Metal Melter

[Onoff]

Got up this morning to find laddo had completed the print. Didn't have much time other than to snap these few pics off. A quick check and the spiral sections on the two halves seem to align nicely:

Not sure why my pictures are so blurry, the phone camera, the lighting in there, very possibly the stark white material...?

I want him to make up a portfolio of what he designs and prints from concept, to CAD, to finished form. Might help if he chooses to go for an engineering biased job rather than the computer science his degree's in...if he ever starts looking! He's definitely more back end, happy pottering away like this and keeps at it. Scared sh!tiless I think of approaching a big concern like one of the banks, internet providers with a CV. I don't know where he'll end up.

 

[Onoff]

Somebody has suggested using butter on the PLA mould might compromise the integrity of the clay.

???

 

[Onoff]

Ready to post! All the test bits too, might be handy for seeing how the clay forms around the hooked rib shape before committing to the big former:

 

[Pete.]

Mate you need to turn that element off before sticking those tongs in the hole.

 

[Kram]

The neon switch on the front controls the element, I didnt dare turn it back on after the RCD cut as they were not too pleased indoors.. *if* I knew/understood the cause I might have continued. I wish it were simple as me poking the element!

Im lost as to why the RCD keeps tripping with this, it seems to be repeatable when very hot, themocouple reading around 650c (cruicible 800-850c, outside about 70c)

Does alumina - tig cups conduct as they get hotter?
Some searches found paid-access papers saying yes but hinting the temperature is nearer 1000c.

High Temperature Electrical Conductivity of Aluminum Oxide

The electrical conductivity of polycrystalline and single crystal AI2O3 has been determined between 1000 and 1650°C. Measuring the conductivity between the outside and inside surface of a thin-walled, hollow tube eliminates the adverse effects of gas or surface...

link.springer.com

Google Scholar

scholar.google.com

And this

Alumina is an electronic insulator, but becomes ionic conductor at high temperature.... Conduction takes place throughout alumina by tunnelling effect.

 

[Kram]

The neon switch on the front controls the element, I didnt dare turn it back on after the RCD cut. Landlord was watching telly and was not pleased! (The VM box takes an age to restart).

Im lost as to why the RCD keeps tripping with this, it seems to be repeatable when very hot, themocouple reading around 650c (cruicible 800-850c, cylinder under 70c)

Do alumina tig cups conduct as they hotter?
Some searches found paid-access papers saying yes but hinting the temperature is nearer 1000c.

High Temperature Electrical Conductivity of Aluminum Oxide

The electrical conductivity of polycrystalline and single crystal AI2O3 has been determined between 1000 and 1650°C. Measuring the conductivity between the outside and inside surface of a thin-walled, hollow tube eliminates the adverse effects of gas or surface...

link.springer.com

Google Scholar

scholar.google.com

2) SCR or SSR in the control box, or bad wiring, leaking.

3) The coil acting as an antenna with radiated energy going to the grounded casing instead of down the neutral wire. I thought that only applies to vacuum tubes

Next upgrade may be replacing insulators or coating in clay.

 

[Misterg]

Does alumina - tig cups conduct as they get hotter?

The electrical conductivity of most glasses / ceramics become dramatically better as they get hotter.

This also applies to alumina (or alumina/zirconia which I *think* TIG nozzles are made of) but they remain fairly resistive (the presence of any sort of binder probably dominates the behaviour) which is why they get used as high temperature insulators.

Many imputities (particularly sodium, potassium, calcium, etc. ) drastically reduce the high temperature electrical resistivity (as they enable ionic charge carrying).

I think you have sodium silicate in there? That becomes very conductive at temperature - probably a few 10's of ohms/square at 700°C and dropping like a stone. Make sure there is no path to earth from the inner face of the kiln (i.e. don't have any metal rods extending from outside to inside. If you have a metal sheathed TC inside the kiln, that will probably need to be floated above ground (and suitably guarded as if it were live).

Even if you eliminated the sodium silicate, you'd likely end up with a similar situation over time due to the fume from melts reacting with the refractory lining.

