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yellowshark

Some questions on perimeters, 100% infill& extrusion width

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Hi all, so having got my new printer setup I am ready to do some serious printing.

I have some confusion over perimeters and extrusion widths and how to produce solid parts. Hoping you guys and gals can help.

  1. Is the density of material laid down by setting the infill to 100% the same as the density of the perimeters?

  1. If not, how do I get a solid object, uniform throughout?

  1. Is the extrusion width the width of material laid down at 90 degrees to the extrusion path?

  1. I assume generally it is best to have the extrusion width equal to, or a multiple of, the nozzle diameter?

  1. I have read that giving the outer perimeters a thinner extruder width can give a better surface finish?

  1. If so, is the an optimum setting for this, e.g. 50% of nozzle diameter, or is it something else?

I am printing parts, not figurines etc. so I need, for the most part to print solid objects. In that context “perimeters” does not mean anything to me. When I see the question how many perimeters do you want, that leaves me somewhat nonplussed, as I guess the answer is none! I just want it to print what I designed in my 3D model. If the depth of my spur gear is 1mm I want it to print it 1mm. If my design does not have a hollow middle then I expect the printer to make it solid.

  1. Should I put 1, or should I ensure my dimensions are multiples of the extrusion width and put, say, 50. (not always possible - spur gear being an example)

  1. How wide is a perimeter? Is it the extrusion width?

Of course if 100% infill gives the same density then I suppose this is not actually problematical. I do note that that you can set different speeds for perimeter and infills and can see that that would be useful.

Cheers

Pete

 

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Pete...

Generally speaking the printer extrudes a constant-width bead of plastic, at right-angles to its direction of travel, and where that width is usually the nozzle width (i.e., 0.4mm for a standard Ultimaker). Some slicers (e.g., Slic3r) allow you to set different extrusion width for different parts of the print, but that can be problematic, since it results in different extrusion rates, and hence head pressure. In general you will get best results printing at a constant extrusion rate (mm³/second). In practical terms, that means always printing at the same linear speed, and same extrusion width. In slicers where you have to set it, I recommend setting the extrusion width to match the nozzle width unless you have a really good reason not to, and you understand the potential pitfalls.

Perimeters are the concentric loops around the outside of each layer that provide the finished 'sides' of the print. They line up with the outside edge of your model, and grow towards the inside of the model, as you make them thicker. These sides can consist of one or more passes of the head. In Cura, the 'Shell Thickness' sets the depth in from the outside of your printed part that is printed in these concentric perimeter passes. Generally, you want the shell thickness to be an exact multiple of your nozzle width. Once the perimeters have been printed, anything else that is 'inside' your model is filled with infill. If you set 100% infill, then that space will be filled with solid plastic. If you set it to zero, the space will be left empty. Between those settings, the space will be filled with some sort of regular pattern. In Cura, it is rectangular pattern; other slicers let you specify other patterns - concentric rings, hexagonal grid, etc.

If you absolutely need it for mechanical strength, you can specify 100% infill. But often that's not really necessary - you can get strong parts with much less than 100% infill, depending on your object and application. The outer surface will still be solid.

The 'shell thickness' sets the width of the outside edges of each layer. The top and bottom thickness is set via the Bottom/Top thickness setting - that needs to be an exact multiple of the layer height, since it is made up of solid layers. Generally 5-10 times the layer height (i.e., 5-10 layers of solid top/bottom surface) gives a good result, depending on the shape, layer height etc.

What slicer software are you using? Some of the questions your asking seem to relate to the more esoteric settings of slicers like Slic3r or Kisslicer. I highly recommend getting started with Cura, which simplifies a lot of the settings, while generally giving good results.

 

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Thank you Simon, really really helpful, it is slotting into place now. And unsurprisingly astute too J. This past week I have been easing myself in with Cura but spent some hours today looking at the Slic3r software and reading the manual.

I am about to spend 3-4 weeks testing and optimising the settings for the various materials I may use. So I have started to look at some of the advanced settings and also begun trying to get a feel for how Cura stacks up against some of the other slicers.

