Smaller dimensions at lower z height

[jbobhutto]

I have a Rostock max v2 machine. When I print an object the actual dimensions of the printed part will be different depending on if it was part of the print measured is near the print bed or further up in z during the print. My prints dimensions will be nearly perfect after about 10 mm up in the z axis. For example a square block that is 25x25x25 mm will be 24 x 24 in the x and y axis below around 10 mm and above will be 25x25 on the same axes. I have uploaded a photo that shows an example of the dimensional discrepancy at the bottom of the print.

[Mac The Knife]

It looks like the part is lifting up at the bed, Once you get that cured, your "shrinkage" should disappear.

So the question for you is, What material are you printing with, what temperature is the bed at, what temperature are you extruding at, and what are you using on the bed for adhesion.

[jbobhutto]

This particular print was HIPS and did lift off the bed. However, it doesn't matter what material i use i still have the same issues with the x & y dimensions being consistently off. My first thought was the lifting also but even with PLA i have this issue. So i am not so sure it all due to the lift.

I printed the HIPS at a bed temp of about 100 and nozzle at 225

PLA which has the same dimentional issue without the lift i print at 60 bed 210 nozzle

Do you think these setting on PLA i would have the same issue?

[Polygonhell]

You an get an effect that looks like your image if you run your bed too hot for the plastic.
It looks kind of like a small hour glass section of the print. I've never really understood the mechanics of why.

[jbobhutto]

Thanks for the help

I reprinted the objects at a much lower temperature and they came out perfect with no lifting or "shrinkage" issues.

However, I have also noticed that dimensions on the internal features of my prints are much smaller than they are supposed to be.

for instance the print I posted above came out exactly 1.5" x 0.5" x 0.75" the semi-curricular cut out also came out at .5" dia. but there are two 0.25" holes in them that are about .040" too small. It seems like this may be a perimeter issue because it only seems to happen on features with Internal perimeters (if that make sense). I haven't searched the threads yet for this issue but if you have any clue on this also that would be great. Otherwise i probably need to open a different Thread for this issue.

[Polygonhell]

Inside perimeters will tend to be undersized, the more circular the more undersized.
There are several reasons why, IMO the primary one is that the plastic is stretched as it leaves the nozzle and as the nozzle follows the circular path it tends to slightly cut the corner, resulting in undersized holes. Do a search for polyhole and nophead, he had a fairly extensive discussion of the issue on his blog, and his proposed solution.
Personally I usually just account for it in the model.