Why isn't there any post-insertion heat shrink tubing?

Technical · Heat Shrink Tube

Why can't heat shrink tubing be applied after assembly?

Heat shrink tubing is a tube that uses the property of shrinking in diameter when heated to insulate and protect wiring and terminals. It is often used for connecting wires and terminating cables. It is threaded onto the object before shrinking, and then heated with a heat gun or dryer to achieve tight adhesion.

While this "shrinking" property is a major advantage, it also comes with the limitation that "it cannot be applied after assembly." We frequently receive requests for "a type that can be applied after assembly."

This article explains why "heat shrink tubing that can be applied after assembly" generally does not exist, from a technical and structural perspective.

Basic Principle of Shrinkage Structure and Why Post-Application is Not Possible

Basic Principle of Shrinkage Structure

Heat shrink tubing is formed into a larger diameter than its original size through "cross-linking → extrusion → cooling" during the manufacturing process. When reheated during use, it returns to its original shape (smaller diameter), possessing a "shape memory" property.

Insulation and Waterproof Performance Will Be Lost if a Slit is Made

If a slit is made in the tubing to allow for post-application, the slit will open when heated during the shrinking process, making complete adhesion and protection difficult.

Reduced Insulation Performance — This results in a dangerous usage where electrical insulation is not guaranteed.
Reduced Waterproof Performance — There is a risk of water ingress through the slit.
Displacement/Peeling During Shrinkage — Pressure can cause displacement at the slit.

"Cannot be applied after assembly" = "Requires attention during the design stage"

Heat shrink tubing is a product designed to be "threaded beforehand" due to its structure. If you plan to use heat shrink tubing during the wiring design stage, the following points require attention:

Design considerations

Always thread the tube before connection.
Estimate the tube's length and shrink ratio in advance.
If post-application is difficult, consider other protective materials (spiral tubes or slit tubes).

What to do when there's no choice but to apply it after assembly?

If it is difficult to thread the heat shrink tubing beforehand, use a tube with a high shrink ratio. For example, our Rokuichi Tube can cover cables with an outer diameter of 5mm, even over connectors exceeding 30mm in diagonal measurement.

Product Page

Rokuichi Tube — High Shrink Ratio, Large Diameter Compatible Heat Shrink Tube

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Summary

Due to its nature, heat shrink tubing generally cannot be applied after assembly.
If a slit is made, it will open when heated, compromising insulation and waterproof performance, creating a dangerous situation.
To maintain safety and performance, it is necessary to "thread it beforehand."
When post-application is unavoidable, selecting products with a high shrink ratio or alternative protective materials is important.

If you are unsure about product selection, please feel free to contact us.