Neither of the links are working for me.

*edit: never-mind found em by searching

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AFK with real life, still checking PMs

odd, they both work for me.

I think you're on to something with this burning plastic bags business. But I accept no responsibility for any injury caused by my encouragement.

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just remembered I made this a while ago:

.

I needed a trowel and had some hdpe pipe lying around. HDPE is the same basic plastic as plastic bags, just produced slightly differently. It might be easier to get an even melt on though - no air bubbles trapped inside.

I'm currently looking at some possible methods for steam casting. there is hope comrades!

so last night I indulged my pryomaniacl side again.

this photo shows afterwards, once all the fire was gone:



Note how I carefully cleared all flammable materials away from the work site. Note also the presence of the watering can. I'd just like to say it was full of water. I'd like to say it, but I'd be lying. I will improve on this next time.

So: four mini molds at the front - bolter, heavy bolter, shotgun and large skull.

pliers over the half can - the plastic will melt onto the pliers and stay there burning and dripping off. Have somewhere for it to drip into.

Cigar shaped thing cooking over the three candles. An investigation into a more controlled method.
It's a plastic bag, wrapped up tightly in sheets of aluminum cut from a beer can. (it was going to be foil, but we'd run out)
The idea was to cook it slowly at an angle until molten PE dripped out then end, letting me do the plastic drop casting without fire everywhere. It didn't work. The molten plastic is sticky and still quite viscous - it'll need to be forced out somehow. I have plans.

anyway: this was what I found when I opened the cigar later:


the pale green on the right never melted, presumably the molegrips acted as a heatsink.
I'm not sure whether to use this in future experiments, or cut the pillar off, set it on a base and use it as a demon's warp rift objective:
close up of the gnarly bits:



OK.
the products of the smaller moulds:


there was quite a lot of shrinkage - for both the heavy bolter and shotgun the arm snapped off due to it.
All molds were overfilled, and all shrank by 1mm vertically. The skull being thicker, shrank much more. Approx value from molten to hard = 20-25% reduction. Where the plastic was touching the silicone, it was much less.
Presumably the molten plastic built up on those bits in thin layers, each setting very quickly.
A reservoir system will be required for large casts.

There are two heavy bolters. The upper one was my very first atttempt, where I squashed a lump of molten plastic into the mould with a ceramic tile. The second was done purely by drops and came out better. Still not production standard though.

The bolter didn't cast. This is due to a terrible mold, not the process. Although I think not visible in the pic, what parts of the bolter were accessible cast perfectly. Without something to force the plastic into the mold, we're limited to push mold types. The plastic sets hard too quickly to flow through gaps.

Especially notable on the skull mold was ash. The burning plastic is just hot enough to ignite the silicone caulk. Thin details on or near the top of the mold suffer. Ones deeper get coated quickly so no oxygen and no damage.

testing will continue.

Couldn't sleep yesterday, so tried another round of experiments:

first off - a comparison test between a normal plastic bag, and the concentrated one from last time.

the concentrated one burnt and melted much move evenly, in a much much easier to control process. I found the best way to place the molten plastic was dragging the stick over the mold a few times, and then waiting a few seconds for it to get back up to heat.

the normal plastic bag had a habit of dropping huge big burning lumps - finally I put a piece of aluminum over them and squidged them flat with a ceramic roller.

Neither piece came out of the mold in one piece - in both cases some of the plastic was very very thin (fractions of a mm) - I'd chalk this one up to oparator error. it's also a slow method.
results are promising though:



I also tried a completely different type of experiment.

For the type of process I have in mind, the silicone caulk moulds will have to be quite thick.
Since they cure from water vapour in the air, the inside areas of a thick bit might take a very long time to cure, making the mould weaker and more floppy then wanted.

one idea I've seen is to mix flour though the caulk. that might affect the fire resistance though.
a second idea is to mix a bit of plaster through -
the curing caulk gives off acetic acid, which breaks down the calcium carbonate, releasing water, which is exchanged for acetic acid and so on...

Start at the extreme case:
I mixed two tablespoons of plaster with 1 tablespoon of caulk and a tiny bit of water. ended up with a bread dough type consistency. completely non sticky too.
tried modelling with it:



you can see some shrinkage cracks appearing this morning.


Finally, this is the plan I'm slowly working towards:


its a method for casting silver.

A clay mold is made around a wax pattern. the wax is then burnt out, heating the clay up to a nice hot temperature.
a puddle of silver is melted into the depression on top of the mold. it doesn't flow into the sprue because the hole is just small enough for surface tension.

a shallow lid (tuna can) lined with damp tissue paper is placed firmly down on the mold. Where the damp tissue hits the hot clay around the metal you get steam. A nice high, short term, pressure forces the metal throughout the mold.

This is what I'd like to do, but with plastic instead of sliver, and a reusable silicone mold instead of clay. Surface tension is lower, viscosity is higher, overall temperature is lower, heat transfer of both plastic and silicone is low.

thoughts?

