Calculating blanket and plate packing is not rocket science, but is technical enough to have a calculator on hand. At our shop we simply have a standard blanket and plate thickness, so there is really little need to sit down and figure this stuff out when it comes time to change packing. However, having an understanding of the math behind it will make you a better pressman. So here goes.
First and foremost, the calculations below assume that your bearers are set properly. Once you have those set, you are ready to start using this formula. Let's start with your plate to blanket setting.
Calculating Blanket to Plate Squeeze
Your desired squeeze from blanket to plate is .005". In other words, that is the total amount of the blanket and the plate being over the bearer height. GATF recommends that the plate be packed .001" over bearer while the blanket is .004 over bearer (or whatever is determined based on your paper thickness).
Formula: (Required plate height over bearer) + (plate cylinder undercut) - (plate thickness) = (required packing).
Let's look at an example. As we established, the required plate height over bearer is .001". Let's say the undercut of your plate cylinder is .020". Your plate is .012. That would give you the following: (0.001) + (0.020) - (0.012) = (0.009). Your packing should be .009".
Calculating Blanket to Blanket Squeeze
For all you web printers out there, here is the formula. This assumes some of the following standard variables:
- Blanket to blanket squeeze is .011
- Paper thickness is .003 - though this a standard, though it could change depending on paper thickness.
- Your system is running blanket to blanket.
First of all, these calculations have a large bearing on how thick your paper is. So we will not use numbers to start with just yet. Here is the formula step by step:
- (required blanket to blanket squeeze) - (paper thickness). Then divide this result by two as we are splitting the difference between the two blankets. This will give us the 'over bearer blanket height'. In this case we have (.011 - .003) / 2 = (over bearer blanket height)
- (over bearer blanket height) + (cylinder undercut) - (blanket thickness) = (required packing for each blanket)
Calculating Blanket to Impression Cylinder Squeeze
This is of course for sheetfed printers. Usually, each sheetfed press will have clearly marked somewhere the exact packing and undercut required for printing. I do not recommend venturing out of these parameters. Unlike web, sheetfed presses will allow you to adjust the impression cylinder based on paper thickness and so there is usually no need to venture outside of the given measurements. Follow the instructions for plate to blanket squeeze and use your impression cylinder adjustment to give you a good quality image with minimal squeeze.
You may have some other suggestions that I would welcome. I certainly do not know everything there is to know about this. Feel free to leave your comments or any corrections you think are necessary.