Why go proprietary? Aside from the obvious solidification of plastic revenue, manufacturers want to ensure a more reliable and consistent printing result by using filament that meets their requirements.
Meanwhile, generic filament 3D printers don't have that advantage. They use plastic filament that's a lot less expensive, but it may vary in characteristics and quality. Typically one looks only at the price, color and diameter, typically 1.75 or 3.00mm.
But a lot more can go wrong.
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