Part 2 Heatshrink the does and don’ts
Part 2 (High) Medium Voltage Heat shrink (Gas Torches)
Heat‑shrink tubing is made from cross linked polyolefin (most commonly), although other polymers like PVC, elastomers, fluoropolymers, and specialty blends exist. The key is cross linking the polymer chains is to chemically bond, so they behave like a network rather than a loose tangle. Cross‑linking is the process of creating covalent bonds between adjacent polymer chains, turning them from long, independent strands into a three‑dimensional network.
In polyolefins (like polyethylene), the backbone is a carbon chain with hydrogen atoms attached. Cross‑linking works by extracting a hydrogen atom from the polymer chain which consists of hydrogen and carbon, creating a reactive carbon radical. This allows the two radicals on neighbouring chains to bond carbon to carbon, producing a stable cross liked which ties the chains together.
This transforms the polymer from a thermoplastic into a thermoset‑like network with memory, strength, and thermal stability. The two methods of cross linking are Chemical (Peroxide) or Radiation gamma beam.
Both methods achieve the same structural outcome but through different mechanisms.
Chemical cross‑linking (peroxide systems)
A peroxide (e.g., dicumyl peroxide) is added to the polymer. When heated the peroxide decomposes forming free radicals which abstracts the hydrogen from the polymer chain, resulting the carbon radicals to bond together forming heatshrink. This method is common in cable insulation, heat‑shrink, and elastomeric components.
Radiation cross‑linking (electron‑beam or gamma)
High‑energy radiation directly interacts with the polymer where radiation removing hydrogen atoms off the chain, bonding carbons instantly and allowing for adjacent chains cross linking without chemical additives.
Electron‑beam cross‑linking is the classic method used by many heatshrink manufactures and produces extremely uniform cross‑link density.
Why cross‑linking matters for heat‑shrink
Cross‑linking gives the polymer give the heatshrink its shape, high recovery force, thermal stability during expansion, resistance to creep and cold flow and dimensional stability after shrinking and recovery.
Without cross‑linking, heat‑shrink tubing would simply melt and deform, it would have no memory and no ability to recover to its original size.
Gas torches and why it is important to use the correct one.
Heatshrink is a cross linked polymer. Once cross‑linked, the material becomes far more sensitive to over temperature. These polymer chains cannot melt and flow like a normal thermoplastic and excessive heat causes surface carbonisation which causes the polymer to become conductive, brittle and mechanically unstable.
The importance of using the correct gas torch is as important as the measurements of the joint or termination you are constructing. Gas torches are controlled thermal input devices, and the incorrect use of this input device can destroy the joint before the installer even realises.
Heat‑shrink needs, even distributed heat so the polymer can recover uniformly around the cable profile. A torch with a soft yellow‑tipped flame provides a broad heat distribution engulfing a large area of the polymer with a wider soft flame envelope. A yellow flame has a lower flame velocity reducing the risk of hot/cold spots on the heatshrink tube, spreading around the tube rather than concentrating in one spot. Controlled temperature rise will have a minimal chance of surface carbonisation. This is exactly why the gas handle torch with a yellow flame tip are the industries preferred tool. The yellow flame profile allows the tube to shrink evenly, without burning, blistering, or creating conductive carbon tracks.
Blue flames are hotter and more concentrated onto the heatshrink. This causes a higher velocity of heat and is very directional and is intense in a small spot rather than being distributed around the circumference. This can cause localise overheating in one spot of the tubes causing the tubes start burning creating surface carbonisation which becomes conductive. Burning the polymer can prevent the tube form shrink evenly with its mechanical integrity comprised. Blue flame is not the correct flame to use and doesn’t really shrink down the tubes evenly it can cause issues with successful heat shrinking.
Heatshrink tubing is designed to recover uniformly without carbonised paths embedded into its outer layer by the incorrect heat source being applied. When a blue flame is used to shrink down polymer tubes, the following can occur:
· One side shrinks faster than the other
· Adhesive or mastic flows unevenly
· The tube pulls into distorted shapes
· Voids form under the tube
· The seal becomes discontinuous
· Stress control layers are misaligned
· Moisture paths are created
· PD initiation points form
This is not a cosmetic issue it is a functional failure waiting to happen.
