Packing and Cylinder Pressure
Packing and cylinder pressure seem to be the number one culprit in printing related problems. It is absolutely necessary for the press operator to check and maintain proper printing pressure between plate, blanket, and impression cylinders. Press manufacturers should be able to supply this valuable information. Most of the time the packing information is located on a plate on the press or in the operating manual. How much pressure is too much? Most operators feel that is .004" squeeze is good, then .008" must be better, and if that doesn't work then an extra .002" - .004" will certainly help. This will without a doubt effect quality and production. Most presses should operate with .004" - .006" squeeze between cylinders. Any more than this can cause problems like dot gain, emulsification, bearing shock marks (streaks), poor register and fit to name a few. It would take several paragraphs to explain how each one of these problems occur due to excessive pressure, but to keep it simple, resetting the pressures to manufacturers specs might eliminate problems that you never knew you had.
The proper tools are essential to doing the job right. A micrometer is mandatory and a packing gauge is ideal to set pressures on printing presses. For duplicator operators most people don't bother checking pressures and some manufacturers don't even give a positive scale or gauges for selling cylinder pressure. It is still very important if you ever plan on printing higher quality jobs.
To set cylinder pressures on printing presses it is recommended to follow the manufacturers directions very carefully. If the press manufacturer is not willing to walk you through this procedure, you can follow 3 basic methods to help check or set proper cylinder pressure. The first and most basic procedure consists of using a micrometer to pack the plate and blanket to specifications listed on the press or instruction book. This procedure is the most basic way to pack the machine to specs. Unfortunately this method does not take into consideration the fact that the blankets compress when tightened around the cylinder. Some blankets do not compress and others might compress .003" - .005". For this reason, using a micrometer alone is not an adequate method for setting pressures.
The second method is much more accurate. This procedure requires the use of a packing gauge in order to measure the relationship between plate, blanket, and bearer height. Specifications are provided by the manufacturer to show the relationship between the bearer height and the cylinder surface for each type of cylinder. This procedure measures plate, blanket, and packing on the cylinder and therefore eliminates blanket compression as a variable.
The third procedure is the cylinder stripe method. This procedure utilizes the transfer of ink stripes between cylinders to a width that is specified by the manufacturer. This method is very easy to perform but should only be used as cross check of the two above mentioned procedures.
The following is the recommended setup procedure for aligning and adjusting cylinders on most popular printing presses:
1. Clean all cylinders and bearers.
2. Pack the plate and blanket cylinders to the specifications provided. (USE A MICROMETER)
3. Confirm that plate and blanket cylinders are the correct distance above the cylinder bearers. Compare to the specifications provided. (USE A PACKING GAUGE). Install or remove packing if needed. A. Zero packing gauge to plate cylinder. B. Measure plate bearer in relation to cylinder body. C. Zero packing gauge on the blanket cylinder. D. Measure blanket bearer in relation to cylinder body.
4. Parallel impression cylinder and calibrate adjusting dials (clocks) to the specifications provided. A. Position cylinder gaps toward the delivery and manually throw the unit on pressure. B. Insert proper feeler gauge between blanket and impression cylinder bearers as mentioned in operational instructions. NOTE: MAKE SURE IMPRESSION CYLINDER JACKETS ARE INSTALLED IF APPLICABLE.
5. If the press prints on bearer contact, it is important to check for proper pre-load on the bearers. This can be done by smearing ink, red lead, or lipstick on the bearers at four different points. All four points should transfer evenly on both sides if the pressure is set correctly. This procedure should be performed by a qualified technician. This is called spotting the bearers.
6. Cross check the above procedures by using the cylinder stripe method.
To check plate to blanket:
A. Ink plate cylinder solid.
B. Position gap toward the delivery.
C. Manually throw the unit on and off pressure (if possible).
D. Inch the machine and check the ink stripe. The proper stripe dimension is provided in the technical book or service bulletins.
To check blanket to impression:
E. Verify proper squeeze between blanket and impression cylinder.
F. Ink the blanket cylinder solid.
G. Feed a .004 sheet of paper on to the impression cylinder.
H. Make sure the impression cylinder clocks are set at zeroing position. Manually throw the unit on and off pressure.
J. Inch the machine and check the ink stripe on the paper. The proper stripe dimension should be available from the manufacturer (normally about 1/4 - 5/16 on presses smaller than 28").
All three methods used in conjunction with each other will insure the proper calibration and ideal starting pressure. It may still be necessary to adjust impression cylinder pressure for different stocks.
The above mentioned tips mechanical adjustments manufacturer for additional are just some of the basics. Care should be taken whenever are necessary. Whenever possible contact the press information.
This article was written by Mike Grego.
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