Heat shrink tubing application and aircraft EWIS components

Heat shrink tubing can be found in nearly every location on aircraft and can interact with aircraft EWIS components. These materials protect wire repairs, provide additional abrasion protection, and prevent the ingress of fluids (and in a pinch, may be used as a secondary harness support when other options are not viable). Although this product can be found everywhere, there are still challenges that are common to manufactures and maintainers.

Product Identification

Currently there are no requirements for marking or identifying mark on heat shrink tubing; the product is identified at the spool level, but not on the product itself. As the product moves through the acquisition and distribution process, a length of tubing may be cut from the spool and sold without identifying marks on the delivered package. As such, the end user of the product may not know the original material source.

There are a couple of reasons why an end user would want the identification at the item level:

• Counterfeiting protection
• Determine the material source

What is seen in the fleet is that, if there is any problems with the product, it is hard to trace back to the source. For example: a heat shrink sleeve is placed onto an aircraft wiring system, and sometime during the service life (far earlier than anticipated), the tubing degrades and becomes unusable. Because there is a lack of identify marks, going back through the supply chain to identify the root cause may be impossible.

Given that the typical application of heat shrink tubing is only a couple of inches, it would be difficult to have all necessary identifying information placed on each segment. The lack of tubing identification may be a problem without a practical solution.

Flammability

For those that are not familiar with the flammability classifications for heat shrink, there are three currently defined:

• Class 1: Nonflammable,
• Class 2: Flammable material, and
• Class 3: Highly nonflammable.

An obvious caution here is that a user may look at class #1 as non-flammable, and class #3 as being highly nonflammable and may think that the class #2 product is a mid-range product in terms of flammability (also, class #2 products are cheaper as they do not have the flame retardant chemicals).

For those in the aerospace industry, an immediate question should be considered: Why is a flammable material included in an aerospace standard? Great lengths are taken to ensure that nearly everything on an aircraft is non-flammable (with the small exception of fuel). Unfortunately, because this material is used widely outside aerospace, the class #2 heat shrink tubing cannot be removed from the standard. As such, there should be an aerospace restriction for Class 2 heat shrink sleeving.

Storage and Shelf Life

It is fairly obvious why there is a maximum storage temperature of heat shrink tubing, but should there be a minimum? The recommended storage temperature for many of the heat shrink tubes is 18-35^oC. The fact is, the impact that cold has on heat shrink tubing can be much lower. The idea with heat shrink tubing is that the application of heat will cause the contraction; lower temperatures do not impact the heat shrink tubing.

As a final point of creating a shelf life on heat shrink tubing is to consider what should be a practical shelf life for heat shrink tubing? From a practical perspective, one must consider the following – if you churn through your stock once every 10 years, the annual usage is likely a low quantity? Except for distributors, a 10 year shelf life is not a large concern.

For everyone else, keep the tubing in a cool, dry, limited light exposed area to maximize the shelf life.