Design Principles of Parting Surfaces in Molds - Longterm Manufacturing Solutions Limited

1. Overview

In plastic injection mold design—a core segment of Mold Manufacturing—and particularly for high-precision fields like automotive mold production, the parting surface is a critical element of mould DFM design (Design for Manufacturability). It refers to the separable contact interface between mold halves in a plastic injection mold, serving as the critical boundary for ejecting plastic components and solidified cold runner material. Ideally, the parting surface should be perpendicular to the injection molding machine’s mold opening direction. Rational selection of the parting surface is a prerequisite for intact plastic part molding in plastic injection mold applications, as it directly impacts the demolding process, mold structure complexity, and overall Mold Manufacturing costs. For demanding scenarios such as automotive mold production—where dimensional accuracy and long-term durability are non-negotiable—a well-designed parting surface not only meets all product performance requirements but also simplifies mold construction, reduces processing costs in Mold Manufacturing, enhances mold structural strength, and extends service life, aligning with the core objectives of mould DFM design.

2. Core Design Principles

Directional Requirement: In plastic injection mold design, the parting surface shall not be parallel to the mold opening direction. Its inclination angle relative to the mold opening direction must be no less than 3 degrees—this is especially critical for automotive mold projects, where tight tolerances demand reliable demolding and structural stability to avoid part defects. This principle is a foundational consideration in mould DFM design to ensure manufacturability.

Surface Extension & Transition: Priority should be given to extending the parting surface on non-textured areas in plastic injection mold design. For regions where direct extension is infeasible (common in complex automotive mold geometries), adhere to Mold Manufacturing best practices:

Use a grid surface and ensure seamless connection with swept surfaces to simplify machining;

Alternatively, first extend 10~15mm for sealing purposes, then perform stretching. Add an R-angle transition at the junction of the stretched surface and the extended surface to eliminate stress concentrations—an essential detail in mould DFM design for long-cycle plastic injection mold durability.

Textured Surface Considerations: Parting surfaces on textured product surfaces (frequently encountered in automotive mold interiors and exteriors) should preferably be designed as butt joints (avoiding alignment with the product’s surface contour). If following the contour is unavoidable, clearly mark the parting line on the surface to guide Mold Manufacturing and inspection—a key requirement in plastic injection mold design for quality control.

Edge & Corner Optimization: Minimize sharp edges and corners (angles less than 80 degrees) on the parting surface. This facilitates precision machining in Mold Manufacturing, ensures accurate mold alignment during assembly—critical for automotive mold dimensional stability—and preserves mold structural integrity, reducing maintenance costs for plastic injection mold operations.

Coaxiality & Alignment Control: For plastic injection mold components requiring strict coaxiality (e.g., automotive fasteners, sensor housings in automotive mold), or features prone to misalignment, position them on the same side of the parting surface. This prevents dimensional deviations caused by mold parting misalignment, a non-negotiable requirement in mould DFM design for automotive applications.

Anti-Sticking Design: Evaluate the risk of the plastic part adhering to the cavity (front mold) in plastic injection mold scenarios. For automotive mold production—where high-volume runs demand consistent demolding—if sticking is likely, design side features (e.g., ribs, bosses) on the core (rear mold) to shift demolding resistance to the core side, ensuring smooth part ejection and aligning with Mold Manufacturing efficiency goals.

Mold Opening Direction Selection: When multiple mold opening directions are technically feasible for plastic injection mold design—especially for complex automotive mold geometries—conduct a comprehensive evaluation rooted in mould DFM design principles. Select the option that minimizes mold complexity, reduces Mold Manufacturing costs, and optimizes mold quality and maintainability, ensuring scalability for high-volume automotive production.