Disolvable core for composite lamination

Hi,

I want to print a "core" or "plug" for a composite part, wrap it in carbon and epoxy resin and then remove as much of the core as possible.


My initial idea was that I could print a shell in PLA (which I have plenty of) and then fill it with something cheap like candle wax to get a fairly solid lump that I can sand and mess about with prior to applying the carbon.

I then thought that I could melt out the wax and get the PLA soft enough to pull out too.

But putting some old scrap prints into boiling water I have found that they are surprisingly tough still, and I have limited holes to get at the cavity through.


Reading up, it seems like caustic soda will dissolve it eventually but it may need quite a while and heating and agitating etc. And I am a little concerned that it may also have an adverse effect on the epoxy resin of the actual part.



Any ideas? Other materials to print with? Other solvents or ways to break up the core?

PVA/ water
ABS/ acetone

Thanks for the suggestions.

I couldn't find much info on the PVA, my only experience of it is as glue where it is very soft which wouldn't really suit my purpose, is the filament or rather the printed part fairly hard and rigid and sandable?

Surely the acetone to dissolve the ABS would be pretty tough on the epoxy too?

Acetone will damage or dissolve most epoxies. Strongly recommend not using it in this application.

I haven't use PVA personally but my understanding is it prints like any other filament. I would suggest starting your experiments with PVA.

My further searching has found the PVA described as "soft" and "like the glue", which would make it difficult for me to sand the surface smooth and correct any blips.
I also haven't yet found a UK source of the filament (?) and I don't really want to spend the money to order a large reel from the US only to find that it isn't suitable, so any clarification or feedback from people with experience of it would be valuable.

Sodium hydroxide, to soften/dissolve the PLA, is described as "attacking" epoxy resins above 70% concentration, but "not attacking" epoxy when below 50% concentration....
What concentration am I going to need to break up the PLA? the successful stories I have found seem to involve a saturated (100% I assume?) solution and lots of agitation and heat for a significant time.
My chemistry isn't up to understanding what would be going on here, and my worry is that either it attacks epoxy or it doesn't, if I need a long period of time and heat then maybe a 50% solution will still attack the epoxy? Or, maybe in the same way that a sub 70% solution wont attack the epoxy, it also wont attack the PLA? (at least with this one I can do a test easily enough on some old PLA).

Any insight very welcome

George

PLA has been used as a template in lost-wax style casting. Perhaps you don't need to use any other material.

[blog.reprap.org]

In the article above, no chemicals were used, just an oven over a long period of time.

Edited 1 time(s). Last edit at 01/14/2013 03:18PM by PeteD.

Thanks for the suggestion Pete but my composite part will be a couple of feet long* so an ordinary domestic oven wouldn't be big enough and the highest temperature I feel happy subjecting the composite part to is about 170 degrees or so (with a high temperature epoxy) and I think that the PLA would take a long time to slump out at that temp...

George

* core will be made in multiple segments and glued together.

Instead of trying to sand PVA, you could probably smooth it with a wet sponge.

If you print your core hollow/very sparse infill, is it actually heavy enough to be worth removing?

Thanks Greg, the idea of sponging it smooth is very attractive I hadn't thought of that, if it saves me a lot of sanding then the expense of the PVA would be well worth while... but would the PVA be stiff enough to hold it's shape while I fill it with wax (or something)?

Also I am still looking for a UK source if anyone knows of one.

The only place I have found is RoboSavvy but they only seem to have the 1.75mm (and I am currently set up for 3mm) and it is £50 which is a lot to fork out when I am not sure the parts are going to be rigid enough.

Konwiddak; my initial thought was to leave a thin shell in there but it actually increases the final part weight by over 50% and I need the space in some places so I really need to get most of it out unfortunately.


I suppose another option might be to print an external shell and then cast a (possibly wax) core inside it, then just cut the external shell away. My worry with this is that ordinary wax will shrink a lot and leave big dimples. A low temperature alloy like woods or field alloy would be ideal but that stuff is amazingly expensive in the UK (though there seems to be relatively cheap stuff in the US)... suggestions for alternatives much appreciated.


George

So I tried making a small test wax core and combined LDPE, paraffin wax and beeswax. The resultant material was nice and strong, relatively hard, and had minimal shrinkage, however it also seemed to need temps way in excess of 100 degrees to re-melt so would be hard to remove at the end without overheating the part (unless I use a high temp epoxy).

Am going to try some sub 50% Sodium Hydroxide solutions on the stock PLA and see how that goes.

Gsport,

I am very curious to know if the sub 50% worked well and managed to dissolve the PLA.
Thanks.

André

Sorry this took me so long to come back on.

Had virtually ZERO success with Caustic Soda/Sodium Hydroxide.

My research on the epoxies suggested that they were fairly happy with up to 50% solution, but even 50% seemed to have very little effect on the PLA. After several days, weeks even, the part was still 90% intact. Prodding, stirring etc seemed to make very little difference.

I am now wondering if I should make a paste extruder and use icing!

PVA still seems to be ridiculously expensive...

George

Have you looked at using ifeelbeta's recipe? [ifeelbeta.de]

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[haveblue.org]

Thanks Have Blue, I hadn't seen that, might be worth a try. The Aluminium Hydroxide is likely to be the only problem. Epoxy is listed as having "good" resistance to it, as long as the temperature stays pretty low (under 22C)...

George