Do some filaments shrink?

I make up test parts for any critical hole sizes that consist of a small part with all the holes & other shapes on a similar surface (horizontal wall, tube or vertical wall etc.) I then adjust the sizes empirically after printing with the same material and slicing with the same parameters as I intend to print the part, and transfer the adjusted sizes to the required part.

Dave
(#106)

I have tested a bit with different perimeter widths and layer heights, to see what form the plastic takes, and what effect that has on the real perimeter size. In order to do that, I printed a simple test piece consisting of just a thin wall. Thin walls are printed with a single perimeter width by Slic3r, the same perimeter width as is used for normal perimeters. Slic3r lays down the perimeter line half a configured perimeter width from the specified side. Therefore, if there is a difference between the configured perimeter width and the real perimeter width, then you will not get correct sizes of your objects.

Here are the results of the test (all have nozzle diameter set to 0.5, and with 20mm/s speed. Printed with the PLA that comes with the machine):

Layer     Configured    Real         Notes
height     Diameter   Diameter
0.25         0.4       0.55          Will be used when you specify 160% as perimeter width (it's 160% of layer height)
0.25         0.5       0.60          The Slic3r default (equal to nozzle diameter), taken when you specify 0
0.25         0.75      0.75
0.25         1.0       1.00
0.5          0.5       0.65
0.5          1.0       1.00

The 0.25 / 0.4 sample also had many small holes in them, indicating that the plastic extrusion cannot keep up with the speed.
The 0.5 / 0.5 is very fragile; the layers hardly stick together. The are some small gaps between the layers here and there as well.

This strengthens my advice to set the perimeter width to at least the nozzle diameter + the layer height.

I also printed a 40x40mm test piece with a 20mm diameter round hole in it, with the 0.25/0.75 setting. The outer diameter of that piece came out as 39.95x39.95mm, and the inner diameter 19.75mm.
So it does seem that the inner diameter is more affected than the outer. But I suspect it may have to do something with the fact that the hole was round, I don't think Slic3r compensates for the extra material that is extruded because of the bends in the line segments that make up the hole. I will try a rectangular piece next.

Here are some photos of the different test pieces, so you can see how the cross sections look like.

One of the things you are supposed to do as part of the more advanced calibration process is to make a single shell print exactly as you did, measure the wall thickness and adjust the extrusion multiplier so that the wall thickness is the same as the setting.

So if you were printing with the default of 0.5mm and your test result was as you have shown (0.6mm, exactly the same as my test result), you should set the extrusion multiplier in Slic3r to .833 Re-slicing will then result in a print with the correct wall thickness.

Dave
(#106)

Quote
dmould
One of the things you are supposed to do as part of the more advanced calibration process is to make a single shell print exactly as you did, measure the wall thickness and adjust the extrusion multiplier so that the wall thickness is the same as the setting.

So if you were printing with the default of 0.5mm and your test result was as you have shown (0.6mm, exactly the same as my test result), you should set the extrusion multiplier in Slic3r to .833 Re-slicing will then result in a print with the correct wall thickness.

Dave
(#106)

These results show that you should set a correct perimeter width before you tamper with the extrusion multiplier. Reducing the extrusion multiplier before having all other settings correct will simply not extrude enough which will have consequences many other areas. This is especially true because the default perimeter width of Slic3r seems to be incorrect.

Quote
Flyskyhy

Quote
dmould
One of the things you are supposed to do as part of the more advanced calibration process is to make a single shell print exactly as you did, measure the wall thickness and adjust the extrusion multiplier so that the wall thickness is the same as the setting.

So if you were printing with the default of 0.5mm and your test result was as you have shown (0.6mm, exactly the same as my test result), you should set the extrusion multiplier in Slic3r to .833 Re-slicing will then result in a print with the correct wall thickness.

Dave
(#106)

These results show that you should set a correct perimeter width before you tamper with the extrusion multiplier. Reducing the extrusion multiplier before having all other settings correct will simply not extrude enough which will have consequences many other areas. This is especially true because the default perimeter width of Slic3r seems to be incorrect.

Why do you believe that a perimeter width of 0.5mm is incorrect?

Dave
(#106)

Quote
dmould

Why do you believe that a perimeter width of 0.5mm is incorrect?

Dave
(#106)

Have a look at the test results posted above. When setting a perimeter width of 0.5mm, you will actually get a perimeter width of about 0.6 or 0.65 mm. That is because the width of the nozzle is 0.5mm, and the plastic oozes out to the side a bit after extrusion.

I have now tried with some square test pieces with square holes. I ran that with speed = 20 mm/s, nozzle_width = 0.5, layer height = 0.25, and perimeter_width = 0.75.

Their measurements where within 0.05mm accuracy both on the inside and outside, and they fit inside each other quite snugly. I am happy :-)
Also, the X and Y measurements were the same, which I guess mean my belt tightening is fine.

@Flyskyhy: short question Did you do a test with a closed topsurface and if so, how did it look? I ask since my test showed gaps in the surface. Unable to continue my own testing, therefor I ask.

thanks

hi,

one question.
why dont use skeinforge?
skeinforge has the strech plugin.

Quote
Marc van Beelen
@Flyskyhy: short question Did you do a test with a closed topsurface and if so, how did it look? I ask since my test showed gaps in the surface. Unable to continue my own testing, therefor I ask.

thanks

Closed top surfaces look fine, no gaps, for me. It might be that you are not extruding enough? Make sure to precisely measure the width of the filament (it is usually not exactly 1.75 mm), and set that in Slic3r.

Quote
Le-Seaw
hi,

one question.
why dont use skeinforge?
skeinforge has the strech plugin.

Well... I just happen to start with Slic3r, as it seems to be the most used slicer around.
The stretch plugin of skeinforge I assume multiplies all measurements by a certain amount? That would probably help with shrinkage of the plastic after extrusion, but I have not seen much shrinkage with PLA. If incorrect measurements are due to incorrect other settings, then I would rather fix those, then to try and fight the symptoms.