Temperature differences across the injection mold cavity wall are a key factor affecting product quality. These temperature differences between different areas of the cavity wall can lead to uneven melt cooling rates, which in turn can cause defects such as warping, sink marks, and weld marks. The occurrence of temperature differences across the cavity wall is closely related to the cooling system design, heating method, mold material thermal conductivity, and product structure. Maintaining this temperature difference within a reasonable range (typically within ±2°C) is crucial for ensuring product dimensional accuracy and surface quality.
An irrational cooling system design is the primary cause of temperature differences in the cavity wall, such as uneven water channel layout, too small a diameter, or excessive distance from the cavity surface. When the distance between the cooling water channel and the cavity wall deviates by more than 5mm, localized high-temperature zones can easily form. For example, the cooling water channel of a certain box-shaped mold is 15mm from the cavity wall at the corner, while the flat area is only 8mm away. This causes the temperature in the corner to be 10°C higher than the flat area, resulting in noticeable sink marks in the corners of the product. A water channel diameter that is too small (e.g., less than 8mm) reduces the cooling medium flow rate and affects heat dissipation efficiency, while a larger diameter increases mold weight and processing costs. Therefore, a water channel diameter of 8-12mm should be selected based on the cavity size, and the water flow rate should be maintained at 1-2m/s to create turbulent flow and enhance heat transfer.
The thermal conductivity of mold materials significantly affects the temperature difference between the cavity and the wall. Common mold steels have widely varying thermal conductivity coefficients. For example, the thermal conductivity of 45 steel is 40 W/(m・K) , while that of H13 hot-work die steel is 25 W/(m・K) , and that of stainless steel is only 15 W/(m・K) . In large molds, using materials with high thermal conductivity can effectively reduce temperature differences. For example, a car instrument panel mold using 45 steel for the cavity experienced a maximum wall temperature difference of 8 °C. Switching to S50C steel with higher thermal conductivity reduced the temperature difference to 4 °C, and part warpage was reduced by 50% . For locally thick-walled areas, beryllium copper inserts (with a thermal conductivity of 200 W/(m・K) or higher) can be inserted to leverage their high thermal conductivity to equalize temperature. For example, inserting a beryllium copper insert in the camera boss of a mobile phone case reduced the temperature difference from 6°C to 2°C.
Improper heating system layout can also exacerbate cavity wall temperature differences, especially in molds using electric heating rods. Excessive spacing between the heating rods or uneven power distribution can lead to localized overheating. For example, a precision gear mold placed heating rods at the tooth root with a spacing of 30mm, while the spacing at the tooth tip was 50mm. This caused the tooth root temperature to be 7°C higher than the tooth tip, resulting in tooth thickness deviation after gear molding. To address this issue, mold flow analysis software is used to simulate the temperature field, optimize the position of the heating elements, ensure uniform power density (generally 10-15W/cm²), and fill the gap between the heating rods and the mold with thermal grease to enhance heat transfer efficiency.
The complexity of a product’s structure is an objective factor contributing to temperature differences between the mold cavity and the wall. The coexistence of thin and thick walls, as well as bosses and grooves, results in varying heat dissipation conditions. For example, in a plastic housing with reinforcing ribs, the rib thickness is 3mm and the main body is 1.5mm. The cavity wall temperature corresponding to the ribs is 5°C higher than that of the main body, resulting in sink marks. To address this situation, additional cooling wells or jet cooling are required in the thick-walled areas. For example, a 6mm diameter well can be installed below the ribs to directly cool the area with high-pressure water flow, keeping the temperature difference within 3°C. Furthermore, the product’s rounded corners and transition structures should be rationally designed to minimize sudden changes in wall thickness, thus reducing the potential for temperature differences at the source.
Controlling cavity wall temperature differences requires adjusting process parameters. If defects are discovered in a product due to temperature differences, the relative value of the temperature difference can be reduced by increasing the overall mold temperature (for example, from 60°C to 70°C), or by adjusting the injection speed and hold time to compensate for the effects of uneven cooling. For example, a product exhibited weld marks due to cavity wall temperature differences. By increasing the mold temperature by 10°C and increasing the hold time by 2 seconds, the weld mark strength was improved by 20%. Regular cleaning of scale and impurities in the cooling water channels is also crucial. Scale can reduce thermal conductivity by over 30%, leading to increased temperature differences. Therefore, the water channels should be cleaned with a citric acid solution every 5,000 molds to ensure cooling efficiency.
