Instructables
Picture of Ultra quick resin cast parts
If, like me, you tend to ignore the instructions given with most materials and try a few 'mad' ideas instead, here's an ultra quick resin casting method using the 'wrong' materials.

For high quality, long lasting moulds, most cold casting methods use an RTV rubber of some kind. These are excellent, and I do use them, but they are expensive and pretty slow to cure. Usually 4 to 24 hours. The polyurethane resin favoured by most small parts casters, such as Easy-Flo or one of the other 50% / 50% mixes are easy to use but need to be ordered in and again can be expensive.

I wanted detailed results VERY quickly so I chose to use Alginate as my mould making material instead. The resin is car repair resin, which I can pick up 5 minutes down the road.

 
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Step 1: Materials

Picture of Materials
The materials needed are:

A pattern with very little undercut
Alginate powder
Car repair resin
Some plastic cups
Stirring sticks (the ones from the coffee shop are best and free)

The problem with Alginate as a casting mould material is that it is not very strong, it cant handle big undercuts and it's considered to be a 'one shot' mould. Dentists use it to cast your teeth. The advantages of alginate are, that it takes very fine details, it very safe to use, and most of all it cures in 90 seconds flat. Also it is MUCH cheaper than RTV

You may have to think out of the box a little when making your patterns. I wanted to cast a set of small tank wheels. I had to fill some of the undercuts with plasticine, then grind them back with a Dremel after casting. I also built the main pattern  so that there were no undercuts at all.

Step 2: Setting up the fence

Picture of Setting up the fence
To fence my wheels I used a cut off plastic cup.

The pattern is mounted using a bit of plasticine, with the main 'face' of the pattern uppermost.

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Southpaw692 months ago

Cool tank! Try to avoid jargon (or at least explain it) I didn't know what "undercuts" or a "Fence" were.

sabu.dawdy6 months ago

this is soo awesome

nerd122 years ago
i use the same resin. i wanted to cast a knife out of it so could you please tell me how strong it is and how chip proof. also do you have and advice on casting?
Please respond
marshon (author)  nerd122 years ago
It would be far too brittle for a thin section knife blade unless you added fibre matt to it. Easier to make the two side seperately and then glue them together.
nerd12 marshon2 years ago
i was thinking of a knife about 3/4th of a centimeter for purely decorative purposes. i still want it to be strong so please tell me.
marshon (author)  nerd122 years ago
yep, I would certainly cast it in two halves reinforced with fibre matt then glue the two halves together.
nerd12 marshon1 year ago
Hello again!
I decided to finally cast some resin, and tried to make ezio's pauldron buckle. It is a simple belt buckle size. It came out ok, but the resin had a lot of pin holes on the side which was in contact with the alginate. Perhaps due to the water, so do you know how i can seal the alginate before casting polyester resin?
Thanks
nerd12 marshon2 years ago
okay. thanks for responding
I'd recommend for safety purposes, using a cup NOT made from plastic, due to the exothermic reaction of the resin and hardener.
A vacuum box will lower the air pressure that will allow the bubbles to escape leaving a much better copy.

marshon (author)  mrthomasfritz4 years ago
True, but then if you're going to those lengths you might as well use RTV and have done with it. Also it increases the time (and expense) of creating the mould. I use one for my RTV moulds, but this is designed to be quick and dirty.
It's terribly easy to put your mold flask onto a smallish board (say 12" square scrap of plywood, cutting board, etc.) and set something like an electric toothbrush, razor, or dremel tool against the board so that the whole thing vibrates. If you chuck a 10d nail into your dremel with a wee bit of a wiggle in it, it'll set the whole thing to vibrating violently enough to shake the bubbles out. Don't go putting a 90 degree bend in the nail or anything, and make sure it's chucked tight, and wear safety goggles - also use a low speed on the drill/dremel if you go that route.

either way, about 10 seconds is enough to get the bubbles up and off the bottom surface of the cup and pattern.

troggo4 years ago
A nice instructable - thanks. 
I have a question, what would be the best material for both strength and flexibility?
I am tryiing to 'cast' some tiny gears - around 12m diameter and 2mm thick. I do not have the equipment to cut metal ones and wonder if casting them would be a practical method?
Mark
marshon (author)  troggo4 years ago
It would depend entirely on the stresses placed on the gears, the proposed size of the teeth, and the speed they need to run at.
I would suspect that resin based gears of this size would be liable to stripping teeth if placed under any sort of load. They would also wear far too quickly.

I would be looking to use ABS or one of the hard nylons (Delrin), but these would need to be machined rather than cast.

Maybe somone else has used cast resin gears. Of course you could always try some backyard metal casting and use aluminium, but they would probably need machining too.
does anybody know how to coat plaster paris molds so as to make them non pourous so that resin can be cast into them?also i read somewhere long ago that adding alum and or borax to plaster paris makes it as hard as marble and outdoor statues used to be made from this mixture in victorian times . does anyone know anything about this?if so please advise.
troggo marshon4 years ago
Thanks - I would prefer ABS to aluminium or metal but will still need to machine it.
I will keep digging.  
marshon (author)  troggo4 years ago
If you know the diameters, but don't have a specific number of teeth in mind there are number of cheap alternatives.
Most model shops, especially RC ones, stock ABS gears in little sets. The ones I bought for a worm gear project were £1.76 GBP for three gears and a worm gear.

