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Duel extruder on the cheap

Now I have finished writing I have a little more time to put towards other projects. You might think that I have many important things to do and you would be right. I want to get into Blender sculpting that looks really interesting. Then there’s my Flsun cube printer that I want to turn into a coreXY. So, I do have a lot of things that I want to do but a silly thought popped into my head the other day and I felt compelled to test this out first.

Now, if anyone wanted to try printing in two colours they would probably buy a dual extruder hot end assembly. There are quite a few to choose from, and they don’t cost the earth. But I wondered if you could join filament together and print something with that. For instance, take something like three different colours of PLA and somehow stick them together so that they acted as one length. How hard can that be?

Let’s find out!

For this, I will be using my substantial inventory of Rigid ink filament. I subscribe to their monthly filament club, so I get three or four 10m lengths sent once month. Which comes in handy. I have PLA and ABS but for this little test, I will only use PLA.

First question How am I going to join them together? Can’t use a glue because that is going to block the nozzle. The only way I can see this working is to heat the ends and join them that way.

I will also need to cut the ends of the filament at an angle rather than straight. I want each end to have as much area to use at the join point.


As you can see above those ends look like they hate each other. Getting them to stay put and be close together is another problem that needs to be addressed. To keep them in place I will hold the ends together with some 2mm shrink tubing, that you use to shrink around wires and electrical components. The idea being that I heat the area at the join and the shrink tube shrinks round both ends keeping everything lined up. That’s the plan anyway.


In a moment of wisdom, well more of a “wisdom wasteland” I decided to use a set of helping hands to hold the two pieces of filament. My thinking behind this was that I could then get access to the back and underneath of the filament. Thus I would be able to heat it up all round and get a good weld. That I should add didn’t work, it dropped and got out of shape too quickly.


A couple of points here worth a mention. The first is that when you are welding filament there is a state of being for the filament that comes before the melting point. This is known as the “glass transition” This is where it softens with PLA this happens around the 50c mark then starts to melt at around 60 to 70c. Then starts to physically move at around 160c. so you don’t really want to go above this temperature range (150c)

The point to this explanation is that having it hanging around in mid-air like I have in the picture above, allows it to sag and deform. It only has to deform a little bit and it will get stuck somewhere in its journey to the hot end. Something I found to my cost on my first print, as seen below.


As a little test, I printed a Benchy. All was going well and then as the yellow filament tried to get through the top of the hot end it got stuck and the print failed. It had a blob of fatness (technical term) that just stopped it passing through to the nozzle.

So I decided to try another way. I remember seeing a guy on YouTube doing something like what I was trying to do. He had stuck it to a table on a bed of Kapton tape. So I thought why not try that.

For my next try I went with the following setup. I stuck some Kapton tape onto my work bench then after cutting the ends of the filament at a nice dramatic angle I stuck those down with tape.


I made sure that both ends were as close as possible and left enough slack to slide an inch of shrink tubing by lifting one end of the filament end up to slide it onto.


Then position the shrink tube so it covers both ends. Try to get the middle point of the shrink tube in line with the two ends of the filament.


The heat source

I tried a few ways of getting heat to where I wanted it. Hair dryer was kicking out too much air force. I then tried some GHD straighteners. This was way too hot, and I couldn’t see what was going on at the critical joint point. I decided to go with my gas-powered soldering gun. It had a little blow torch thing going on half way up the stem. I found that if I turned it down a little it was much more controllable.

So, I applied the heat source around the area that needed the heat. If you want to try this yourself these are the things to lookout for.

When you start applying the heat, the shrink tubing will start to pull tight. You may witness the ends of the filament budge inside the tubing. This is okay the tubing will bring the ends together. Keep moving the heat around the area all the time, don’t pause in one place for too long. Make sure you give some heat to the back as well.

DO NOT heat it up too much. you will warp the filament or put a kink in it. This is trial and error. It will all depend on how hot your heat source is. Use some off cuts of filament to test joining before going into mass production. In the picture below look at the white filament just as it comes out of the shrink tube. This is warping it means I have given it a little too much heat. This is about the most you can give it, more than this deformation and you will get problems getting it past the extruder motor or the top of the hot end.


When you feel you have got a good enough join allow it to cool down. This is important, let it cool on its own the most you want to help it with is to gently blow on it. Leave it for a minute then gently cut the shrink tubing away from the filament. As seen in the picture below.


If you tested before trying this you will have an idea of what a good strong joint looks like. If you didn’t test then the picture below is a good joint.


This was taken from a small desktop microscope that I have, but you can see that both ends have successfully melted into each other. Don’t be scared to give them a good pull. If the joint is good, it will withstand a fair amount of force.

Another way of checking if the tolerances are within the 1.75mm limit. I had a Bowden tube kicking around, so I pulled the end off and after each weld, and when it was cool I slid the Bowden coupler over the joint just to make sure it was able to get past the joint without snagging.


That’s about it. When you have enough in length to do a print give it a whirl. Print at the normal temperature you would for PLA on your printer. Keep an eye on the filament to make sure it doesn’t coil up and kink. Remember, you have joined two pieces of filament, but you have also introduced a weak point it can still break easily.

Now the big question. Is it worth doing? Well, it’s fun in a “I tried that” sort of way. I can see that you could maybe use this is a vase print if you didn’t mind all the pre-printing work that you would have to do to get a load of filament long enough to make a vase. I can see where you could turn a vase or that type of print, into a psychedelic masterpiece.

The problem is that it’s not easy to figure out when a colour will start. There must be a way of calculating at what point a hot end will be in relation to the G-Code being run. Then I suppose you would then say at this point, 2.5 meters of filament has been used and I now want to start using another colour. But then it might be as easy to say “I’ll buy a duel-extrusion setup from say E3D.

Here is a Sine vase that I found on Thingiverse by Chompworks, and yes, I know there are some holes but that’s because I had the speed up to high and I wanted to get it done so that I could finish this post. Any way’s it does look okay, and I was pleased with the result. If anything, it makes me want a duel-extrusion setup now, but I have other things to do first so that might have to wait. Below this is another Benchy that looks really nice too.


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