Why nichrome wire?

It’s a simple question, what’s so special about nichrome (nickel chromium alloy) wire that makes it suitable as a heating element?

The important factors are its high electrical resistivity, its high temperature corrosion resistance and high melting temperature of 1400 degrees c.

Nichrome wire is irreplaceable in situations such as hair dryers and spaceheaters as other metals will quickly oxidise and degrade at elevated temperatures. However, at the relatively low temperatures (<300C) used for reprap heated beds other materials such as common copper could be used provided some precautions are taken.

The high conductivity of copper could be overcome by using a very long length of wire of a very small cross sectional area. Also being a near pure element in use with a melting point of just over 1000 degrees C it will have a stable microstructure during use. That is providing there is a good thermaly conducive path to a heated bed and so its not allowed to glow “red hot” at any point. The problem of corrosion could also be eliminated provided the copper is removed from oxygen, such as when covered by fire cement or polyimide tape.

So why go to all this trouble when nichrome wire is so cheap and widely available? Well there is one thing that nichrome wire isn’t; printable. Making a heated bed with nichrome wire will always be a labor intensive and possibly dangerous part of making a reprap, more so if mains power is used.

However, now that great strides are being made with producing   single sided PCBs on mendels there is the possibility of “printing” a heater bed element.

Imaging a blank PCB that is slightly smaller than the print bed. Then print on a square spiral or zigzag patten with each trace as thin as possible to create a single, tens of meter long ‘wire’. Etch the PCB, cover with a layer of polyimide tape and attach to you print bed material of choice. Then you would have a thin evenly heating heated print bed that is, in a way, printable.

Thoughts?

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About Richard

I am a Materials Engineering working in the field of Magnetic Materials in Melbourne, Australia. This blog covers my personal interest in all things CNC.
This entry was posted in A Reprap Project and tagged , , , . Bookmark the permalink.

8 Responses to Why nichrome wire?

  1. nophead says:

    Yes what you describe is pretty much what the Makerbot heated bed is.

  2. stephen george says:

    Hi Richard

    What a great question. I never thought of using copper as it is such a good conductor. But you are right, making the diameter of the wire small will increase the resistance. How about using Aluminium which is less conductive (more resistance) and due to a low melting point (but greater than 300c) might be more printable.

    regards

    Stephen

    • Richard says:

      Hi Stephen

      I’m a little confused, are you suggesting that Aluminium could be directly placed down as a wire? Or are there blank aluminium clad circuit boards available that could be etched in the same way as I described in the post?

      Something worth keeping in mind is that Aluminium is a relatively low temperature metal. So although its melting temperature is over 600 degrees C, some alloys will become very soft or undergo microstructural changes at much lower temperatures.

      Another option could be to use metal cored printed circuit boards (MCPCB) such as the ones used for power LED’s. However trying to find a blank the size of a mendels printer bed could be very difficult.

  3. Santosh Kumar says:

    hi richard

    I want to know why is nichrome and tungsten used as a heating element?

    • Richard says:

      Both Nichrome wire is used for heating as it has a high resistivity, high melting point and does not oxidise (rust) at high rapid temperatures like most metals. I imagine tungsten is used for similar reasons, although I believe it is not as resistant to oxidation, hence the need for an inert atmosphere inside the old style incandescent light globe.

  4. how can hot spots be detected in Teflon coated Nir A 80/20 wire

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