Heat shrink tubing rarely fails in service when it's the right product, correctly installed. When it does fail, the cause is almost always one of four installation errors. Recognizing the symptom tells you the cause — and the fix.
Failure #1: Bubbles and Delamination
Symptom: Raised blisters on the tubing surface, or layers peeling apart at the ends.
Cause: Heating too fast or in the wrong sequence. When the middle of the tube shrinks before the ends, air is trapped between the shrinking section and the object. The trapped air expands with continued heating and pushes the wall outward, creating a bubble.
Prevention: Always heat from one end to the other in a continuous sweeping motion. Never start in the middle. For long installations (>200 mm), use two heat guns working from opposite ends toward the middle.
Failure #2: Cracking or Splitting
Symptom: Longitudinal cracks along the tube, or splitting at bend points.
Cause: Overheating (the polymer degrades and loses elasticity) or low-temperature installation (the tubing is stiff and brittle when cold). In adhesive-lined tubing, cracking can also occur if the adhesive layer doesn't fully melt and distribute.
Prevention: Control the heat source temperature. Polyolefin needs 90-120°C — a propane torch on full is 1200°C. Keep the flame or hot air moving and maintain 50-100 mm distance. For cold-weather installations, pre-warm the tubing and the cable to above 10°C before shrinking.
Failure #3: Loose Fit or Sliding
Symptom: The tubing rotates or slides along the cable after installation.
Cause: The tubing's recovered ID is larger than the object's diameter — there's simply not enough shrinkage to create a grip. This happens when the size is selected based on the AWG wire gauge rather than the measured outer diameter, or when the tubing isn't heated to full recovery.
Prevention: Measure the actual object with calipers, not the nominal wire size. Select tubing where the recovered ID is at most 80% of the measured diameter. During installation, apply heat until the tubing stops shrinking — don't stop as soon as it "looks tight."
Failure #4: Moisture Ingress
Symptom: Corrosion or green discoloration at the cable end, water dripping from the tubing, or intermittent electrical faults after rain.
Cause: Using single-wall tubing where a waterproof seal is required. Single-wall tubing grips the cable mechanically but doesn't seal the interface. Moisture wicks along the microscopic gap between the tubing and cable surface.
Prevention: Use dual-wall adhesive-lined tubing (SRS or DWT) for any connection exposed to moisture — outdoor, underground, marine, or washdown environments. Confirm the adhesive has flowed by checking for a small bead at each end.
Quick Reference Table
| Failure | Symptom | Root Cause | Fix |
|---|---|---|---|
| Bubbles | Blisters, delamination | Trapped air from heating middle first | Heat end-to-end |
| Cracking | Splits, brittle surface | Overheat or cold install | Control temp, pre-warm |
| Loose fit | Sliding, rotation | Wrong size or incomplete shrink | Measure, 80% rule, full heat |
| Moisture | Corrosion, water entry | Single wall in wet environment | Use dual wall adhesive |
FAQ
My tubing looks shrunk but still rotates — is it defective?
Probably not. It's likely not fully recovered. Continue heating — the tubing will continue to shrink until it reaches its recovered ID. If it still rotates after full heating, the size is too large for your application. Check the recovered ID spec against your measured diameter.
Can I reheat tubing that has bubbles?
Sometimes. Reheat the bubbled area gently with a heat gun (not a torch) and smooth the tubing against the object with a gloved hand. If the bubble has already caused delamination or wall thinning, replace the piece.
How do I know if I've applied enough heat to dual-wall tubing?
Look for the adhesive bead at each end. When the inner adhesive layer reaches its flow point (approximately 110°C), it melts and flows. Excess adhesive squeezes out at the tubing ends, forming a visible ring. No bead means the adhesive hasn't fully activated.
Written by Long, Sales Engineer at ZZ Heat Shrink. For product specifications, see our product range. For installation guidance on specific products, contact our engineers.