tips for resin casting?

[Titus Wells]

I did a lot of casting for a living some years back for the movies. We dusted the surfaces of our silicon molds with baby powder.

Take the foot of a nylon stocking or pantyhose and pour a half cup in. Tie it off. Tamp it against the surface gently. Then give it a gentle blow. Don't worry, the resin will soak up the powder. The powder actually draws the resin into details. That's why it works. It also breaks up the surface tension.

Doesn't that mean you can only achieve a matt finish though, unless you varnish it to hell afterwards?

[Professor Z]

It has no effect on the finish of the cast part. Give it a try.

Another hint that I'll pass along is re-using old silicone molds. You can grind them up with a meat grinder and use them as filler in new molds. It's best to use a non-filled layer against the part to be cast. Brushed on. Let cure. Then fill the rest of the box with a lumpy mixture of old and new.

There's a great book for do-it-yourselfers...I have it in a box somewhere,m The book has all sorts of cheap alternatives to professional materials. They make molds with hot glue, bathroom silicone and even Bondo. I'll try to dig it out. I'm sure the title would be a lot more usefull.

[silastic armor fiend]

thats a really good idea. i never wouldve thought of that.

[Titus Wells]

Back before I started workin professionally I made a copy of the Haaskekah swords from Dark Crystal and used a window sealant to add decoration to the hilts.... it took two months to dry for some reason!

[Rev. Marx]

Though not a Steampunk site, there is a great site I visited a lot when I first got into casting and mold making. It is for making DIY castles like the ones sold by Dwarven Forge.

http://www.hirstarts.com/index.html

Check out their forums and tutorials. There are lots of tips and tricks for casting and mold making.

[Titus Wells]

There are some nice pieces out there, and available as moulds. Actually those are not as nice as many I've seen.

I'm thinking of producing my own range over the next few months, scenery will be available in cast form initially, maybe then as moulds. Any ideas/things wargamers might want (specifically things not already available) let me know!

[Rev. Marx]

Actually, I have a question of my own.

I use polyurethane resin for casting, and sometimes I have trouble getting paint (even primer) to stick to the pieces after they are demolded. I suspect that it is a residual layer of silicon from the mold release spray, but I'm not 100% sure. I have tried washing the pieces in TSP and also in acetone (and sometimes both). Sometimes it seems to help, but sometimes they still won't take paint.

Has anyone else had this problem? If so, what do you do to get around it?

[Titus Wells]

Sometimes it can take a couple of castings before the paint will stick. Usually by the third casting the residue has gone, although I appreciate multiple casting is a pain. Sometimes a simple degreaser like waship up liquid or swarfega will work but if you've used a wax release spray (as I sometimes do) the resulting layer can be hard to remove. Boiling water sometimes works.

[Gryphon]

Wash cast polyurethane parts in a strong solution of dish detergent and hot water to remove the silicone mold release, let dry then follow up with a wipedown with alcohol and the part should be ready for paint.  A for-plastic primer coat is a huge help as well, but the best thing of all is to pigment the mix before you pour the casting. 

[silastic armor fiend]

I'm having trouble mixing the pigments that i got specifically for the resin I'm using. Whenever I try to mix them, the part comes a kind of muddled brown. Any idea what's causing this? I've had good luck with this alcohol based ink that I have, problem is, I don't have much of it.

[Gryphon]

The two most likely explanations are either that the pigment is reacting with your resin's catalyst; or, your resin is getting too hot as it cures exothermically and it's "baking" the color out.  Many pigments burn brown.  Some types of pigment CANNOT be combined with each other in a single piece as they are reactive with each other, that could be what's going on as well.  Contact the manufacturer.

[silastic armor fiend]

thanks for clearing that up. but im not that concerned about it, i have pigments that work and i could always get more   

[Random Number]

just as another note, the brown color in the casting is mostly just due to the resin type. Polyester resins, and the curing agent MEKP, tend to make them sort of amber brownish in color. There's hundreds of different types of epoxies out there, though.

I do a lot of work with carbon fiber and solar cells and such, and when the aero team gets bored, they use the excess resin in casts to make things. The standard west system epoxy (really good, cures clear, but somewhat harder to find than most resins) is what we use, I believe 202 and 209 is the epoxy/hardener, they take a while to cure, but they usually come out effectively bubble-free. if you're doing parts you are painting anyway, then it's a bit of a moot point to try to get clear resin, and any surface imperfections can be sanded down and/or filled with a bit of bondo. Sanding them helps paint to stick as well.

If you're looking to get parts to be clear, it's more expensive, but try to find optical epoxy. It's what we use to encapsulate solar cells with, and is specifically designed to be runny when it's mixed (so bubbles can move freely out of the resin), and dry very very clear, and not yellow with age. Again, it's much more expensive, and hard to find, but it is out there. If you have an air compressor, a pressure chamber will do wonders for minimizing bubbles. to make one, just get a big PVC pipe, put a port on top, put your piece inside, and compress the hell out of it. There might be bubbles, but they'll be very small.

