The injection barrel temperature is a crucial process parameter in the injection molding process, directly affecting the fluidity of the plastic melt, the plasticizing effect, and the quality of the final product. The barrel temperature setting requires precise control based on the type and properties of the plastic, as well as the structural characteristics of the product. Excessively high temperatures can cause plastic decomposition, discoloration, and even the formation of bubbles. Excessively low temperatures can lead to inadequate plasticization and poor melt fluidity, resulting in insufficient mold filling or surface defects such as cold spots and silver streaks. Therefore, properly setting the barrel temperature is crucial for smooth injection molding production.
The barrel temperature is typically set according to the principle of “staged, gradual temperature increase.” From the feed port to the nozzle, the temperature gradually increases to meet the plasticizing requirements of the plastic at different stages within the barrel. The feed section is typically set at a lower temperature, primarily to prevent premature melting of the plastic and the resulting “bridging” phenomenon, ensuring smooth entry into the compression section. This section’s temperature is typically set 10-30°C below the plastic’s glass transition temperature or melting point. For example, when processing polypropylene (PP), the feed section temperature can be set between 150-170°C, while when processing high-temperature-resistant plastics like polycarbonate (PC), the feed section temperature should be set between 220-240°C. The compression section temperature needs to be further increased to ensure thorough melting and mixing of the plastic under the shear and compression of the screw. It is typically 20-40°C higher than the feed section. The metering section temperature must reach the plastic’s optimal flow temperature to ensure good melt flow and stability for mold filling. This section’s temperature is typically 10-20°C higher than the compression section.
Different types of plastics have significantly different requirements for barrel temperature, which is determined by their molecular structure and thermal properties. Crystalline plastics such as polyethylene (PE) and polypropylene (PP) have a clear melting point. The barrel temperature must be set above this melting point to ensure complete melting, but not above the decomposition temperature. For example, the barrel temperature for PE is typically controlled between 180 and 230°C, while that for PP is between 200 and 260°C. Amorphous plastics such as polystyrene (PS) and polyvinyl chloride (PVC) do not have a fixed melting point, but rather a relatively wide softening temperature range. The barrel temperature must be set within this range to achieve proper fluidity. For example, the barrel temperature for PS is generally between 170 and 210°C, while PVC, due to its poor thermal stability, must be strictly controlled between 160 and 190°C to prevent decomposition and the production of toxic gases. For some engineering plastics such as polyoxymethylene (POM) and polyamide (PA), the setting of the barrel temperature also needs to take into account their hygroscopicity. If necessary, they need to be dried before processing and the barrel temperature should be appropriately increased to compensate for the effect of moisture on plasticization.
Barrel temperature adjustment also needs to consider the structural characteristics of the product. Thin-walled, complex-shaped products require higher barrel temperatures to improve melt fluidity and ensure that the material fills every corner of the mold cavity. For example, when producing thin-walled plastic parts with a thickness of 0.5-1mm, the barrel temperature is typically 10-20°C higher than for products of conventional thickness. For thicker-walled products, the barrel temperature can be lowered to reduce cooling time, improve production efficiency, and avoid defects such as shrinkage cavities and bubbles within the product. Furthermore, the type and size of the gate also influence the barrel temperature setting. Small gates require higher melt fluidity, so the barrel temperature needs to be increased accordingly. Large gates, however, require lower melt fluidity and can be cooled.
In the actual production process, monitoring and adjusting the barrel temperature is an important measure to ensure production stability. Operators need to regularly check the actual temperature of each section of the barrel to ensure that it is consistent with the set value. The deviation should generally not exceed ±5°C. If abnormal temperatures are found, the heating coil, thermocouple and other temperature control components should be checked in time to ensure that they are working properly. At the same time, the barrel temperature needs to be dynamically adjusted according to the quality of the product. For example, if the surface of the product appears dull and matte, it may be due to the barrel temperature being too low and insufficient plasticization. In this case, the barrel temperature should be appropriately increased. If the product shows discoloration or burnt spots, it may be due to the barrel temperature being too high and the plastic decomposition. In this case, the barrel temperature needs to be lowered. By continuously optimizing the barrel temperature parameters, product quality and production efficiency can be effectively improved, and production costs can be reduced.
