Silicone defects fall into a handful of families — flash, short shots, air traps, flow lines, warpage, and discoloration — and each has a specific cause and fix. Flash (excess material at the parting line) is the most common, because silicone flows like water at molding temperature. Most defects are not a mystery: they come from tooling, process parameters, or material — and a good factory can tell you which in minutes from a photo. This guide lists every common defect so you can spot them in samples, understand the cause, and judge whether your supplier actually knows how to fix them.
The defect families at a glance
| Defect | What it looks like | Primary cause |
|---|---|---|
| Flash | Thin excess material at the parting line | Low clamp force, worn tooling, over-packing |
| Short shot | Part not fully filled, missing edges | Insufficient material/pressure, trapped air |
| Air traps / bubbles | Voids or bubbles on the surface or inside | Poor venting, fast injection, no de-gassing |
| Flow lines | Visible swirls where material fronts meet | Slow fill, low temp, poor pigment dispersion |
| Warpage | Part bent or distorted after molding | Uneven cooling or wall thickness |
| Discoloration | Yellowing, browning, or color streaks | Overheating, contamination, poor pigment |
| Burn marks | Dark spots or scorched areas | Trapped air compressing and igniting |
| Sink marks | Dips or dimples on thick sections | Thick walls shrinking unevenly |
Flash: the most common — and most controllable
Flash is a thin web of excess silicone that squeezes out at the parting line. Silicone's low viscosity means it flashes more readily than rigid plastics.
- Cause: insufficient clamp force, worn or damaged tooling, or over-packing the mold
- Fix: proper clamp pressure, well-maintained shut-off surfaces, correct shot size
- Why it matters: flash requires deflashing (an extra step) and, if severe, indicates worn tooling that needs repair
For a deep dive, see How to Reduce Flash in Silicone Molding.
Short shots: the part that isn't full
A short shot is an incomplete part — the material didn't reach every corner of the cavity.
- Cause: insufficient material, low injection pressure, trapped air blocking flow, or a cold mold
- LSR-specific: an improper mix ratio of the two-part liquid silicone can also cause it
- Fix: increase shot size and pressure, improve venting, warm the mold, verify the LSR mix ratio
Air traps and bubbles: the invisible flaw
Air traps show up as surface bubbles or internal voids — sometimes invisible until the part is cut or stressed.
- Cause: poor venting, injection speed too high, or air trapped in the material
- Fix: add or enlarge vents, slow the injection, use vacuum de-gassing, or add breathing cycles in compression molding
- Why it matters: internal voids are a structural weakness and a common cause of field failure in seals and gaskets
Flow lines and knit lines: cosmetic but telling
Flow lines are visible swirls where two material fronts meet (knit lines are where they weld together).
- Cause: slow fill, low mold temperature, or pigment that doesn't disperse evenly
- Fix: adjust injection speed, raise mold temperature, improve pigment dispersion
- Why it matters: usually cosmetic, but a knit line can also be a weak spot under load
Warpage and distortion: the part that won't sit flat
Warpage is when a part bends or twists out of shape after it leaves the mold.
- Cause: uneven cooling, uneven wall thickness, or demolding while the part is still too hot
- Fix: even out wall thickness, control demolding temperature, use uniform cooling
- Why it matters: a warped seal won't seal; a warped lid won't close
Discoloration and burn marks: the heat-related defects
- Discoloration / yellowing: overheating, contamination, or poor pigment. Use a heat-stabilized compound and control cure temperature.
- Burn marks: trapped air that compresses and scorches the material — a venting and injection-speed problem.
Sink marks: dips on thick sections
Sink marks are dimples that appear where a thick wall shrinks more than the surrounding area.
- Cause: thick walls that shrink unevenly as they cool
- Fix: core out thick sections, or keep wall thickness uniform (a DFM issue)
Is it the design's fault or the factory's fault?
When samples come back with defects, the first question is who owns the fix. Use this quick test:
| Situation | Likely owner |
|---|---|
| Impossible undercuts, uneven walls, poor venting geometry | Design (needs DFM correction) |
| Flash, short shots, contamination on a sound design | Factory (process or tooling) |
| Discoloration from overheating | Factory (process control) |
| Sink marks from thick walls | Design (wall thickness) |
A good factory doesn't just trim flash and ship — it identifies the root cause and fixes it. Ask for a first-article inspection (FAI) report and a corrective action plan when defects appear. A supplier that can't explain the cause of a defect is guessing, and guessing leads to rejects at your door.
FAQ
Q: What is the most common silicone molding defect?
A: Flash — excess material that squeezes out at the parting line. Silicone flows like water at molding temperature, so it flashes more easily than rigid plastics. It is also the most controllable with proper tooling and clamp force.
Q: What causes a short shot in silicone molding?
A: A short shot — where the mold isn't fully filled — is usually caused by insufficient material, low injection pressure, trapped air, or a cold mold. For LSR, it can also be an improper mix ratio of the two-part material.
Q: How do you fix air traps (bubbles) in silicone?
A: Improve venting, reduce injection speed, or add vacuum de-gassing to the mold. For compression molding, proper pre-forming and breathing cycles help release trapped air.
Q: What causes silicone discoloration or yellowing?
A: Overheating (scorching), contamination in the compound, or poor pigment quality. Yellowing over time is usually UV or heat exposure. Use a stabilized compound and control cure temperature.
Q: Why is my silicone part warped or distorted?
A: Warpage comes from uneven cooling, uneven wall thickness, or removing the part from the mold too hot. Even wall thickness and controlled demolding temperature reduce it.
Q: Can silicone parts have flow lines?
A: Yes. Flow lines appear as visible swirls where material fronts meet. They're caused by slow fill, low mold temperature, or uneven pigment dispersion. They're cosmetic and often fixed by adjusting injection speed or pigment dispersion.
Q: How do I know if a defect is the factory's fault or the design's?
A: Run a first-article inspection and a DFM review. Design problems (impossible undercuts, uneven walls, poor venting) are design issues. Flash, short shots, and contamination on an otherwise sound design are usually process or tooling issues the factory should own.
See our Quality Documents page to learn what inspection reports a real factory provides, or send us a photo of your defect for a root-cause analysis.