Heat shrink tubing is the most-used protection component in harness work — insulating joints, bundling, strain relief, marking, sealing — and also the most casually specified. The wrong ratio, material or grade turns a protection feature into a field failure point. This guide organizes the selection logic.
Shrink Ratio: The First Specification
Shrink ratio describes how much the tubing shrinks: 2:1 means it recovers to half its supplied diameter. Selection logic runs on two diameters — supplied ID must slide over the work (connector bodies, solder joints) before shrinking, and recovered ID must grip the finished joint tightly. The classic mistakes: a 2:1 tube whose supplied size passes a connector but whose recovered size is too loose on thin wire (no grip, no protection), or a 3:1/4:1 tube specified where 2:1 suffices (cost and bulk). The rule: size the recovered diameter to the substrate first, then check the supplied size clears the bulkiest point.
Materials by Duty
- Polyolefin: the general-purpose workhorse — flexible, flame-retardant grades standard, -55 to 125°C class. Default for most harness insulation and bundling.
- PVC: economical, softer recovery, lower temperature class (~105°C) — fine for light-duty consumer and appliance work.
- Fluoropolymer (PTFE/FEP): high temperature (200°C+), chemical resistance, thin-wall precision — aerospace, medical, engine-zone duty.
- Silicone: extreme flexibility, high temperature, dielectric strength — flexible joints and high-voltage insulation, mechanically delicate compared to polyolefin.
- Elastomeric: stays flexible at low temperature — cold-environment and flex-critical applications.
Adhesive-Lined vs Standard
Adhesive-lined ('dual-wall') tubing melts an inner adhesive layer during shrinking, bonding the tube to the substrate and sealing the ends. It is the required answer wherever the joint must be sealed: outdoor exposure, washdown zones, underground and marine duty, and any spliced connection in a wet path. Standard single-wall tubing insulates and protects but does not seal — using it where sealing was needed is a classic latent failure that passes every bench test. Our waterproof harness guide shows where adhesive-lined is standard practice.
Other Selection Variables
- Wall thickness: thin-wall for flexibility and space, medium/ heavy-wall for abrasion and mechanical protection.
- Color and printability: identification duty — printed or laser-markable grades serve labeling (our harness labeling guide covers marking practice).
- Flame rating: UL 224 VW-1 class where the tube is part of the assembly's flame compliance path.
- Recovery temperature: matters near temperature-sensitive components — low-temp grades shrink at lower heat.
Application Practice
Proper shrinking needs even heat and patience: a heat gun with reflector for production, moving the gun continuously to avoid scorching, shrinking from one end or center-outward to avoid air traps, and full recovery (no hazy, half-shrunk sections — an under-shrunk tube's grip and sealing are both compromised). Solder joints should cool before sleeving slides over; sharp edges (connector shells, splices) get smoothed or padded — tubing punctured at install is a wasted component.
Kaweei Tubing Practice
Kaweei specifies and applies heat shrink as a controlled process item — material and grade per drawing, sized by recovered-diameter logic, adhesive-lined where sealing is specified, applied with calibrated heat and inspected for full recovery. Contact our engineering team for harnesses built with tubing specified to the duty, not the default.