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I've been Resin printing for a while, but was considering getting an affordable FDM printer for large simpler models, and for when the mess of a resin printer is just too much of a pain to be bothered with.

I was considering buying the Elego Neptune 2 FDM printer, then I saw that for about $60 more I could get a version with two extruders.

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Other than the obvious printing in two colors, is what can I do with a dual extruder that I can't do with a single one (Regardless of whether it's cost effective). Anything practical, anything cooler?

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As an owner of a dual extruder printer, an independent dual extruder machine (IDEX), I can shed a bit of light on the topic. One of the better features of an IDEX but not necessarily capable of the printer you've selected is duplication and mirroring. By using both extruders simultaneously, print time for "production" quantities is halved. As noted, this is not possible for the selected printer.

That leaves wash-away support as the other feature besides two colors. Instead of printing support material as with a single extruder printer, the second feed is available to use PVA, which dissolves in warm water. You can have models with internal supported cavities that would be nearly impossible to remove in single material prints, but need only to be placed in water and agitated to provide a cleared model.

Using PVA support material also eliminates the marks on a model from support structures, reducing post processing labor. My experience with PVA is that it's challenging to print, requiring tuning the print parameters. It also tends to carbonize in the nozzle if temperatures are a bit high, but that again follows the tuning aspect.

As referenced in the comments, a dual extruder machine can also create parts with interlocked material. I have seen, for example, wheels constructed with a PLA hub and rim and TPU for the tread. I've not created such an item (yet) and overlooked that option. Typically, you'd want to combine materials with similar temperature requirements, but with certain constructs, the mechanical components can accommodate some disparity. Thanks to R.. GitHub STOP HELPING ICE for the reminder.

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    $\begingroup$ Aside from supports, you can also do interlocked dual-material prints - for example, incorporating TPU joints into a print-in-place object mainly made of some rigid material (PLA etc.). $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Aug 28, 2021 at 20:37
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    $\begingroup$ To add to what is here, one other advantage of dual extrusion is one material, but two different nozzle sizes. With a small nozzle for detailed walls (.25mm-4mm), and a large nozzle (say .8mm) for infill or top and bottom layers, you can cut down on print time dramatically while having fine layers and line width for the walls. $\endgroup$
    – ChinchillaWafers
    Commented Aug 29, 2021 at 6:44

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