Hot runner injection molding requirements for plastic raw materials
As a highly efficient and high-precision molding process, hot runner injection molding places specific and stringent requirements on the properties of plastic raw materials. These requirements directly impact the stability of the hot runner system, product quality, and production efficiency. Compared to traditional injection molding, hot runner processes require raw materials to remain molten within the flow channel for extended periods of time. Therefore, the raw material’s thermal stability, fluidity, cleanliness, and shrinkage must be compatible with the hot runner system to fully realize the advantages of this process. A thorough understanding of these requirements is fundamental to selecting appropriate raw materials and optimizing process parameters.
Thermal stability of plastic raw materials is a core requirement for hot runner injection molding. Because the residence time of raw materials in the hot runner (typically 3-10 minutes) is much longer than that of traditional injection molding (1-2 minutes), insufficient thermal stability can lead to thermal degradation, resulting in defects such as bubbles and burnt particles, and even gate blockage. Thermal stability can be assessed through thermogravimetric analysis (TGA). At operating temperature (e.g., 260-280°C for PA66), weight loss within 30 minutes must be ≤0.5%. For example, PET raw materials are susceptible to hydrolysis in hot runners due to high temperatures. Therefore, specialized grades that have been dried (moisture content ≤0.02%) and supplemented with antioxidants should be selected. Otherwise, viscosity loss and discoloration of the finished product may occur. For heat-sensitive plastics such as PVC, strict hot runner temperature control (≤190°C) and a short residence time are required. Furthermore, sufficient thermal stabilizers (such as calcium-zinc composite stabilizers) must be incorporated into the formulation to ensure compatibility with hot runner processes.
Raw material fluidity and stability are crucial for hot runner filling. Typically, the melt index (MI) is required to be within the range of 5-30 g/10 min (190°C/2.16 kg), with batch-to-batch MI variation ≤10% to ensure uniform filling across all cavities. Excessively high fluidity (MI > 30) can easily lead to flashing and drooling; too low fluidity (MI < 5) requires increasing the hot runner temperature or injection pressure, increasing the risk of degradation. For example, when using PC in a hot runner, the MI should be controlled between 10-20 g/10 min. If the MI is too low (e.g., 5 g/10 min), the hot runner temperature must be increased to 300-310°C, significantly increasing the risk of PC degradation. Furthermore, the material's melt viscosity must be moderately sensitive to shear rate. Excessive shear thinning can lead to excessive pressure loss in the late stages of filling, compromising pressure retention. Therefore, shear-sensitive materials like PP require multi-stage injection speed control to stabilize the filling process.
Raw material cleanliness requirements are far higher than those of traditional injection molding. Because the hot runner system’s narrow runners (3-8mm diameter) and tiny gates (0.5-3mm diameter) can easily cause blockages and production interruptions due to impurities in the raw materials (such as metal shavings, unmelted particles, and dust). Raw materials must undergo rigorous filtration (screen size ≥80 mesh) and magnetic separation to ensure that the number of impurities per 10kg is ≤5, and the maximum size is ≤0.3mm. For example, when producing optical-grade PMMA products, raw materials must be packaged in a dust-free environment to prevent contamination during transportation and loading. A 120-mesh filter should also be installed at the hot runner inlet to prevent even small impurities from entering the runner. For glass-fiber reinforced plastics (such as glass-fiber reinforced PA), the glass fibers must be uniform in length (3-5mm) to prevent excessive fiber entanglement and clogging the gate.
Raw material shrinkage and its uniformity affect product dimensional accuracy. Hot runner injection molding typically achieves 10%-20% lower shrinkage than conventional injection molding due to its superior pressure retention. However, this requires stable raw material shrinkage (batch-to-batch variation ≤0.2%); otherwise, process adjustments will be difficult to compensate. Crystalline plastics (such as PP and PE) have higher shrinkage (1.5%-3.0%) and are significantly affected by cooling rate. Therefore, it’s important to select low-shrinkage grades or add nucleating agents to keep shrinkage within 1.0%-2.0%. Furthermore, mold temperature uniformity should be improved (±1°C) to minimize local shrinkage variations. Amorphous plastics (such as PS and PC) have lower shrinkage (0.5%-1.5%) but are sensitive to hot runner temperature fluctuations. For example, PC’s shrinkage increases by approximately 0.1% for every 10°C increase in temperature. Therefore, hot runner temperature fluctuations must be controlled within ±2°C to ensure stable shrinkage.
of raw materials must be strictly controlled, especially for hygroscopic plastics (such as PA, PBT, and PC). Moisture can vaporize at high temperatures in the hot runner, causing defects such as bubbles and silver streaks, while also accelerating raw material degradation. Before hot runner injection molding, the moisture content of PA6 must be ≤0.03%, PC ≤0.02%, and PBT ≤0.01%. This is typically achieved by drying in a dehumidifying dryer (dew point ≤ -40°C) for 4-8 hours. For example, if PA66 raw materials are exposed to an 80% humidity environment for one hour, the moisture content can rise from 0.02% to 0.1%, directly leading to the appearance of numerous silver streaks on the surface of the part during hot runner molding, requiring re-drying to restore normal appearance. Furthermore, the volatile organic compound (VOC) content of the raw materials must be low to prevent volatilization in the hot runner, forming air plugs that affect melt flow. Therefore, for raw materials prone to generating volatile compounds, such as ABS, low-VOC grades should be selected, and hot runner venting should be enhanced.
