Strong strings in printing covid shields  

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tadeasekf
(@tadeasekf)
Active Member

Hello,

I have problem with my Original Prusa i3 MK3S - I am printing this Prusa covid shields https://www.prusaprinters.org/prints/27267/files (4pcs) and in printing it makes somewhere strong strings. Sometimes it makes strings only on one shield and sometimes on 3 shields. 

Can somebody know, how can i fix it? Thanks for help         

 
Posted : 08/04/2020 12:29 pm
bobstro
(@bobstro)
Illustrious Member

What type of filament are you printing with? I had identical problems with AmazonBasics PETG. That problem is caused when there is insufficient adhesion between extruded filament layers. As the nozzle moves across, it creates tension which is sufficient to pull away a few strands, creating that "strong string" effect. Good news is that you can easily clean it with a knife after printing, and for some reason, it only seems to affect that outer band which is away from the wearer's skin.

I found that increasing the print temperature helps. If you're printing PLA with the part cooling fan enabled, turn it down or off. Finish isn't as critical on these parts and hotter printing temps will also improve layer bonding and reduce holes. If printing PETG with no cooling fan, increasing temps 5-10C helps.

In my case, I was printing the NIH version of the Prusa frame with a front shield. With hotter temps or less cooling, I had bad sagging on the underside of the shield. I did the following:

  1. Increase PETG print temp to 60C. This works well with a 0.6mm nozzle at 60mm/s for external perimeters, 100mm/s for all other features.
  2. Changed Filament Settings->Cooling->Cooling thresholds->Enable fan if print time is below to 45 seconds. That way, the fan stays off when printing the lower part of the frame (the entire Prusa RC3 part) but turns on gradually as it hits the thin shield part at the top.

I'm getting consistently good results. This print takes 2h8m with PETG and a 0.6mm nozzle. You can see a tiny bit of that stringing in the top visor. That bit cleans up easily with a heat gun.

I've got notes on my COVID printing workflow here that might be of interest. 

If you have problems, feel free to DM me. I have done Google Hangouts chats with several people helping them with issues.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 08/04/2020 3:03 pm
tadeasekf
(@tadeasekf)
Active Member

@bobstro

I am printing with original gcode from Prusa, it is PETG and temperature from Prusa is 265 for my 0,4mm nozzle and 90 °C for bed.

I tested this gcode several times on my printer and on other same Prusa i3 MK3S printer (filament from same manufacturer, same gcode), it makes this strong strings only on my printer.

It seems like that the printer doesn't follow shape of shield. As you wrote, it affect that outer band which is away from the wearer's skin and it doesn't look good.

Thank you

Posted : 08/04/2020 6:46 pm
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