By Laura Wishart
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.
Figure 1: The figure shows the cross-section of a sample of heat shrink tubing both before and after unrestricted shrinkage. To verify the dimensional life of the sample at any time, a measurement of inner diameter is recorded followed by unrestricted oven shrinkage of the sample. After the shrinkage, the inner diameter is re-measured and the wall thickness is measured.
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 dimensional life is important to consider, anyone who has had to fit a wire bundle through an undersized heat shrink tubing will say otherwise. Typically, specific parts will be identified for harness build fabrication. If the tubing does not fit, there are two likely actions:
- consult engineering about the defect, or more likely
- use the next larger size of heat shrink tubing.
Putting numbers behind option #2, if a M23053/7-111C replaces a M23053/7-110-C (1.050-inch and 0.963-inch inner diameter, respectively), this adds 9% more heat shrink weight. Slowly, this additional 9% stealthily erodes weight budgets and is a hidden cost of not addressing heat shrink tubing dimensional life.
All things have their limits, heat shrink tubing is no exception. The latest revision of AS23053 includes information on this and should be considered when selecting heat shrinking procurement type and quantity.
Contact Lectromec with any questions regarding the dimensional life of your heat shrink tubing. Our ISO 17025 accredited lab is happy to offer consultation for the selection of components in your application.
Lectromec has been a leader in wires and wire interconnect systems since 1984 and has provided expertise to the aerospace industry, both military and commercial. They help organizations worldwide improve the safety and readiness of their air fleet and reduce the costs associated with aircraft maintenance.