Casting Detailed Parts: Prosthetic Fingers (That Glow, Change Colour With Heat, and More...)

This is a guide about casting small, complex parts - cheaply. It should be said I'm no casting expert, but as necessity is often the mother of invention - some of the processes here have worked well. I met Nigel Ackland at Future Fest in London, and we got talking about his bionic hand. Check out his Wired Talk also. LINK.

He was frustrated that his fingertip rubber grips had worn out and become irritating to use, as the rubber flapped around, making the grip unpredictable. Worse still, although Nigel could send for a replacement part, he would probably not be given a 'loaner arm' so would be without an arm for some weeks.

I realised that the fingertip grips were a injection moulded rubber, over a hard plastic. So the plastic was fine, but the rubber could probably be replaced by Sugru. I happened to work there at the time, so suggested he try it, (see picture of me holding the first attempt, followed by later variations...).

From this Nigel and I got talking about what other enhancements might be fun for his fingertips. Naturally I wanted to experiment with his fingertips, but as I didn't want to risk damaging his original plastic fingertips, I figured I could cast copies in resin, and then try out some ideas for different functional grips.

More on this in a second Instructables LINK. But here, I wanted to share what is a generic approach to casting small mechanical parts, when you otherwise could not easily CAD/3D Scan them. Indeed, 3D Printing is strong, but some resins can be far stronger. So I hope this is a useful guide.

And as shown in the main photo - I didn't stop at just casting in black coloured resin - I went beyond this an made them Glow in the Dark*, change colour with heat (Thermochromic), and a few other tests.

*Thanks for voting if you liked it!

Disclaimer: It is likely prosthetics companies don't encourage this. This Instructable was created in my own time, and is not endorsed by Sugru. It is shared in good faith as an inspirational project, but no liability or responsibility can be accepted for this process, or what you ultimately use it for, be it in prosthetics or otherwise.

It's important to point out that often 'solid' looking shapes can have hollow cavities.

In this case I wanted to keep things lightweight, so casting this 'hollow' was important, but as these are flexible moulds - it's worth pointing out that I could have cast the fingertips with the two halves together, and the resulting cast would simply 'ignore' the void, and be a solid part, based on the external form.

Clean each part in warm soapy water to remove any grease, or use IPA if needs be. Oils can disrupt the silicone casting.

I'm sure it goes without saying, but do check that whatever you are casting is not going to stick to the silicone mould. Most plastics, metals, woods, etc. are fine, but sometimes porous things like rocks, foams, etc, may want a 'release agent' or a lacquer over the top first.

Please disassembled your prosthetics only at your own risk. Small-prints may advise against it!

When casting a large, simple shape, you may not need to worry so much about air bubbles - but with small, intricate parts, you need to allow space for the air to escape (even if using a vacuum chamber).

Runners should usually try to fill from the lowermost part of the object. (When inverted in the mould, not as shown here). Risers should come from anywhere where there is a chance for air to collect. (When inverted).

The orientation of the part is also worth considering. I wanted the best cosmetic finish on the tips, so put these at the runner (lowest filling point), so any bubbles would rise away from it.

1. As I was using syringes to cast, I used these as pouring funnels / stands, and filled small cut-off sections with glue-gun glue. Then inserting the Runners in. This was using ABS plastic tube (~3mm diameter).

2. I used a lighter to warm the ABS, and bend into a 'u-bend' shape.

3. Spray Accelerator on the object you want to attach. (It's better this way around). This is optional, but helps speed it up. Ordinary Superglue is fine.

4. Apply a tiny amount of super glue to the Runner, then quickly attach to the object. If you really can't use Superglue, use a gentler glue like UHU, PVA, or even Blutack, but this may be less strong for very small parts.

5. Orientate so that the object and the runner do not touch sides. Allow to dry/stick.

6. Add some Risers on the other side of the part, so as air escapes (remember this will be turned upside-down to cast!). Mount on some Form Board.

Create a box around the part using Foam Board and Glue Gun.

Ensure that the part does not touch the sides, but ideally keep as close as possible (say 3mm away). This simply saves volume of your silicone. And if it's your first attempt, you may even want to have enough for a second attempt!

TIP: Any duff/old moulds you have, chop into small pieces and add while still liquid, it will recycle, and make no difference to cast quality - just be careful not to push in so hard you dislodge your mould object!

I used a off-the-shelf 'general purpose' casting silcone, GP3481-F by Polycraft. It's under $10/£7 for a small kit. LINK.

1. With your moulds ready. Mix as per instructions, to the required ratio.

2. A great trick I was taught when casting parts at university, was to 'pour high'. This is a little messy when not used to it, but worth at getting the knack of it (as you can see, no spillages!). Start by pouring a blob as high as you dare, so you know the trajectory. Then keep slowly pouring, but lift your arm up. The thinning of the stream of silicone bursts bubbles, not only as it pours, but also as it lands. Pouring in a few blobs leaves a lot more air in.

