The design of the parting surface position is a key step in injection mold design that determines the quality of plastic parts and production efficiency. Its position selection must follow a series of general principles to balance the appearance of the plastic part, dimensional accuracy, demolding difficulty, and mold manufacturing costs. A reasonable parting surface position can simplify the mold structure, reduce the generation of flash, and ensure smooth demolding. Inappropriate position design may cause plastic part deformation, dimensional deviation, or even make it impossible to produce. For example, a mobile phone casing mold had a parting surface position that was too low, resulting in difficult-to-remove flash on the edge of the plastic part. After adjusting the position, the flash defect rate dropped from 15% to below 1%. The design of the parting surface position must comprehensively consider factors such as the plastic part geometry, appearance requirements, and molding process. Through systematic evaluation, the optimal solution is determined to lay the foundation for subsequent mold manufacturing and production.
Ensuring smooth demolding of the part is the primary principle in parting surface design. This ensures that the part remains on the movable mold side (or the intended demolding side) after mold opening and can be smoothly removed by the demolding mechanism. The parting surface should be located at the part’s largest contour, determined by the fluidity of the plastic melt. This location provides sufficient space for the part to separate from the cavity during mold opening. For example, the parting surface for circular parts is typically located at the circumference with the largest diameter, while for rectangular parts, it is located at the edge with the largest circumference. If the part has undercuts or lateral protrusions, the parting surface must be positioned to coordinate with the core pulling mechanism. For example, for parts with side holes, the parting surface should be positioned away from the side holes to ensure sufficient space for the core pulling mechanism to avoid interference with the part or other components during core pulling. Furthermore, the parting surface must be positioned so that the part adheres more strongly to the movable mold than to the fixed mold. This can be achieved by increasing the surface roughness of the movable mold cavity ( Ra 1.6μm) or by installing undercuts to prevent the part from adhering to the fixed mold and preventing demolding.
The design of parting surface location must protect the plastic part’s appearance quality and avoid leaving parting marks on visible or functional surfaces. For plastic parts with high appearance requirements (such as home appliance housings and automotive interiors), parting surfaces should be located in concealed areas or on non-appearance surfaces. For example, they can be located on the inside or bottom of the part, or on the mating surface with other components, so that the parting line does not affect the visual effect. If all surfaces of the plastic part are appearance surfaces, the parting surface should be located in an area with minimal curvature change, or parting marks can be eliminated through subsequent processing (such as grinding or polishing). For example, a smartwatch case has the parting surface located in the groove where the strap connects, which conceals the parting line without affecting the overall aesthetics. Furthermore, the parting surface should be located away from mating surfaces and areas with high precision requirements, such as gear tooth flanks and bearing bores, to prevent the parting surface clearance from affecting dimensional accuracy. The higher the dimensional tolerance requirements of the mating surface, the further away the parting surface should be from this area.
The design of parting surface locations should simplify mold structure and reduce manufacturing costs, avoiding increased mold processing difficulty and cost due to complex positioning. Planar parting surfaces are preferred, as they are simple to machine and cost-effective, making them suitable for parts with simple structures. For parts with complex curved surfaces, adjusting the parting surface position to convert the curved parting into a stepped or combined parting can significantly reduce processing difficulty. For example, a curved part originally designed with a full curved parting surface required a five-axis machining center, which was costly. After adjusting to a combined “flat + short curved” parting surface, it can be processed on a conventional milling machine, reducing mold costs by 30%. The parting surface location should minimize the use of core pulling mechanisms. If core pulling mechanisms are necessary, ensure that the core pulling direction is coordinated with the parting surface location to avoid the complexity of the mold structure caused by multi-directional core pulling. Furthermore, the parting surface location should facilitate mold assembly and maintenance. For example, placing the parting surface at the edge of the mold plate facilitates the installation of positioning components such as guide pins and guide bushings, and allows for quick removal of the cavity and core during mold maintenance.
The design of the parting surface position must adapt to the requirements of the molding process to ensure smooth melt filling and uniform pressure distribution. The parting surface position should make the melt flow path in the cavity as short as possible, reduce pressure loss and filling time, for example, the parting surface of a large plastic part should be set on the side close to the gate, so that the melt can diffuse from the vicinity of the parting surface to the surrounding area, avoiding insufficient filling caused by long-distance flow. For multi-cavity molds, the parting surface position must match the cavity layout to ensure balanced filling of each cavity. The unbalanced arrangement of cavities can compensate for the difference in runner length by adjusting the parting surface height. The parting surface position also needs to consider the exhaust effect. The parting surface should be set in the area where the melt is last filled, and the parting surface gap should be used to discharge the gas in the cavity. For example, the parting surface of a box-shaped plastic part is set at the edge of the box mouth, and the exhaust groove can effectively discharge the gas in the corner to reduce bubble defects. In addition, the position of the parting surface needs to be coordinated with the cooling system to avoid water leakage caused by the cooling water channel passing through the parting surface and ensure stable mold temperature control. For example, the parting surface can be set above or below the cooling water channel so that the water channel is completely located on the movable mold or fixed mold side.
