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Avi

Tue Jan 28 2014, 02:51PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

I have this cheap quadcopter with custom irreplaceable parts, and i need to recreate one in order to repair it, attached is a photo of the part in question, and how it attaches (yes that is a screw thread in the end).
I have the shaft part but i need to recreate the plastic part (i have tried polymorph already, it snapped under rotational/vibration stress.
I have made a mould of it in paraffin wax (which does not include the screw hole as i will drill that out or something), and a jig to insert the shaft into the mould at the right position and angle.
What substance is recommended to fill the mould with that starts off in a liquid state (that i can easily get in the hole, and not have too much trouble with air in it), and hardens to a state that would hold up to knocks and rotational stress, and that wont melt the paraffin mould when drying/hardening.

Ash Small

Tue Jan 28 2014, 03:05PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Maybe epoxy? (preferably not a 'fast cure' one)

StormInABottle

Tue Jan 28 2014, 03:15PM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

Perhaps you could put the mold in a bucket of ice and fill it with lead? I know lead is very cheap

Avi

Tue Jan 28 2014, 03:29PM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Ash Small wrote ...

Maybe epoxy? (preferably not a 'fast cure' one)

doesn't that heat?
and how slow is "not fast"?

Ash Small

Tue Jan 28 2014, 07:41PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Avi wrote ...

Ash Small wrote ...

Maybe epoxy? (preferably not a 'fast cure' one)

doesn't that heat?
and how slow is "not fast"?

Epoxy does heat, but it depends on the amount used as to how hot it gets (volume/surface area). Your component is quite small (around 20mmx6mm) so will not heat much. I imagine the 'five minute epopxies' get a lot hotter than the slow cure ones. Wax is used as a 'release agent' when using epoxy in molds, so I imagine you'll be ok, but maybe try a test first, so you don't destroy your mold.

Epoxy is usually used with a re-inforcing fibre (glass, kevlar, carbon, etc) but I have used it on it's own before and it's strength is 'reasonable'.

I can't give any guarantees though, as epoxies vary from one manufacturer to another, and without knowing the forces it will be subjected to, etc. it's impossible to say, but compared to most 'plastics' it's 'quite strong'.

Can you not machine the part out of aluminium, or something? The 'hexagonal' end could be achieved with some careful filing. (scribe lines at 60 degrees along the length of the bit to be filed first, maybe?)

Patrick

Tue Jan 28 2014, 11:16PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im still trying to figure out what to say, but im going to guess youll need 30 minute epoxy with some fiberglass strands chopped up and thrown into the wet epoxy.

however, i fail to see what is broken from your pics.

Avi

Wed Jan 29 2014, 01:04AM

Registered Member #580 Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410

Ash Small wrote ...

Epoxy is usually used with a re-inforcing fibre (glass, kevlar, carbon, etc) but I have used it on it's own before and it's strength is 'reasonable'.

what is it? just powdered form you mix in? available at a hardware store?

Ash Small wrote ...

Can you not machine the part out of aluminium, or something? The 'hexagonal' end could be achieved with some careful filing. (scribe lines at 60 degrees along the length of the bit to be filed first, maybe?)

no, i do not have such equipment or knowledge. The local hackerspace doesn't really want to help me out either unless i am prepared to 'get involved' on a long term basis with them. (something about they are not a cheap fab shop to be used, compared with industrial services)

Patrick wrote ...

im still trying to figure out what to say, but im going to guess youll need 30 minute epoxy with some fiberglass strands chopped up and thrown into the wet epoxy.

hm, same as above, is this something typically found in a hardware store?

Patrick wrote ...

however, i fail to see what is broken from your pics.

This is a photo of a good one, i have already disposed of the broken one and kept the shaft part

Andy

Wed Jan 29 2014, 01:25AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Hi Avi
I would try to make a mold for Al, you can buy Al plate and rod that could be used to melt down. You might have to check the tensile strength(or more elasticity, Al is good for metal), from plastic to Al. You might loss a fraction for the weight of Al compared to plastic, but fibre glass coating from a hardware store, you could make the blades narrower

If you want plastic,

,most manufactures use plastic balls which they force into a mold that is hot, might be batter than Al, and you might beable to order it online from the bank.

Ash Small

Wed Jan 29 2014, 01:26PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

You can get tubes of small amounts of epoxie at most hardware stores. It comes in two tubes lijke toothpaste tubes, or sometimes in a ;'double barreled syringe'. You mix equal amounts together, and then use it.

The stuff you get in hardware stores is quite viscous, but it does flow, especially if warmed a bit.

You can get larger amounts from places like boatyards, or order it online. This stuff is less viscous, but you don't need much, so the hardware stuff will probably be a better option for you.

The big tradename around here is 'Araldite', but this may not be the case where you are. There are loads of different brands.

I usually have to hunt around a bit to find one that isn't 'five minute epoxy', though. The five minute stuff will probably get hotter, and you won't have as much time to mix it and pour it into the mold as you'd have with the slower setting stuff.

Even the five minute stuff takes 16 hours to reach full strength, but it starts to set after a few minutes, so you'd not have much time to get it into the mold and let any air bubbles get out, etc.

As Patrick pointed out, you'd get a stronger part if you can get some fiberglass matting and get it into the mold as well, but this would be difficultt to achieve, I think (it could prevent the epoxy from filling the mold, or it would at least be more complicated to achieve good results)

You can get fibreglass 'kits' at car accesoruy shops for body repairs, which come with matting and polyester resin, which is similar to epoxy, but not as strong, but I'd try epoxy first, although I can't guarantee that it will be as strong as the original part, although it may well be stronger.

Patrick

Wed Jan 29 2014, 01:32PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

there is some cheap e-glass sold in the boat or vehical section at walmart or hardware stores. Not sure what you have in the Australia.

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