Mouldable plastic that will be hard-wearing?

Take a look at this stuff

cold (room temp) it feels rather like nylon, but it becomes like putty when immersed in boiling water. I use it for all sorts of things. Don't know how hard wearing it is, but it can be softened and re-moulded as much as you want.

Does it have to slide on the splines, or are they just there for assembly and to provide torsional strength?

I would be looking at cast epoxy, or even polyester resin. Readily available with a mineral filler in the UK, we call it "car body filler".

With some resins you might find you get enough expansion on curing to provide a suitable running clearance even if you want a sliding joint. Methyl methacrylate comes to mind, turns into what you would call plexiglass when polymerised.

You could wrap the spline with aluminium foil, packing it into the grooves before moulding or pouring epoxy. Then it would hopefully slide off OK and you could unpeel the foil to provide running clearance.

I'd start by doing some experiments!

Yes, it slides.

On the high end, Moglice (not exactly the way they expect it to be used, though you could get closer to what's expected by turning an "outside" part with a keyed/undercut area bigger than the splines for the Moglice to grab - and you could to the same for the solution below, which is basically cheap wannabe imitations of Moglice, depending what you mix in.

Lower than high end - epoxy and filler (aka filled epoxy - buy it that way or buy components and make it that way) with mold release (wax) on the splined shaft. Devcon plastic steel (and plastic titanium, if you want to spend more money for something that is not as strong as the lowest grade of non-plastic steel) is one common "premixed" filled epoxy system. There are others. Rolling your own is not too hard either - mix metal or plastic or ceramic (or molybdenum disulfide) powders or small balls into thin (low-viscosity) epoxy. From the title of your post I was going to suggest tungsten or silicon carbide in epoxy, but I think that would probably ruin your splined shaft (and be difficult to machine) upon reading the inside of your post.

Epoxies tend to work a lot better the more fussy you can be - ie, mix them right, which involves clean measuring utensils, careful measuring, and careful and complete mixing. If you have access to a vacuum chamber, degas after mixing - if not, use a slow, thin epoxy to get as much air out naturally as possible.

...and cure them right, which involves long times or high temperatures or a bit of both, in most cases. Rushing the cure is not a good plan. In many cases a "slow cure" epoxy is stronger than a "fast cure" epoxy.

Go to

and read their how-to-do-it's on making and pouring molds for lots of good advice. I've used a few of their polyurethanes like the 780 rubber (shore 80A so it's like a firm pencil eraser) and the 70D plastic (shore 70 D so it's very hard, sounds like what you need), both worked well. They also have silicone and other products. Anyway several other companies sell similar products, I'm just a happy customer of smooth-on.

I may have to make an internally-splined sleeve to fit a shaft. Is there a plastic that I can pour around the shaft so it has the splines where it needs them; I can then turn the outside to fit. Original is supposed to be nylon, might not be obtainable anymore (I'm looking though). This is not a high-load application.

Maybe heat the shaft and press it through a hole while hot?