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I have a few kg of 3 mm filament when I only have use for 1.75 mm.

How can I make 1.75 mm from 3 mm filament?

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    $\begingroup$ Note: I believe this will not be worth the cost/effort but am very curious to see what useful suggestions are provided. $\endgroup$
    – kaine
    Jan 12 '16 at 20:00
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In theory, making filament is easy. You take a 3 mm hotend with a 1.75 mm hole, and extrude the 3 mm (sometimes actually 2.85 mm) filament, let it cool, and then reel it up.

In reality there are a lot of pitfalls to making filament - if the pressure isn't even, the hole not perfect, the temperature uneven, you can end up with oval filament, filament with bubbles, or worse. If you are over temperature you may damage the filament and it could look good, but not melt correctly when used. If you reel it too fast you may thin it out more than the intended diameter, or too slow and you may thicken it. A lot of hotends use steppers, which may result in ripples in the filament, so you may end up building a nearly custom filament machine.

Resolving all these problems is probably not worth simply selling or giving away the filament to someone that can use it, and buying the right size for your machine.

If you are still interested, though, you might as well go all the way and build a full filament extruder that accepts raw plastic feedstock (usually pellets) as well as your filament, and convert it that way, then continue using it to create your own filament.

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    $\begingroup$ Your "theory" is wrong, the size of the hole through which you push the plastic only has a small influence on its size. Generally it will swell a little to become larger than the hole through which it was extruded, but by pulling on the plastic you can stretch it out. $\endgroup$ Jan 12 '16 at 22:57
  • $\begingroup$ @TomvanderZanden Which is why I tried to make the distinction between "theory" and "reality" starkly apparent. The die size influences the filament diameter, but there are many factors, as stated, and you'll have to make adjustments to make good filament at a given diameter. $\endgroup$
    – Adam Davis
    Jan 13 '16 at 12:35
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The best option is to find somebody in need of 3 mm filament and trade them for it (either in exchange for 1.75 mm filament or in exchange for legal tender with which to buy said filament).

The next best option would be to cut it into small pieces, and feed those into a filament extrusion system such as the filastruder.

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  • $\begingroup$ Exactly. Sell it, exchange it, or I have even contacted my US based provider and simply exchanged it. If you buy if from a physical location or have more direct contact with the provider, they may be willing to exchange it for little or no cost. $\endgroup$ Jan 14 '16 at 4:58
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You could build a machine that has a nozzle with 3 mm input hole and 1.75 mm output hole, based on some designs for filament making machines. Or you could just cut the filament into little peaces and use them instead of the granulate in an original filament making machines.

There are some open designs for such machines you can build, or you could buy one, such as Filabot.

However, as mentioned by kaine, this is very unlikely to be worth the cost/effort. Best option for you is to try to sell the 3 mm filament to someone who has a use for it, take the money and buy some 1.75 mm filament instead.

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