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Tuesday, May 21, 2013

Shrink Sleeve Overheating: What is the Effect?

Question:  My crews have overheated a shrink sleeve.  What do I do now?

Answer:  I wish the answer were a little simpler!  There are many different levels and degrees of overheating.  I will try to address three below.

    First of all; as seen here - a shrink sleeve is supplied with a cross hatch pattern in the backing.  As the sleeve is properly heated (the backing temperature must reach 267F in order for the cross linked PE to reach its crystalline melt point and revert back to its original size), those cross hatches disappear and are replaced by a smooth PE backing. 

    For the purposes of this discussion, I will limit the possible extent of overheating to three different groups:  surface crazing; crazing with smoke and/or flame; and splitting.

     Surface crazing can occur when an area of the shrink sleeve gets too hot.  This is most often caused by the installer failing to keep his torch continually moving in a circumferential motion.  Keep in mind, a propane torch might operate at several thousand degrees.  In order to shrink a shrink sleeve, we are trying to get the temperature of the backing up to 267F.  Installers MUST keep their torches moving!  In a case where a sleeve has cooled, and there is an area with light surface crazing (often accompanied by a dull area lacking in shine) this is not a problem.  Crazing does not effect the integrity of the sleeves (in fact some studies would actually indicate the crazing increases the fracture toughness of a polymer! That doesn't mean we recommend intentionally creating crazing in your shrink sleeve).  In a case where crazing has or is occurring; simply STOP heating that area and move on to the other sections of the shrink sleeve; always moving the torch in a circumferential motion. 

      Sometimes, this surface crazing can be accompanied by smoke or even flame.  The fact that the PE backing of our shrink sleeves have been cross linked means that the backing will no longer melt as a typical PE will.  The backing can still burn, however, and will burn if the installer is not continually keeping his torch moving.  In a case where a sleeve begins to smoke; the installer should simply stop heating that section of the sleeve!  In a case where a sleeve begins to flame; the installer should immediately put the fire out (this can be done by simply removing the torch and quickly patting the flame out with a gloved hand).  As long as this has been caught soon enough; again - this causes no serious damage to the shrink sleeve and no repair needs to take place.

     Finally, there are times that an installer seriously, seriously overheats a shrink sleeve.  I have seen hundreds of shrink sleeves installed and I've never actually seen this happen in person.  To actually burn completely through one of our shrink sleeves; an installer would have to be incredibly distracted by something; leaving his torch open full bore while concentrating the flame on a small portion of the sleeve.  Even then; the sleeve itself would give him many warnings as it crazed; smoked; caught fire; and ultimately burned through.  Once the backing has been compromised, it will be obvious as the mastic material will begin to flow freely through the backing and the backing will continue to split wide open.  This is the kind of thing that could not possibly escape the attention of even the most inexperienced inspector.  In a case like this; there would absolutely have to be repairs made.  Repairs are discussed in depth here.  

     One other thing I would mention.  Every wrap around shrink sleeve requires a closure strip to hold the sleeve in the shape of a tube during the installation process.  That is the closures only job; it has no corrosion prevention or sealing duties.  At times, particularly when it is dog eared; or slightly raised; the closure strip can flame up when overheated.  In such a case, the torch would simply be moved away; the installer would 'pat out' the flame (patting down the closure in the process) and then installation would resume.  This would not be a problem at all and would require no repair.  

     As always, we are available for consultation.  Any time someone might have a question about overheating; my first bit of advice would be to take some photos and email them right away to - I will get back to you quickly. 

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