[silastic armor fiend]

thanks for the help. ive been having much better luck lately using fiberglass resin. the polyester resin i was using sometimes took DAYS to cure completely, but the fiberglass is very quick.

[Titus Wells]

Good news. Just watch you don't try and cast in too large a volume with it or it will crack and burn and that does not smell pleasant and is not good for your general health!

[silastic armor fiend]

already learned the hard way, but thanks for the heads up

[Titus Wells]

Also it can be quite brittle when thin as it's designed for use with fibreglass strands.

[Gryphon]

There are three resins commonly used to wet out fiberglass for layup - polyester, vinylester and epoxy.  Polyester resin is by far the most common and least expensive, and comes in a wide variety of grades and types for specific purposes (casting, general-purpose layup, marine layup, foam lamination, peel layer, high-wax finish gelcoat, etc.)  My guess is that your original resin was a lower grade of polyester, and your current resin is just a better grade of polyester (you're probably better at getting the catalyst ratios right by now as well.)  Polyester is very brittle when thin and unreinforced, plus it shrinks and heats up as it cures; the thicker it is, the hotter it gets as it cures.  If you are casting a large volume of polyester, you must significantly decrease the amount of catalyst added, or it will overheat as it cures - browning and cracking and possibly bursting into gouts of cyanide-laiden smoke.  MUCH better to lay up large pieces with fiberglass than to cast them solid anyway.

A good trick to minimize shrikage/distortion when laying up fiberglass with polyester - lay up the first layer of 'glass in your schedule and let it set up completely before you lay up the rest of the reinforcement layers.  For a really dimension-critical part, use epoxy resin.  WEST and System 3 are both excellent layup epoxies.

[silastic armor fiend]

very helpful post gryphon. thanks a lot. i just picked up some epoxy resin but have yet to use it and dont know much about the quality of it. i will be using it soon and will post the results when im done.

[Adan Shepard]

Here's an easy question for someone in the know with resin casting. I am thinking about making a British Zulu era Uniform and found an other ranks button kind of pricey for one. I am thinking of duplicating it with an RTV mold and resin cast buttons to keep the cost down. I've never worked with either RTV or resin. What sort of RTV material would work best for making a small mold that might have some under cuts? Can I add bronzing powder or just powdered pigments to the resin or would liquid dyes work better? Would it work to do the shank as part of the cast or would I be better off embedding loops for the shank in the body of the buttons?

[Titus Wells]

I'd definitely dip a loop into the resin to make the shank, then you only have to worry about a 1-part mould (with an open back). You can get some good silicone putties (mix two parts and press onto a surface) which would give you a decent mould quickly and easily and I tend to use por-a-kast or ultrakast resin, either would do you. I haven't used filler powders with them, but I'm pretty sure you can, just check the manufacturer's website. You could use a polyester or epoxy layup resin to cast as well as I'm sure your buttons aren't very large. You can definitely pigment and add filler powders to layup resin.

[Gryphon]

faux brass military buttons are fun to make.  Borrow as many of the appropriate buttons as you can and press their backs into a strip of modeling clay, then build a dam around the strip of bedded buttons and spray mold release over the entire thing.  Use RTV silicone to make a small gang mold of the buttons.  After the rubber sets (leave it in a warm place for a couple of days) remove the mold, wash the mold release off the buttons and immediately return them to their owner - make sure to check the backs and remove any little strings of RTV that may be clinging there.  Mix up a small batch of your preferred resin and stir powdered brass or bronze into it - the more the better up to about 70/30 (70% powder by volume,) as long as you can still press the paste into the mold well enough to pick up all details.  Once this sets up, you will have a lovely set of brass- or bronze-finish cold-cast button faces that can be buffed lightly with 000 steel wool then polished like the real thing.  I fill each button cavity full and bed wire loops into the backs of mine, but I know folks who prefer to make thin shell faces and epoxy them onto commercial button backs.

The quick-and-dirty way is to use fiber-reinforced automotive body putty and spraypaint; a platoon's worth of buttons can be done in a weekend this way, but they won't look nearly as good as the cold-cast ones.

[Adan Shepard]

Hi folks thanks for the input! I am looking forward to trying this.

[Ravenson]

Depending on the resin and the paint you are using you might want to try painting the inside of you mold with a thin coat of auto body primer before you cast the resin.  When the casting has cured wash it with Dawn dish soap and warn ( not hot) water gently.  Your paint should stick much better.  Note this works real well with urethane Resin and acrylics in a Silicone mold.

Jeff

[groomporter]

Had a great tip from my silicon supplier a while back when making two-part molds from an original master. When pouring the second half of the mold use a mixture of petroleum jelly disolved in naphtha as a mold release to keep the silicon from sticking to the first half. I was having problems pulling apart the two halves with some of the aerosol mold releases, it doesn't dry as fast the aerosols, but it has been more reliable. I don't have an exact recipe, just enough petroleum jelly so when the naphtha evaporates there's a slight greasy residue.