By The Metric Maven
The first job I had which required employing technical expertise was operating an offset printing press. We all know that oil and water don’t mix. This is the simple principle behind offset printing. But when putting this property into practice, and printing images onto paper, the devil is in the details. There is a knob which controls the amount of water which is fed to the water rollers, and another knob which controls the amount of ink fed to the ink rollers. The first time I tried to run a press, like many rookies, I ended up with water in my ink rollers, and no acceptable image. The balance between water and ink is important to master.
Offset printing plates at that time were aluminum sheets with image areas made of lacquer. The aluminum areas of the plate are able to take up water, with the help of a mixture of chemicals, which are acidic and slowly etch the aluminum. The areas of the plate which readily take on ink become the printed image. In Figure 1 the basic parts of a printing press are shown. The ink rollers and water rollers contact the plate cylinder and produce an inked image. The plate cylinder then transfers this image to the blanket on the offset cylinder. This reverses the image to be printed. Paper then passes between the offset and impression cylinder. The image on the offset (blanket) cylinder then is transferred, or offset, to the paper and reversed back to the original image on the plate.
The situation is actually a more complex than Figure 1 shows. The offset cylinder has a sheet of rubber material wrapped around it which is called a blanket.
Figure 2 shows the image as seen on the plate and its reversal when transferred to the rubber blanket on the offset cylinder. The image image is again reversed as it is printed by the blanket onto the sheet of paper passing between the blanket and the impression cylinder. This all appears straightforward, but it turns out the accuracy of the pressure required to accomplish this is very, very, important to creating a quality image. The plate has plastic sheets behind it known as plate packing. The blanket also has packing behind it. The amount of packing behind the blanket controls how much pressure the blanket will press down on the paper with the impression cylinder behind it. The plate packing and the blanket packing control how much pressure is between the printing plate and the blanket on the offset cylinder.
The Graphic Arts Technical Foundation refers to Offset Press Cylinder Packing as “one of the most exacting operations required of the press operator.” If one over-packs the plate, it can “smash” the blanket on the offset cylinder. The blanket is compressed enough that the pressure between the blanket and the impression cylinder is reduced. If the blanket is “smashed,” the paper passing between the blanket and impression cylinder does not have enough pressure to produce an image. Taking the thickness of the paper into account is also critical. The balance of packing the plate and blanket is absolutely critical to obtaining a quality printed image. The range of packing which will produce a quality image is about 10-30 μm. Yes, 10 to 30 micrometers!
One evening I accompanied my father to a printing company run by a friend of his. The owner of the printing company had never operated an offset printing press before. He had always used a letterpress with hot metal type. With letterpress, the relationship is simple, more pressure—darker image. The offset press was printing blotchy ugly images on the paper. The owner of the small company had been working all day trying to get a better image, without improvement. My father inquired what the thickness of the blanket packing was. It seemed about right. He then asked what the plate packing thickness was. The owner was uncertain, so a measurement was made. My father indicated a couple of sheets of packing should be removed. One could see an immediate improvement in the image, and surprise on the face of the owner. Eventually the packing was adjusted so the image was of high quality, and the job was printed.
A number of years passed. I went on to become an Engineer and only thought about offset printing when I spoke with my father. Then, unexpectedly I spoke with a fellow by the name of Jon. As we talked I was mostly mentioning the lack of the metric system in the US, but then he inquired—seemingly out of the blue: “did you used to run an offset printing press?” I was thunderstruck, something I said apparently revealed this part of my background. Slightly stunned I said “Yes–yes I did, a long time ago. I ran a Royal Zenith, and a Davidson 500.” Jon said “I have a metric story for you.”
Jon told me that he had been visiting another printer who had recently purchased a Japanese 5 color press. A press of this type is essentially five offset presses all put together so a single sheet of paper can be printed with five colors in one shot. This allows for four color process printing of color photographs. The printer asked Jon if he wanted to purchase the press–at a considerable discount. Jon asked why, and the printer said “I can’t get it to print a good image.” Jon then explained to me that Japanese [and German] presses are considered the best available—by far these days. He related surprising innovations of which I had never heard before. Jon said it stood to reason that the press was well designed. Clearly there must be a way to fix the image problem.
Jon purchased the Japanese five color offset press, had it moved, and then set it up with standard test patterns. He encountered the same low quality images as the previous owner, but was sure the press had to work. He contacted the Japanese manufacturer for technical help. No matter what he did the test images were unacceptable. Jon was completely frustrated, and after numerous discussions, the Japanese technical representative cryptically said: “we think you should look into metric.”
Jon thought about it, and contacted his graphic arts supplier. He asked if it was possible to obtain blankets with a metric thickness, along with metric packing sheets and plates. The supplier indicated no one had asked for metric before, but maybe he could find some supplies in Europe and obtain them from there. The supplier eventually found metric blankets, packing sheets and a metric packing gauge. There are many other important settings on a press that I have not mentioned—Jon obtained every tool he needed for implementing and verifying these press settings in metric. Jon told me that the first thing which struck him was how much easier using the metric tools and gauges was, when compared with common imperial ones. The tools also allowed him to use the metric values presented in the manual for the printing press directly, without any need for conversion.
I asked if it helped. Jon smiled with satisfaction, and said the images were just beautiful perhaps the best he’d ever seen. He could see why the Japanese presses were considered the best available. I slowly realized I was standing there slack-jawed with amazement–almost unable to speak. I knew how important plate and blanket packing are, and of course all the other press settings should be adjusted to meet the design requirements for the press. For me it was an incredible story. I realized that the available packing sheets and blanket thicknesses, made in the US—in inches, could possibly keep one from finding a combination which would actually work with a metric printing press. The use of inch gauges for settings could preclude ever obtaining the required metric values. Converting from metric to imperial would introduce opportunities for error and inaccuracies. Yet, US distributors only carried inch based products. Because we have not changed to metric in this country, all of the press supplies in the US are in inches, yet German and Japanese offset printing presses are all designed with metric specifications and parts.
The real punchline to this story is that there is no US manufacturer of offset printing presses any longer. In other words no more inch based printing presses are being manufactured—period. The US graphic arts suppliers still offer only inch based blankets, packing sheets and gauges for inch based printing presses which are no longer manufactured—at all. Only the legacy presses which had been manufactured in the US remain. When people challenge the estimate that it costs each person in the US $16 per day because we have not switched to metric, it is stories like this one, that convince me the estimate is probably low.
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