Thin film on surface of casting

[bucknast]

Hello all, I'm new to castings and to this forum so bear with me.

I'm attempting to make water clear castings of spinal vertebrae for an art project, and am using a two part platinum cured silicone mold, polyester resin with MEKP catalyst, and stoner rocket release.

The issue I'm getting is that there's a thin film that forms on the outside of the casting that folds and buckles when opening the mold and adheres quickly to the sticky surface of the casting, creating a nasty surface finish.

http://i.imgur.com/ry7OOjb.jpg

The mold interior has a nice smooth surface finish, so I ruled that out.

My thought is that this is caused by shrinkage and heat issues during curing, and am attempting to find an optimal ratio of resin to catalyst.

What am I missing here?

Thanks for any advice!

[johncast]

Hello bucknast

Welcome to the forum , sorry to hear about your problems with the casting .
As I have experience the very same problems on my first resin castings. Years ago

The problem is a incapatability with the rubber and resin . the Polyester resin is reacting with the silicone rubber , a problem that is called triping, as the texture of the sticky resin resembles the tripe you buy from the butcher shop. the casting will set eventually once taken out of the mould but with the poor surface texture.
Talk to your resin supplier and get a clear urethane resin, I'm sure that will give you better castings. your mould should be OK and not damaged, Give your mould a wash in soapy warm water
first before casting again.

Good casting, keep us posted on your results

Johncast

[bucknast]

Johncast thanks for the response! Any recommendations on a urethane resin? I'm thinking of trying smooth on's crystal clear 202. Also, would you happen to know of any resource that discusses triping?

[Fleafa]

I hate polyester resins
Smooth On is great though expensive depending on your location. Where are you?

[bucknast]

I'm in LA

[Fleafa]

Then Smooth On are probably not too expensive; go for it. Over here, in the UK, we pay a premium for their products.

[mangozac]

Wow Johncast thanks for sharing that! I wasn't aware of such incompatibility!

Bucknast a polyurethane will definitely be the way to go. The Smooth On Crystal Clear 202 should work well, but make sure you have a good read of the datasheet: that material can be a severe irritant (you definitely want to wear some breathing apparatus). Also note that for maximum physical properties it needs to be post-cured (in a low temperature oven for 8 hours or so). Is it a short or long term installation? Is maximum strength required? If not then you can probably get away without the post-curing...

[bucknast]

hey guys quick update. thanks for all your help so far.

Playing around with catalyst to resin ratios and warming the molds has helped a lot! the surface comes out with no triting.

I picked up some smooth on crystal clear 202 and have made a few castings with it. A couple things I'm struggling with: 1) bubbles, lots of small bubbles around the outside of the casting, which i've been told can only really be taken care of with a pressure pot, and 2) surface haze, the finish is great, nice and smooth, but it's not all that clear, a bit milky. I want to work with smooth on crystal clear because i've been told by several sources that when you get it right, it's definitely the most optically clear and white resin out there, is that a fair statement?

maximizing physical properties aren't all that important to me, optical clarity and purity are paramount. thanks for any advice!

[Pinselknecht]

Hey, did you heat up your molds?
If not, that is the answer!

All cristal clear resins do need heat to cure! RTFM!

[bucknast]

Yes molds were warmed prior to pouring.

[Pinselknecht]

did you use the right mixing ratio? is your balance 0.001g accurate?