(I know I've said it before, but please treat everything in contact with the hot face of the liner as live & interlock accordingly: As well as the possibility of a serious shock - even if you don't directly contact the element - there's also the issue of what happens to whatever you're holding when you get the shock. /preachy - Playing with glass and electricity for 30 years.)

 

[m_c]

Easiest option to avoid RCD issues, would be to add an isolation transformer.
1500VA continuous can be had at Screwfix - https://www.screwfix.com/p/carroll-meynell-3000va-intermittent-isolation-transformer-230v-230v/320hv

 

[Kram]

Thank you, answered tbe question and solved it.

!!





I had assumed that glass and ceramics were good electrical insulators ay temperature. Yes I used sodium sillicate but again assumed that set, dry sillicate was insulating.
The live and neutral pass directly through the 3" perlite/sillicate mix with a bit of clearance, its an 8mm hole. The central two inches I expect 500c+

Porcelain would be ok, if I can get some tubes?

Theres a massive surface area between the central liner/perlite, and the grounded casing.
Looks like an isolation transformer would do the job, though storage would be a pain. Thanks for the link.

 

[Onoff]

PLA's a good insulator.....

 

[Misterg]

I used sodium sillicate but again assumed that set, dry sillicate was insulating.

It is at room temperature.

The live and neutral pass directly through the 3" perlite/sillicate mix with a bit of clearance,

I can't see that being a problem as neither are earthed (unless you mean you're getting a hotspot between them).

The cooler parts (I guess less than 3 or 400 °C) won't be electrically conducting enough to worry about - it's only the hot face.

Isolating transformer is a good shout.

 

[Onoff]

Made a twisty, handle thing for it that might aid winding it in and out of the clay:

I'll hopefully be sticking in an InPost parcel locker PM for Hermes to do their worst with it!

 

[Onoff]

Damn...just realised we should have chamfered the ends of the spiral...

You'll be trying to force a blunt end through the clay. Maybe do do yourself with a small block plane etc. Might need some filling thereafter if you break into the infill.

 

[Onoff]

It's in the post @Kram Hermes.

Over to you now!

 

[Kram]

https://core.ac.uk/download/pdf/38912631.pdf

I fell asleep trying to read this

Landlord is going out soon so I may have another go tonight and try measuring some things while its hot, try to solve the problem.

No problem with the blunt end. Twisty handle looks good! Thanks!

How did you get on with the cows face?

 

[Onoff]

How did you get on with the cows face?

She's sitting next to me on her pc...

 

[Onoff]

Seriously now...the cow's face...as alluded to in our PM's I took a random photo off the net, turned it greyscale, then ran it through the online imagetostl.com.

I think I set the height too high at 10mm. Face on its vaguely cow like but the extrusion effect is maybe too deep. From any angle it's dire.

Lad did trace round the original image it and made a "cookie cutter" for his sister to have a go in clay...work in progress.

Doing another face right now at 5mm high, again in brass coloured PLA.

 

[Misterg]

https://core.ac.uk/download/pdf/38912631.pdf

I fell asleep trying to read this

Welcome to my world!

You want figure 2 (page11) or table 4 (page 46).

Sodium silicate corresponds to roughly 50 mol% Na2O, so its resistivity follows the red line I've drawn on the graph from figure 2 in the paper:

I can't find the units they've used for resistivity anywhere in the paper, but it would usually be 'ohm metres'

So at 100°C, the resistivity is about 10^6 (1,000,000) ohm m, about 10^2 (100) ohm m at 400°C, and extrapolating a bit, about 10^0.5 (3 or 4) ohm m at 700°C

The resistance of a bar of material (measured from end to end) is [resistisivity] x [length] / [cross sectional area], so for a cube, the resistance between opposite faces is equal to the resistivity.

Hello...

Hello... ??

HELLO ??? Anyone still awake?

 

[Onoff]

Cheers @Misterg , slept like a log!

Cow face No.2 on the left. This one only 5mm high. (The 10mm high one on the right).

The nostrils and eyes are at least better defined. Might try 2.5mm extrusion height later.

Then I need to get my lad to add a fancy, fluted/holed surround in Fusion, add lettering etc.

Still need to go and buy that investment powder...

 

[Pete.]

Could you not have chosen a different colour it looks like you've printed a cow's face out of cow pats