We have been getting some questions on our forum about glueing White, Strong & Flexible. So we decided to research the matter and test out our results. The material that we call White, Strong & Flexible is also called Nylon 12 or Polyamide 12. Polyamide 12 is a flexible thermoplastic that enables live hinges and is generally used for flexible tubing and piping. When it is used in rapid manufacturing or 3D printing it comes from a powder. Polyamide 12/Nylon 12 materials used for rapid manufacturing therefore always have a powdery feel. Each manufacturer has its own version of these materials, EOS for example calls it PA 2200. If you would like more information on the material you can check out our materials page and the acompanying data sheets.
Selective Laser Sintering is the process that turns these materials into parts and 3d models. SLS works by having a laser scan a layer of fine white powder. It then fuses the material where the material needs to be built up. The build tray with powder is lowered and a new layer of powder is applied and the laser continues scanning and fusing. Because of this SLS does not have any support material, the powder that is not fused acts as a support material and can be easily taken out of the model.This glueing advice is meant to be for our White, Strong & Flexible material, not our other materials.
We would advise people to not glue parts unless you have either broken one or want to build something that exceeds our SLS build envelope of 20 by 25 by 23 cms. For this little demo we have taken my little SLS stress ball. I’ve carried it with me for the last month or so to show people our materials. The ball itself is based on a design by Shapeways member George Hart, you can check out his site here. It was my favorite Shapeways model for a while because you could bounce it. The structure of it is very mathematical and George’s page explains it well. So, I carried it with me every day and was continually bouncing it on the floor and street as I walked and off the walls of my office when my boss was out. Then one day it broke.
Breaking rapid prototype parts is either easy or hard, it really depends on the design and the material. If you make a thin tail-like structure such as our monkey’s have, then a good push with your thumb will break off the part. If the structure is less limb-like a hammer might be needed to break it. SLS is a strong Nylon and it is resistant to impact. When it does break it tends to shear. The rhomboid structure used in the ball meant that this was a robust enough structure to withstand weeks of bouncing on the streets of Amsterdam and Eindhoven but sadly all that wear and tear built up. The coup de grace was an unfortunate incident of some careless oaf stepping on it. 
The picture to the right was the result, my dear little ball had become pacman. If you get the felling that I might very well have become quite attached to my ball then you would be correct. Thankfully our research told me how to repair him. There are two very main stream glues that one can use to repair Selective Laser Sintered Nylon: Super Glue and Araldite.
Super Glue is also known by its much less marketable name of Cyanoacrylate. The glues of this family are ethyl-2-cyanoacrylate, n-butyl-cyanoacrylate and 2-octyl cyanocrylate. Ethyl-2-cyanoacrylate is most commonly marketed as Super Glue(secondelijm(NL), sekondenlijm(DE)). The other two families are used as medical glues so you will probably not find them at your local stores. All of these glues can be used to repair or stick together Nylon 12 parts.
The second type of glue that you can use is Araldite, an epoxy resin. You can check out the Araldite website here. We chose to use Super Glue for our little repairs. Please take into account all the warnings on the packaging such as using a well ventilated room and keeping these products away from children. And don’t glue your eye, because that would just be terrible.
I applied glue sparingly and removed any visible left over traces with a paperclip. The glue of course becomes a different colour to the ma
terial so any excess glue is ugly. In repairing a complicated face or a large flat face you might want to sand crucial contact points. Generally however the glue bonds very well to the part and this is not necessary. Cleaning a surface beforehand though does improve the bonding of the glue. As you can see from the picture the ball healed well. There was a tiny hairline fracture visible but generally you did not notice it. Indeed my Nikon lens was unable to pick it up on this picture but you might be able to see it on the left.
There are two important things to consider when using glue though. One of these is the material properties of the glue itself. Nylon 12(or Polyamide 12) is flexible the glue , not so much. This means that although the glue repaired the part well and indeed restored the bouncy ball qualities of my object, sadly this did not last. After a half an hour of bouncing, the glue pressured by the forces acting on the sphere let go. So even though super glue works well to repair ornamental parts there are limitations as to how much functionality can return to a part. So, I would not assume that things such as live hinges and springs can be returned to their full previous functionality once broken. 
For the second thing we have to paint the model. Paint, what did he just say paint? Yes, later this week we’re going to give you some more advice on painting your models. But, to show you this one thing we had to give you a sneak peek. You see, once you paint a model that has been glued in some cases the paint accentuates the hairline fracture and the glue itself. If you look at the picture of my pretty red ball it looks great.
I painted it using a standard Tamiya Colour Spray paint. It took seconds and was fun to do. However when using paints the paint sticks to visible glue left overs. Strands of glue that you did not see previously will also become very visible now. It is very important that you use very little glue and have next to no left over glue before you paint a model. On my model my near invisible fracture now looks like this: 
Some delicate work with an unfolded paperclip meant that I could quickly remove the paint that had built up in the openings. The roughness on the outside though is still visible and will take some sanding and several coats before it is covered up.
So I would recommend that you use glue: only when necessary, as little as possible on visible parts, take care of leaving behind glue residue. I would also warn you that it is difficult to glue live hinges and springs while maintaining their functionality and to be extra careful with glue when you intend to paint the part afterwards. As a last piece of advice I would also warn against bouncing a recently spraypainted ball off the walls of your office. In some cases small red splotches of paint might appear.