Introduction: Ultra Quick Resin Cast Parts

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.

Step 1: Materials

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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

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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.

Step 3: Pour the Alginate

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Mix the alginate 1 part powder to one part water by volume and give it a good stir with a stick.

once it is unifirm, carefully pour the alginate into the bottom of the cup. Ensure that the pattern is completely covered to a depth of not less than 25mm or 1 inch.

Using this method should ensure that the bubbles rise to the surface and don't cock up the pattern mould.

Step 4: Wait 90 Seconds

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After a minute and a half the alginate will have cured into a strange wet rubbery substance.

Turn the cup over and press out the mould. CAREFULLY (I use tweezers) draw the pattern out of the mould. The mould shape should mean that it is self supporting.

Step 5: Pour the Resin

Picture of Pour the Resin

The car repair resin comes in two parts. The resin and a hardener. The mix is very forgiving, as long as you don't go mad either way (too little or too much hardener) you are pretty much guaranteed a quick cure. 12 minutes to de-mould is the time I generally get.

Be sure to mix the resin until it is a uniform colour before pouring. I mix it in a plastic cup using another coffee stick.

Step 6: Demould

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Using tweezers carefully remove the cast. If your mould has very little undercut you shouldn't damage the alginate too much.

The heat of the curing resin tends to 'leach' a little water from the alginate into the mould. This is as bad as getting bubbles in the rubber. Just blow the mould out with a little air (I use an air can for this).

Step 7: Repeat

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Using this method I managed to get 10 good casts before the alginate began to fail.

Step 8: Cleaning Up

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The parts need to be worked with the Dremel to complete them. Here are a few I have started on.

The entire process including casting 10 wheels took 2 hours.

Step 9: The Finished Article

Picture of The Finished Article

Here are some finished wheels on a scratchbuilt M39


jtobako (author)2017-03-22

Fiberglass resin, i was thinking Bondo when you were talking about car resin

Southpaw69 (author)2014-09-07

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

sabu.dawdy (author)2014-05-06

this is soo awesome

nerd12 (author)2011-12-03

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)nerd122012-02-03

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 (author)marshon2012-02-03

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)nerd122012-02-05

yep, I would certainly cast it in two halves reinforced with fibre matt then glue the two halves together.

nerd12 (author)marshon2013-11-22

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?

nerd12 (author)marshon2012-02-05

okay. thanks for responding

jumpup_dnb_dj (author)2012-12-17

I'd recommend for safety purposes, using a cup NOT made from plastic, due to the exothermic reaction of the resin and hardener.

mrthomasfritz (author)2010-05-20

A vacuum box will lower the air pressure that will allow the bubbles to escape leaving a much better copy.

marshon (author)mrthomasfritz2010-05-20

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.

OrionBlade (author)marshon2010-12-11

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.

troggo (author)2010-05-20

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?

marshon (author)troggo2010-05-20

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.

afridave (author)marshon2010-10-13

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 (author)marshon2010-05-20

Thanks - I would prefer ABS to aluminium or metal but will still need to machine it.
I will keep digging.  

marshon (author)troggo2010-05-20

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 (author)marshon2010-05-20

Thanks - some good ideas to work on there.

corradini (author)troggo2010-10-05

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 (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.)

troggo (author)corradini2010-10-05

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

corradini (author)troggo2010-10-05

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, maybe -- they have a lot of acetal gears down in that size range.

IronBuddha (author)2010-07-22

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.

heathbar64 (author)2010-05-20

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)heathbar642010-05-20

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.

heathbar64 (author)marshon2010-05-20

 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)heathbar642010-05-21

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)heathbar642010-05-20

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.

jsjgreen (author)2010-05-20

Mario - It should just be a polyester resin, best place to get lots of it from would be a boat builder/yard.

mario59 (author)2010-05-20

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.

marshon (author)mario592010-05-20

Italian supplier of alginate at reasonable price:


marshon (author)marshon2010-05-20

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)mario592010-05-20

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.


curbowman (author)2010-05-20

How strong can these casts be? I think they could be useful for building guitar bridge pins.

marshon (author)curbowman2010-05-20

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.

emcelhannon (author)2010-05-18

Where would I find the cheapest source of alginate powder?

bucklipe (author)emcelhannon2010-05-19

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)emcelhannon2010-05-18

Ebay is always a good place to look. I get mine from a local Dental supply service who offer very good rates.

DIY Micah (author)emcelhannon2010-05-18

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.)

dolabil66 (author)2010-05-17

how durable is the car resin ? what exactly is it ? bondo ?
Very nice instructable, it`s given me something to think about

marshon (author)dolabil662010-05-18

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 ......

kcls (author)2010-05-18

Nice job! Quick and simple. I like it!

MrRetardo (author)2010-05-18

Another "Cool" fact about Alginate:  As it dries out, it shrinks.  Fill a mold with sand to retain the 'negative space', let the alginate dry.  If you do this with a life casting of a face you can literally make a shrunken head ;)

Chowmix12 (author)2010-05-18

That is really beast!! especially the M39! 5/5* Keep it up!!

Kryptonite (author)2010-05-18

That's really neat! So fast! Brilliant job!

caarntedd (author)2010-05-17

This is exceptional. Excellent 'ible with endless uses.

Hiroak (author)2010-05-17

How well would this work to make LEGO mini figs or their gun and what not?  I know LEGO's are PVC but is this close?

M4industries (author)Hiroak2010-05-17

 Legos are actually ABS.

Similar, but different.

marshon (author)Hiroak2010-05-17

The system should be able to cast any small parts. The trick is to be able to get the pattern out of the alginate without destroying the mould in the process.

Alginate will not stick to anything, including itself, so it is possible to make two part moulds by pouring the first half and wait till it cures then pour the second part.

As long as there is no undercut it will work.

nickodemus (author)2010-05-17

Whoa. This would be good for casting guitar knobs. Great work.

About This Instructable




Bio: Untidy, disorganised and a bit silly. I am a photographer, artist, body artist, sculptor, prosthetic maker, model engineer, and general idiot who likes making stuff ... More »
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