Water marks on 8 percent of cast parts traced to a leak inside the mould, and the pressure test dropped from 8 bar to 4 bar in twenty minutes. Dye penetrant found a crack at a cooling channel corner, and the die was being clamped 20 percent over its rating.
This senior case guide covers cooling channel pressure testing, crack detection and the clamping stress that opens channels, with the checks that prevent leaks.
Common Mistakes and How to Avoid Them
| Mistake | Why It Happens | Practical Fix |
|---|---|---|
| 1. Water marks blamed on spatter | Leak hidden inside | Pressure test the channels |
| 2. Clamp set over rating | Tensile stress opens cracks | Clamp per die calculation |
| 3. Sharp channel corners | Stress concentration cracks | Radius of 3 mm or more at corners |
| 4. Test skipped after repair | Repair unproven | Pressure test every repair |
| 5. Water quality ignored | Corrosion erodes channels | pH 7-8.5, low chloride |
| 6. Crack found, cause not | Same crack next insert | Trace stress and geometry |
| 7. Hardness too high | Brittle channel walls | 44-52 HRC window |
| 8. Water quality not logged | Corrosion repeats unseen | Log pH and chloride monthly |
| 9. Leak tolerated | Steam defects and downtime | Stop and repair at first sign |
Best Practices
- Pressure test cooling channels at 8-10 bar with a 30-minute hold
- Clamp every die to its calculated force
- Keep channel corner radius at 3 mm or more
- Control cooling water pH and chloride
- Test after every repair and quarterly in service
- Record clamp force and test results on the mould card
Implementation Roadmap
| Step | Frequency |
|---|---|
| Pressure test | Quarterly and after repairs |
| Check clamp force | Every set-up |
| Inspect channels | Per service, corner radius |
| Control water | pH weekly, chloride monthly |
| Log results | On the mould card |
Leak trace flow
- Test - 8-10 bar, 30 minute hold. (A drop of 4 bar in 20 minutes is a leak.)
- Locate - Dye penetrant, then section. (Cracks hide at sharp corners.)
- Cause - Clamp stress, radius, water. (Fixing the crack without the cause repeats it.)
- Prevent - Clamp per die, radius, water control. (Quarterly tests catch the next crack early.)
Over-clamping turns normal cooling pressure into crack growth. | A 3 mm corner radius removes the stress concentration.
Cooling channel integrity data
Reference values for mould cooling channel safety.
| Parameter | Reference | Why It Matters |
|---|---|---|
| Pressure test | 8-10 bar, 30 min hold | No drop allowed; 4 bar drop means an active leak |
| Clamp force | Projected area x 500-700 bar (zinc) | Over-clamping cracks channels |
| Corner radius | 3 mm or more at bends | Sharp corners concentrate stress |
| Die hardness | 44-52 HRC | Brittle steel cracks from the channel |
| Water pH | 7.0-8.5 | Acid corrodes from inside |
| Chloride | Below 100 ppm | Chloride drives pitting and cracking |
| Test cadence | Quarterly plus after repair | Catches cracks before steam defects |
Channel pressure test log
| Date | Test pressure | Hold result | Action |
|---|---|---|---|
| Before failure | 8 bar | Dropped to 4 bar | Repair, find cause |
| After repair | 9 bar | No drop, 30 min | Return to service |
| Quarterly | 8-10 bar | No drop | Log and continue |
Case 1 - leak traced to over-clamping and a sharp corner
Scenario. Water marks appeared on 8 percent of cast sliders, and steam pitting showed near one cavity. The plating line was blamed first, but the casting surface told the real story.
Action.
Result.
Case 2 - chloride water corroded a channel from inside
Scenario. A tropical plant found a slow leak in a zipper pull mould; the clamp and radius checked fine, and the crack surface showed corrosion rather than overload.
Action.
Result.
Frequently Asked Questions
How do I know a mould leaks?
Water marks and steam pitting on cast parts, plus a cooling pressure test that will not hold.
What pressure for the test?
8-10 bar with a 30-minute hold; any drop means a leak, and 4 bar in 20 minutes is active.
How do I find the crack?
Dye penetrant on the suspected area, then sectioning to confirm depth and direction.
Why does clamping crack channels?
Over-clamping adds tensile stress on top of cooling pressure; sharp corners concentrate it.
What clamp force is right?
Calculate from projected area and cavity pressure, then verify on the machine at every set-up.
Why a 3 mm corner radius?
Sharp corners concentrate stress; the radius spreads it along the channel wall.
Can cooling water cause this?
Yes - acid or chloride water corrodes channel walls until pressure opens them.
How often should I test?
Quarterly in service and after every repair; the test that caught this leak was overdue.
Can a crack be welded?
Only with proper procedure and stress relief; most plants replace the insert.
What Would You Like to Solve?
Tell us your leak pattern, clamp settings and cooling water quality, and we can work through the pressure test and clamping checks for your moulds.