Conducting many forensic investigations, 95% of the failures are caused by the installers using the incorrect gas torches and other issues which I will touch on in future blogs. When conducting forensics the incorrect shrinking techniques can be seen by the uneven shrinkage and wall thickness, burnt or blistered polymers, adhesive starvation on one side, carbonised outer surfaces, poor bonding to the cable sheath and cold spots on medium and heavy wall tubes. This can be contributed to poor shrinking and the possibility of the using a blue flame torch rather than a yellow flame one.
Medium-high voltage joint investigations, heating tubes incorrectly show major defects which are the main contributor of Partial Discharge (PD) activity, moisture ingress and tracking of components. Once PD, tracking and moisture ingress the joint will suffer failure modes which can be seen in examinations and autopsies of blown joint bodies and terminations.
MAP (MAPP) gas.
MAP (MAPP) gas is a quick way of destroying heatshrink. MAP (MAPP) gas (Methylacetylene-Propadiene Propane) was first designed in 1940’s by Linde and is used for soldering, brazing and welding due to its high heat flame temperature. The flame is an extremely directed hot blue flame not suitable for heatshrink polymer.
MAP (MAPP) gas (Pro) is a high‑pressure propane, producing a narrow high velocity blue flame, localised heating and not centrifugal, rapid surface heating and causes carbonisation of polymers. Once the polymer is carbonised in any MV or HV application, surface tracking and PD initiation points will occur. This will cause accelerated breakdown of insulation and premature failure.
Forensic investigation of a MAP (MAPP) Gas shrunk joint, or terminations exposes PD activity, tracking, thermal runaway, moisture ingress due to the tubes walls being shrunk unevenly and joint failures. The root cause of many forensic investigations starts with the shrinkage of the components, and the use of incorrect gas torches. Incorrect shrink down the polymer tubes is the first indication of the installers heating techniques or incorrect torch selection.
Below is a table for the types of flames which should be used and should not be used.
Torch Type
Flame Colour
Heat Profile
Suitability
Propane (standard)
Yellow
Broad, soft
✔ Ideal
Butane
Yellow
Moderate heat
✔ Suitable
MAP/MAPP
Blue
Narrow, high‑velocity
✘ Never use
Uneven shrinking of the polymer tubes
Using a yellow flame provides broad, even heating essential for uniform shrinkage of the polymer tubes. It allows for uniformity when shrinking down heatshrink tubes. Avoid using blue flames (MAP Pro gas) due to its high temperature and directional heat which will shrink the polymer tube unevenly and may cause over shrinkage.
Position the flame centrically around the tube heating the tube evenly by holding the torch so it wraps around the full circumference of the tube, maintain a consistent distance to avoid hot/cold spots by rotating the torch with a 360° heat coverage over the heatshrink and object which is being shrunk down.
Uniform heating ensures the polymer recovers symmetrically by moving the flame steadily along the tube length avoiding holding the flame in one spot which can cause localised overheating. Whilst heat shrinking the polymer tubes, watch for even wall collapse or shrinkage around the object ensuring there is no thick and thin spots. The tube MUST be heated and shrunk evenly for the best result.
Whilst the polymer wall is shrinking down monitor even shrinkage and stop immediately if uneven shrinkage appears and correct your flame direction to apply heat.
Prevent over shrinking as this can cause brittleness moisture ingress and insulation loss to the polymer tube. When shrinking of the tubes is finished, verify the tubes have a uniform wall thickness ensuring no gaps, there are no voids, hot or cold spots. Heatshrink tubes are designed for uniform wall thickness and it extremely important this wall thickness is even all the way around the object which the heatshrink has been applied.
The correct heating sequence for each tube type.
Heatshrink components in MV–HV joints are application‑specific engineered polymers, and do not interchangeable sleeves. Their behaviour under heat depends on the polymer type (Polyolefin, elastomers, EPR blends and double, triple layer composites and crosslinking density. Heatshrink wall thickness are also a contributing factor with recovery rations being 2:1, 3:1, 4:1which requires a lot of heat for recovery of the tubes on the joint or termination or cable repair. Adhesives and mastic types and volume impregnated onto the tube can also influence the polymers reaction to heat. One of the main functions of these tubes is to insulate the energy from the outside world, sealing against moisture ingress creating a moisture barrier and for stress control of MV-HV cables. Because of this, each tube requires a different heating strategy and torch control technique.
Medium or heavy wall tubing is thin and long usually wide with adhesive. It has a shrink ratio of 3-1 or 4-1. The heating is complex with the tubes being large and shrinking down a long way, the gas torch required to adequate to do the job is a yellow tip flame. A yellow tip flame engulfs the heatshrink and distribute the heat around the heatshrink as it collapses and bonds on the object. Jointers use many techniques, but moving the gas torch around in a circular motion usually works and the tubes shrinks.
