In the domain of precision injection molding, the parting line is far more than a simple separation plane—it is a fundamental architectural element that determines the structural integrity, manufacturing feasibility, and aesthetic quality of both the injection mold and the final plastic component. SANDO, we engineer molds based on a foundation of established engineering principles. For parting line design, these principles are systematic rules that guide optimal decision-making. Here, we present the ten key principles that govern professional parting line design.
Principle 1: Maximize Parting Surface Simplicity
The parting surface should be as flat and geometrically simple as possible. A planar or single-curved surface significantly reduces mold manufacturing complexity, lowers machining costs, and facilitates easier grinding, fitting, and mold polishing. Complex 3D parting surfaces increase the risk of flash, complicate venting, and demand higher precision in mold assembly.
Principle 2: Ensure Unobstructed Part Ejection
The parting line must be positioned to allow the molded part to remain on the moving side (core) and be ejected without obstruction. This principle dictates analyzing part geometry to identify features that would “lock” the part in the cavity side. The design should leverage draft angles and strategic undercuts to ensure the part reliably releases from the core side upon mold opening.
Principle 3: Minimize or Eliminate External Side Actions
Whenever feasible, position the parting line to avoid or minimize the need for external side-action mechanisms (slides). While slides and lifters are essential for complex undercuts, they increase mold complexity, cost, maintenance needs, and cycle time. Clever parting line placement can often transform an external undercut into an internal one that is resolved by the core itself.
Principle 4: Optimize for Draft Angle Application
The parting line directly defines which part surfaces require draft. It should be located such that necessary draft angles can be applied to non-cosmetic or functional surfaces, preserving the intended aesthetic and dimensional integrity of critical visible areas.
Principle 5: Locate on Non-Cosmetic Edges
Place the parting line on an edge, corner, or other natural break in the part’s geometry where the witness line will be least visible or easiest to remove. The flash line is inevitable; strategic placement minimizes its visual and functional impact, reducing secondary finishing operations.