Parting Line Design Guide — Where the Mold Separates

The parting line is the plane where the two halves of the injection mold meet. It's the most consequential design decision in any mold because it determines: where flash can appear, how the part orients in the machine, whether slides or lifters are needed, and how the gate and ejection system are arranged.

This guide covers parting line types, selection strategies, flash management, and what importers need to verify before the mold design is frozen.

Types of Parting Lines

Flat Parting Line

A single, planar surface separating A and B halves. Simplest to machine, easiest to maintain, lowest risk of flash. Best for parts where all features can be oriented in one draw direction. The flat parting line is the default choice — if your part design forces a non-flat parting line, that's a design constraint you should know about before quoting.

Stepped Parting Line

Multiple flat surfaces at different heights, connected by vertical step walls. Used when the part has features on different planes that must all be in the same draw direction. Stepped parting lines require careful alignment — each step transition is a potential flash point and requires precise machining (typically ±0.01mm on step depths).

Contoured / Curved Parting Line

Follows the part surface contours. Used for parts with complex shapes like automotive fascias, medical housings, or consumer products with aesthetic curves. Contoured parting lines are expensive to machine (5-axis CNC or extensive EDM), hard to polish perfectly flush, and prone to mismatch over time as the mold wears.

Parting Line on an Angled Plane

The mold halves separate at an angle relative to the machine platen. Rare but necessary for parts that cannot orient straight-pull on a flat plane. Requires special interlocks and angular guide pins — adds 15-25% to mold cost.

Parting Line Selection Criteria

1. Cosmetic requirements: The parting line leaves a visible witness on the part. For consumer-facing products, hide the line on an edge, bottom surface, or inside a recess.
2. Flash control: A flat parting line under uniform clamp pressure produces the least flash. Stepped and contoured lines concentrate clamp force on the highest points, leaving lower areas more prone to flash.
3. Gate placement: Submarine gates must be placed at the parting line. Three-plate molds allow center gating but add complexity. Your gate type constrains the parting line.
4. Ejection direction: All features must open in the core/cavity draw direction relative to the chosen parting line. Undercuts require slides or lifters, which may change the parting line decision.
5. Venting: The parting line is the primary vent path for air escaping the cavity. A stepped or contoured line provides natural vent paths along its features, while a flat line needs dedicated vent grooves cut into the land.

Flash Mechanism and Prevention

Flash occurs when molten plastic escapes the cavity through the parting line. The main causes:

• Insufficient clamp force: The injection pressure exceeds the force holding the mold closed. Solution: larger machine or higher clamp tonnage.
• Parting line mismatch: The A and B plates don't seat flush, often due to thermal expansion differences or warped plates. Solution: re-grind the parting surface.
• Steel crush at the parting line: Over time, the high-pressure contact zone at the parting line compresses, creating a gap. Solution: surface hardening or hard-facing the parting line land.
• Oversized vents: Deep vent grooves allow material to flow. Standard vent depth for most materials: 0.02-0.05mm. For thin-flow materials (PP, PA), as small as 0.01mm.

Parting Line Maintenance

Over 100,000-500,000 cycles, every parting line develops wear. The mating surfaces compress, the edges round off, and flash appears. Maintenance options include:

• Parting line re-grind: Remove 0.1-0.5mm from the parting surface and re-establish the land. Cost-effective but reduces cavity depth.
• Welding and re-machining: Build up worn areas with TIG welding and re-cut the parting line. Permanent fix but expensive.
• Hard inserts at flash-prone areas: Small replaceable steel inserts at high-wear parting line sections. Easy to swap and maintain.
• Preventive clamp force reduction: After initial break-in period (first 10,000 shots), reduce clamp force by 5-10% to minimize future wear while maintaining part quality.

Questions to Ask Your Mold Maker

1. Show me the parting line CAD overlay on the part — where exactly is the witness line?
2. Is the parting line flat, stepped, or contoured? What drives this choice?
3. What is the parting line land width? (Standard: 10-25mm. Too narrow = rapid wear; too wide = excessive clamp force requirements.)
4. How will you manage flash at the step transitions?
5. What interlocks or alignment features prevent parting line shift? (Taper locks, side interlocks, or straight guide pins only?)

Related Guides

• Mold Design Overview
• Mold Gate Design Guide
• Draft Angle Design Guide
• Mold Venting Guide