If they can fit your part into their MJF printer, then it's perfect. The process is high-detail and generally accepts paint/primer. I think it may be slightly machinable, too.
So then I did a bit more shopping around, and the prices keep getting lower! I found a firm called Midlands3d which can print my model in PETG for only £19 (not including tax, but even so...). This sounds too good to be true doesn't it?
… The website says that PETG doesn't shrink, has a glossy finish and is strong so I'm wondering whether ABS would have any advantages in this application.
PET-G tends to be
very glossy — like
this glossy. It's also rather hard to paint, since it's waterproof and resists virtually everything. Sanding/machining? Good luck with that … removing support materials from PET-G is hard enough.
But the price is doable: I'd say they'd be making a small but adequate profit on £19 for your job in PET-G.
ABS, though pretty good for injection moulding, is too much work to correct for shrinkage in large 3D printed models. I'd forget it as an option.
Oh and one more thing - there is an option to select different amounts of "infill", 20%, 40% etc up to 100%. This adds slightly to the cost - but what is it, and do I need it?
Okay, I don't think this applies to MJF, but for most filament printers, objects aren't 100% solid. Please see
this — but excuse the tentacles¹ — for examples of 0%, 20% and 50% infill. Prints are formed by a (typically) 0.4 mm nozzle creating layers from 0.1 – 0.3 mm high. If you think of 3D prints being made up of blocky wider-than-they're-tall MODE 2 pixels, you're on the right track.
You don't need 100% fill for strength. Because your model has thin walls, it will end up being a slightly higher fill %age because:
- vertical walls will typically have two 0.4 mm "shells", so your 2.6 mm (?) case will have and 0.8 mm inner wall, a 1 mm mostly hollow bit, and an 0.8 mm outer wall. You can see the walls on the last link. For strength, you might want to specify 3 shells, which would give you 1.2 mm walls and a 0.2 mm hollowish bit in the middle. Changing the infill %age will only affect how much fill goes into the "cavity wall" bit
- Top and bottom layers are usually solid for the first/last millimetre or so to ensure a smooth and solid surface. That means that most of your model will be solid anyway, with maybe an 0.6 bit in the middle — two or three layers — that aren't solid.
If strength is your main objective, I'd specify 3 shells and maybe 30% infill, just because of the shape of your model. Thicker layers also make for stronger prints, as each layer is a possible place for the model to delaminate and crack. Thick layers are also much faster to print. If you don't mind visible ridges, I'd recommend going with 0.3 mm layers. The printing place will thank you, as it's much quicker, too.
---
¹: the samples were supposed to have 0, 20 & 50 in raised letters on the front, but guess who accidentally made the raised letters float 0.1 mm off the front of the cube? The tentacles are the remains of the letters valiantly trying to stick to the cubes anywhere they can … 3D printers are only as smart as their users.
) and the result is this:
, please. We have very distinct spelling: we have colour like 
, tire (as in 
, not 🥱) like 
, but yogourt like no-one else. I guess I once knew how to spell draft your way, as when I was living in 
(up to 2002) I was the typesetter for the Collins Gem English Dictionary.
) I discovered that you can saw it with a junior hacksaw. Takes time though. In terms of strength, I took the roughly 2mm x 2mm x 10mm bit that I'd sawed off and tried to break it with my fingers. I couldn't do it.
