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.
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.
Remove these ads by
Signing UpStep 1: 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.
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.
Visit Our Store »
Go Pro Today »
Please respond
either way, about 10 seconds is enough to get the bubbles up and off the bottom surface of the cup and pattern.
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
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.)
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
Try SmallParts.com, maybe -- they have a lot of acetal gears down in that size range.
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.
I will keep digging.
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.
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.
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.
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
Alginato
Fibreglass resin
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
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.
Very nice instructable, it`s given me something to think about
How durable? Well they make ocean going yatchs from this stuff ......