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Vacuum (from old project)

A project log for Fighting Oxidation with Vacuum

Melting metal in low vacuum to reduce oxidation without shielding gas

Dominik MeffertDominik Meffert 03/31/2021 at 14:260 Comments

Some time has gone since the last update.

In the last few weeks I had not much time to work on my projects because I moved to a new location, but the work is almost done and I can continue working on stuff.

So I will continue with this project and will try to fix the problems which I had not solved in the past, which are:

- Oxidation, because of the lack of a controlled inert atmosphere.

- High EMI noise, what made the use of sensitive electronics like sensors and microcontrollers almost impossible and required elaborate signal transmission over optical cables from a distance or complicated shielding of all parts.

- Going 3D, with a SLS style recoater and piston sytem.

- Control of all parameters, with an inverter based power supply and a few sensors.

I recently had an idea that could solve these problems:

Vacuum

I learned that the breakdown voltage of a gas decreases in vacuum described by Paschen's Law.

So if the pressure and electrodes distance would be low enough and the applied voltage would be high enough an arc would ignite without the need of an EMI emitting high voltage source and as there are vacuum arc furnaces out there it should also be possible to reach high enough temperatures to melt the metal powder.

IGBTs and MOSFETs which can switch up to 1000V are also easy available.

I think the challenging parts while building such a vacuum arc printer would be the construction of a large enough vacuum chamber which can fit a 3D printer and the problems caused by the vacuum like the reduced breakdown voltage and outgassing what would make the use of electronics like sensors, stepper motors and some materials like grease and maybe rubber impossible.

My current plan is to 

use a piece of large construction steel pipe (>500mm inner diameter with 5-10mm wall thickness) with machined acrylic or aluminum top and bottom,

or welding of a steel or aluminum box with 5-10mm wall thickness,

or less likely glue together an acrylic box with >20mm wall thickness,

everything in combination with a good vacuum pump that can achieve less than 1mbar.

To solve the breakdown and outgassing problem I thought about bringing the motion from every stepper motor placed on the outside of the chamber into the inside by the use of a shaft sealed by a mechanical seal.

For detecting the limits of the axis I would use Trinamics StallGuard/Sensorless Homing feature.

I think if everything should work it could be a very interesting machine at the end.

So that's the plan and now the work can begin.

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