Silicone mold screwup #2

kaho · #1 02-08-2011, 02:46 PM

So this Sunday I bought a new 1 pound kit of Alumilite RTV and set it up to do three molds. (A wheel rim, thanks to stratos75 by the way!, and two tires) It's been a year since I did my last mold and when measuring the catalyst to the specified 10:1 ratio, I thought it looked a little too much for the amount of RTV I was using. I ended up pouring back some catalyst into the bottle and mixed the rest with the silicone.

At first I didn't trust my electronic kitchen scale, so I used an analog one, which I ended up didn't trust as well. I now know that it was a very stupid idea! I did keep track of the final ratio before mixing though, According to the readings from electronic scale this batch of mold has a ratio of 12:1. It's been 38 hours since they were mixed, and it is still sticky. The cup that used to hold the RTV still has overly soft silicone, although not flowing. If everything went well they would have cured in 24 hours.

Do you think 12:1 is way off? How long would you expect such a mold to cure? If this batch of silicone does end up curing, what would be the negative effects of using these molds?

white97ex · #2 02-08-2011, 10:29 PM

what is the weather like? specifically temperature and humidity?

Also, what is the precision of your scale? I use a small scale that reads in grams when I mix my silicone molds

Plumberboy · #3 02-08-2011, 11:22 PM

I'm pretty sure it is to be mixed by volume not weight. 100 grams of silicone to 10 grams of activator. That's the HSII silicone. Very expensive stuff!

kaho · #4 02-09-2011, 01:27 AM

The silicone is being cured in a house at 16'C. I am sorry I do not know the humidity of the house.
The catalyst is meant to be mixed by weight. It was specifically 177g silicone to 15g catalyst.

Anyways, it is hardening by the hours, although very slowly. It has gotten less sticky on the surface. It might be okay after a week. I just hope it doesn't take 6 months to dry with a little deviation in catalyst application. It would suck to have to stop all my projects because I have 3 pails of pink liquid with my parts in them, sitting on my work table.

Plumberboy · #5 02-09-2011, 08:21 AM

I don't know what I'm thinking! It is mixed by weight. The stuff I use everyday is mixed by weight too. I pay about 8 bucks a pound for my silicone but buy it by a 44 pound kit. Hopefully it will cure. If it does not fully cure hopefully you will be able to remove it from all of the detail of your parts. I have found that my silicone does not like to be next to a part that was detail painted with acrylic paint. It causes curing problems. Maybe the link below will help you out some.

http://smcbofphx.proboards.com/index.cgi?board=rs

white97ex · #6 02-09-2011, 11:12 AM

thats about 60 degrees F. Seems a bit cool, that could be slowing down the process

MAXIMA2278 · #7 02-09-2011, 03:09 PM

did you mix the catalyst separately before mixing the base with that catalyst. I know Alumilite you either have to vigorously shake the catalyst or stir it. But it sounds like there was too little catalyst in there, that can cause the silicone to cure slower and in some places not at all. It could also be your mixing container was contaminated with a petroleum base product of some sort. There is a number of things that can cause it to cure very slow.

MAXIMA2278 · #8 02-09-2011, 03:10 PM

My Background is in making silicone hands and fingers....

jano11 · #9 02-09-2011, 03:48 PM

16 degree Celsius is way to cold so don't expect it to cure any time soon, especially if you also mixed less catalyst than needed.

CFarias · #10 02-09-2011, 06:10 PM

It does seem that it's too cold to cure quickly. Try placing the mold into a food dehydrator if you have one or behind your TV set if it can fit. Don't put it in an oven or give it direct heat, like from a hair dryer, as the concentrated temperatures may actually melt the part being molded.

kaho · #11 02-09-2011, 09:25 PM

Thanks for the advices and suggestions guys! Luckily the molds are still curing. By this evening the silicone surfaces no longer leaves permanent marks upon being touched. It has cured to the familiar feel I used to get from all the other times. I am pouring in the second halves of these molds tonight.

ratpackv8sonly · #12 02-09-2011, 11:23 PM

Is it the pink one or the tan one? I have had the pink one take almost a month to cure and it was mixes right.

kaho · #13 02-10-2011, 02:11 AM

Mine is a pink one. HSIII and much more pink than the HSII I was using before. Like I said they are supposed to be fully cured by 24 hours so I'm not sure if you mixed it right.

ScaleTuned · #14 02-10-2011, 02:24 PM

I use Alumilite HSII all the time and it normally will cure way less than 24 hours but I let it sit overnight anyways. Its much easier to use the scoop that comes with the pink catalyst. 1 scoop of catalyst for every 2 ounces of rubber gives you perfect ratio each time. I've poured pounds of rubber and have not ever used a scale. Just alter this ratio depending on how much volume of rubber your project requires. As in, if you need 4oz of rubber use 2 scoops of catalyst. or 5 oz of rubber and 2.5 scoops of catalyst.

rsxse240 · #15 02-11-2011, 04:05 AM

I personally don't find the alumalite stuff to be very good for long term use. I was only able to get about 10 sets of wheels out of mine then the mold broke down and was to stiff to be able to get the parts out of. plus the shrinkage of the resin and rubber made for inaccurate parts over time.

I just went in a little while ago and found that my wheels have actually shrunk in size to the point the tires are only barely touching the rim any more, where as when I first built the model, the tires were stretched fairly tight to the rim.

I started using the resin available through Micro Mark (but they're gone now, I believe) and it was VERY good. The silicone I used was purchased from an ebay seller that I can't recall right now, but I had to buy a gallon (2 half gallons) and most of it went in the trash because it got to old and hardened on me in the bottle. But it was MUCH better than the Alumilite and after 4 years, I can still pour resin into the molds and have exactly the same part as the first one I made.

Long story, I know, but I just wanted to let you guys know about alumilite and there are MANY more, much better casting options available. You just have to do a load of research to find out what is right for your purpose. Most times, just a few pops to make a couple simple parts, the alumilite is good.

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