I ran a few experiments today to try and get at the erosion problem.  

First, I got my film to adhere nicely to the mylar Puretch film by using a lot less mineral spirits.  I just dampened the end of a finger and rubbed a light coating on the mylar before I placed the image film.  After rubbing out the air, I found a tight bond with no tendency to slip around. This made it much easier to expose properly.  I exposed all plates for 12 seconds.

 

My first attempt was at 5 volts and about .6 amps. All looked OK until 40 minutes and then the ground (Puretch resist) started to crinkle.  I thought by reducing the voltage, I would eliminate this problem.  I can etch deep but could not find a way to get rid of the erosion as the ground seems to release after 30 minutes.

 

I decided to just move on and experiment with a plate that had the positive image used as the photo film.  The twiggy tree limbs were masked and the body was open for etching.  At 5 volts the amps were 1.3. I figure this is because of the increased copper surface exposed.  I decided to run the system in 5 minute intervals and then stop and inspect.  I found that a scum covered the exposed copper and it was helpful to wash the plate in cold tap water and to lightly rub the plate with a soft bristle painting brush. I found that the plate cleaned nicely and upon reentering the etch process, the amps would go back up from .7 to 1.4

 

I stopped at 20 minutes as I felt the etching was sharp and wanted to inspect before any erosion occurred. I was very pleased with the etched surfaces. There was no sign of the Puretch lifting or curling. The lines were very sharp.  I will repeat the experiment tomorrow and check at 5 min intervals until I find the time that erosion takes place.  This timing will probably just have to be the limitation of this system until I can get the resist film to not breakdown.

 

The image above looks like black on copper but the twigs are actually the original shiny copper surface and the background is the dull matte surface of the etched region.