I have previously raised the issue of poor technical/structural design consideration, but this time I want to raise the issue of failings in the technology systems themselves, or is it the operatives?
Bear with me here this might be a bit boring for some of you. I am referring to the variable condition of ‘cured’ resins. Let’s try and understand some basics first. Resins when they build are built in layers, but that layer would look like a regular grid of ping pong balls if magnified. Add many more layers, and you create a structure with a grid of passageways containing liquid resin. Now keep this in mind, and add in the fact that the resin contains a photo-inhibitor to improve build definition.
The build process is mostly very reliable, but the resultant build is a semi-cured structure, also containing liquid polymers that need to be cured. If liquid polymers remain in the structure, then in the early stages of the burnout, that liquid migrates into the surface of the mould, eventually hardening at higher temperatures. Next the resins starts to contract and with the mould surface now bonded to it, pulls the mould surface away. This results in loss of form and detail, but at least the surface is raised so could possibly be re-worked. The burnout continues and all the resin is burnt away, but those bits of the mould surface that were pulled away now remain as particles, loose in the mould. When the metal flows in these particles will end up wherever in the casting, usually at the surface, resulting in depressed defects. Neither of these issues are porosity, they are a result of the variations in resin processing.
After the build the resin needs to go through further ‘curing processes’. The recommended procedures are first a heating cycle to evaporate some of the liquid polymer, but that will only effect the build to a shallow depth from the surface. Next the resin is subjected to hours of exposure under UV light to fully cure the resin. However, remember the resin has a photo inhibitor so the UV cannot penetrate deep enough to cure completely. I have also heard that some people put the resin in some water and microwave it! Please don’t. It may work for thin sectioned items, but anything normal like a ring, it rarely works.
But on the plus side, it is also clear that some people are more successful at applying these processes than others. We know that properly cured resins will cast perfectly and can produce results like a beautifully machined piece of metal. Then there are other users who just cannot seem to get it right.
So what is the point of this article, first to enlighten and educate all users of these technologies? It is clear however, that the providers of these technologies need to do more research into post-build processing that will give predictable and consistent results. To that end I, and no doubt other casting companies, are working with technology providers to try and find a solution.
When it comes to casting resins, don’t automatically assume the casters can’t do their job – 90%+ of failures/casting defects found in resins are caused by incomplete curing, and sometimes poor structural design leading to mould failure. Contrary to what you may think from this article, I actually think CAD/CAM is great, a wonderful opportunity to diversify, be creative, and offer bespoke design services. But educators and trainers, please, we need many more real traditional crafts people to help process the results of this modern creativity. Castings don’t finish, assemble, polish, and set themselves.
This feature first appeared in the February 2017 issue of Jewellery Focus