How would you melt the bags in the depression? or would you light them above and drip them into it. From the instructables links it looks like pretty viscous stuff, so will be interesting to see if the steam method has enough push to force it through something with a small enough in gate to initially hold it in place with surface tension. Depending upon the shape of the part being moulded and the hardness of the silicone you might also find that inside parts of the mould deform a bit when having viscous stuff forced in under pressure?

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when burning / melting, it's pretty much the same consistency as burning marshmallow - if you've that experience.

I'm not a huge fan of having to burn the stuff to be honest. let alone the fumes, it just seems silly to waste half the material. not to mention the risk of making the result too brittle.

I'd got as far as considering a ceramic top plate above the mold with nichrome wire running through it.
The problem is since plastic is such a poor conductor of heat, it's possible to char the 1mm touching the hot plate and have the top 5mm still solid. It's the reason some of the inscrutable methods melt the PE in oil.

Bringing the temperature up very slowly would do it, as would melting the plastic elsewhere and just pouring it into the depression just before casting. I don't really want to risk ruining our kitchen oven. some things the wife would not forgive.

What about putting the plastic in a hot-glue gun (instead of glue, alternatively, just doe hot-glue casting), that would melt it and inject it, something like this http://www.instructables.com/id/How-to- ... ittle-to-/

My lead casting moulds get too hot to touch both externally and internally after say 15 pours, would think it should be possible to get the mould reasonably heated.

Microwaves? I realise you can't normally melt plastic in a microwave unless sufficient heat is conducted by water jiggly stuff like food, but if you can melt glass and metal in a microwave, why not plastic?
http://home.c2i.net/metaphor/mvpage.html
http://www.popsci.com/diy/article/2003- ... -microwave
http://www.metacafe.com/watch/1004040/m ... er_bottle/

Oh, and edit: i am of course in no way encouraging you or anyone reading to damage yourself with molten plastic fumes nor encouraging testing on GW components. Both very bad

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Interesting experiments but isn't burning plastic meant to be a really bad neurotoxin? I have the worst memory of anyone I've ever met and I sometimes wonder whether it's related to the household rubbish of all kinds we burnt on our fire to keep warm in our council house as a teenager (we had no central heating and didn't realise at the time it was bad).

Following Apoc's suggestion, I bought a small hot glue gun.

I wanted to be able to use it normally, so I’d need to convert my plastic bags into 5mm extruded rods. Fortunetly, I had a piece of aluminium pipe that fit exactly – it came off one of those wind chimes people insist on hanging at my head height.

My first attempt was just stuffing the bag into the pipe using a paintbrush handle. Not very successful. Let’s approach this creatively.

First step – the bag was torn in a spiral pattern to get one long ribbon. This was completely by fluke, but worked out extremely well. If your trying this, try and make sure the ribbon is the same width along it’s length. It helps later.

Then, I needed to twist the ribbon up tightly to make a thick dense cable. I’d have preffered to use a drill, but the cousin’s borrowed it. So one of the household fans was volunteered. Note how nice and even the ribbon thickness is.



What the photo doesn’t show is the very confused cat that came in to watch.

Stuff the twisted thread into the aluminum pipe, ram it as tightly as possible; heat briefly on the stove, ram more in and so on.
Heavy heat proof gloves would be good for this. I used an oven glove but it did get a bit singed.
When the plastic has cooled a little, push it out and voila!
Well, that’s the theory.


From right to left you can see fairly rapid improvement in density and length of the plastic. And then I bent the pipe. Oops.
If I can get the rest of the process working, I might get a steel mould made.

As for the rest of the process…

Well, I tried a PE stick in the hot glue gun this morning – it either doesn’t get hot enough or I can get enough pressure on – plastic refused to leave the heating chamber. Testing will continue, but I’m not desperately hopeful of this avenue.

And it was looking so good until that last paragraph heh. Great idea with using metal tube to make the rods.

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Did you get a high or low temperature Glue Gun? I have a dual, with settings at 140 deg C and 170 deg C. Might make a difference.

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well, the note on the gun said can operate at temperatures up to 300deg Farenheit - which google tells me is about 150deg C.

Bear in mind the burning plastic method locally heats to maybe double that. The instructable that was linked worked, but maybe he was using a different lower temperature plastic - I think fishing baits aren't rigid like the PE i'm trying to use.

on the plus side, I've made some more moulds - primarily to test making thick pieces.

OK so you need a thermoplastic which melts below the temperature of the glue gun, but is solid at room temperature.

Wikipedia has a list of thermoplastics, unfortunately not many have melting temperatures listed.
http://en.wikipedia.org/wiki/Thermoplastic
However, PE is supposed to melt at 105-130 celsius. A glue gun whose element is 150 celsius is probably not sufficient to actually extrude PE as the heat will dissipate from the element, but theoretically it should be not too far off. Maybe there are different molecular weights of PE too? My knowledge of this is limited, but e.g. bin bags do seem different to shopping bags, maybe more stretchy. Worth a try?

I did also learn that Friendly Plastic/Polymorph is made of polycaprolactone:
http://en.wikipedia.org/wiki/Polycaprolactone

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