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PLA gears not printing correctly

Posted by mrShrimp 
PLA gears not printing correctly
July 30, 2013 05:41PM

I have been working on a 3D printing related project with a club at my school that requires us to print gears to drive experimental paste extruding mechanisms. Our printer is a RepRapPro Mono Mendel, and it has been working surprisingly well for the moderate amount of calibration we did. We have the bare glass bed leveled, and have been getting great adhesion without having cleaned it with acetone for probably 10 prints now. We are not using a fan to cool the print, and there is no visible curling of PLA on the print bed, although that has been a problem in the past.

There does, however, appear to be some minor curling of the edges of prints that is causing the gears that we print not to mesh at straight angles. I attached some pictures of the particular gears that we noticed this problem with. My best guess is that the minor curling at the very bottom of the gear teeth is translating throughout the entire height of the teeth, causing the tooth profile of the gear to be angled inward. I am stumped though, as to how to fix the problem. If this is the effect, what is the cause?

The print settings we used for the gears in the attached photos are:

- Hot End: 200 degrees C for the first layer, 195 degrees C for all other layers
- Heated Bed: 65 degrees C for the first layer, 50 degrees C for all other layers (this should be below the Tg of PLA, but it takes a while to drop 15 degrees, so maybe this is a problem)
- 0.3mm layer height
- 0.5mm nozzle diameter
- 1.75mm diameter filament
- 4.5mm/s print speed for the first layer, 30mm/s for everything else, 60mm/s for infill

Any help would be greatly appreciated!
open | download - Optimized-20130730_125302.jpg (286.1 KB)
open | download - Optimized-20130729_155715.jpg (355.7 KB)
open | download - Optimized-20130729_155546.jpg (371.7 KB)
Re: PLA gears not printing correctly
July 30, 2013 11:17PM
what surface are you printing from if it is glass surface then leave the bed at 50deg c constantly. the glass expands at different temperatures, and can shrink even with the 15deg c change. glass expands when heated, and shrinks when cooler. and i think you mentioned that the temp is cooling to 50deg from 65deg which is above the Glass transition temp.

on another note 200 deg c is really high temp for pla without cooling. do you have active cooling on print?

Here is a suggestion to try and see if it works. 50deg all the time for bed, and active fan on all layers. reduce flow rate by 2-5%

this should prevent deformation of bottom 3-5 layers.

others may have more advice on this. ymmv.
Re: PLA gears not printing correctly
July 31, 2013 02:57AM
What does a thin wall or hollow straight-sided vase look like?
Re: PLA gears not printing correctly
July 31, 2013 05:12PM
use some watered down PVA glue on the glass to hold the print down.

I run bed at 60 C for the whole time

Also i run PLA between 185 - 230 C (230C for all Metal Hotends)

Also i dont like 1st layers being different other then the speed which i run at 10mm/sec
If your using a normal hot end i would tyr following settings.

PVA coated Glass bed at 60C (put the PVA on while at 60C)
Hot end at 190C
60mm/sec in all settings (other then bridges normally around 10 - 20 but would need to be tested)
dont have slicer do 200% on the 1st layer i see no point in this with PLA

also a Fan with PLA is a must with a PVA bed you can get away with a large fan as the print will very rarly lift.
I go with a 120mm fan blowing across the bed (but of tape across bottom so its not cooling directly on the bed) failing that just get a desk fan and point it at the print. (i have it blowing all the time again no 1st layer nonsense)

good luck
Re: PLA gears not printing correctly
August 02, 2013 08:07PM
Sorry for the late reply. I have not been able to fix this yet, but I should have more access to the printer now to test your suggestions. It seems a consensus among your replies is to use a fan, so that is what we will do. We have some 40x40x10mm fans on hand, but it wouldn't be hard to find a 120mm fan lying around. I found some fan mount designs that have already been made for RepRapPro Mendel on Thingiverse, but I have some questions about them.

Here they are:
1: http://www.thingiverse.com/thing:87112
2: http://www.thingiverse.com/thing:75078
3: http://www.thingiverse.com/thing:96710

The first and second designs both use 40mm fans, which is convenient, but the first one looks like it doesn't focus the airflow that well (Would the bed receive too much cooling?), and the comments on the second design say that the airflow is not strong enough to provide optimal results. The third design uses a 120mm fan, which is fixed to the frame instead of the X axis. Would this cool the bed down too much, or do fixed fans usually work as well as fans mounted on the X axis? Which of these designs would be the best to try out?

Thank you!
Re: PLA gears not printing correctly
August 03, 2013 04:42AM
The first design is mine. You can control the speed of the fan in Slic3r to get exactly the amount of airflow that you want. I find that it produces a nice breeze across the print area and run it at 100% all the time. You can see the tree frogs printed in the background which have a lot of overhangs. As it only costs a couple of quid to print it, why not give it a try?

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