Got Resin?

[Umi_Ryuzuki]

I have a question about the molds. I have RTV Silicone and I have latex mold-building compound. Can I make two-piece simple molds from latex too? Will it hold up to being demolded?

Thanks!

Kenny

Yes you can make two piece molds using latex molding compound.
Back when that was all that was available, that's what everyone used to
use. Latex is stiffer than the silicone, but as long as you have simple
pieces with few undercuts, it whould work well.

[Spacephrawg]

a correction to my previous post:


I interned at a sculpture casting studio a few years ago and they used Smooth-On exclusively, but the boss admitted to me that only one kind of paint would stay on it long-term. The said that he'd tried model paints and such and after a decade tehy would mostly have flaked off. He said the smooth-on resin secretes oils or something.

After I finished my internship there I didn't have a chance to try the stuff out for myself so I didn't think about it much. More recently, on another forum, I inquired about alternatives to Smooth-On and someone mentioned Vagabond. "Its what Verlinden uses! Its great!" or something like that.

Never got a chance to try that stuff either.

And then just recently it occurred to me that perhaps my internship workmates simply weren't washing the mold release off of the castings. Could that be the case?

I pondered this for a while and mentioned it to someone else I know who is in the sculpture biz and he said "hmm. all i know is I don't like smoothon products in general. They off-gas and break down over time."

Well now I'm just not sure of anything.

What do you guys think?

[Glorfindel]

I know this has already been covered in the past, but are there any new adhesives that will bond resin to styrene quickly without damaging both materials? What are the products used now?

[Space Garbageman]

Any type of CA adhesive (super/crazy glue) should bond resin to styrene.

[Squall67584]

I'm confused... I was reading the info on Smooth-On's website, and the differences between silicone molds, urethane molds, and all that. It was saying that silicone has to be put into a vacuum chamber to get all the air out of it and then it can be poured. But then when you look at their starter kits, they have silicone (Oomoo it's called) rubber that doesn't have to be vacuumed. It left me a little confused since they said all silicone had to be vacuumed, but then they sell stuff that doesn't...

Has anyone used their starter kits? I'm looking at the pourable one to cast a few small bits and pieces.

[Sparky]

Yes you will probably need to vacuum the oomoo, it is so thick post mix pre-cure that you will experience loss of detail on most [sci-fi] modeling applications if you don't vacuum the RTV after you mix it, and use some special pouring techniques to cover your masters. I used the oomoo in the beginning and was unable to mold this part even after trying the brush on first technique:
http://www.kc6sye.com/images/images_02_ ... _clean.jpg

Also if you plan to pressure cast the oomoo will have lost of air pockets in it that will kill the mold within a few pressure casting cycles, also castings will have distortion in them due to these collapsed pockets (when under pressure). With I had happen to me as well with early molds, since I got the pressure pot after I started molding and before I got a vacuum pump.

you may try the bombs away method of degassing and pouring and see how this works for you, I haven't tried it personally. This method is shown on the T.A.P. plastics website.

[Squall67584]

Hmm, I don't really have anything big in mind to cast, just a few small pieces here and there. I don't plan on casting anything bigger than a couple of inches, so I don't really want to invest in a pressure pot and all that stuff. I'll have to look into that bombs away method.

Is there a type of molding medium that's easy to use and not have to be vacuumed?

-Edit-

D'oh ! I see there's a sticky for molds... I've spent too much time with the model glue.

[Ziz]

Ok, what am I doing wrong?

Got some SmoothCast 305. I've tried casting several pieces with mixed results. Thicker pieces, around 3/8" thick, take a while to cure but eventually end up pretty solid. Some flat panel pieces, no more than 1/8", remain rubbery and flexible no matter how long I let them sit for. And this is from the same mix batch, not separate mixes. I'm guessing that maybe the proportions of A to B aren't exact because I'm trying to mix very small amounts - half ounce or so of each - so maybe it's more sensitive than a larger volume would be? Either that or I'm not mixing it enough before pouring.

Suggestions, opinions?

