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Slowing a print

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Hi folks.

I'm experimenting with some very small parts, the one I'm currently having a slight issue with is a small sphere (around 10mm diameter) on a cylinder which is around 6mm diameter. A ball on a stick if you like.

Up to the ball things print ok, then as the sphere is being built, layers are being put down before the material (in this case PLA) has cooled enough to be stable, so I get a growing wobbly blob. I have to say, at the end of the print it's actually not far from spherical, but not quite good enough to be useful.

The final part will be nylon but I'm expecting the same problem. Yes I can slow down the printing, which I'm more than happy to do, but what suggestions would you guys and girls have for such a situation?

Many thanks.

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My computer is rendering out an animation right now and cannot start Cura, but there is a plugin called (Something like this) Change at Z.

Another option is to make the cylinder separate from the sphere. Place them in proper orientation and make sure the picot points are the same location.

Bring both objects into Cura and then use per object settings to set speed independently for each object and then go to Edit: Merge Models.

Then each part will have its own speed.

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One common solution is to print more than one of the objects at the same time.

There’s also a setting under Cooling that is named Minimum Layer Time (or something like that), which shokld allow you to automatically decrease the print speed if layers are too small

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Slowing down will probably not be enough: if the nozzle stays too long in the neighbourhood of the already printed areas, the radiated heat will still prevent the part from cooling down. Since I often have small models, I tried all sorts of different solutions.

For such small objects, now I always design and print a dummy model next to it. The dummy needs to have a sort of "inverse" or negative shape of the real model, so the total printing time per layer is equal: this is important for good quality. Sudden differences in layer printing time (and thus in cooling time) do show up as horizontal lines in the print.

One way you could do this, is make a cube that is slightly bigger than your model, move a copy of the real model into the cube, and subtract it. So the dummy cube now has a hollow of the same size as the real model. Then create a bottom layer of 0.5 mm under the hollow part in the cube, to get good adhesion to the build plate. And make sure that the dummy is at least one layer higher than the real model (typically 0.1mm or 0.2mm).

When printing this dummy, sausages of filament will sag down into the hollow part, but this does not cause problems in my experience, as long as there is a solid bottom layer under the void.

Printing this at the lower edge of the temp and speed range of the material, also helps. For PLA you could get down to around 190°C and 20mm/s.

Putting a desktop fan in front of the printer also helps for PLA, but is likely to cause warping for other materials.

Yes, a dummy wastes some material. But less than you would waste when you don't get any usable models. You could also recycle the dummy cubes and use them as "houses or hotels" in Monopoly, or as pawns in other games...

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