In the design of plastic composite mold products, optimizing the runner system is a core aspect of improving filling uniformity. Its design requires comprehensive consideration of the synergistic effects of material properties, product structure, and molding processes. Plastic composite molds typically involve co-injection or stacking of multiple materials. Different materials exhibit significant differences in rheological properties. An unreasonable runner design can easily lead to inconsistent melt front advance speeds, resulting in problems such as incomplete filling, obvious weld lines, or product deformation. Therefore, the runner layout must aim to balance melt flow resistance. By rationally allocating the length, cross-sectional shape, and dimensions of each branch runner, the synchronous filling of different materials within the mold cavity can be ensured.
The cross-sectional shape of the runner directly affects the melt flow state. While circular runners offer the lowest flow resistance, they have higher processing costs. Trapezoidal or semi-circular runners are widely used due to their ease of processing, but flow uniformity needs to be optimized by adjusting the runner depth-to-width ratio. For composite molds, if a cold runner system is used, differentiated runner cross-sections must be designed to account for the viscosity differences of different materials. For example, high-viscosity materials require larger channel cross-sectional areas to reduce flow resistance, while low-viscosity materials can have their cross-sections appropriately reduced to avoid material degradation caused by excessive shearing. Furthermore, the surface roughness of the channel needs to be controlled at a low level to reduce friction between the melt and the channel wall, preventing uneven filling caused by excessive local resistance.
The branching structure of the channel is a key factor affecting filling uniformity. In multi-cavity molds, the channel needs a reasonable branching design to achieve balanced melt distribution. Traditional single-level branched channels tend to cause lag filling in the end cavities, while using natural balance channels or rheological balance channels can effectively improve this problem. Natural balance channels ensure consistent melt arrival times in each cavity by accurately calculating the length and cross-sectional area of each branch channel; rheological balance channels compensate for differences in flow resistance along different paths by adjusting the channel cross-sectional shape. For composite molds, if multiple layers of material are involved in co-injection, a buffer section needs to be set between the main runner and branch channels to coordinate the flow front velocities of different materials and avoid interlayer separation or uneven mixing due to velocity differences.
The gate design serves as the bridge connecting the runner system and the mold cavity; its location, number, and size play a decisive role in filling uniformity. The gate location should avoid weak areas of the product, such as abrupt changes in wall thickness or stress concentration points, to prevent product deformation due to melt impact during filling. For composite molds, if a sequential co-injection process is used, the injection volume and propulsion speed of different materials must be controlled by adjusting the gate size to achieve precise control of the interlayer interface. For example, in laminated molding, the gate size of the material injected first should be slightly larger than that of the material injected later to ensure that the first injected material forms a stable support layer and prevents the later injected material from penetrating or mixing.
Temperature control of the runner is a crucial means of optimizing filling uniformity. Different materials have significantly different sensitivities to temperature. Uneven temperature distribution in the runner can easily lead to fluctuations in melt viscosity, which in turn causes differences in filling speed. For hot runner systems, precise temperature control of each branch runner is required through independent heating units to ensure that the melt maintains a stable flow state within the runner. For cold runner systems, optimizing the mold cooling water channel layout is crucial to reduce the temperature gradient between the runner and the mold cavity. This prevents premature cooling of the runner, which could lead to melt front solidification and hinder subsequent filling.
Simulation analysis plays an irreplaceable role in runner optimization. Computer-aided engineering (CAE) software allows for visual simulation of the melt flow within the runner, predicting potential issues such as velocity differences and uneven pressure distribution during filling. Based on simulation results, iterative optimization of the runner layout, cross-sectional shape, and gate parameters can be performed, reducing the number of trial runs and lowering development costs. For example, adjusting the runner branch angle or increasing the buffer section length can significantly improve the uniformity of melt filling within the mold cavity.
Adjusting process parameters in actual production is a supplementary method to runner optimization. Even with a well-designed runner system, improper setting of molding process parameters (such as injection speed, holding pressure, and mold temperature) can still lead to uneven filling. Therefore, it is necessary to determine the optimal process window through experimentation, taking into account the characteristics of the runner design. For example, for high-viscosity materials, the injection speed can be appropriately increased to enhance melt flowability; for heat-sensitive materials, the mold temperature needs to be reduced to decrease the risk of degradation. Through the synergistic optimization of runner design and process parameters, high-quality molding of plastic composite mold products can ultimately be achieved.
