Getting imagepac flat in a floating glass exposure unit

What is a floating glass exposure unit?
This type of machine was invented by Asahi and the technology has been used in flexographic plate making machines and stamp mahines alike. The upper sheet of glass is supported loosely behind brackets and is allowed to fall with gravity onto the bearers to achieve two planar parallel sheets to expose within. 

Why it is difficult to compress an imagepac on this type of machine?
In a floating glass exposure unit the glass just rests under its own weight, rather than being forced down as in European style exposure units which have springs or clamps.  The downward force on the pack is not large and achieving a flat plate is consequently difficult.  Creasing can occur or you may have to leave the imagepac under for a long time to level prior to exposing.

1. How to compress imagepac easily on these exposure units without modifying the machine

i. Clean Glass
Clean the upper and lower glass to prevent them from sticking to the plastic. Clean the brackets supporting the upper glass. These can often get gummed up with congealed polymer and will stick as they come down.

ii. Remove existing bearers and apply lower ones
Ensure that the bearers are the same size as the imagepac, the depth of the negative and removed imaging side of the imagepac will then ensure adequate compression.
 
iii. Warm sachets up first
If the sachets are above 20 degrees C the resin will flow easily to the corners, below this temperature it will be slow and may never make it.   Every increase in 10 degrees C cuts the viscosity of the photopolymer in half.  The more easily the resin moves, the quicker and more completely the sachet will become level.  You can pre-heat the box in a dryer or low temp oven (do not go above 60C).

2. How to modify the exposure unit
By far the fastest and most certain way of using these machines is to dam off the lower glass with insulation foam outside the vacuum point. We use draft excluder tape from the hardware store for this. If you make this border outside the vacuum lines the inside will become a vacuum chamber and pull the upper sheet down onto the bearers with force. Take care to release the vacuum before lifting. This is the way we make all of our large output machines and is guaranteed to deliver sufficient force to get a flat imagepac in seconds.