I wasn't expecting my first resin casting (mis)adventure to go to plan, but it went better than I expected. I did get something vaguely resembling the original master, at least!

I used fast cast PU resin:
http://www.cfsnet.co.uk/acatalog/Fastca ... Resin.html

This should demould in 15-30 minutes. However, the resin has still not completely set even after 36 hours. It seemed to reach a certain point quite quickly (about 30 minutes) and then stop curing. The resin could be demoulded but is still quite flexible and, most notably, sticky to the touch. It also still smells.

Any ideas as to what went wrong? As far as I know I followed the instructions correctly, I weighed the parts on a 0.1g scale and shook them in their containers first. The resin was delivered less than 2 weeks ago and the temperature was about 23 celsius on the day.

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Probably didn't mix it correctly.

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Like E&C stated, it's probably a mix problem.

However, sometimes you can cause a chemical reaction often refered to as 'cure inhibition' if your resin/silicone comes into contact with a reactive substance. (did you use a mold release, was it something the manufacturer recommended) This usually happens if you attempt to use a sulfur based clay when making a mold with silicone.

In addition, many newly prepared molds need to cure before you attempt to make a casting.
As they 'dry' they release alcohols and other solvents that can cause inhibition of the resin cure.
In this case the resin usually will cure, it will just take longer than the ideal.

If it reached a certain cure and stopped, then it is likely that there wasnt enough catalyst to fully cure the resin, this could be because:

Incorrect proportions, did you have the right instructions?, doesnt help if you follow them if they gave you the wrong instructions.

Expired, Is there a use by date on the container? resin does expire and failure to cure is a common occurance among very old resin, it wouldnt be the first time a supplier had a batch hanging around too long.

I had a similar problem with my initial cast using alumilite RTV molds, then I realized that I hadn't heated the mold in the oven to 140 degrees F (stupid instructions making me read...). After that they all cast very solidly.

Either you got the proportions wrong when mixing the resin or you didn't stir it enough. Both quite common first-time mistakes.

I think I can rule out quantities. It is a 1:1 mix and I used a .1g scale to measure (I wanted to be sure). The silicone should also be fine as the mould is over a week old, and has cured well at room temp. I didn't use a release agent. There is no date on the tin, it just says use within 6 months (useful!)

I used plasticine, but the worst affected areas are the back, ie only in contact with the air. The only other materials involved were plastic cup and knife for mixing. I'm guessing not enough mixing is the prime suspect given some parts are Ok and others aren't. I'm surprised as I thought I'd been quite thorough and it was a uniform colour (beige), but seems the most likely explanation from your suggestions.

I will do another one (without mixing too much this time!) and see what happens.

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Yep, As said above probably not the right mixture or poor mixing I have had the same trouble on some occasions especially when using a new resin, the problem been im crap at working out mixes and the other is the mix stage, as if you use a fast demould time resin if you spend to long stirring it starts to go of to soon.........

The resin your using seems like a good price though and if you can get used to it and get decent results it may be worth sticking with that resin, I myself use easy flow white, its a simple equal quantity mix and gives very good detail, the only draw back is the price at £18.00 plus p&p for 1kg and it does not go very far with a few miscasts, waste etc

" You posted the above post before i posted yes defo not mixed propally if its a 1 to 1 ratio, also i use three clear cups mixing, one for each liquid and the final one for pouring both in and mixing, make sure you stir well for a least a minute and make sure you get the edges mixed in, try not to pour down the side of your mixing pot. Also i have left 1 to 1 mixes for a few days and they have gone hard, only once have i had a cast that stayed like rubber. regards epic

Regards epic

Also, is the ratio 1:1 by weight or by volume? The resin I use has different densities for each part and is meant to be mixed by volume - when mixed 1:1 by weight it comes out soft.

Was just thinking that, will have to find out. The kit is sold by weight, but it doesn't say specifically.

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Yeah, because they're different chemicals, they very likely weigh differently. You might want to pick up some graduated mixing cups to make sure everything is equal.

I had the same thing happen to me a few weeks ago - after dozens of perfect casts, all of a sudden, failure. I thought my resin went bad. Nope, apparently I was slightly off on my eyeballing of my components. I had a crunchy shell with a liquid center - it was pretty gruesome.

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Sounds delicious.

Toxic, but delicious.

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It seems most people eyeball the quantities. At the quantities we're talking about (IIRC 45g or so) are differences in density going to be more than any inaccuracies due to eyeballing?

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Scratch my last post, it's irrelevant. I didn't spot it in the data sheet before, but it is meant to be mixed by weight anyway.
http://www.cfsnet.co.uk/acatalog/CFS364TECH.pdf

When I get some time next week I'll have another go with lots of mixing.

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