I have another question resulting from one of your recommendations...

“In practical terms, that means always printing at the same linear speed, and same extrusion width. “

One of the components I want to make is spur gears. The dimensions are fixed apart from the no. of teeth (and resultant diameters) and I have also made the inner weight saving holes smart and linked them to the inner diameter. It is important that the teeth are well defined so I was thinking of printing them at a slower speed than the body of the gear. So if I set the shell thickness to 0.8mm and reduce the print speed but increased the infill speed that I think would do it for me.

Do you think for this application my approach in changing the speed would be valid?

Cheers

Pete

 

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One of the issues with printing different parts of the print at different speeds (whether that is in terms of linear head speed, extrusion width, layer height, or some combination of two or three of those) is that it will require a different volumetric extrusions rate (in mm³/s). As you print faster and/or fatter, the volume of plastic extruded through the nozzle per second increases. In order to do that, the pressure in the head is increased (assuming the nozzle opening stays a constant size).

There are limits to the extrusion rate that you need to be mindful of. It varies with material and temperature, but somewhere around 8-10mm³/s seems to be a good guideline for both the UM1 and UM2 when printing PLA in the 220-230º range. If you exceed that level, you will get under extrusion, and may get head blockages if the high pressure causes molten plastic to squirt back into cooler parts of the head, and then solidify there. So that's the first thing to be mindful of - whatever speed changes you make, you always need to stay within that range.

The bigger challenge, especially for detailed parts like small gears, is what happens when the speed changes. When printing fast infill, the pressure in the head rises, to squeeze out the higher amount of plastic per second. If you then change to printing the perimeters at a slower speed, the pressure cannot adjust instantly. Instead, you get over-extrusion at the start of the perimeter, as the excess pressure bleeds off, and adjusts to the new lower-speed, lower-pressure steady state. Extra plastic oozes out as the perimeter prints, until the internal pressure is equalized. (Then when the head goes back to printing fast infill, you get under-extrusion for a bit, until the pressure builds up again - but that's probably less of a concern on the inside of the print).

Depending on your speeds, and geometry, it might not be a huge issue. But if the surface finish of the gear teeth is important, and I expect it would be, then you might want to just go with the same speed for perimeters and infill. (You might even argue that, since the infill will consist of longer, straighter lines, while the perimeters will be detailed shapes with lots of small segments, you might even want to print the infill a bit slower than the perimeters, since the head will probably move slower than its nominal speed on the perimeter, as there will be lots of deceleration for sharp corners. But that's probably overkill).

As with all prints, its all about finding the balance that works for you. There's no such thing as the 'ideal' setting - every print is different, and there are numerous tradeoffs between finish quality and speed. The challenge is not to find the 'best' setting, but to find the one that gives you a print that is 'good enough' for your needs, while giving you that result as quickly as possible. Go too fast, and the quality isn't good enough for your purpose; deliver higher quality than you absolutely need, and you're just reducing productivity by taking longer over each print than you need to.

So, I'd start with mid-range speeds (50-70mm/s), medium-thick layers (0.15mm?), and a middling temperature (220 for PLA) and see what you end up with. Do a few test prints, keep notes, and see what happens. Generally speaking, slower, cooler, thinner, will give higher quality prints... but it really depends on the specifics of each print. Do a few tests, and home in on a setting that works for you. While the finer points of Slic3r et al will give you more control, the actual improvement in quality for most of those settings is going to be small - certainly compared to the fundamentals of speed, height and temp. So focus on those first, and don't worry too much about the refinements.

(If you're going to be using Slic3r, be sure and see my http://www.extrudable.me/2013/11/03/slic3r-strange-defaults-causing-qu-bd-woes/. It does some odd things, left to itself. (That was written in the context of QU-BD printers, but the same principles apply for the Ultimaker.))

 

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If the gears don't have to *look* good but they need to function perfectly you can probably print very fast - say 150mm/sec and hot, say 240C. If you want them to look perfect you should print very slow, say 20mm/sec and cooler, maybe 200C.

If you care about function over look, then you can usually print quite fast.

 

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