Cheap ways of colouring polyurethane resin?

[Matthias Gladstone]

I'm currently working on a project which, i'm afraid, must remain secret for now; it involves casting polyurethane resin. I've seen you can buy various tinting dyes, all of which are too expensive for me (the budget is tight as is). I was wondering, would black food dye be suitable? Would it weaken the resin at all? All we need is a solid black colour.
-Matt

[IGetPwnedOften]

I can certainly tell you how not to do it, as I went through the same dilemma. I tried various media, and one thing is certain - anything water based will fail miserably. Firstly, it doesn't like to mix into the resin, and even if you do manage it, the resin will never cure. I left one batch to dry for over two weeks and it was still like jelly after that time, so I threw it away.

I don't know if food dye is water based, but I would suspect it is. You could mix up a tiny amount and see what happens. I use cheap plastic shot glasses as disposable mixing vessels for small amounts, because I can just bin them when I'm done.

[Narsil]

Laser printer toner should do the trick. You will want to mix it to a paste with a small amount of resin first and gradually add resin to the paste.

With all resin pigments and fillers you should mix the resin up first before you add the catalyst and allow to stand for a while to allow bubbles to escape. If you need several batches to be exactly the same colour it is best to mix up as much as you will need in total and mix is with catalyst in whatever quantity you want to use at any one time. If this is impractical then make a careful note of exactly how much (by weight) filler/pigment you use with the first batch.

[Matthias Gladstone]

Cheers guys; if toner works, do you think powdered charcoal would? It's a toss up between black and brass currently, but will let you know.
I remember when I first tried resin casting I used alginate jelly moulds. The results were distastorous; the resin never set thanks to all the water. I was not impressed.
-Matt

[Herr Döktor]

Humbrol enamels (the little metal tinlets) work very well with "fast cast" style polyurethane resins- if I want lots of things of a similar colour, I mix a tinlet or two into the part A of a batch of resin, and go from there.

[Maets]

Would brass dust work?  Or steel dust?

[Matthias Gladstone]

Would brass dust work?  Or steel dust?

Indeed, that's what i'm referring to. You powder the mould with the dust, then apply a gel coat which is heavy in metal powder. Backfill the rest, buff, and you should have quite a realistic looking piece.

Humbrol enamels (the little metal tinlets) work very well with "fast cast" style polyurethane resins- if I want lots of things of a similar colour, I mix a tinlet or two into the part A of a batch of resin, and go from there.

That's what I like to hear, nice and cheap.

[Fat Spider]

There are special dusting powders available for surface colouring resin, the method is to brush the surface of the mould with the powder beforehand, the powder is then simply absorbed into the surface of the resin, I'm only aware of these been available in metalic finishes but you could try using other powders like those designed for cake decorating or even try using make up, eye shadows etc.

If you need a supplier of Polyurethane Resin or RTV silicone at a reasonable price just google "Tomps"

If you need any help with Resin Casting or advice on mould making drop me a PM, or contact me via www.fatspider.co.uk

Reply Edit: sorry was a bit late posting this last night (or rather this morning ) so not really awake, missed your bit about dusting powder and gelcoats, also listed my website instead of my email, just replace the www. with alan@

My good lady dabbles in cake decorating so I may borrow a few supplies and do a little experimenting the next time I'm casting.

[elShoggotho]

I wonder if pigment powders would work. Check at an art supplies store, they should have some knowledge on the topic.

[J. Wilhelm]

I've tried mixing brass powder with epoxy (quick set) as an experiment.  I got strange results.  The mixture looks dark grey -green!  There is a complex process of refraction and reflection whereby different colours are refracted or reflected differently in different directions as well, depending on particle size and density, so watch out if you expect it to look like real brass (or copper; I also tried that).  I have heard of "cold casting" and I have seen successful attempts using bronze, but perhaps the particle size is much smaller than the powder I was using and they use  really dense mixture...

