related to multiple factors, including raw material properties, injection molding process, and mold design, and requires targeted measures to address them.
First and foremost, strict control of raw material drying is fundamental to preventing gate air marks. PC materials are highly hygroscopic. If the raw material contains moisture, it will undergo a hydrolysis reaction during the high-temperature melting process, generating gases. These gases cannot be discharged promptly during mold filling, resulting in air marks near the gate. Therefore, PC materials must be thoroughly dried before injection molding. Typically, a hot air circulation dryer is used to dry the raw material at 120-140°C for 4-6 hours, keeping the moisture content below 0.02%. During the drying process, ensure that the hopper is properly sealed to prevent the dried raw material from reabsorbing moisture. For recycled materials, the proportion of recycled materials should be strictly controlled and a separate, more thorough drying process is required, as recycled materials often contain more moisture and impurities, making them more susceptible to air mark defects.
Secondly, optimizing the injection molding process parameters is the key to solving the gate air marks. The melt temperature has a significant impact on the fluidity and gas discharge of the PC material. If the temperature is too low, the melt fluidity is poor, turbulence is easily generated during mold filling, and air is drawn in to form air marks; if the temperature is too high, the PC material is prone to thermal degradation and gas is generated, which will also cause air marks. Therefore, the barrel temperature needs to be set within a reasonable range, generally controlled at 260-300℃. The specific temperature needs to be adjusted according to the thickness of the product and the mold structure to ensure that the melt has good fluidity and does not degrade. At the same time, the mold temperature also needs to be appropriately increased, usually set at 80-120℃. A higher mold temperature can slow down the cooling rate of the melt, which is conducive to the discharge of gas and the fusion of the melt, reducing the generation of air marks.
Controlling injection speed and pressure is also crucial to the formation of gate air marks. If the injection speed is too fast, the melt velocity at the gate suddenly increases, easily causing jetting and entrained air, which can form air marks. If the injection speed is too slow, the melt takes a long time to fill the mold, leading to premature cooling near the gate, which prevents subsequent melt from smoothly merging and forms air marks. Therefore, a staged injection process should be adopted, with a lower injection speed near the gate to prevent melt jetting. The injection speed should be increased after the melt passes through the gate to ensure efficient filling. Sufficient injection pressure is required to ensure that the melt fills the mold cavity. However, excessive pressure can increase shear stress in the melt, leading to degradation of the PC material and gas generation. Generally, the injection pressure is controlled between 80-120 MPa and fine-tuned based on the molding conditions of the product. Extending the holding time can also help reduce air marks near the gate.
Optimizing mold structure also plays a crucial role in addressing PC material gate air streaks. Gate design is crucial. If the gate size is too small, the melt will flow rapidly and experience high shear forces, easily generating turbulence and gas, leading to air streaks. Improper gate placement, such as directly facing the cavity wall or core, can cause the melt to impact the cavity surface, forming vortices and entraining air. Therefore, the gate size should be appropriately increased. For thick-walled products, fan-shaped or flat-slit gates can be used to ensure smooth melt entry into the cavity. The gate should be located in thicker areas of the product or in areas with a smooth melt flow path to avoid direct melt impact on the cavity structure. Furthermore, the mold’s exhaust system must be unobstructed. Vents should be installed near the gate and in cavity corners, where gas is likely to accumulate. Vents should typically be 0.02-0.05mm deep and 5-10mm wide. This ensures that gases generated during filling are promptly discharged, minimizing the formation of air streaks.
Finally, strengthening production monitoring and raw material management can also effectively prevent gate air marks. During production, the stability of parameters such as barrel temperature, mold temperature, and injection pressure should be regularly checked to ensure that the equipment is in proper working order. Attention should also be paid to the purity of the raw materials to avoid contamination with other plastics or impurities. Because different plastics have different melting characteristics, mixing can easily cause bubbles and air marks in the melt. For products exhibiting air marks, the cause should be promptly analyzed and addressed by adjusting process parameters or improving mold structure to prevent further defects. Furthermore, regular cleaning of residues in the barrel and mold to prevent degraded material from contaminating new material is also an important measure to prevent gate air marks. Through comprehensive control and optimization, the problem of gate air marks in PC injection molded parts can be effectively resolved, improving product quality.
