Silicone Curing Problem

[becks]

Ok quick question for yall.
As you can see from the thread in my sig, we were working on pulling a mold off our clay (non-sulfur) molded helmet for casting. After pulling a couple casts we realized that the print layer seemed to have no cured properly and thus, upon pulling it away from the cast took chunks of silicone with it and made some bad marks on the castings.

So we regrouped and tried again, one side cured fine on the print layer but the other side remained quite tacky and would borderline wipe off. Now I could attribute this to the fact that the side that didnt cure was facing a cooler breeze that was blowing into the garage, but no such breeze existed in the first molding.

So any ideas? Bad mixing? Bad batch of silicone? Need more heat? Certainly getting frustrating at this point that we have a finished helmet just waiting to be cast, but this time I want to make sure all of the layers are properly cured before we move onto the next, lest we waste silicone.

 

[TyrDropProps]

I'd guess bad batch of silicone but it very well could be mix or weather related.
Did you mix the same each time you did a layer?
How much did the weather vary while the silicone was curing?

 

[Arioch]

Are there any other chemicals coming into contact with the mold? SOme chemicals react badly with silicone. I think latex tends to ruin it. There may be others.. if youre using a mold release, make sure its ok with silicone. Another thing is maybe if youre not using a mold release, you should try one. It will put less stress on your mold.

 

[becks]

The weather went from sunny and warm (75) to cloudy and cooler before a thunderstorm. The side that didnt cure was facing out and getting the breeze and I suppose possibly increased humidity. For this one it was the same batch for the print layer for both sides. We scrapped the print layer when we saw this as to not continue till we had a good cured one.
That being said, it doesnt explain the original mold print layer which was done indoors and still didnt cure well.
No other chemicals came into contact and no mold release was used.
If it was mixing, any suggestions on how to best measure out silicone to ensure equal parts of each compound? Bit of a PITA given the viscosity of the stuff.

 

[becks]

Bump. Any ideas or suggestions for better mixing? Would like some input before we burn more silicone in a futile effort.
Thanks.

 

[Roadwarrior]

A long thick-ish paint brush is good for mixing(make sure you only put the handle in and not the brush) I measuring using two red Dixy cups and it also uses a 1:1 ratio and It's not too finiky if you have a little two much of either part. You may want to wash the model and see if that helps.

Also, how long did you wait for it to cure the second time?

 

[TD2253]

I use a 10:1 mix ratio of silicone (10 parts) and catalyst (1 part) from Aeromarine Products. I've been using their product for about 5 yrs. now and had no curing problems whatsoever. From what you've described, it seems that you either didn't mix your batch thoroughly or your mold is going bad.

 

[becks]

two red Dixy cups and it also uses a 1:1

Same method we used as well. It sat for a couple hours on just the print layer and one side was perfectly cured at the other, while set up, basically wiped off. I didnt think about washing the model, thats a good idea. Also think I may switch to a new jar of silicone and see if that makes a difference.
Thanks!

 

[ShadoKat]

What kind of silicone are you using? Rebound, Dragon Skin, other? I had a curing problem for my first batch of silicone, too, and I will give you the same advice the Smooth-On rep gave me, which is to clean the model with a very light application of mineral spirits to remove any potential contaminants. As for the potential environmental factors you talked about, I'm no expert, and this is just a suggestions, but you might try not curing it outdoors, if at all possible, and do it where you have slightly better control of the environment. Sure, silicone is not entirely non-toxic, but in a well-ventilated room you should be just fine. If you must be in the garage, close the door, get a space heater, and/or enclose it in a large box or something after you're done applying the silicone, so it's protected from the environment. If you do this, though, I would also ventilate the box. Even though I'm pretty sure most silicone molding products don't air-cure, I would think you don't want the fumes to collect under there too much.

 

[active84u]

Avoid ANY contact with anything at all that has touched latex. Contact with latex (including latex gloves) will inhibit its curing and if there is a lot of contamination, it can prevent it curing at all.

I work with both latex and silicon and have seperate equipment for using each.

 

[becks]

It was rebound 25 and no latex was ever anywhere near it.
Good idea about the mineral spirits, I will have to give that a shot.

 

[BLACKULA727]

Are you using rustoleum primer? If so, I have heard that the fish oil used in Rustoleum primer can inhibit the silicone from curing properly.

 

[MightyJohn]

Becks,

I'm having the same problem.
I've tried 4 times now: I thought I messed up the ratio, so I made a second one. I thought, maybe the the model wasn't clean, so I tried a third one. Now after BLACKULA727's info, I just tried a fourth, using the same paint I made my first (good) molds with. No good.

My final assessment: I bought my rebound 25 about 8 months ago and this is what happens when it goes bad. I was hoping it had a longer shelf life; as I will be throwing away a little less then 1/4 of a gallon.

Hope this isn't your problem too. Good luck.

Anyone know the expected shelf life of Rebound 25? Couldn't find it anywhere.

 

[ShadoKat]

Anyone know the expected shelf life of Rebound 25? Couldn't find it anywhere...

I think the recommendation is to use it within six months after it is opened.

My final assessment: I bought my rebound 25 about 8 months ago and this is what happens when it goes bad.

Was it full and unopened in the intervening 8 months between when you bought it and when you used it, or did you use some of it right away, and let the rest sit for the 8 months? I had an unused trial kit of Rebound 25 sitting on a shelf in a bedroom with no temperature control for over a year, and I didn't have that problem, cured just fine. That is actually a very good question... what DOES happen to silicone when it goes bad? Perhaps more importantly, is there a way to tell whether it is bad before using it?

 

[MightyJohn]

Thx for the info ShadoKat

I bought and used most of it 8 months ago. Less then 1/4 of a gallon was left to sit for 8 months in my basement.