General Guidelines for Part Designs

I want to start getting some printed parts hosted but need to know a couple of things first, and there all to do with what can be expected from the average RepRap.

1. Is there any ability to create support material and if so what are the limitations?

2. If there is no option of support material is the max overhang angle 45 deg?

3. What is the dimensional accuracy that people tend to be able to print at?

4. Are there any other limits that should be adhered to? eg. build area being 200 x 200 x 100 (mm)

The forum is so fast moving I don't really know what the current machine standard is! I know sometime ago I saw about support material being added as an option in Skeinforge but haven't seen anyone seem to use it. I also saw that people could span large distances without the need for support material by using a fan to cool the plastic as it exits the extruder, but again there still seems this problem of minimum unsupported angle, which is very limiting considering that being able to print in any orientation should be possible with a 3d printer, else you don't really have a 3d printer, you just have 2.5D (or something like that).

If there are any threads on support material or bridging large gaps without horrendous sagging please let me know where they are!

1+2. Skeinforge and SFACT can add support material, so overhangs are possible. The angle of unsupported overhang variest a bit with layer height, but 45° isn't a bad representative number. Depending on printer settings, support can be difficult to remove and will leave scars on the model face. I try to avoid using support because the cleanup is a pain and it is a waste of plastic when there are design alternatives.

3. Accuracy and precision vary widely depending on calibration and build quality. If you're going to pay someone to build parts, get guaranteed values. If you're going to run your own machine, you can achieve 0.3mm accuracy or better. You may not find it easy.

4. That's a good representative build volume for a Prusa Mendel. A single part that large can have severe warping problems due to uneven shrinking, unless the printer has a heated chamber. A more common use for the large volume is printing multiple small parts simultaneously. Material selection is important. Some materials are stronger, others warp less, etc.

What would you say are warp-less maxes? PLA doesn't contract much, but I do start to see a little bowing at around 100x100 with a cold bed.

I've never printed ABS so i can't speak to it's properties.

It's impossible to say where the limits are for warping. It depends a lot on the part geometry too. Some parts won't show it, others will be obviously distorted. Parts that are fairly flat on the heated print bed won't have much trouble. Parts with enough thickness to get away from the heat of the bed will show problems. Basically, if the part is large enough that significant areas are away from the bed or nozzle heat, there will be warping.

That's ABS on a heated bed, which I should have mentioned in my previous post. I've never printed PLA, so I'll have to defer to you experience with that.

yellow_fish Wrote:
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> 1. Is there any ability to create support material
> and if so what are the limitations?
>
> 2. If there is no option of support material is
> the max overhang angle 45 deg?

If you look at the designs people are publishing at Thingiverse nowadays, virtually no one is making designs that require support material. The Skeinforge's ability to print real world designs with support material is pretty theoretical and very few people have multi-material printers. So the "standard" is pretty much to just constrain the design to max overhang of about 45 degrees, plus horizontal bridges, which can be several centimeters long (see Prusa X axis motor mount).

I have also made a design where an empty vertical cylinder becomes smaller without a 45 degree cone by making a really fancy bridging manoeuver where the layers are first "windowed" in one dimension, then in another, and then the smaller diameter starts to print. That worked perfectly even while I was thinking "that's never going to work"

> 3. What is the dimensional accuracy that people
> tend to be able to print at?

If I design things which must fit one inside another, I'll leave a 0.4mm gap between them (in each gap), and that works pretty well. I believe I have an average or above average printer in the accuracy department (Orca v0.2).

> 4. Are there any other limits that should be
> adhered to? eg. build area being 200 x 200 x 100
> (mm)

200x200x120 would be something that all the midsize printers (Mendel-class) are able to print.

Please,
Could you post a link to an image of that cylinder trick ?


Thomas
(orca 4, ordered, waiting :-)