The 1st exposure time will make the floor or support, the 2nd will make the detail by shining light through the negative.
If you are using your exposure machine for the first time that day, warm the bulbs up for three minutes before exposing. The critical time in plate making is the 1st or back exposure. Bulbs give out much more light when hot; the back exposure is done first and is the shortest, so cold bulbs have the largest effect on the depth of floor. A small variance in the 1st exposure time has a much greater effect than a similar change in the 2nd exposure. Cold bulbs often give low plate floors which lead to wobbly lines or text falling off.
Use spacers (bearers) to ensure even compression. These should be the imagepac thickness that you are using plus the negative thickness (usually 0.15mm) and the front film (0.06mm). For a 2.3mm (90”) thick imagepac use 2.4 – 2.5mm spacers. For a 2.55mm imagepac (100”) use 2.7 – 2.8mm spacers.
Ensure that the glass bed is clean and dry. The negative should be dry and with the black dark enough density to stop UV. Lay your imagepac sachet on the negative, making sure that you can read the printed “imagepac” on the edge of the sachet. Close your exposure unit. The upper glass should exert enough pressure to remove all wrinkles and flatten the sachet. If you are using your existing exposure unit then you should use the same exposure times as you were using with liquid resin.
Different exposure units have different light configurations producing different light intensities, also, as bulbs age they produce less light, often less evenly distributed down the tube. For these reasons it is not possible to state exact times to expose a perfect imagepac on every machine. As a rough estimate try 20 seconds and 300 seconds as your first test.
This method will help you set the exposure times accurately when you use imagepac. It will also help you perfect plate quality if you are finding it difficult to hold fine relief or you are filling in fine reverses.
Set your current back exposure time, to save cost you can use A7, A8 or A9 sized sachets, lay the sachet on the negative on your exposure unit with ‘imagepac’ readable. Black-off completely around the negative. Expose the sachet for 30 seconds on the back only- do not use the main exposure. Remove the sachet, cut the front plastic with a scalpel not scissors, and wash it gently as it will be delicate without relief. Measure the floor, aim for a floor depth that is just under half the total plate thickness (35-45% is ideal). Adjust this time accordingly to achieve the desired floor thickness, this is your correct back exposure time.
Now calculate the main exposure time, if you only have one set of bulbs or the upper and lower lights are the same, then the main exposure will be, as a rule of thumb, about 8 x the back exposure time you have just set. Use a negative that has fine relief and reverse on it. Ideally, use a specially made test negative. Place the negative on the unit and the sachet on top of it, imagepac readable. Expose for the floor exposure time that you have just set, then expose for the main exposure time through the negative for 8 x that time.
In some cases you should adjust your main and back exposure according to your artwork. Calculating the correct main exposure time depends upon how deep the floor is, how much of the negative is black and the fineness of the artwork. Remove the sachet and without cutting it open, press it between your finger and thumb. The relief should be firm to the touch surrounded by liquid resin.
If the plate is under-exposed the relief will not be attached to the floor. In this case if you squeeze the sachet, the text will move freely in the resin. To rectify this increase the main exposure time by 30 seconds and repeat the process until the text holds. If the plate is over-exposed, the pools of liquid resin will be shallow or non-existent and the fine reverses will have filled in. The plate will feel rigid with very little liquid resin present. To rectify this, decrease the main exposure time by 30 seconds and repeat the process until the text is surrounded by liquid resin. When you have perfected the main exposure time, wash this plate out, post-expose it and check that you are happy with the fine reverses and reliefs in detail using an eye glass.