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marcv

Raft settings

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I'm trying ABS for the first time but i've got trouble with warping. Now I want to add a raft but when I use the default settings for it, it will only give me a raft of 2 layers..

Is it possible to add layers to the raft? I thought I had seen it in previous versions of Cura but I can't find it in the version of June 27 2013.

thnx !!

 

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I haven't printed with ABS but there are lots of ways to get it to stick better. Google "how to make ABS stick to print bed" or something similar and see what you get.

Certainly I recommend "brim" more than "raft". When plastic lifts up off your print bed it starts at the corners due to pulling/cooling of upper layers. If your print has corners the problem is much worse. If the print has rounded corners it isn't as bad but if you have the brim feature turned on it helps even more and keeps the corner from lifting in the first place. Once the corner starts to lift it's easier for more of your print to start lifting.

Read about "glue sticks" and "abs glue" and other techniques.

 

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Is there an answer to the original question asked... does anyone know how to adjust the thickness of the raft? I want to print about 5mm of raft to completely remove the warping and shrinking that happens on corners, whether or not it stays stuck to the build plate.

Once that's answered (is it simply just not possible?). Are there then other ways to reduce the warping and shrinking?

We have the UM2 with heated bed, with larger rectangular parts, even if they stick all the way round to the build plate, we're finding it still creates a necked in section for the first 5mm or so - especially round the corners.

 

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I looked around in the settings and I can't see a way to make it thicker. However, based on what you describe I don't think a thicker raft is what you want/need. You mention that the print isn't lifting (this is what is referred to as warping) but it has a necked in section that is 5mm. By default the fans are set to go full blast at 5mm, I think this is the reason. So I would go into the expert settings and change that value to something lower (I like 1mm).

Also, what is the bed temperature you guys are using? The default is something like 70-75C IIRC which IMHO is way too high and can cause issues like you describe, try reducing this to 60C and see how that goes.

 

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Ok thanks. Yes sometimes the warping does cause the corners to lift as well - we haven't tried anything other than the stock gluestick on the glass. Plus we've been running it at the standard 70-75deg. I'll try lowering the bed temp and see if that helps first.

This particular model has failed 10 times now, so I was thinking I'd just stick a raft on that was deep enough to get past the point that it starts going nice and straight, which seems to be over about 5mm deep. It's a hack, but it would get it done!

 

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I forgot to add that you should enable brim if you're getting lifting in the corners, it helps quite a bit.

I've been using regular white wood glue mixed up with water and then spread onto the bed. That seems to work well for the prints I've been doing. However, I haven't done any big square flat things so I can't say how it works on those types of prints.

 

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we're finding it still creates a necked in section for the first 5mm or so - especially round the corners.

 

This is very common when the bed is too hot (70-75C). It's not exactly warping but the fact that the PLA is above the glass temp when the next layer above is being laid down and it pulls it inward on the corners as the filament is more like a rubber band has it is cooling but still liquid and coming out of the nozzle. The solution is to keep the solid layer below at or below glass temp (around 50-60C) and setting the bed to 60C is a good compromise.

Raft is an old feature that isn't used much anymore. It helps greatly with lifting corners but "everyone" has slowly learned better ways to deal with it and now it's semi-obsolete.

As far as getting large parts to stick better...

Rounded corners help quite a bit. Square corners have all the lifting force on one tiny .4mm square of PLA at the corner. If you round the corner the force is spread out. But if you use brim you don't have to round the corners.

Brim is important even if you have rounded corners. Use lots - maybe 20 passes if you can and make sure it is squished down nice and flat into the glass. The glass needs to be>40C to stick well so 60C is a good temp for this reason also. This is so the PLA flows nicely leaving as few air cracks as possible. Look at the brim as it is laid down and make sure there aren't gaps that allow air to get in and the corners to lift.

Cleaning the glass first is good in case there are any oils from your skin or whatever on there. Soap and water works fine but a glass cleaner is better because it dries faster. (windex)

Glue stick works great. To improve it you can add some water and spread it around a bit thinner. Alternatively take any old elmers or any brand "wood glue". Mix with water 5-10 parts water for 1 part glue. Shake it in a jar really good and lay it down with a paint brush on the glass. Wait until it evaporates (which is much faster with the glass at 60C). It will be almost invisible. The ratio of water and glue is not critical as you evaporate the water anyway. You can always later add water and spread it around again. This coating lasts for dozens of prints.

FINALLY, if none of these glue techniques work, you can always *raise* the glass temp to 75C or so and keep the fan down to 40% or so (enough to get it to spin). This will give you worse quality prints that exhibit the warpage you speak of *but* it will keep the bottom layers above glass temp and it will not come off the glass as it can deform slightly and absorb the shrinkage forces.

Or use a raft :)

 

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Do all this properly and it's possible to get the part to stick so well you will damage the glass when you remove the print! :)

 

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