Conductive Casting Resin,

[BrianGar]

Involved with a tablet stylus pen.

A run of 'looks like', and if possible 'works like' items are required.

I am making them using vacuum casting/pressure method pouring into silicone moulds.

I had found a research paper stating that doping casting resin with 20% graphite power works fine and will provide enough conductivity.

The resin I'm using is easyflo60 which is a two equal part polyurethane plastic.

So far Ive tried 20% graphite by weight, 50% and then lastly 100% with no effect.

I cannot even get cured material to show minor conductivity with two meter probes placed into the material just 2mm apart.

Im guessing the graphite power(very fine) is getting encapsulated and thus insulated by the resin. (On another test I used some left over carbon nano tubes I had with same negative results)

Do I have any other options on this in pourable format or is it a case of further in house r+d?

Last resort will possibly be carbon fiber yarn placed in the tool - contacting tip and exiting where the user would hold it.

Hoping not to have to go there yet though.

Any opinions?

Brian,

(ps, on the production run they are intending to injection mould these from a cosmetically pleasing conductive polymer - is this practical and possible given that its highly cosmetic?)

 

[Pud]

Carbon fibre is conductive at 20% for sure. 3mm start length for fibres. Never have heard of graphite being used though. (I remember talking to a aluminium flake techie and he said that they could never get it conductive no matter how much they added to polymer - "dollar shaped" flakes).

Never have seen or heard of "cosmetically pleasing" electrically conductive. Usually black. Fibres need to be on surface for obvious reasons. Stainless steel fibres looks even worse.

Chrome plated ABS looks nice....

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[BrianGar]

Harry,

Thanks for the response. So your saying setup the yarn chopper for 3mm and mix in, or should I mix in 3mm - 10mm lengths?

They intend going with piano black finished items(shock horror to all mould polishers) - looks like this is not going to happen based on what you are saying re-plated abs, unless 'black chrome' exists? Oh ya...it has to flex too in some areas...

Im thinking design will have to be altered to include an intool conducting internal frame/wire, and use conductive pads on stylus.

Brian,

 

[Pud]

Hi Brian,

3mm is the approx length of the pellets used for injection moulding (nylon in our case). Remember you need "ends" and "overlaps". 3-10mm lengths sounds sensible. Make sure fibres are not coated with anything though!

Piano black is ok as far as polishing goes, btw. It's getting the effect in plastic. Usually a transparent base material with intense black dye rather than black pigment.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[BrianGar]

Harry,

Thanks for that - will try a small batch of resin now and report back.

Brian,

 

[D2S]

Find a conductive grade of carbon black. 15% by weight should work fine. Graphite is not usually used for conductivity. It's dirty to work with.

 

[1gibson]

You're assuming that electrical contact to the fingers is required, but it isn't. I've got a pen on my desk with some sort of elastomer tip on the back that works with capacitive touchscreens (phone, tablet, etc) with no direct contact from the skin. I'd guess silicone, and "capacitive silicone rubber tip" seems to come up with good google results.

Not sure if it would work with a resistive touchscreen.