Double or triple‑layer tubes used inside MV–HV joints are completely different in design and function and require different techniques to successfully shrink the tubes down without compromising their integrity. These tubes typical, include 2-3 layers which include EDPM, Polymer layers and outer semiconductive layer. These layers require a large amount of heat to shrink down and following manufactures instruction on how to shrink the tubes down MUST be followed. These tubes have been designed to control electrical stresses, smoothing the field around the screen cuts, prevent PD initiation and bond mechanically and electrically to adjust components. Because of this, precise heating sequences is required.
Stress tubes under the triple layer tube requires lower controlled heat input reducing the risk of carbonation and creating a perfect symmetry around the cable core. When heating stress tubes DO NOT BURN THEM. Turn the yellow flame torch down, this allows for an even heat. These polymer tubes are made from carbon, with a lot of heat, may burn them so be cautious in shrinking these tubes down and take your time.
Double and triple‑layer tubes are not just “shrinking plastic” it is an electrical component. Incorrect heating destroys its stress‑control function and introducing a failure mode.
A blue flame will not work in shrinking these tubes as it distorts or burns the tubes, creates discontinuities, causes uneven surfaces which can lead to a failure.
How to visually identify if the wall thickness is correct
A visual identification of correct shrinkage of the heat shrink should be conducted whilst shrinking the tube down. If this is done after the tube has shrunk down and you notice the tube is not uniform in wall thickness, it is too late. Applying more heat to try and get the wall thickness to increase will only make the heatshrink brittle and it may burn. The wall thickness should be even all the way around and there should not be areas where the wall is thick and thin. Cold spots occur when the tube you have shrunk down has flat spots in the heatshrink. This is an indication the heat has not uninformatively been applied. If these spots are seen on the heat shrink outer, softly apply more heat to that spot and the cold spot should disappear. Post heating an outer tube is always a good idea, when the heatshrink has shrunk down. This process should take about 1-2minutes feathering the tube allowing for any hot adhesive to fully melt and bond to the surface to which is being shrunk onto.
Visual identification MUST happen during the shrink, not after. Once the tube has recovered, the polymer memory has already locked in place. Any attempt to “fix” uneven wall thickness afterwards will only overheat the tube, make the tube brittle, damage to the adhesive bonding and a possible risk of burning the polymer tube.
When visually watching the tube shrinking, watch the tube is collapsing around the cable profile and the there is no ununiform radial collapse around the full circumference. Identify any areas which are collapsing faster or slower than other sections of the polymer tube. When this occurs correct the flame position immediately for an even spread of heat.
Uneven wall thickness indicates uneven heating and MUST be corrected immediately.
· Thick sections = overheated or over‑shrunk areas
· Thin sections = underheated or cold spots
· If seen early, adjust flame angle and rotation to restore uniformity
· If seen after full recovery, do not attempt to fix by adding heat
Avoid Post‑Shrink Correction Attempts NO recommended
Once the tube has fully recovered, its polymer memory is set and cannot be reshaped safely, do not attempt to thicken thin areas by reheating as additional heat will make the polymer brittle, it increases the risk of scorching, blistering or carbonisation.
Perform Controlled Post‑Heating Recommended
Post heating ensures adhesive flow and full circumferential bonding to the object which the heatshrink has collapsed onto. After full recovery of the tubes, feather the fame over the tube for 1-2 minutes using a soft yellow flame and avoid overheating. This allows for any hot adhesive to melt uniformly and bond down onto the object. This method will also confirm that there are no voids, gaps or unbonded areas remain which could have been missed when originally shrinking down the tubes.
A final note about this subject, Polymer memory locks immediately after recovery. Once the tube has collapsed, the crosslinked polymer has returned back into shape, and any attempt to reshape it afterwards is destructive and is not corrective. Heatshrink is a cover and when recovered it forms the shape it has collapsed onto, and moving a hot heatshrink joint can also create voids and failures. Uneven heating causes problems which include moisture paths, distorted stress control layers, PD activity in MV-HV joints.
This is the end of part 2,
Part 3 will continue with, correct tools for removing critical layers
An explanation of
Why semicon removal must be controlled, not hacked
The correct use of:
Semi conductive scores
· Semicon shaving tools
· Knife blades
· Hook blades
· Abrasives