Using heat shrink tubing to prevent zippering

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Lugnut56

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Anybody else tried using heat shrink tubing on Kevlar to try and reduce/eliminate zippering? I've put it on most of my rockets (all LPR) and have not had one zipper yet , but I've not flown any one rocket more than a couple times so I'm not sure that is a big enough sample group. I also use it on the first couple inches of Kevlar by the MMT. I know heat shrink tubing is NOT flame or high heat proof and will char, but my thinking was it will add some additional life to the cord.

Heat shrink 1.jpgHeat shrink 2.jpg
 

manixFan

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I use heat shrink on Kevlar anywhere it makes contact with an edge. Kevlar is not very resistant to abrasion and heat shrink is a good way to protect it. For model rockets I can see how it would help reduce the odds of a zipper. Also seems prudent to use it by the engine mount. On my larger mid-power rockets I pull the kevlar through a cork to prevent zippers, it has worked very well over the years.


Tony
 

Mike Haberer

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I've started doing this in my builds over the past two years. I don't have a large body of data yet, but I think it works. It won't prevent a zipper if a flight goes sideways (literally, like a bad weathercock in high'ish winds). I had a Super Big Bertha zipper despite the heat shrink when it went horizontal into the wind and then ejected at speed. For flights that don't violate the 40 degree weather-cocking cone in Rocksim, I think it will prevent almost all zippers.
 

dr wogz

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I do like the idea.

And I would go one further: wrap tape of foam or .. around the cord at that particular point where it meets the tube edge, then add the heat shrink (a larger size). The idea being to make that cord-tube-edge contact point as big as possible to further ensure it can't zipper..
 

BEC

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I have been doing this, more to prevent a CA-hardened upper motor tube lip from sawing through the Kevlar after a couple dozen flights than to prevent zippering. So far it's been working OK. I have one model with 40 flights on it done this way and the Kevlar hasn't worn through and won't for some time. This particular model hasn't suffered any particularly egregious zipper-inducing incidents but it has had a few not-quite-optimum flight profiles including one windy-day snapback early in its life last spring that caught a piece of the body tube in the base of the payload section, inside-out. See second picture.

This is what it looks like right now.
IMG_4204.jpg


IMG_2927.JPG
 
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hball55

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LOL, I’ve never had a zipper on any LPR. Their lack of any appreciable weight I would think would rule out zippers.
 

Mike Haberer

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I do like the idea.

And I would go one further: wrap tape of foam or .. around the cord at that particular point where it meets the tube edge, then add the heat shrink (a larger size). The idea being to make that cord-tube-edge contact point as big as possible to further ensure it can't zipper..
What I do is build up the section with several layers of heat shrink of increasing size.
 

BrendanH69

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I now put heat shrink on all my Kevlar shock chords, mainly to prevent abrasion to the Kevlar rather than anti-zipper.

What I've started to use (for shock chords that are not wide/flat) is Kintsuglue flexible epoxy putty. I knead some up and put a good long blob of it around the shock chord as my anti zipper strategy. Stays flexible but strong.

1615111724960.png
 

prfesser

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It just occurred to me that foam blister tape might be a good candidate for such work. Wrap as many layers as needed around the shock cord and top it with heat shrink. Think I'll give it a try at the next launch.
1615124878724.png

Best -- Terry
 

Greg Furtman

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I now put heat shrink on all my Kevlar shock chords, mainly to prevent abrasion to the Kevlar rather than anti-zipper.

What I've started to use (for shock chords that are not wide/flat) is Kintsuglue flexible epoxy putty. I knead some up and put a good long blob of it around the shock chord as my anti zipper strategy. Stays flexible but strong.

View attachment 454009
It's interesting that this LocTite product isn't available in the US. Hmmm...
 

dhbarr

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It just occurred to me that foam blister tape might be a good candidate for such work. Wrap as many layers as needed around the shock cord and top it with heat shrink. Think I'll give it a try at the next launch.
I already just roll on regular fabric medical tape, hadn't considered the thick stuff.
 

Old School Doug

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LOL, I’ve never had a zipper on any LPR. Their lack of any appreciable weight I would think would rule out zippers.
While I have no pics to offer for proof I can guarantee that LPR rockets can and do zipper. Several repair jobs on early builds validates that statement.
 

RobertH3

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Learned the heat shrink from a New Way kit and I like the idea of using it at tube / nose cone join!

Cheers / Robert
 

Buckeye

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Anybody else tried using heat shrink tubing on Kevlar to try and reduce/eliminate zippering?
The heat shrink tubing alone will not reduce zippering. The diameter of the cord + heat shrink is not much greater than the kevlar alone, and it can still cut through the cardboard like a knife. You need more surface area. Bulk up the cord with tape, cork, foam, putty, etc.

See the Giant Leap Fireball as an example of the concept to its extreme:

 

BABAR

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I really liked @BlaineS ear plug integration. Worked well on my Interceptor E

 

Dane Ronnow

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I'm using the Flent earplug for my MPR build:


But rather than using the coffee straw/stirrer, I pull the Kevlar through the plug with a large darning needle. Single overhand knot above and below the plug.

The only question that remains is how the foam plug holds up to heat from the ejection. I'm thinking of applying heat shrink over it for a little more protection, but I haven't decided on that yet.
 
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manixFan

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Here's an example of the Kevlar pulled through a cork that I use on larger body tubes. I've never had a zipper with this method. In spite of initial concerns the cork might bind in the tube, that has never happened either.


Tony

cork.jpg
 
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bschultz32

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Buckeye is right, just 3 words of advice: Giant Leap Fireball:

FIREBALL 20200910.jpg
 

Tractionengines

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I know heat shrink tubing is NOT flame or high heat proof and will char, but my thinking was it will add some additional life to the cord.
This stuff will protect the first few inches by the ejection charges. I use this at work a lot for protecting brazed splices in magnet wire used for induction heating coils we manufacture. It's rated for 260C (500°F) use, and takes 350+C (660°F) to fully shrink it.... I have blasted it with a butane torch and it does almost nothing to it. (Note DO NOT BREATHE THE FUMES OUT GASSING FROM THE PTFE [ generic Teflon] if you test this...)
 

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Mike Haberer

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The heat shrink tubing alone will not reduce zippering. The diameter of the cord + heat shrink is not much greater than the kevlar alone, and it can still cut through the cardboard like a knife. You need more surface area. Bulk up the cord with tape, cork, foam, putty, etc.

See the Giant Leap Fireball as an example of the concept to its extreme:

I use multiple layers, starting with the smallest diameter and adding until it is several times the diameter of the kevlar. Seems to work.
 

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