Mold parting surface Design Principles

2026-06-17

In a mold, separable contact surface that allows removal of product or cold runner condensate is called parting surface. parting surface should be oriented perpendicular to injection molding machine’s mold opening direction. Choosing right parting surface ensures a flawless molding of part, impacting not only part demolding but also mold structure and manufacturing costs. A well-designed parting surface not only meets all product performance requirements but also simplifies mold structure, reduces processing costs, improves mold strength, and increases mold life.

parting surface Design Principles

1 parting surface must not be parallel to mold opening direction, and its inclination angle with mold opening direction should not be less than 3 degrees.

2 Extended (non-grained) parting surfaces are preferred. Mesh surfaces should be used where extension is not possible, with swept surfaces blending in. Alternatively, extend 10-15mm before sealing and then stretching. A R-shaped glue transition should be applied at junction of stretched and extended surfaces.

3 Grained surfaces should ideally penetrate the designed parting surface; do not extend along product surface. If this is unavoidable, a line should be drawn along parting surface.

4 Avoid sharp corners and edges on parting surface to facilitate machining, positioning, and mold strength. Note that angles less than 80 degrees are considered sharp.

5 Place parts of part that require coaxiality or are prone to misalignment on same side of parting surface.

6 Check whether parting surface tends to stick to front mold. If so, it is recommended to place side faces on the back mold to prevent sticking.

7 When multiple mold opening directions are available, consider all factors to choose simplest mold structure to reduce costs and improve mold quality.

Design Example

parting surface Design Example

Design on the top cannot extend outward along product’s surface, as it would create sharp steel points regardless of which direction it extends.

Design on the bottom can completely extend outward along product’s surface.

For design on the top, if part’s original surface can’t be extended, it’s acceptable to pull surface directly outward. This is because rear mold already has an angle greater than 180 degrees, which can be processed by CNC machining, and no corner cleaning is required.

For design on the bottom, if part’s original surface can’t be extended, try pulling it outward as close to 180 degrees as possible. If you pull along red line, rear mold will have an angle less than 180 degrees, which cannot be processed by CNC machining, and EDM corner cleaning will be necessary, increasing costs.

When R-radius is half on each side of part, deviations in machining and assembly can cause misalignment, resulting in a step. Normally, upper mold is exterior surface, and step should not be visible from front. Therefore, R-radius of part on the back mold is generally reduced by about 0.1mm to ensure that step is limited to non-exterior surface. However, 3D files are generally difficult to modify, and some complex surfaces can take a long time to create, so margins are usually left to programmer.

It is recommended to specifically indicate programming department when issuing document to avoid programming omissions.