In hardware plastic mold accessories, the sprue bushing serves as the primary channel for the plastic melt to enter the mold cavity. Optimizing its structure is crucial for reducing flow resistance. The inlet structure must be addressed first. Conventional sprue bushing inlets often have right-angle or sharp-angle designs, which can cause turbulence when the melt enters. Optimization requires a smooth, curved or tapered shape. This allows the melt to flow along a gently curved surface after exiting the injection molding machine nozzle, avoiding vortices caused by sudden cross-sectional changes and thus reducing resistance during the initial flow phase. Furthermore, the inlet bushing must fit tightly and seamlessly with the injection molding machine nozzle. Gaps can not only cause melt leakage but also allow air to enter, creating air resistance. Therefore, precision machining is crucial to ensure the flatness and coaxiality of the mating surfaces, ensuring smooth flow of the melt into the sprue bushing.
The design of the ratio of the inner diameter and length of the sprue bushing for hardware plastic mold accessories is equally critical. An excessively large inner diameter can slow the melt flow within the channel, prematurely cooling the melt and increasing flow resistance. An excessively small inner diameter, due to the narrow cross-section, can subject the melt to excessive shear forces, also affecting flow efficiency. Optimization should consider factors such as the plastic material's fluidity and the product's wall thickness to determine an appropriate inner diameter range. Meanwhile, unnecessary channel length should be minimized. An overly long sprue bushing lengthens the melt's flow path and increases resistance along the way. Therefore, while meeting the overall mold structural requirements, the effective length of the sprue bushing should be minimized to allow the melt to reach the gate more quickly and reduce energy loss.
The surface quality of the channel's inner wall cannot be ignored in its impact on flow resistance. A rough inner wall creates significant friction with the melt, hindering melt flow. During optimization, precision grinding and polishing processes should be employed to improve the inner wall's finish, significantly reducing the friction coefficient between the melt and the wall during flow. At the same time, defects such as scratches and dents on the inner wall must be avoided. These defects not only increase frictional resistance but can also cause melt retention, impacting subsequent product quality. In some cases, special coatings can be applied to the inner wall to further reduce the friction coefficient and create a smoother environment for melt flow.
For hardware plastic mold accessories, the connection between the sprue bushing and the main runner and branch runner is also a key optimization point. If there are steps or misalignments at the connection, the melt will experience impact when turning or entering different channels, creating localized resistance. During optimization, it is important to ensure that the axes of the connection are coaxial and the cross-sectional transition is smooth. For example, the outlet of the sprue bushing and the inlet of the main runner should be designed with the same taper. This ensures that the melt flows seamlessly into the main runner after exiting the sprue bushing, avoiding sudden changes in flow direction due to structural abrupt changes. Furthermore, the wall thickness at the connection should be uniform to prevent sudden pressure drops during melt flow caused by localized thinness, further reducing flow resistance.
The taper design of the sprue bushing requires flexibility for plastic materials with different fluidities. For materials with poor flowability (such as PC and POM), the taper should be appropriately increased so that the channel cross-section gradually expands from the inlet to the outlet, reducing extrusion resistance during melt flow. For materials with better flowability (such as PE and PP), a smaller taper can be used to avoid flash caused by excessively wide channels resulting in excessive melt flow. The taper design must ensure a uniform inclination angle across the entire channel wall, without localized abrupt changes. This ensures stable melt pressure during flow and avoids additional resistance caused by pressure fluctuations.
Optimizing the cooling structure of the sprue bushing can also indirectly reduce flow resistance. If the channel's outer wall temperature is too high, the melt temperature will increase and viscosity will decrease during flow. While this may reduce resistance in the short term, it can easily lead to melt degradation in the long term. If the temperature is too low, the melt viscosity will increase, increasing flow resistance. Therefore, a well-designed cooling water channel should be designed outside the sprue bushing. By controlling the water temperature, the sprue bushing should be kept within an appropriate temperature range to ensure stable melt viscosity during flow, avoiding increased resistance due to excessively high viscosity and compromising product molding quality due to excessively low viscosity. Cooling water channels must be close to the outer wall of the channel without compromising structural strength, ensuring uniform temperature control and avoiding flow anomalies caused by localized overheating or undercooling.
The venting structure design of the sprue bushing for hardware plastic mold accessories is crucial. Air can be entrained during melt flow. If this air cannot be promptly expelled, it can form air blockages within the channel, increasing flow resistance and even leading to defects such as bubbles and material shortages. During optimization, a small venting groove should be installed at the sprue bushing outlet or where it connects to the main flow channel. The groove's depth and width should be determined based on the characteristics of the plastic material to ensure smooth air evacuation without causing melt leakage. The venting groove should be connected to the mold's overall exhaust system to form a complete venting channel. This allows gases generated during melt flow to be promptly discharged outside the mold, fundamentally eliminating the impact of air blockage on flow and further improving melt flow smoothness.