I get most of my Delrin nylon gears from old printers, the last HL4 I stripped offered up a dozen gears of various sizes, two silver steel rods 300mm long and 2 aluminium rods of the same length (and an LED laser to boot).

The ABS and Nylon can be machined successfully with a Dremel and some patience and imagination. That's how I finish my cast wheels too.
troggo marshon4 years ago
Thanks - some good ideas to work on there.

You might try ThermoSteel (available at any auto parts store) -- it's an epoxy resin with steel powder used for high-temp (up to 2400°F) metal repair. Comes in a cup the size of a film canister, about US$8-9. You just stir it up into a thin paste and apply. It should cast pretty well, and I think it's fairly strong. I've used it for repairs but not casting. (I do know a lot about casting metal though - see my Instructable on making your own metal lathe....)

Another option might be FastSteel (any home store near the epoxy) - a stick of epoxy putty and steel powder filler that you knead together (it's about the consistency of a TootsieRoll). It doesn't shrink, and can be sanded, painted, machined, etc. Website says it's good for rebuilding small engine parts, making handles, etc. It's not what I'd call "castable" -- it's not a liquid -- but you could make a plaster mold and press it in. Color is grey but it's paintable with a metallic paint, obviously, and should be strong enough.

Similarly, you might try JBWeld (any hardware store) - one of my favorite "miracle products" (like WD40 and duct tape). It's (again) a 2-part epoxy that you can repair a tractor engine block with (no kidding, says so on the package). I've never cast anything with it, but I've made parts with it and it's STRONG. Again, it's machinable, sandable, drillable, paintable, etc.

eHow has several articles on using 95/5 solid lead-free solder (basically casting pewter - any hardware store), and carving a mold from cuttlefish bone (any pet store sells 'em for birds to nibble) -- you could use plaster (any Michael's crafts store). You can melt it on the stove (be careful!) or a camp stove, BBQ, or with a propane torch (any hardware store). You can also cast pewter or any of the "white metal" alloys in RTV rubber molds made from actual parts.

Finally, there're a bunch of low-temperature metal alloys that might be exactly what you want (they're just not all as cheap or easily available). Search Wikipedia for Wood's Metal, Rose's Metal, and Field's Metal. You can buy low-temp melting casting alloy from McMaster.com (they're amazing, and fast!), search for Bismuth. You can melt some of these alloys in boiling water, and you get over a pound for US$35-40. (Note - they contain lead - toxic. Some have cadmium - highly toxic. Standard disclaimer: you may die, you've been warned, I'm not responsible.)

(Also search McMaster for "castable" and look at all the options for both casting and mold materials.)
Thanks corradini,

I did try various steel epoxy as well as JBWeld but it did not have sufficient strength on the tiny teeth and they sort of just crumbled after a some tests..
I gave up on that idea after that - hopefully I can source some proper gears somewhere, sometime when I gety back on that project again.
Thanks for all the good info though.

Cheers - Mark
From your comments -- I don't think it's likely that you'll be able to cast gears (a) at that size, which (b) have the mechanical strength you need, without (c) a lot of skill and practice, and (d) using higher temperatures than you're comfy with.

Try SmallParts.com, maybe -- they have a lot of acetal gears down in that size range.
IronBuddha4 years ago
I learned a trick from a guy I know that sells mold casting supplies and has years of experience. To keep the resin from leaching the water from the alginate, mix the resin and stir till it is just about to set, then pour into the mold. This resin will set quickly preventing it from leaching the water. Just experiment with it till you get the timing right. Another trick with the resin is that if you time it just right, you can pour the resin and it will set as it is being poured from the cup.
heathbar644 years ago
for a quick and dirty flexible mould, I've had very good sucess with unflavored gelatin. Where the instructions say one tablespoon with two cups water, I use a ratio of one tablespoon to 1/4 cup water. it comes out thick and rubbery.  I'm not sure I could get 10 copies from one mold, but you can easily re-melt it and make a new mold with the same batch of gelatin.
marshon (author)  heathbar644 years ago
I use gelatin for my prosthetics using a mix of glycerine, honey, water and gelatin crystals. Much stronger than Alginate and will take quite a heavy undercut too. Yep it's re-meltable ..... but.

It is far too hot for some plastics, especially thin styrene which I use a lot, and the heat tends to melt thin plastic cups too, plasticine gets deformed on the surface by the heat.

