I develop products that require integrating 3D printed elements with casting resins of which there are three types: polyester, polyurethane and epoxy.
I have cast a nylon bolt into polyester resin. When cured I turned the bolt out and discovered workable threads in the cast that being the objective I wanted to determine. Learning about adherence with other types of material used to print is of considerable interest to me and I surmise others.
Using resins that may not bond will require building keys into a part which is easy of course. With the creative crowd here I think many will be interested in this application and I think it both interesting and important to determine what kinds of surface bonds if any the resins can, will or won’t make with respective filament material types, shrinkage, cracking adjacent parts and so on.
Let’s go with some experiments. I will be casting more polyester resin into a nylon piece this week that will be domed with polyurethane and will comment when cured. Doming explained (google: meniscus chemistry)
Comments or questions from others interested in experimenting with casts and prints?
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Xeno
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Your idea is great, I have done this for a long time, I used PVC materials, as most resins don’t adhere to PVC.
But there is a solution that makes it even more simple, release agent 
spray it, paint it or dip it, there are all sorts of release agents.
Good to know Zeno but bass ackwards so to speak; what I (we) want to learn is how to use casting resins to bond to printed objects not release. I’m thinking a poured on welding metaphor. We pour the resin onto the surfaces of a print and a bond at the surface is created by some catalyst between the resin and the material and form that was printed. Your thoughts on this?
Xeno
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If the material is porous enough the bond will be good, but most resins just stick, and not really bond, also consider the shrinkage and heat element of resins, bigger parts could contort because of added shrinkage of the resin, or deform because of the heat. A lot, if not all polyurethane resins won’t vulcanize with itself, and just sticks, and when force is applied it breaks off, that is because some of the catalyst sweats on the surface when curing, this prevents vulcanization.
Polyester resins can have high shrinkage rates. A casting epoxy resin would offer a better solution for filling the voids of your prints, introducing keys or overhangs in the printed parts void will help to lock the cast resin in place if it does seperate from your nylon piece
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I concur and have recently used epoxy resin to cast a bolt and it does not really noticably shrink however I continue to work with polyester because it can be buffed and polished whereas epoxy does not like to be buffed and polished.
In my experience with casting the polyester resin really only shrinks on the surface exposed to air as I’ve discovered presumably due to the presence of nitrogen perhaps. The worst part about polyester resins are the dangerous VOCs emitted so working anywhere in one’s home is out of the question and I would not even use it in a shop without ventilating exhaust equipment; I like using the garage as the car and all the other sh!t out there have no complaints as of yet. lol