3. It's not essential to use a degassing chamber, but if you have one do. It worth noting that if you use a slow catalyst, then this allows bubbles to escape. A very fast catalyst means less will escape, and you also have to be more confident in doing the casting before it sets.

4. If you are using the vacuum chamber to degas, ensure that your walls are high, as I have shown (at least 2x your part height). This is because the silicone rises like a muffin, until all the air has escaped, then it slumps back down again. (see last two pictures).

5. Leave to cure.

WARNING: Silicones (and casting resins especially!) can be dangerous! Use all advised safety protection and ensure good ventilation. Smooth-On is one of the best companies I've used, and they are very patient in helping people with all manner of tech/safety questions. (I'm not endorsed by them, they are just excellent. Props.)

As you can see, the silicone has gone all the way up the sides of the inner walls, before slumping down again to the natural height of the volume.

The Foam Board can be broken off, though if you are feeling eco-friendly, reusable mould boxes can be used, but the may not be a snug. So it's a trade off.

Trim off any edges, and your ready to take out the object...

I suggest using a 'claw-like' scalpel blade, like this. (LINK). As it not only creates a edge which 're-joins' nicely, but it also is less likely to cut off chunks (which will mean your cast has those defects). It also has a good point to access those fiddly bits!

Gently cut the side, following down from the funnel. As you go, pull apart the silicone. This makes the scalpel cut through better, and means you can see what you are doing.

Cut closely to the object, and then pull out as you best you can. Remove any Risers also.

Because of this cutting style, you'll see that it 'joins' back together.

NOTE - you are not cutting the mould fully in half, but more like a hot-dog bun, you want a 'hinge' side left, as this makes casting much easier.

I used a general polyurethane resin to cast the copies of the fingertips. Polycraft K2. (LINK). It's worth noting that this is yellow, but can be coloured (even to white), using pigments (LINK).

WARNING: Polyurethane Resins are dangerous when in their liquid (unmixed and uncured) states, and while curing (setting solid). Extreme care should be taken: gloves, ventilation, masks, goggles, etc. If you have not worked with these chemicals, do read the safety information provided with them, or read here. (LINK).

1. Weigh out the B-part first, and add pigment if you require it. Have moulds ready - with elastic bands around moulds, not too tight, but to ensure they are nicely together.

2. Add A-part seconds, and mix thoroughly. Be aware that you may only have 1 minute to mix, and another minute to suck up with a syringe (20ml shown). So have everything ready and waiting. And remember H&S.

The part looks smaller than a full Syringe, so I weighted out 50% and 50% of this max volume, but if in doubt, make say 20% extra, for mistakes.

In all honestly, this is one of those instructions where experience is no substitute. As ambient temperature varies, this part may happen sooner or later, but what you want to wait for is when you feel a slight change in temperature of the resin, this is it starting to react. If you inject too soon, it's like water, and flows out. Too late and it does not flow into all the gaps. Safe to say more runny is better, but there will be more wastage. By a couple goes, you'll get the idea!

3. Inject the mixed resins into the mould (be very careful not to squirt any into your eyes, etc.). Allow some to flow out the Risers. Extract the syringe slowly, and squirt out a little extra into the 'Funnel' - as this keeps the pressure on the moulded cavity, as the level will go down as some air escapes, and even some small leakages. So this acts as 'reservoir' to ensure a fully cast part. Ignore it at your peril ;o)

4. Repeat. I suggest doing no more than 2 moulds in one go, as this takes practice.

5. Leave to 'cure' in a well ventilated place. Not indoors!

Gently pull apart your mould, and remove the resin copy.

As you can see, these moulds are a little 'fresh' and if you want to leave them to fully cure, it's good to given them 24 or 48 hours for their full hardness to develop (or whatever is advised on the label).

Carefully trim off the 'flashing' (over-run), Runners/Risers, and even sand with some very fine emery paper.

Leave to fully cure in a safe place, like a improvised peg-board, as shown. It sounds patronising, but do also label which part is which, and when it was cast. It's easy to get confused when doing many variations of different mixes, etc.

As mentioned, I was exploring creating new prosthetics augmentations/features for Nigel, and I realised I could add functional powders to PU casting resins, with no major deterioration to the performance of the resin.

I tried the following:

Glow in the Dark (LINK)

ThermoChromic Pigment (LINK)

and some less good ideas...

Magnetite Powders (LINK) - dunno what I was thinking, but was curious. Didn't work well!

As mentioned, I was exploring what 'functional additives' I could add to PU Resins to give Nigel and range of different experiences. I used pigments which changed colour with temperature, and glowed in the dark (spoiler!), but if you'd like to see these in more detail - check out the second part of this series of Instructables. LINK.

If you liked this, please vote, and check out more at judepullen.com or follow on @Jude_Pullen