Searching for the perfect casting resin

Hvae you tried adding more resin after the shrinkage to fill the void?

Eric

snipped-for-privacy@bigfoot.com spake thus:

I don't think you can do that after the part is cast. Think about it.

Go to

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Look at the liquid Plastic products, I've used the smooth cast 305 with great success and noticed no shrinkage. Also when I didn't mix enough cast for the molds, after the hardening stage I just made another batch and poured it into the mold over what was already in there.

If you didn't measure the resin proportions very exactly, it can result in a pour which is softer than it is supposed to be.

There are several casting related groups on Yahoo Group - you might get some good advice there.

Also, I recommend that you post your question at:

are very helpful there!

Peteski

I was using two identical disposable pipettes -- one for each of the two components -- and they were filled to the same level to within a small fraction of an inch -- by eye probably only a few hundreds of an inch difference which was easy to see since the liquid level was just above the bottom of the bulb. I don't think that was the problem here.

HOWEVER --- each pipette full is approximately 1/8 oz and two possibilities remain: 1. While they were filled equally, they might have not emptied equally because of the differences in viscosity of the two components 2. The actual casting uses only a minute amount of even the 1/4 oz that I prepared and it's possible that incomplete mixing might have left some of the resin with different proportions. Since it's a 5-minute from liquid-to-solid resin, you don't have much time to do the mixing before you start to loose it.

Norm

" snipped-for-privacy@bigfoot.com" wrote in message news: snipped-for-privacy@t31g2000cwb.googlegroups.com... | Hvae you tried adding more resin after the shrinkage to fill the void? | | Eric

Actually that's not as far-fetched as it sounds since this is a clear plastic resin which allows you to embed objects by doing exactly that.

What I have tried is to overfill the mold, leaving a thin layer of liquid resin on the surface. I haven't demolded it yet, but either that surface layer evaporated or it really shrank because there are pieces in the mold that are still not full. I have to conclude that this product isn't suitable for castings this thin.

Norm

I tried using the Castin'craft Clear Liquid Plastic again. This time I overfilled the mold, leaving a thin coat of liquid resin on the top surface, figuring that it would prevent shrinkage -- or at least complement it.

WELL ... not quite. Two observations: 1. The remaining unused resin in the cup was clear and hard as a rock ... but the stuff in the mold was almost uncured soft. 2. I still had shrinkage in some channels

I have to conclude that there's some kind of chemical incompatibility here -- perhaps between the casting resin and the mold release since there's no release in the cup and that cures perfectly.

SO ... I'm going to try a third time, this time I'll clean the mold thoroughly and I won't use any releasing agent -- the unused slug comes out of the cup well enough that I should be able to de-mold it from the RTV mold without too much trouble.

Makes me wonder about the problems I was having with the Alumilite stuff too and since that's a 5-minute product maybe I'll try that first.

I'll post the results later.

Let me summarize by saying that I was having serious problems casing some very small parts. I've partially isolated the problem to a chemical incompatibility between the resins I was using Castin'craft's Clear Liquid Plastic Casting Resin Alumilite Super Plastic (white) and the mold release spray Mann Release Technologies "Easy Release 2000" wihch seems to be preventing the resin from completely curing.

I thoroughly cleaned the mold with both Acetone and Alcohol and then I repeated the casing with the Alumilite (5-minute) stuff with no release spray. Curing of the unused slug left in the cup seems to be completely uniform and that resin is very hard. Parts de-molded seemed completely cured but were much more flexible than corresponding parts from yesterday that were left in a closed box on the desk overnight.

SO ... I think there's definitely an incompatibility with the release spray and the casting resins I'm using. Since the primary purpose of a release agent is to prevent curing at the surface junction between the RTV mold and the part being cast, it's possible that the extreme thinness of these parts -- some of which are only .025" thick -- may just be preventing the part from curing at all.

BUT ... the difference between the hardness of the slug in the cup and the parts I demolded leads me to believe that the RTV is partially at fault as well. I hate to get caught in the "You can only use my resin with my RTV!" shtick that manufacturers sometimes pull by creating minor incompatibilities between their product and their competitors' stuff, but there may be some of that going on here. For the record, the RTV I'm using is MicroMark's 4-hour One-To-One which seems to me to create a rather poor mold anyway -- corners aren't as sharp as they might be and the rubber is very fragile.

But the adventure continues. More later.

