DIY How to Fix / Join PLA Prints

Joining PLA models can have many uses. Say you want to print something humongous in several pieces, or you cut your model to print it without supports, or maybe your printer jammed 3 hours into an 8 hour print and so you printed the last 5 hours separately and need to join them. Whatever the need, here's how you can get it done.


There are a few ways of joining PLA:

1. Glue. Cyanoacrylate (aka superglue) doesn't work too well with PLA. I don't know the exact science but I have had it fail on me with much less stress than it should have held. As far as I am aware, there is only one specific loctite glue that works and it is hard to find (sorry, can't recall the name)

2. Friction welding. As demonstrated here: http://makezine.com/projects/make-3...-and-post-processing-your-3d-printed-objects/ . Basically, you use a dremel tool to spin a bit of filament into your part so that the heat from the friction melts everything together.

3. Solder welding. This is the method I prefer and will detail for everyone the strongest type of bonding method I have found.


In my case here, I printed two tower support bases for an automotive project, but forgot to mirror the second one, so I printed out 2 identical, left sided pieces. These were printed with 60% infill and took 3 hours each (with a .25 layer height and 100mm/sec speed so they are ugly...) and I didn't want to reprint anything. Since the only difference was the base of it, I just sawed off the bottom of one of them, printed a new bottom "slice" and joined everything together.

First, plug in a cheapo soldering iron so it can heat up during your prep. Then:

Step 1: Prepare your prints to be joined

In the first photo you can see my full height tower on the left, my cut tower on the right, and the wedge bottom piece I printed to correct my initial mistake. Test fit everything and make sure all adjoining surfaces will rest flat together. Good surface contact is critical for a strong bond.

Step 2: Melt your prints

That's right, melt them. I have found that it is easy to not only join the outside of prints together with welding, but that you can also melt the adjoining surfaces together. To do so, move a torch over the faces to be joined until the plastic goes from plasticy gloss to high melty gloss (in the photo you can see part of my print is starting to look different). The trick is that you only want to melt the top layer, but to do so you have to slowly heat up the entire print underneath it. You need to move the torch constantly so that the print heats up uniformly. If you heat up the top layer too fast it will delaminate from the rest of the print and warp. If you hear plastic clicking/cracking, slow down. If you do it right you should hear almost no noises as it heats up. Just take your time and you will be fine.

Step 3: Join your prints

As soon as both joining surfaces are melty glossy you can push them together. PLA retains heat fairly well so you will have about 15 seconds to put them together before the layers cool and are no longer melty. When you push them together you will notice that the entire heated part of the print is kind of squishy. Don't worry, its just heated up and will not deform permanently. I like to really press my melted parts together for a good interior bond. At this point your parts will be pretty well joined. If you don't need structural bonds and want to keep the outside looking pretty, you can stop here.

For those of you who need structural bonds, keep reading

Step 4: Create a welding channel

Now that your soldering iron is heated, run the tip along the line where your two parts meet to create a notch between them. Doing this not only further melts your parts together, but creates a valley that you can fill with welding plastic for an even stronger connection.

Step 5: Weld

Using a spare piece of filament and your soldering iron, you can begin to fill in the channel you just created. To do this, lay the filament along the channel, melt one end so that the filament doesn't move and then melt the rest of it. You don't have to move or drag the filament as you go, just melt it in there. When you get the hang of things, you can push or pull the filament along to vary how much you melt down if you want to. Obviously, the more plastic you ad, the stronger the connection becomes.

Step 6: Clean up your weld

My welds almost always look terrible at first. Just sweep your soldering iron along the weld to smooth it out. It probably won't look pretty, but it will look better.


That's it! Now you have a structurally joined, bonded part that is as strong, if not stronger than if it was printed that way to begin with. These things are ugly little babies, but they are strong and solid.

 

I much prefer Weldon4 for either PLA or ABS

 

I knew you guys would know which glues worked

I just don't get enough peace of mind sometimes when I glue things together. Sometimes you need a strong print with no seams or fault lines and that's what this tutorial is for.

 

I use some CA glue and then "spin weld" the seam and grind it down for PLA. Most of the time I skip the glue and just use filament. I've tried to come up with a plastic welding using a soldering iron.. but no joy.

Spin weld to me is the Dremel with a piece of filament in it. It takes some practice but once you get it down, you can fix or attach about anything.

I joined a 100mm cylinder in a structurally relevant part using spin welding.

There are several joining glues for plumbing that work well too.

 

Hot glue gun should work. There are basically 2 varieties... 140c and 250C... or thereabouts. 250 C would probably work great...
I stole one from the wife.

heat break is too large... Need a tube in there.. I have a peek tube that MIGHT work.. TBC