Monday, February 25, 2013

Delft Casting.

Delft casting is simply a different name for sand casting.

The sand has been mixed with an oil, allowing it to keep it’s shape, much like a dough.

I have never had the need to use this system, because I am OK with spin casting.

With the Palindrome Cannon project however, I don’t need that accuracy of casting a finely detailed model that spin casting provides.

Rather, I needed a quick rough and ready casting system that I cast 20 to 30 grams of metal in twenty minutes, modifying  the piece as I go along from cast to cast until I’m happy.

And I also wanted to cast cartridge brass, because that’s what the Palindrome is made of.

So there is no colour difference.

I use one millimeter brass sheet, so I can recycle my off cuts.

I had heard quite bad reports of casting sheet brass, but after a couple of casts I get the idea.

Unlike spin casting, where generally the temperature of the metal is married to the lowest fluidity point that will give you the level of detail you require, with cartridge brass ( 70% copper, 30% zinc ) I have found that the temperature to be as high as possible with a reducing flame, until the zinc JUST starts smoking.

What I also like about Delft clay is that it’s quite possible to cast a very rough wax, cast it and then finish the metal result off and that metal model becomes the master for the rest.

Metal masters are much nicer because you can bash them harder into the sand than wax.

Breaking a wax model when you pushing it into the sand is a major mission.

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The stuff is about UE30 a kilo, so it is not really cheap if you cast big things, but for small things it’s perfect.

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At the end, I didn’t use this model,--- too fugly and impractical.

I use it as an illustration of Delft casting then.

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I could have made a special frame, or bought the frame the company made, but instead I just used some vulcanizing frames I had lying around.

Here the sand is packed in and I have pushed the wax model into it.

I used Ferris purple carving wax because it’s hard and it’s a nice carving material.

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Talcum powder so nothing sticks.

I push with my fingers and tap it into place if the model is made out of metal.

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In this picture of another model, some of the corners of sand have broken off when I removed the wax model. This is because there were areas where the wax was not properly shaped for removal, so I modified them.

Delft casting does not allow for under cuts, unlike rubber moulds.

Also, the detail is not spectacular, so fancy engraving and fine detail is not possible.

Rather, what I am doing is to cast the basic shapes for the Palindrome Cannon as I go along and then finish them off after casting.

For that purpose, I have found it exemplary.

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I tap the top part down, not to hard, not to soft.

This was one of my first casts, so my locating pins were some what rough and ready…ahem.

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How not to use a steel ruler.

No worry, this one has been designated as sand trimmer, cheese cutter, screw driver, chisel and scraper.

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Normally I use a Stanley knife to carve a conical shape in the sand for the metal.

This was my first try and it was not very successful.

The mould did not fill all the way.

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So then I made a kind of volcano shaped entry, and I also increased the sprue diameter to about ten millimeters.

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This worked well, although I suspect that the increase of the diameter of the entrance from 7mm to 10mm had the most to do with success, rather than the volcano do da.

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Anyway, I replaced the first one with the top two, which I also cast.

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Here I only needed two copies, so I carved a wax model and cast two. The whole thing took about two hours, from wax carving to finish.

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It holds the front gun platforms in place.

600mm ruler.

I made this picture double the  size when you click on it.

1a

Video grabs of melting and casting etc.

The green flame is from the  Boric acid that I use as a flux.

The torch flame looks hot in the picture, but actually is is a very reducing flame because of the zinc in the mixture.

2

Casting.

Check the green flame is still there, so my melt is still under flux as I cast.

Zinc  cooks easy, so you got to look for a temp where there is a little bit of smoke, and you swirling it in the crucible, and then  a smooth gentle pour, all the while pulling the flame backwards.

 

3

Then there is this candle like flame after casing. Very romantic.

4

The mould is not hot, so you can pick it up no problem.

5

Then I tap the piece out of the top part of the mould.

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Flames a bit.

7

That’s the top of the mould with the button tapped through.

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Here is a semi finished cast.  It is still getting a thread tapped in, so the nut will not be needed.

1 comment:

Sandra Graves / Isis Rising said...

My DH has always wanted to do sand casting. It's nothing that has really attracted me because I'm all about the detail, but who knows? We may end up playing with this down the line. I love what you're doing with the cannon...

- Sandra