Norm

"Norm Dresner" wrote

(snipped)

| BUT ... the difference between the hardness of the slug in the cup and the | parts I demolded leads me to believe that the RTV is partially at fault as | well. I hate to get caught in the "You can only use my resin with my RTV!" | shtick that manufacturers sometimes pull by creating minor incompatibilities | between their product and their competitors' stuff, but there may be some of | that going on here. For the record, the RTV I'm using is MicroMark's

4-hour | One-To-One which seems to me to create a rather poor mold anyway -- corners | aren't as sharp as they might be and the rubber is very fragile. | | But the adventure continues. More later. | | Norm

Hi Norm, I'm not adding anything to this discussion because I know very little about mould-making, but seeing as you seem to have a good knowledge of it I wondered if I could ask you something - is there any reason not to use ordinary silicone sealant to make moulds with? I mean the cheap stuff that is used to seal gaps in the bathroom etc. I know what RTV mould making silicone is but it seems awfully expensive and fairly difficult to get hold of.

Ivor

that is used to seal gaps in the bathroom etc< Because that stuff is really a silicone glue! As it is anyway with molding silicone making sure the master doesn't stick can be a problem.

Proper proportion is critical especially if the final quantity is small. I tried using pipettes or straws but because the time is limited and the viscosity of each part of the resin is different, even if you measured the correct amount, different amounts will be left stuck to each pipette's wall.

I have used a Smooth-On brand of clear resin and I too notice that thin cross-secions are somewhat flexible.

If you want a stiffer part, try using Alumilite White or Regular (tan). They are stiffer (if mixed properly).

Also, small cross section pours benefit from heat curing. I pour my parts and then I place them in a food dehydrator (around 100 deg. F). That speeds up the cure and makes the cast parts stiffer.

Peteski

Alumilite does not requre any mold release when used with Silicone RTV molds (like Dow Corning 3110). I've cast dozen's of parts and the mold is still ok.

Alumilite sells several types of Silicone RTV rubbers. Like I said, I use 3110 and it picks up the most minute details from the master.

Like I mentoned - visit Alumilite's website and check out their products and the forum.

Peteski

Go here. They stock most everything that has to do with the type casting you're doing , including release agents , mold materials , master wax etc ,etc. They also have a great CD that covers many aspects of the molding and casting process.

Ken Day

Just a thought. Many casting resins and other casting materials mix by weight.......some mix by volume , so you need to check that.

Ken Day

Since I've determined that the release agent was the problem, I'm certain you're right.

BUT ... it sure would have been nice for Alumilite to have included that in the full 8.5x11" sheet of instructions -- they certainly had the room for it. They may make a fine product -- and I'll probably use the bottles I have until they're empty ... but I won't purchase Alumilite again because they don't care enough to include complete instructions with their products.

Norm

You can use just about anything as a molding agent if it has these properties: 1. It retains it's shape under pressure 2. You can remove the masters from it when making the mold 3. You can remove the resin from it after casting. I've communicated with some people who have used Sculpey oven-hardening clay as a molding agent. In my experience, the hardest thing is to get the master out of it. I've been told that Vaseline works but I've never tried it. OTOH, I've used Sculpey to make masters for molding.

Norm

If you read my OP, you'll see that one of the products I was having problems with was Alumilite White.

What I found was that a little extra thickness made a significant difference. By overpouring a tad -- winding up with a .008" layer of flash over the whole mold -- I made what looks like perfect castings. And for Alumilite, it seems to make its own heat. If you've ever felt the slug left in the cup about 5-10 minutes after mixing, you'll understand what I mean.

Norm

Alumilite provides two, identical, small, (1 oz) measuring cups with instructions to measure the same "amount" in each. I'm pretty sure they mean volume here.

The Castin'craft product comes with a chart by thickness of final part with instructions on how many drops of hardener to add per ounce of resin. Again, I'm pretty sure from the illustrations they're talking about volume.

But yes, you're right, there are some products that need to be mixed by weight.

Norm

I've been using Alumilite for close to 20 years now. I've never heard of an incompaibility with any release agants.

As far as literature goes, I've been using it long before they had a website. I didn't feel that there wasn't enough information included with the product.

And now they ahve a forum on their website where most posts are answered directly by their technical people! How much more can you ask for?! Anybody can create an account on those forums. You don't have to be a customer.

If you are unhappy - get in touch with them - they'll make it right. Yes, I'm a satisfied customer. They have helped me on several occasions.

And to answer your other post - I am well aware that Alumilite gets warm when it sets. But the small quantity you have in the mold doesn't have enough volume to warm up. Mold is acting like a big heat sink. Like I said - try heat cure. Again, I'm talking from experience.

Peteski