Hi folks
I cast a small tapered pyramid in Glasscast 50 with base dimension 13 X 13mm and top dimension 6 X 6mm, with a total height of approx 65mm. The mould was made from Polycraft GP-3481 condensation silicone. It was cured for 48 hours before use. The mould master pyramid was 3d printed using standard PLA resin, which was then sanded down to create a smooth surface.
I mixed resin and hardener using a precision digital scale with resolution 10mg and confirmed the scale calibration using a reference weight first. I mixed for several minutes to be quite sure of uniformity given the small size of the cast, but made approximately twice as much resin as I required to ensure that I would have a uniform mix for the cast.
The resin was left to cure at approximately 20 degrees for a week. At this point it turned out very nice and bubble-free and was left to cure in air for a further couple of days. However since that point it's never really fully hardened. It feels hard but will slowly deform under pressure. (i.e it will bend slowly if bent by hand). It also can't be properly polished up to the optical finish I require. 

The excess resin, which was mixed in a disposable plastic cup, has however hardened perfectly. This makes me wonder if I have an incompatibility between the resin and the moulding compound, which uses a tin-based catalyst. Should I use an additive silicon product instead, or should I have cast in two smaller pours?. Or should I wash out the mould e.g with isopropyl alcohol or something prior to use?. 

I don't know GlassCast50 very well, so I'm sure EC staff will comment when they're back at work on Monday. However in the meantime, I do have experience with casting resins that are available here in Aus, and generally with poor cures on epoxies there are three things to think of: mix ratio/thorough mixing, cure time, and working temperature. My first suspicion would be that you have cast a very small quantity of resin that simply didn't have the mass to generate enough heat to cure properly in the working temperature and time given. Did you have more left over resin in the cup than you poured into your mould? There shouldn't be any problem using epoxy in a condensation cure silicone. You can be fairly sure that the mix ratios were ok and the components were well mixed, as the left over in the cup is ok. Typically any of these issues will present themselves as a tacky or cloudy surface, so it doesn't sound like it is anything like that.

So that leaves cure time/working temperature,  and I feel it is likely a combination of these two. These thick casting epoxies are designed to be extremely slow reacting, so that when they are cast in a very large mass at 20 degrees (ie, 5-10kg at a time in a river table application) they don't generate too much heat in the exotherm and get a thermal runaway. So when used in smaller castings, they can take a very very long time to cure unless the temperature is very warm.

My suggestion would be to heat the casting up as best you can for as long as you can. 8-10 hours or so at 60 degrees would be ideal, but if you can't achieve that then try for several days at 30-40 degrees. Be aware that because it has already been demoulded, there's a good chance the surface finish won't be excellent and there's a chance it will distort. However at least if it cures fully hard, then you know what the issue is and you can either do an elevated temperature cure prior to demoulding your next cast, or you can use a faster curing epoxy (GlassCast10 seems like a good choice for this size project, but I'll let EC staff confirm).

Thank you very much for taking the time and trouble to respond. You are correct in that only a small amount of resin was mixed, but the amount left over in the cup would have been comparable to the amount actually in the mould, albeit that it would have had a considerably higher surface area exposed to air, of course. I'm guessing maybe 20g mixed, 10g left over.
It's probably about three weeks since the resin was demoulded, though, and so it seems like the cure process basically stopped, but at the rather odd point where you'd swear it was hard - it'll even sand and - more or less- polish to a dull burnish - but it clearly isn't fully cured because pressing a piece of plastic fibre optic cable into it for a few hours deformed it (that was just an accidental discovery when I mounted the cast pyramid in the project it was intended for). It was not sticky when demoulded, either, it looked properly cured.
I did consider glasscast 10 but I was unsure that the total depth of the pyramid (65mm) meant that I would be exceeding the limits of the resin, even though the other dimensions are of course much smaller.
I have ordered some additive moulding compound just to see what happens and plan to also recast into the existing condensation mould but this time (a) wash and bake the mould to ensure any surface traces of catalyst are removed and (b) after the resin fully gels, bake the cast itself as you suggest at maybe 50-60 degrees or so, and see how that works out. I am not at all experienced in casting so this is effectively my apprenticeship in mould making and casting and you have to chalk up some failures to experience. I had to also learn 3d printing (and design) to get to this stage, so it's been a steep learning curve but incredibly rewarding. And the basic quality of the resin is superb. In particular, it has excellent light transmission capabilities even at the short end of the spectrum (405nm) where a lot of casting resins have terrible turbidity and excessive absorption. So the resin itself is obviously quite suitable for optical components. This is great because I was able to prototype using UV cure resin, which allowed me to experiment with moulds and design and get an instant cure (using a 405nm laser diode) but that resin - not surprisingly - absorbs and fluoresces at that wavelength - so it's only good for a quick proof of concept, although it also does cure water-clear.

The GlassCast range has been tested in condensation and addition cure silicones and both were curing without problem.  We have not tested that specific silicone however, so there could be some kind of issue with that brand. Usually with cure inhibition, it is the surface in contact that does not fully cure so for the whole thing to be soft is a little strange.

It does sound like it has stopped curing at about 95% cured. Is the surface tacky at all?  Typically if there is an imbalance of hardener and resin, or poor mixing you would get a slight tack to the surface.  Very small batches can cause problems in terms of small errors being proportionaly magnified and even things like the corners in a mixing cup which has unmixed resin in can have enough volume to upset the mixing ratio.  Also that size makes it harder to build up heat  which is needed to cure.  So it could be that or a combination of factors having an effect here.

The fact the cup went off ok does lean potentially to an issue with the silicone as a significant factor so something to look at for sure.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

I may have missed it somewhere but what quantities did you mix together (weight of resin and weight of hardener) - I can see that you mixed 20g overall which is a very small quantity in reality but if you could let us know the quantities of each, we can confirm that as well.