Silicone mold screwup #2

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?

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

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!

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.

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

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

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.

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

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.

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.

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.

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.

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.

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.

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.

There aren't lots of choices in terms of silicone where I live. I was lucky my LHS had these in stock actually. When you talk about your wheels shrinking did you mean the resin shrunk or the mold swelled? If it is the resin, I am planning to try some Smooth-On resin once my tube of Alumilite is used up. Apparently the Smooth-On cures in 10 minutes rather than Alumilite's 3 minutes. That will give me some more work time to rid some bubbles.

Anyone tried making their own vaccuum chamber out of a vaccuum cleaner for casting?

I found out that their Quick Set silicones are really fragile. I casted one seat and the mold already broke at the seat belt slot under the head rest. The HSII on the other hand worked well. I never needed to cast 10 sets of wheels though so I probably haven't used the molds to their limits.

Sorry do not agree. This is not to say you did anything wrong, but what I find most when folks are having issues resin casting is user error. Micro Mark didn't go anywhere, they are still open, have a thriving business, and can easily be found online. Micro Mark sells rebadged Smooth On products. I use Alumilite white exclusively along with Dow Corning HSII purchased from Alumilite and have easily gotten 50-60 clean pulls from every mold I have which are a mix of 1 and 2 piece molds. I have seen zero shrinkage after a year on my molds and resin parts. I also have Alumilite resin which is 3+ years old that is still good enough to pour parts with. To the OP good luck with your resin projects no matter which products you use.

I just pulled a part out an hour ago and it looked decent. To my surprise the shirinkage was minor enough that my resin rim fits into the resin tire snugly! Resin used was Alumilite Amazing Casting Resin. However I did end up with more bubbles than I wanted, especially the rim where the thickness is right at 1mm in some places. A bubble curing in those places left me a thin film a push from breaking apart. If anyone can give me some tips on making a vaccuum chamber affordably it would be great.

Hi Kaho, can you post pics of your resin rim so perhaps I can try to help better? Are you using 1 piece flat mold? Or, 2 piece mold? Also, what product did you use to make the mold? I do not use pressure or vacuum and have no bubbles. There are steps you can take to prevent bubbles so I'd like to know more and see the pics to see if I can give some advice.

I had a pic of some parts in my Suzuki WIP thread. but here are some more with the rims cleaned up. You can see one of them had a huge spot missing, but that's probably my bad going too fast on covering the top mold piece. The rim is a 2 piece squeeze mold and the tire is a 2 piece injection mold.

http://i19.photobucket.com/albums/b168/kaho4g94/Vitara/DSC02935.jpg

I have some varying successes depending on what part it was. The cone and the brembo came out flawless (one piece molds). I had trouble casting the steelie with Super Plastic but once I switched to Amazing Cast it came out worlds better. The lug nuts are still a problem area though and in this photo I lost a lug on that wheel. As you can see the seat is a total nightmare. It is a 2 piece squeeze mold dividing at the seat bolsters like how it was made by Fujimi.

http://i19.photobucket.com/albums/b168/kaho4g94/Vitara/DSC02936.jpg

The right rim is the casted piece. You can see vaguely there is a bubble between 2 of the 5 lug holes. It's these kind of tiny bubbles that I have a hard time ridding of.

http://i19.photobucket.com/albums/b168/kaho4g94/Vitara/DSC02937.jpg

Tires came out way way better than I expected due to the intricate tread patterns. Some bubbles underneath the tread blocks but I think it won't be seen once painted. Problem is that I left flat spots where I cut off the sprues. It's going to be painful trying to carve out the treads there.

http://i19.photobucket.com/albums/b168/kaho4g94/Vitara/DSC02938.jpg

Hi

Here is my input, after casting for about 10 years.

16C is too cold. Anything to increase the temp a bit during curing will help cure faster.

I tried the pink Alumalite recently and found it to be stickier than my previous which was called MoldAll(green rubber). But it cures eventually. It just takes longer. I only prepare about an ounce of rubber(old film cannisters), so half a cup of catalyst. Just eyeballed half a cup!

With the green stuff I used to just eyeball the amount. More cures faster, less slower. Way to little seems like it will never cure and I have had one of those. Way too much, molds get brittle and crack and tear easily. If rubber is too soft and you will need for rubber around the outside of the part (to maintain mold stability), and harder obviously less.

One trick I have learned it to pour the rubber slowly in as very fine string. It helps to remove bubbles as it leaves the mixing container and in the mold as the fine filaments overlap and the rubber flows better into the parts features under its own weight.

Somebody mentioned resin. I have found after a while it goes off, particularly the dark half. I think it is the air to resin ratio in the bottle that impacts so as the amount in the bottle gets lower and you have more air it goes off faster. Unless you know you are going to use it all soon, I would suggest buying a smaller rather than a larger quantity.

Kaho, what exactly is a "squeeze" mold? Are you just doing a 2 piece mold in which you pour the resin in and lay in (on) the top half of the mold? Thus there is no pour and vent hole? If not please explain or show picture of mold. Personally I would still do a 2 piece mold on the rim but add a pour and vent hole in proper area's on the side (where inside of tire would hide, just like your tire mold). This could easily be sanded clean anyways. As for the tire, something you can try to prevent distortion of the tire tread with the pour/vent holes is remove a section of the mold with a sharp razor which extends down PAST the tire tread. KEEP this section (plug) as it will have your tire tread for that section at the bottom. After you pour your resin in, replace this plug/section and hold in place until the resin cures. When you pull your tire out there will be no visible pour hole and very little clean up. If your careful with cutting out this section of the mold in between the tire tread design, etc you can have a very clean piece afterwards. To eliminate bubbles caused by the mold rubber, for finely detailed items mix your rubber and simply "paint" it on your master first. This ensures no air bubbles are on/next to your master. After you paint your rubber on the master, do as person above stated - Pour your rubber HIGH at least 12 inches above the mold to create a "drizzle" effect. This thin stream breaks a majority of any air trapped in the rubber during mixing as it drizzles into your mold. Further, during mixing of the rubber, try not to lift your mixing utensil from the bottom of the mixing cup. Keep it touching the bottom of the mixing container/cup as much as possible. This also will reduce air entrapment. To extend life of mold, ensure your using a mold release after every 5-8 pours. You will also want to dust your mold with baby powder prior to pouring resin. This relieves surface tension which can cause tiny air bubbles on finished piece.

I've had great luck doing seats using a 1 piece mold in "split glove" manner. Simply stick your seat down to your work surface like you would any 1 piece mold, paint on your rubber (to ensure no air bubbles - optional), pour your rubber into your mold. When mold cures, flip it over so you see bottom of the seat. Now take a new Xacto blade and CAREFULLY cut the mold (where the back of the seat would be) and cut 1/3 to 1/2 half of way up. This will allow you to easily remove your master and any resin seats you pour from the mold. When its time to pour resin, ensure the area that you sliced is aligned properly and place a couple rubber bands around mold to hold everything in place. Pour your resin in from the bottom, let cure, once cured remove rubber bands and pull out your seat. Should have very minimal clean up on back of seat where the cut is.

Understood on most of these. It would be nice if you could explain more on the tire plug part. How do you put a plug back into the mold while there is resin inside?

I will post pic when I get home of one of my tire molds to help explain.