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A trick for stretching sprue

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  • Member since
    September, 2012
A trick for stretching sprue
Posted by GMorrison on Monday, September 24, 2018 2:40 PM

I've never been able to get useful results stretching sprue, until now. There just seem to be too many parameters- the plastic, the thickness, the heat source.

But I found a video that included a technique I would not have thought of, and it's working like a charm.

Hold the sprue over the flame until it catches on fire and separates. Blow out the flames. Mash the ends back together. Then pull apart.


  • Member since
    May, 2009
  • From: Poland
Posted by Pawel on Monday, September 24, 2018 5:08 PM


I've also heard about that "on fire" trick - but I never use it. My way of doing it is to hold a piece of sprue on one side only and to turn it while heating it over a lit Zippo lighter. When I notice the length of sprue bending I support the other end and rotate it some more to accumulate some more heat. After two or three more seconds I lift the sprue away from the flame and then I start pulling the ends apart. If you do it slowly and carefully you can get a rod in the vincinity of 1-2 mm in diameter (not too much of it, though!). If you pul very fast, you get up to your arms reach of very fine fibre. This fibre will break if you try to pull some more after it cools down, meaning you didn't heat the sprue enough.

Hope it helps, have a nice day


All comments and critique welcomed. Thanks for your honest opinions!

  • Member since
    May, 2011
  • From: Honolulu, Hawaii
Posted by Real G on Monday, September 24, 2018 5:09 PM

Just keep in mind a house smoke detector will pick it up really fast!

My first stretched sprue was when I was in 6th grade.  My dad supervised me in the living room (on mom’s coffee table!) and following the short blurb on a Tamiya instruction sheet, was able to produce a usable antenna for my M3 Lee.

Playing with fire and melted plastic was fun so I got a lot of practice!  But outside, and away from combustible surfaces!

  • Member since
    January, 2013
Posted by BlackSheepTwoOneFour on Monday, September 24, 2018 10:35 PM

GM - that method is the oldest trick in my book. I've been stretching sprues like that since the late 70s early 80s. Stick out tongueCoolBig Smile 

  • Member since
    September, 2012
Posted by GMorrison on Monday, September 24, 2018 10:51 PM

Now you tell me!

Yeah, I've never been able to pull off the way Pawel describes.

And yes, it makes toxic smoke, so I do it in the kitchen under the stove hood thing.

  • Member since
    June, 2018
Posted by ARDVARK003 on Wednesday, September 26, 2018 11:07 AM
I'll have to give that a try. Thanks for the tip. AARDVARK
  • Member since
    May, 2013
  • From: Indiana, USA
Posted by Greg on Wednesday, September 26, 2018 11:34 AM

Me too, appreciate the tip, thanks. Yes


  • Member since
    May, 2011
  • From: Honolulu, Hawaii
Posted by Real G on Wednesday, September 26, 2018 5:50 PM

Practice and patience.  And you will join the Brotherhood of Sprue Stretchers.  Stick out tongue

I use an alcohol burner inherited from my sister.  It tends to have an explosive flare-up before settling in to a steady flame.  Steely nerves and intense concentration are required to prevent flinching and setting my eyebrows on fire!  I took it to a local IPMS meeting to do a sprue stretching demo, and after mentioning the little quirk, the burner suddenly burped out a ball of fire as if on cue.  The guys huddled around the table all backed away like when Dan Akroyd switched on his backpack for the first time in "Ghostbusters".

But seriously (but the above story IS true!) learning to control heating the sprue will allow you to consistently stretch different diameters.  If you start with sprue carved in different cross sections, the post-stretched sprue will have a similar cross section.  I made some knobs by cutting deep grooves around the circumference of a sprue, heated and stretched it, then took thin slices to make the knobs!  And stretching hollow tubing will yield fine hollow tubing.  To cut these without crushing, insert a small drill with the same diameter as the inside, and roll on a hard surface while pressing down with a sharp hobby knife.

  • Member since
    June, 2014
  • From: New Braunfels , Texas
Posted by Tanker - Builder on Tuesday, October 02, 2018 4:45 PM

Hey " G " ;

   Ask the missus what a Chafing Dish is .They used to use cans of Sterno to keep things warm at Fancy Buffets . Now there are what were called Chafing candles or Medium large Devotional candles .This is the best thing to use .

 I have been doing this since the 50s . Works great too . Don't use the Sterno though ! The flame is too spread out and "lazy " ?


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