[modelnutz]

You need to be pretty close on the 1 to 1 ratio....the smaller the batch...the more critical it is.
For small batches, I like to use low cost graduated medicine cups that I get from CVS.

Also, make sure that you pre-mix part A and part B very well.

On thin pieces, temperature matters...a cold mold will slow the cure.
You may want to heat the mold for the thin parts before you pour...a half hour in a very low temp. oven should help.

[Johnnycrash]

How long are you letting the cast sit before demolding?? SC305 is the SLOOOOOWWWW set stuff. It calls for 30 minute demold time, leave it for at least one hour. As modelnutz mentioned, temp is a factor. As well as something called cross sectional area. Or, in layman's terms; the thicker the piece, the better/faster is will cure out. Or, backwards... The thinner it is, the longer it will take to fully set up and become solid. Adding heat after it has set (low over time), will help speed up the curing time. Polyurethane two-part resin is an exothermic chemical reaction cure system. So, the thinner the parts, the less heat is can create internally, and that affects the cure time/firmness of the part at demold.

The mix ratio is forgiving, but with side effects, as you have seen. Too much "B", and it becomes brittle, too much "A", and it goes milky clear, and rubbery. Neat to know if that's the effect you want/need.

Get some of those medicine cups. I have tons of them, and I USE them too. In some cases, you may need to mix up MORE than what you need to stabilize the mix ratio. Have a large mold, are a bunch of little molds, and fill those. I have a bunch of greeblie molds and thing like that that I fill when I mix too much, or need to mix more than needed for the mix ratio to be right.

[Ziz]

I have some graduated cups but they start at 1 oz, not fractions of an ounce under 1. I'll check CVS and RiteAid tomorrow.

I'm letting it sit for over an hour. I even tried putting it under a desk lamp with an incandescent bulb to try and heat it a little after pouring but still no go. That's why I'm figuring that either the ratios are really sensitive or I'm not mixing it enough. I'm just starting to get my feet wet with this, so I've only got 3 or 4 molds I've made for test purposes of random parts from the greeblie box.

Instructions do say to heat the mold at about 150*. What's an easy way to do that with standard kitchen equipment? I'm in an apartment so I'm doing this on the kitchen table for now. As such, I don't have access to fancy casting stuff like dedicated heaters and vacuum canisters.

[Johnnycrash]

I have some graduated cups but they start at 1 oz, not fractions of an ounce under 1. I'll check CVS and RiteAid tomorrow.

Yeah, those are not going to work. You need the graduated ones. The ones I have 1cc increments.

Instructions do say to heat the mold at about 150*. What's an easy way to do that with standard kitchen equipment? I'm in an apartment so I'm doing this on the kitchen table for now. As such, I don't have access to fancy casting stuff like dedicated heaters and vacuum canisters.

Toaster oven. They usually offer lower temps, with better control. And you don't need to heat the whole oven up for a small mold. And you can get a second hand one. Hey! It's not like you are going to put food in it, so it's past history is irrelevant.

[Sparky]

Here is what I used to push the clear cast to cure. Even the fast set stuff will not fully cure, if you are casting consoles and stuff, so it is better to get the slow set stuff and push the post cast cure, other wise you will have a lot of lost casting due to bubbles. (Fast set jells very fast preventing it from flowing into dial faces and levers, but won't continue to the hardening stage when casting thin parts.)

Be sure and get permission to retire the crock-pot before attempting:
http://www.kc6sye.com/images/images_04_ ... _resin.jpg

I filled the pot with sand as a heat transfer medium, and something that would retain some heat. Put in a ceramic dog food bowl then put a square of ceramic tile I got from home-depot (for back sanding parts).

I pre-heat the ceramic tile/bowl setup just before I start the casting process. After the cast has sat in the pot for several hours >2. I pull the casting and transfer it to the setup, at that point I turn off the crock pot and let it sit (usually over night). Be sure and let the parts cool completely before pulling. Alternately you can pull the part early form it to a curve or what-not.

[PetarB]

I ordered some resin which cures a bit too fast for my liking.

Does anyone have any experience with retarding cure times for resin? I'm mainly interested in preventing the onset of the gel like phase....

