Prusa i3 Hephestos: PLA doesn't stick to bed.

First time printer, new poster. I have also posted this in the Prusa and Mendel section. Not sure which is the right one.

I built the Prusa i3 Hephestos from a kit (http://www.bqreaders.com/gb/products/prusa-hephestos.html). This doesn't come with a heated bed, as I believe it is not needed for PLA.

When I try to print, after 30~ seconds the printer pushes the print, and so it starts sliding off of the glass bed. I believe this happens when the bottom layer of PLA cools.
I have levelled the bed so that a sheet of paper can slide under the five points.

Can someone explain in relatively simple terms why this happens and how to fix it?

Thanks.

The bottom layers of the PLA will cool as the print progresses, contract slightly and free itself from the bed.

Many people have success printing on blue painter's tape. The surface seems to be just rough enough to give the PLA something to grip as it cools.

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If you have issues with blue painters tape not working then a fine mist of hairspray on the tape does wonders, I have issues getting even ABS to come off after it cools. Be careful with the hairspray and hot electronics as well as find a way to reduce the overs pray from the hairspray as it makes dust stick to everything.

You don't mention any adhesive on your glass, are you using any?
You can use painters tape, but just plain purple Elmer glue stick works great, and you dont need to constantly replace damaged tape.

I've tried diluted PVA which didn't seem to work. I have also applied masking tape to the surface, which I've seen suggested and that gave worse results than the normal glass. Is an Elmer glue stick different from any bog-standard one?

Don't know why, but even the white stick one from Elmer don't work the same as the purple? ?? I don't dilute it, and to put it this way: I have lost 3 glasses to this glue. I don't use ABS juice, nor kapton or painters tape, just plain glass, this glue and the most important part: a perfectly leveled bed.

This last part that I make for a customer broke one of my glasses, and as you can see it doesn't have too much surface to grab on.

I can confirm that purple glue straight on glass works wonders. But I only use ABS so im unsure on PLA

Hi I use pva solution with no problem like half water half pva apply on cold glass heat glass up 80 or above till all water has evaporated , start printing

Have you got a fan, and is it activating on the first layer. I use hairspray with no probs, but found that i had to activate my fan in slicr after the first couple of layers were layed as it was cooling to quickly and warping. Also i noticed a big difference in room tempature. During the day when it was nice and warm...no worries. Just recently it has cooled down at night...same settings and kept warping off. So I heated up my room to about 33 degrees (from 24) and was fine again.

I have started using the purple glue and man does it hold. I also found that printing with a brim helps as well. In regards to having trouble removing the print with the Elmers purple glue, try putting the glass plate with the print in the freezer for about 10 of 15 minutes after it has colled a bit and it will start to release.

Not all masking tapes work - I found that genuine branded "Blue Tape" works very well, but generic masking tape generally doesn't work very well, whatever colour it is.

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Follow my Mendel Prusa build here: [julianh72.blogspot.com]