shrinking prints

I am having trouble with my prints distorting about 1.5mm above my heated bed. I am using a Prusa Mendel reprap with a .5mm hot end. I print on glass smeared with ABS juice. My bed is running about 108 to 110 degrees C. I print with a hot end temp of 215. On larger prints the print adheres well and prints well but by the time it gets finished, the layers about 1-1.5mm from the bottom seem to shrink and make an hourglass effect on the print. any help eliminating this would be appreciated.

Could you post an image?

Print in PLA

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The bed is too hot if you get a severe hourglass effect. I drop the temperature after the first layer to get good adhesion without this effect.

I always get it to a very minor extent regardless of what plastic I use. The first layer is stuck to the bed so can't shrink. The second layer contracts and pulls inwards as it cools. What I have never understood is why after a few layers it starts to get bigger again.

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[www.hydraraptor.blogspot.com]

The only time I see significant variation in the bottom layers of a print is when the print is small enough that I'm effectively printing each of the first few layers at different temperatures as the nozzle and bed cool from the higher first layer temperature.
On larger prints I generally don't see it. Though I still have some slight Z Wobble on the printer I'm currently using and it could be hiding it.
Since I seem to have no problems with adhesion since I started using ABS Juice on the PET tape that was already on the bed, I keep meaning to experiment with printing without raising the temperature of the first layer at all and keeping the temperatures constant throughout the print.

I got it long before heated beds and I currently get it with PLA and I am using constant temperatures for bed and plastic at the moment. I guess a fan would improve it by hardening the layer below but I can't see how it can be completely eliminated without a heated chamber. It would imply zero thermal shrinkage or infinite stiffness.

Edited 2 time(s). Last edit at 07/27/2012 03:35PM by nophead.

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[www.hydraraptor.blogspot.com]

Quote
nophead
The bed is too hot if you get a severe hourglass effect. I drop the temperature after the first layer to get good adhesion without this effect.

I always get it to a very minor extent regardless of what plastic I use. The first layer is stuck to the bed so can't shrink. The second layer contracts and pulls inwards as it cools. What I have never understood is why after a few layers it starts to get bigger again.

I realize this is a very old thread but I was wondering if you ever figured it out? I get it where it reaches a point of maximum shrinkage then starts to expand out again and I have no idea why.

I have tried all sorts of head and bed temps but I can't conquer it. Seems odd it would do the hourglass effect instead of just continuing to shrink as it goes up.

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I await Skynet and my last vision will be of a RepRap self replicating the robots that is destroying the human race.

The part is stuck to the bed so the bottom stays relatively same size. Heated bed caused it to shrink as it goes up squeezing the middle inward but the further away you get from the heated bed the effect diminishes so it returns to proper size and that is why you get the hour glass look.

80c is now the recommended heated bed temperature. Some people say 100c for first layer then drop to 80c. I run mine at 85c all the time now and my sides are perfectly straight.

These photos show my early attempts to solve this when I just would turn off my heated bed after the first layer

Quote
tmorris9
The part is stuck to the bed so the bottom stays relatively same size. Heated bed caused it to shrink as it goes up squeezing the middle inward but the further away you get from the heated bed the effect diminishes so it returns to proper size and that is why you get the hour glass look.

80c is now the recommended heated bed temperature. Some people say 100c for first layer then drop to 80c. I run mine at 85c all the time now and my sides are perfectly straight.

These photos show my early attempts to solve this when I just would turn off my heated bed after the first layer

Ahhh, those photos are yours? I see them on the trouble page.

Yeah I do 100c then drop to 95c but 80c I can left my prints on the boro glass using glue stick or extra hold hair spray.

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_______
I await Skynet and my last vision will be of a RepRap self replicating the robots that is destroying the human race.

Quote
tmorris9
The part is stuck to the bed so the bottom stays relatively same size. Heated bed caused it to shrink as it goes up squeezing the middle inward but the further away you get from the heated bed the effect diminishes so it returns to proper size and that is why you get the hour glass look.

80c is now the recommended heated bed temperature. Some people say 100c for first layer then drop to 80c. I run mine at 85c all the time now and my sides are perfectly straight.

These photos show my early attempts to solve this when I just would turn off my heated bed after the first layer

I'm guessing the temperature is too high hence the deposited plastic is melting, sometimes happen when the print is too small for the layers to cool off.

Try printing multiple parts or a slower printing speed.