I was thinking about making a small recess along the edge of a part that I need to glue to another part. I was going to make it 1mm in from the edge, 2 mm wide and 1 mm deep, with a fillet to prevent overhang issues. Both parts would have the same recess. The purpose is so I can put enough glue where I am confident the parts are well sealed, and when the parts are pressed together, any glue that seeps out will go into the recess instead of spilling out of the edge of the two parts. I think it might even make a stronger bond between the parts depending on how much glue seeps into the recess.
If you want the prints to fit together, I would plan a 0.2mm gap for the case you mentioned. You need to try it out anyway, I think. For large parts, you may need a slightly larger gap in the area where the parts are joined together. 0.1mm difference in size could then be too little. It also depends on the print settings. When you then assemble the parts, they will not fit together 100% exactly, there should be a small gap (or individual gaps) that you may only be able to see against the light (about 0.1mm).
You can design parts in such a way that you have to press them together with a lot of force. Instead, deliberately choose a slightly larger gap, as already mentioned, so that you can put the parts together more easily, so you have better control over any glue that escapes. If the parts can be put together without too much force, it should be fine. Then use an adhesive that fills the gap (these adhesives are slightly thicker). However, you must always be prepared for the adhesive to leak out at the edges. You must be careful here when joining the parts and remove any excess adhesive immediately. You can of course also make the planned gap slightly larger in the depth (into the object) so that there is more air there and excess adhesive escapes into the depth rather than outwards. It is difficult to say whether this will work. Adhesive could still escape from the edges of the parts. It is also not certain whether the adhesive will completely fill the gap in the depth, so that there will be air pockets in some places.
Remember that if you trap air while assembling the parts, the adhesive will not be able to escape and will look for another route. It may then leak out of the seam after all.
I guess the answer to that also depends on whether you’re intent on using actual plastic glue, such as Cyanoacrylate, or plastic cement. In the case of crazy glue, you’re adding material to create the bond so a channel might help. In the case of cement, you’re using a solvent-based material that will literally dissolve the two materials and weld them together. So in that case, you want to do the opposite and have not only no channel but print on a smooth plate where the bonding surfaces will mate.
If you’re using glue, then I suppose the amount of channel needed would be a trial and error. 0.1mm sounds as good as any place to start, but then again, will the resolution of the filament produce a pure channel or just a slight indent?
Personally, I have found that glue and even epoxies perform very poorly with plastics. Cement, on the other hand, produces a true chemical weld. I have conducted experiments with both. When you bond two items together, the cemented bond is actually stronger than the surrounding material being bonded. I’ve tested it, and when you try to pry two cemented materials apart, the surrounding plastic fails before the bond does, just like a metal weld would.
The material I found to be the best for PLA, PETG and PC is the following:
However, if you ever try to adhere dissimilar plastics or even attempt to bond TPU to plastic, through trial and error, I found this glue.
It works because it is designed to bond dissimilar plastics together. Although it does not claim to support PETG, I have successfully used it on Silicone/PETG and TPU/PETG combinations. One such project was to create a silicone wiper similar to the one used in the A1 for my P1P. It actually bonded so well that the bond was stronger than both the silicone and the PETG that supported it. The silicone tore first, leaving behind a patch of bonded material. This proves that the glue was stronger than the product it was bonding.
This part won’t really work for printing on the glued part, and it doesn’t have to be too strong, so I think glue would be enough. I will likely try using cement in the future.
I tested making a channel at the edge of a part I was joining and no glue came out of the sides, so it appears to work well. I put the channel on both parts.
The adhesive used always depends on the application. If I need to bond something, I first consider which adhesive is suitable for the material. Then I consider whether the bonded area can withstand the expected load. After answering these questions, I usually have a favourite adhesive. The properties of this adhesive determine the design of my interfaces of the parts that are to be joined. With 2K adhesives, for example, dovetails and similar constructions make sense for me to join the parts.
For quick joints with PLA, superglue works fantastically for me: it bonds the two parts firmly in a few minutes; if I don’t have any glue, I buy it from the discount store over the road (quick availability).
For joints that need to be watertight, when gaps need to be bridged and where the adhesive (also as a filler) becomes very hard so that it can be sanded and further processed in other ways, I use 2-component adhesive: regardless of the printing material for my object. I only have to pay attention to the strength of the adhesive after curing, there are differences, some 2K adhesives become harder than others. Here, too, there may be a favourite, depending on the conditions under which the adhesive is to hold the parts together.
I’ve done a few more tests and it works very well. I can put enough glue to ensure I have a really good seal and glue coverage without worrying about it leaking out the sides.
I have glued parts with enough glue that it would have surely leaked out the sides of I didn’t make the channel, and there is not even one small leak.
Might above what most would consider but use this trick often to repair broken plastic parts.
Like engine covers or such…
I use some stiff wire mesh.
Have the two parts aligned with a gap big enough for the wire mesh to pass through.
Heat the mesh with a blow torch, put in place and push the two parts together.
Should just enough heat be left in the mesh to melt the plastic around the wires.
If the gap is too big use the blowtorch on the exposed mesh above or below the joint and keep pushing the parts together until you get gap free fit.
Cut the remaining mesh into strips and bend them in alternating fashion over either side of the joint.
I then use a wood burning soldering iron with the tip cut off to heat the wire and push into it into the plastic.
For smaller parts I just use short staples.
Drill a tiny and not too deep hole for one pin.
Use some tweezers of needle nose pliers to hold the staple and heat the OTHER pin wit a blowtorch.
Push in and let rest while you grab the soldering iron.
If in doubt offer some support using tweezers or such so it goes in straight.
The last bit is always easy, so be careful not to push it through and create a big hole…
If you have a 3D printing pen you can also melt the joint a bit with the tip of an otherwise not used soldering iron.
Cut the ridges off and fill with the pen at a hot setting.
Gives a joint that usually is stronger than joined parts…
For removable parts or low stress joints I also use silicone seal. It doesn’t stick well to PLA but does gap fill and provides kind of a mechanical “bond”. Great for lightbox lenses, etc. and they can be easily removed if desired.
I use enough that it squishes out but with it I don’t remove it immediately since that can smear it into other surface textures and make it hard to remove. But after it cures it just pulls off without a trace.
I used this in the past for more flexible joints.
Creating an intentional gap, usually in spherical form, with the silicone filling it.
Quitr nice for things like phone mounts for the bike and such.