[Will Howard]

I've tried mixing brass powder with epoxy (quick set) as an experiment.  I got strange results.  The mixture looks dark grey -green!  There is a complex process of refraction and reflection whereby different colours are refracted or reflected differently in different directions as well, depending on particle size and density, so watch out if you expect it to look like real brass (or copper; I also tried that).  I have heard of "cold casting" and I have seen successful attempts using bronze, but perhaps the particle size is much smaller than the powder I was using and they use  really dense mixture...

Remember that copper alloys (including brass) will oxidize rapidly under some conditions, especially if they are in a powder or dust form.  This is a greenish oxide called verdigris, & it is poisonous.  So if your casting turned green or grey-green, DON'T use it for anything such as an eating utensil or a food storage container!

[J. Wilhelm]

The colour was acquired instantaneously upon mixing - I'm not sure it turned green by oxidation, unless the acid in the hardener was responsible directly or indirectly for that reaction (the colour is dark grey with a very slight olive-greenish hue).  This was just a test over a piece of wood.

[Matthias Gladstone]

The secret project revealed:
http://brassgoggles.co.uk/forum/index.php/topic,39675.0.html

[Narsil]

It's also worth noting that some dry pigments can react quite violently with some resins with potentially dangerous results.

If you are trying a new combination the safest way is to mix a very small sample batch. Ideally this should be done outside in a safe area. Keep a metal bucket to hand to cover the sample if it shows any signs of getting a bit explody. In extreme cases it can catch fire, releasing toxic smoke and spit hot droplets of resin which can cause very serious eye damage.

Quote :
I've tried mixing brass powder with epoxy (quick set) as an experiment.  I got strange results.  The mixture looks dark grey -green!  There is a complex process of refraction and reflection whereby different colours are refracted or reflected differently in different directions as well, depending on particle size and density, so watch out if you expect it to look like real brass (or copper; I also tried that).  I have heard of "cold casting" and I have seen successful attempts using bronze, but perhaps the particle size is much smaller than the powder I was using and they use  really dense mixture..

You do need quite a high ratio of metal powder to resin to get good results. Exactly how much depends on the characteristics of the mould, the particle size, the type of resin and and other fillers, pigments or additives used.

The powder used also needs to be relatively fine, metal filings won't give the same result.

If you mix the powder with the resin you will usually need to cut back and buff the surface of the finished mould using a fine abrasive to get a good effect, this will also reduce the 'plastic' texture of the casting. Done right the effect is virtually indistinguishable from real metal, at least for satin or polished finishes.

If you dust the powder on the inside of the mould you will get more of a metallic surface straight out of the mould but this will be quite a thin layer which can be lost if you need to apply any finishing processes to the casting.

[Kevin C Cooper Esq]

I've used brass powder to make small cold cast objects (cane handles etc) with polyester resin. You do need a high proportion of metal powder to get an authentic metallic finish. Metal powders are expensive so you just mix enough to give a good coating to the mould, usually latex moulds in my case. Then once it's gone off you can fill the shell cast with resin and a cheap filler like talc.
As Narsil said you then need to rub the surface of the finished cast with fine steel wool or similar then finish it off with metal polish. Gives a very good result.

[Nicolai Bardimus]

sorry to drag an old post up from the dead, but try fiberglass gel coat pigments. Gelcoat is basically resin and pigments are easy to come by (in the US) at a TAP Plastics store.

[SirGawain]

Laser printer toner should do the trick.

A capital idea! Although toner typically contains some wax and co-polymers along with the carbon, they shouldn't be a problem for the vicious solvents in polyurethane, if used sparingly. So, I tried it on a black electric guitar body with a few dings....

I mixed a smooth paste of toner and a little G4 polyurethane before adding sufficient PU to produce a 4:1 mixture (1 teaspoon PU + 1/4 teaspoon toner). Although the toner tended to subside to the bottom of the bottle, it painted easily and coverage was convincing, if the mixture was stirred with every dip of the brush.

The resulting colour was close enough to the original, but the toner-impregnated PU refused to polish up shiny like the surrounding surface, remaining matt regardless of polishing methods. I use successive grades of wet water paper down to 1200 grit, and then switch to Autosol, Brasso, and finally liquid car polish. However, a clear coat of polyurethane over the toner stuff worked perfectly, making this a definite contender for affordable PU pigment.