Otherwise an excellent alternative if your pattern can take the heat.
 OOH! that's interesting. Can you give the proportions for that mix? Where do you get glycerine? is that the same stuff as the glycerine soap they  sell to melt and pour into molds? I could steal some of that from my wife. 
marshon (author)  heathbar644 years ago
Try this:

You can buy gelatine from theatrical suppliers, but I wanted off the shelf stuff. Go to the bakery section of your local supermarket and buy the packet stuff sold as crystals or powder, not the sheets.
Almost any pharmacy will sell you glycerine, it's just a pure form of syrup. Most Internet sites call for sorbitol, but I use a honey and water solution instead.

Mix 1 part warm water with 1 part of honey in a plastic jug, add 2 parts of glycerine and stir. Warm this mixture in a microwave for 20 seconds and stir in 2 parts of gelatin. Stir until it all dissolves, if there are still crystals of gelatin present, then give it another 20 seconds in the mike.

DO NOT LET IT BOIL or you will break down the protien polymers of the gelatin and it won't set properly. Keep stirring and microwaving until all the ingredients are dissolved and the mix is uniform. Place the jug in the fridge for four hours.

The resulting 'rubber' is very tough and will take rough handling. Pull the puck of 'rubber out of the jug, put it in a food bag and freeze it. Lasts for months, when you want to use it, just put it back in the fridge for a day, and then gently melt it in the microwave until liquid, again don't let it boil.
marshon (author)  heathbar644 years ago
The one other problem with gelatine is the time it takes to set. Even in the fridge it takes around an hour to set properly.
jsjgreen4 years ago
Mario - It should just be a polyester resin, best place to get lots of it from would be a boat builder/yard.
mario594 years ago
Hi!
I wonder which type of car resin you use. The one I've bought at the local store comes in a PASTE can, with the hardner tube like a little toothpaste tube, same consistency of the toothpaste.
Is not the red fluid you've used.
Also I wonder where you get the alginate... For what I know is a Dental Speciality powder, and is not all that cheap, at least here in italy... and can be bougth ONLY trough their distribution chains...
Anyway, it's a thing that cannot be found on the shelf, at the local "grocery" ;-)
I AGREE with you on the way easier and FORGIVING approach of these substances, but, I think, are hard to get as well....
Please let me know.
ciao!
Mario
 
marshon (author)  mario594 years ago
Italian supplier of alginate at reasonable price:

Alginato
marshon (author)  marshon4 years ago
Italian supplier for the resin. Second one down, it's a kit but you may be able to find just the resin somewhere:

Fibreglass resin
marshon (author)  mario594 years ago
Hi Mario,

I've had a number of questions about this. I use David's FAST GLASS resin. It's designed for fibreglass applications and is a liquid. It's designed to be brushed onto fibreglass matt layups.

Look here

Do a search for Fibreglass resin and you should find it, or any similar glassfibre polyurathane resin will do. I use it because most auto dealers sell it here in the UK so it's easy to get. There is loads on ebay as well. Over here it works out at about £1 GBP per 100 Mls.
It uses the same 'toothpaste' tube of hardener as your paste, that's where the red colour comes from, our hardener is red.

Alginate costs around £3.50 GBP per lb (455 gms) from my dental supplier, or about £5.00 per lb from ebay suppliers. If you can get the chromatic type it changes colour to help you identify whaich stage you are at. Most art suppliers, theatrical suppliers and craft shops sell it. As a last resort you can get it in kits to cast babies hands and feet with.

A 455gms bag of alginate will make around 10 moulds of the size in this 'ible.

Hope that helps.

Simon
curbowman4 years ago
How strong can these casts be? I think they could be useful for building guitar bridge pins.
marshon (author)  curbowman4 years ago
The cured resin is strong enough to be drilled, machined, filed etc.

It does not have the inherant strength of fully fibreglassed parts because it can be brittle if it is cast too thin in section. I wouldn't want to use it standalone at less than about 1/4" or 6mm thick.

Having said that, if you introduce some fibreglass tissue to the casting it becomes almost unbreakable. It's the same stuff they make boats out of.
emcelhannon4 years ago
Where would I find the cheapest source of alginate powder?
Just a note about alginate powder; if you go into the average art & crafts store and ask for alginate powder you will get confused looks. However if you ask where the mold making materials are they will take you to where you can get "Instamold" by Activa Products. It is probably not the cheapest since it is sold in 12 ounce cans, I'd check online for larger amounts. However if you are making a quick project it could be easier to get the 12 ounce can.
marshon (author)  emcelhannon4 years ago
Ebay is always a good place to look. I get mine from a local Dental supply service who offer very good rates.
It depends where you are located. Typically, art stores will have alginate powder. You can also order it online from various art stores and I have even seen it in some Halloween stores (as it is very useful for making a mould of ones face for mask making as well.)
dolabil664 years ago
how durable is the car resin ? what exactly is it ? bondo ?
Very nice instructable, it`s given me something to think about
marshon (author)  dolabil664 years ago
I think that Bondo is a paste, similar to Isopon P38 in the UK. The resin I use is for glassfibre layup, just don't use the matt. It's much more liquid than Bondo would be. Most Auto-Parts outlets sell it.

How durable? Well they make ocean going yatchs from this stuff ......
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