Heat bed flatness?  

Heat bed flatness?

Hi all-

I’m trying to get a perfect first layer for a project I’m working on and I’m running out of ideas.

I’m using a MK4S in a Lack enclosure.

The material that’s giving me trouble is Form Futura Clear ABS.

.40 mm nozzle @ 260ºc, bed is 100ºc

I had the ABS on hand, and vapor smoothed it with acetone.

The build was making a small box with text on the front and LEDs inside to back light it.

The build plates were all cleaned and wiped with isopropyl alcohol before printing.

The sign surface has 2 layers (.4 mm total height) of clear, and 1.5 mm of cutout text in black ABS.

I profile my filaments using this method and have gotten excellent results with it.

https://guides.bear-lab.com/Guide/Extrusion+multiplier+and+filament+diameter/8?lang=en

In this case using the extrusion multiplier I got from that method resulted in a lot of rippling in the first layer, around the edges mostly. 

Reducing the extrusion multiplier by about 20% helped a lot, but didn’t clear up the problem completely.

Making adjustments to the nozzle and bed temps also helped a little but the results remain similar to the included picture.  The picture shows the first layer on a satin sheet, but the results were the same on textured and smooth sheets: Rippling around the edges and thin in the center.
It seems to me that the heat bed isn’t flat, but I’m at a loss as to how correct it.

Thanks for any insights you can offer.