[Umi_Ryuzuki]

Put it out in the cold garage...

[LobsterOverlord]

I've been using Alumilite for years. The set time is a few minutes. What is the general set time of the "good" resins?

My employer is having me look into decent quality stuff for manufacturing parts (garage kit kind of stuff) and we're trying to figure out what's going to be good for production, but we're not sure if we should look for the faster setting stuff, or go with something that takes a couple hours to cure. Is there something generally worse about the quicker set times?

Thanks,

John M>

[Sparky]

Its mainly about 'pot time,' if you are pouring into a lot of molds or a big mold you might want to go with the longer pot life so that the larger batches of resin aren't kicking in the mixing cup. There are also some resins which cure to a very hard plastic that can be milled and cut. If you will be doing slush casting Id think the longer pot life is a better idea than fast set resin.

[MillenniumFalsehood]

I'm trying to cast a small clear part. I don't need a bunch of them, so the Smooth on stuff is too expensive to justify purchasing it. I'm thinking about using Envirotex, but I am worried that it won't set up right. My piece is about 3/8" thick at the maximum. Here's a picture of it:

http://i1183.photobucket.com/albums/x47 ... C06882.jpg

It's the one on top.

I also thought about getting the pint size of this resin:

http://www.delviesplastics.com/casting_resin.htm

Does anyone know anything about this stuff? I've heard bad things about polyester resins, but the price is very attractive for a hobbyist like me.

[Sparky]

I've had issues with the envirotex setting up in rtv molds, the oomoo is a definite no. And I've had it both work and fail in some other alumiltie rtvs, real pain to clean out took a few castings to get all the sticky out.

take a look at smooth on's new color match. Last time is saw a casting of their new formula it was milky white rather than pine tree sap brown. Not crystal clear but good for some lighting fx.

[junglejim65]

I've been using Smooth On products for a few years now, mainly because there's a local dealer and it's priced right.
-Oomoo - very short shelf life I've found once it's opened.
-I'm using Mold Max 10 currently with a dash of Silicon Thinner. Seems to be pretty forgiving with the 10-1 mix.
-Smooth Cast 321 is my go to resin, pressure cast at 60 psi. I used to have problems with an oil-like seepage occasionally, but found that I wasn't mixing it long enough and probably had the mixing ratios out a bit. I stir it for at least 2-3 minutes now.

Jim

[gsb5w]

I've been using Alumilite regular, Allumilite White and Alumilite RC3. The regular has a pot life of 1.5 min, the White is 2.5 min and the RC3 is 3 min. I would recommend the RC3 or the White if you really want as few air bubbles as possible. I've also used Aeromarine products and found they are good too. Never tried Smooth on products.

[MillenniumFalsehood]

If I degas it and put in a good mold release, can I cast epoxy glue?

[Lt. Z0mBe]

If I degas it and put in a good mold release, can I cast epoxy glue?

I would think so. I "cast" some once into a flat sheet in order to cut some wingtip navigation lights out of. I just poured a bit into a little tray and let it set up. I *think* I also used it for some clear instrument panels but I could be wrong. The upside was it was simple to do. The downside is that it remains relatively soft and seems to be tacky for a long, long time (believe me, I mixed it well). Also, it's kind of thick so I don't know if the bubbles would escape before it starts to cure for you. That would be a problem if you're doing this to have a clear cast part.

This all begs the question, though: assuming you don't need a clear part, why not just use epoxy putty pressed into the mold? Any flaws can be easily repaired with more putty. If it's a complicated mold with lots of undercuts, you're gonna be doing a lot of repair work anyway if you use epoxy glue.

I hope this helps.

Kenny

[MillenniumFalsehood]

It'll have to be a two-part mold, I'm afraid. I'm copying an old engine dome for the Enterprise kit, and it is a rather complex shape. I'm thinking about simply coating the interior of the cavity with degassed epoxy and then pressing the two halves carefully together to make a whole part. The part does indeed need to be clear, so I can't just pour resin into a mold and copy it that way. I don't think the relative softness of the epoxy will be that big a deal since it's not a structural part and it's rather thick.