Parting Line Guide
for Silicone & Rubber Molding
The parting line determines where flash appears. Its placement can make or break part quality — especially for sealing surfaces and cosmetic faces.
What Is a Parting Line?
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In compression and injection molding, the mold consists of at least two halves. The interface between them — the parting line (PL) — is where a small amount of excess material (flash) escapes during molding. After curing, flash must be removed by manual trimming, tumbling, or cryogenic deflashing. Flash on a sealing surface = leaks. Flash on a cosmetic surface = rejected parts. Communicating the parting line explicitly on your drawing is one of the highest-value DFM actions you can take. |
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Parting Line Placement Rules
| Rule | Why | Recommendation |
|---|---|---|
| Never on sealing surfaces | Flash on a sealing surface causes leaks — even micro-flash that cannot be fully removed | Place PL on non-sealing face |
| Avoid cosmetic surfaces | Flash removal leaves visible witness marks even on transparent silicone | Place PL on hidden or non-visible face |
| Prefer flat parting lines | Complex 3D PL increases mold cost and flash risk | Use simple flat PL whenever geometry allows |
| Communicate explicitly | Mold makers will guess if you don’t specify — and they guess wrong half the time | Draw PL as a bold line on ALL relevant drawing views |
Flash Removal Methods
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After molding, flash must be removed. The image shows parts before and after flash removal, demonstrating the clean edges achieved through cryogenic deflashing. Method selection depends on volume and part complexity: manual trimming for low-volume simple shapes, cryogenic deflashing for medium-high volume complex shapes, precision jig trimming for high-value parts requiring tight flash control. |
Flash Removal Methods
Manual TrimmingCost: Low |
Cryogenic DeflashingCost: Medium |
Media TumblingCost: Low–Medium |
Precision Trimming (Jig)Cost: High |
Silicone-Specific Notes
- LSR injection: Flash is minimal — PL witness marks are very light. Often acceptable on non-critical surfaces.
- Compression molding: Flash is significant. PL placement is critical and must be communicated clearly on the drawing.
- Overmolding: The PL of the overmolding mold must not interfere with the rigid insert. Discuss insert geometry early.
- Multi-color / two-shot: PL is where colors meet. Place PL where a color transition is acceptable.
How to Specify Parting Line on Your Drawing
- Draw the parting line as a bold centerline on all relevant views
- Add a note: “PARTING LINE — FLASH REMOVAL REQUIRED ON THIS SIDE”
- Mark critical surfaces: “CRITICAL SURFACE — NO FLASH OR WITNESS MARK ACCEPTABLE”
- If a surface can tolerate flash: “NON-CRITICAL — LIGHT FLASH ACCEPTABLE”
Other DFM Guides:
| Wall Thickness → | Draft Angle → | Undercut → | Shrinkage & Tolerance → |
Not Sure Where to Place Your Parting Line?
Upload your drawing — we’ll mark the recommended PL placement before tooling begins.