MK2A changing bed height while heating up

Hey Guys, I have a RapMan 3.2 double head. I am experimenting with a MK2A attached to the factory acrylic bed with standoffs in the 4 corners. On top of the MK2A I have a 1/4" Fire rated piece of glass held down with small aluminum brackets secured to the acrylic bed in 4 places. Now here is the problem, almost immediately upon turning on the power the bed begins to rise as much as .040 at 130 C. I set a dial indicator in the center of the bed. (See attached pics) Anyone have any ideas on how to solve this problem???

First off, acrylic is a bad matterial to attach a heated bed to, will definetly warp with heat, source an aluminum plate. Heated bed should be mounted with springs to the bed. I have a 5-7mm gap between my heated bed and aluminum plate, with automotive firewall insullation in between the aluminum and heated bed.

Thermal expansion is unavoidable, hot end lenthens with heat as well. zero your nozzle height with everything at print temps.

You should also remove the clamps you have going from the bed to the top of your glass, the glass should be clipped directly to the heat bed only.

Edited 1 time(s). Last edit at 12/27/2012 03:14PM by Dirty Steve.