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Author Topic: Plastic Welding Kit  (Read 98 times)

Col. Craig

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Plastic Welding Kit
« on: October 24, 2017, 05:57:48 PM »

Long time ago I got myself a 'Plastic Welding Kit' from Harbor Freight Tools.  The kit I got is no longer available but here are a couple of newer versions:

800 Watt Plastic Welding Kit with Adjustable Temperature


1300 Watt Plastic Welding Kit with Air Motor and Temperature Adjustment

I can not give a review of the present models but I will share my overview of the concept, process, and value.

Just like welding metal, the concept here is no different.  The 'welder' is a hot air gun which you use in a manor like flame torch welding whatever the fuel gasses may be.  The skill required depends a whole lot upon three variables.

1)  The kind of plastic you are attempting to weld.  Some plastics will weld easy while others are nearly impossible.  In my limited experience attempting to weld plastics I have seen some plastics actually scorch black before melting.  This kind of plastic may not be impossible to weld but it is beyond my skills.  If I see the plastic scorch when I heat it I just stop right there and look for another option.  Other plastics have a microscopic open cell structure like a sponge.  These will melt away very easy and can be welded.  The problem is they will melt away very easy.  So if you see the plastic nearly vanish or shrink away quickly then you will need to focus the heat onto the filler rod and start adding it quickly.

2)  How well the filler rod matches the base material in composition and those attributes that go with it.  The same kind of plastic in two different colors will weld differently.  The plastic welding kit will come with three or four kinds of plastic filler rods but none will be a perfect match.  Your best hope is for a filler rod that has a lower melting point than the base material.  This will result in the easiest weld job.  Also it will never happen.  Seems like the filler rod always takes more heat than the base material making for a very challenging weld to perform.  In extreme conditions you can weld with hot glue sticks.  The hot glue will have a much lower melting point than the base plastic (most of the time).  So you first get the hot glue into position in the usual way, with a hot glue gun.  Then you use the plastic welding torch to chase the hot glue deep into the repair job.  This will get the hot glue to penetrate far better than it ever will coming out of a hot glue gun.  Also, after hot glue sticks have been heated then cooled the glue is far more dense, rigid and strong than what it was in 'stick' form.

3)  The air pressure - the lower the better.  The lower the air pressure the better.  Say it with me - low air pressure is better!  My welder came with a regulator that tops out at 15 psi.  The kinds of repairs I make require about 2 psi or less.  4 psi is enough to blow molten plastic away as if I am 'gouging' a trough through the plastic. In practice I have to run two regulators to get the final air flow down to just 2 psi.

The process is very similar to gas welding or brazing.  The first and last thing to do is get the air flowing so the heating element doesn't get energized without air flow.  If you plug in the welder without air flowing through it I suspect the heating element will burn out in about three seconds!  Start the air flow first and stop it last. 

Now plug in or turn on the heating element.  After energizing the heating element it will only take a few seconds to come up to temperature.  Be sure the wand is in the holder and the nozzle is pointing is a clear direction - treat this just like a flame torch.

Read the directions to determine what kind of plastic you are working with and then select a proper filler rod to match (cough - cough, it will never match but come close).  Using the torch, slowly bring the filler rod close to the torch tip and watch for signs of the plastic beginning to deform.  This will give you a sense of what to expect with the base material.  Now do the same with the base material.  When you sense the base material starting to deform, back off a little.  Now bring the filler rod to the base material and heat both together.  Watch to see which one will melt first.  If the filler rod melts first you are golden - go weld.  But if the base melts first then check your options for a softer filler material.  If you have to weld with a filler rod that melts slightly hotter than the base this will require a lot of skill but is not impossible to do.  Just focus the heat onto the filler rod, rolling it as you do, and allow the blow-by to heat up the base material.  When the filler melts dab this into the base and focus the heat onto the dab of filler to wash it into the base.  Repeat this in connecting beads to form a row.  Go back over the row to wash the weld puddle further into the base if needed.  Use a lot of filler and sand it down later if you want it to look better.

The value of welding plastic is that you do not have to throw away a cheap product that is otherwise still in serviceable condition.  I can't remember all the little shit I have welded in plastic.  Plastic welding isn't something I do very much of.  Sometimes I go a couple of years without using the tool and other times I get on a welding streak and find myself fixing up a pile of junk that has been waiting in the wings for me to get a round to-it.  The most recent item is my boot & glove dryer, similar to the one on that link.  95% of the device is just plastic.  To make it smaller to package they made the base in two pieces.  I put my heavy wet boots onto the boot dryer, then add on top one fat cat and the base broke in half at the coupling!  $40 out the window (with the fat cat)... or not?  I fixed the break by welding the plastic bits together to form a solid one-piece base.

When I got this plastic welder I was only hoping for it to break even at the best.  But over the years I have made enough repairs that I am now certain it has paid for itself many times over.

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