The geometry of heat shrink tubing: Part 2

To finish right, start right

In Part 1 of the geometry of heat shrink tubing, I explained the difference between the diameter of tubing and its flat width. Without repeating the reasoning here, the short answer is that you determine the flat width by taking the diameter and either dividing it by 0.637 or multiplying it by 1.571.

But once you know the correct diameter or flat width of the tubing, there are two other very important questions to answer to make sure that you start out with the right size tubing in the first place. These are:

  1. How big does it need to be to go around what it needs to?
  2. How much does it need to shrink?

Make sure it shrinks enough

One very important thing to know about heat shrink tubing is that the final size it shrinks to has to be smaller than or equal to its starting size minus 75% of the total amount it will shrink.

To put it into simpler terms, if you start with 1 inch diameter tubing with a 2:1 shrink ratio, this means that its minimum size after shrinking is 1/2 inch. This also means that the amount it shrinks is 1/2 inch.

So the largest size you can safely shrink it to is 75% of the amount it shrinks, or 3/4 inches. Multiply 3/4 by 1/2 and you get 3/8. Subtract this from the starting size of 1 (which is also 8/8) and you 5/8. This is the largest diameter you can safely shrink it to, giving a range of final size from 1/2 to 5/8ths of an inch.

The reasons for this are that it will prevent the tubing from shrinking more later as well as avoid the problem of uneven shrinking, which can alter the effectiveness of the heat shrink.

This rule is even more important if you’re using adhesive lined tubing.

One size does not fit all

The most important part of any heat shrink project is making sure you start with the right pre-shrunk size in order to obtain the optimum results once you’re finished. To do this, you have to start at the end and work backwards.

In other words, the important measurement is how small the heat shrink tubing is going to be once you’ve applied the heat to it.

Unfortunately, there’s not a simple formula to plug in a number and get the right size. However, we can give you a guide. Find the final size you need, then see where it falls in the first two columns in the table below. The tubing size you want is in the right hand column.

2:1 Ratio

Wire Size 

(inches)

FROM

Wire Size

(inches)

TO

Recommended

Tubing

Size

1/16 1/16 3/32
1/8 1/8 3/16
3/16 1/4 3/8
5/16 5/16 1/2
3/8 3/8 5/8
7/16 1/2 3/4
9/16 5/8 1
11/16 13/16 1 1/4
7/8 1 1 1/2
1 1/16 1 5/16 2
1 3/8 2 3
2 2 5/8 4

3:1 Ratio

Wire Size 

(inches)

FROM

Wire Size

(inches)

TO

Recommended

Tubing

Size

1/16 1/16 3/16
1/8 1/8 3/8
3/16 3/16 1/2
1/4 5/16 3/4
3/8 7/16 1
1/2 5/8 1 1/2
11/16 7/8 2