Wiring Harness News May-Jun 2022 - Page 58


Dimensional Life of Heat Shrink Tubing

By Laura Wishart Lectromec ___________________________

We are told that low humidity and cool temperatures are the best way to extend component life , but even in such environmental conditions not all components can sit on a warehouse shelf indefinitely awaiting the day of installation . One of the more susceptible EWIS components to aging on the warehouse shelf is heat shrink tubing . Here , we examine the concept of the dimensional life of heat shrink tubing .

The function of heat shrink tubing is straightforward : to shrink from an initial expanded diameter to a smaller target diameter when heat is applied . When manufactured , the tubing is made at the target diameter and is then expanded as part of the processing .
In other words , the “ shrunken ” target diameter is the original relaxed state of the material . This provides confidence that the tubing will remain in the shrunken state throughout use without re-expanding ; the material tends to “ want ” to shrink back to its original dimensions .
Even in controlled storage conditions heat shrink tubing will begin to gradually shrink back to its original size over time . This is a very slow process ( on the order of years ) but is still important for any heat shrink products held in storage for an extended amount of time .
Dimensional life is the minimum amount of time that a heat shrink product will maintain its dimensions within a defined limit and remain acceptable for use ; it is similar to the idea of a product ’ s shelf-life .
Concepts of Dimensional Life
The detailed specification of a tubing product defines dimensional limits that the material must maintain for proper use . These dimensional limits apply to the tubing ’ s inside diameter
( both as supplied and after unrestricted shrinkage ) and its wall thickness ( figure 1 ). As long as a heat shrink product aligns with these limits , it is considered acceptable for use .
The heat shrink product ’ s dimensional life is the minimum duration the product will remain within these limits as verified by the manufacturer ( and / or qualifying activity ). Verification can be made at any time within the identified dimensional life by measuring the inside diameter and wall thickness of the tubing as specified in AS23053 .
While the dimensional life defines the minimum duration a product is rated to maintain acceptable dimensions in controlled storage conditions , this is not the same as an expiration date . The tubing can be used at any time past the dimensional life so long as its dimensions are within the specified limits .
Extension of Dimensional Life
The manufacturer or distributor may extend a product ’ s dimensional life up to an additional half of the initial value if it can be verified that the dimensions of the product reliably remain within the specified limits in the detail specification for this additional duration . The maximum limit for dimensional life is 12 years , with or without extension , but the purchaser may continue to use the heat shrink product beyond that point so long as the dimensional limits are met .
Impacts on EWIS While it may not seem like the