I have already build the MPCNC and am thinking about upgrading, but want a better material than PLA. The biggest thing is rigidity, so is a plastic with a higher then PLA Youngs Modulus OK? Like some carbon fiber or glass filled PETG? Something like that? Is there a different variable I should look at?
Welcome to the forum. I’m not an engineer, but from my layman’s reading, Young’s Modulus seems like a good measurement for stiffness for the plastic used in the MPCNC. Note you may not get much benefit by going with an exotic material. And even if there is some benefit, you might be able to achieve something similar with PLA by changing 3D printer settings (perimeters, infill percentage, infill type…). I’ve bumped into many conversations on this forum concerning proposed improvements to the MPCNC. My main takeaway is test, test, test. Many suggestions that look good at first provide little benefit once tested…or even hurt performance.
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You also have to look at where the “floppiness” happens. Is it the printed pieces? The conduit/tubes? Belts? Do you have a 200mm Z axis? Homemade spindle? 
The bigger thing is that pla deforms overtime and is not very heat resistant. I just don’t want to degrade performance ya know?
I hear your concerns. My guess is that, unless you plan on having the machine live in a very hot environment, you will not notice a difference. When you select your filament I would be interested in the result of a stiffness test between it and PLA.
All plastics deform over time with heat. I would think cf petg would be the way to go. I’m running with pla for now though. It’s not had any issues out in my unairconditioned barn so far this summer, or the past several summers.
If your only intent is to improve rigidity, then it doesn’t make much sense to reprint everything just to get a very marginal improvement on these parts. It will be mostly a waste of good filament.
Do you have actual issues with rigidity in the first place? What are you trying to do?
Remember that people manage to cut aluminum and even steel with standard PLA parts, so unless you have some really specific problematic rigidity is likely not to be critical
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You could always pay the extra and get proto-pasta HTPLA. If part warping is your main concern. 
Well, if we’re looking at ideas to improve material stiffness, strength and heat resistance…
Playing Devil’s advocate here… What about lost PLA casting?
Basically the idea is to make molds from ceramic and plaster using the PLA printed part, then filling the mold with molten aluminum or other relatively low melt metal. I imagine that bronze or pewter would also work fine, but be somewhat heavier.
Upside: You have an aluminum part in place of PLA printed plastic. This is going to stand up to any reasonable, or even unreasonable temperature.
Downside: It’s a real pain in the ass to get the mold right, and if anything goes wrong, you have to start over from zero. Oh yeah, there are some really dangerous parts to the casting process, like dealing with all kinds of scenarios where you have molten aluminum.
I tried some lost PLA casting a few years back, actually to make some silver jewelery pieces, some of which turned out ok, and some not so much. Those were relatively simple shapes and small, which is all I could manage with the tiny kiln I made. The process works quite well, you print the PLA part with as low an infill as you can, and then add some flues for pouring and melt runout. Mix up your ceramic and cover the whole part as well as you can. Fire in the kiln until all of the PLA is gone, then melt your metal and fill the mold. Cool slowly then break all of the ceramic and plaster out of the finished piece, cut off the flues and check for other issues. The most common is if the mold cracks where it’s thin, usually a hole which you can sometimes fix with a drill. Oh, and I mentioned that this is potentially dangerous?
I tried at one point to make an extruder that could print paraffin wax with the idea of using it for lost wax casting, which is really much simpler, and safer as well, though it results in rougher surface textures, this isn’t likely to be critical when replicating 3D printed objects. Unfortunately hopper based extruders aren’t very good. I might one day come back to that concept.
If increased rigidity is important for you, you might consider decreasing the work area and z height? I think that will be the most influential factor. (unless you need a big area as possible though…)
My biggest thing is that PLA tends to creep and deform over time, regardless of temperature. I am not looking to a magic material that will make the machine preform way better, just something that is slightly better than PLA and will not hurt performance. CF PETG or GF PETG would probably be the way I would go if I choose to upgrade. Also any amount of stiffness increase is a upside. Even if the there is still play somewhere else, making any part stiffer is a overall benefit.
I had my MPCNC for something like 4 years, in harsh conditions (cold winters and extremely hot summers), it did not noticeably creep or deform 
I did however found a few cracks here and there after a few years of use, but it was still working fine and I could have reprinted just these parts.
PLA is a really good plastic actually, I wonder why people are so mad at it?
Correct me if I’m wrong, but PETG is less stiff than PLA. It bends easier. So that’s not a good choice if your goal is to improve rigidity. Using PETG will improve wear resistance and weather resistance. So that would be a good choice maybe if you plan on leaving your MPCNC outside.
Not really, forces follow the weak link chain, so almost all the forces will escape through the easiest spot. If you have play somewhere forces will escape throught there.