Though a pressman doesn't need to know everything about how offset printing blankets are constructed, he should understand a few basic characteristics that affect the printed product. Here I offer seven properties that directly affect how well a blanket will run on an offset printing press.
- Tensile Strength
- Solvent Resistance
- Surface Release
1. Tensile Strength
Blankets should only be tightened around the cylinder with just as much force as they need to not move on the run, nothing more. The reason being is that the tensile strength, or the ability of the woven fabric to withstand the pull around the cylinder, is only so strong. The basic idea is to have the blanket stretch as little as possible for two reasons:
1.) The blanket can rip.
2.) Blanket height is lost as more torque is applied to the mounted blanket.
2. Solvent Resistance
Each press room has its own arsenal of chemicals. Another important property of a blanket is that it must resist the tendency to swell, crack or distort when coming in contact with those chemicals. These can creep easily into the sides of the blanket and cause distortion in the image. Additionally, some inks or fountain solutions can actually effect the face of the blanket and cause it to swell. Good communication with your blanket manufacturer will keep this problem at bay.
|Proper gauge for checking caliper.|
4. Surface Release
Typically, the smoother a blanket is, the better the image it will reproduce on the paper. The problem is that the smoothness will not release the ink as easily as if it had a rough surface. There must be a balance. If you observe that you get too many wraps on the start up or you notice that the paper rides up the blanket before releasing to the next printing unit, you could have more than a tension problem. Additionally, the problem could also be a chemical one as the ink may physically bond more readily to the rubber surface. Work with your manufacturer in getting the right product.
Most offset printing presses use compressible blankets as they give more flexibility and produce sharper images. A compressible layer is built into the blanket which literally acts like a sponge, allowing up to 7% compressibility. This property though is slowly lost during the life of the blanket. Most printers nowadays get from 10 to 15 million impressions from these blankets until they must be replaced.
There is usually at least one layer inside the blanket that consists of a sewn fabric. The is literally the strength of the blanket that holds it together. It is sewn in such a way that the most strength is given to the circumferential property. When tightening the blanket, a certain amount of stretch will occur as these threads pull on one another. However overdoing it will cause your blanket height to go down and possibly rip the blanket.
Nobody is perfect. Blanket manufacturers will sometimes send blankets that are not completely square. Or possibly, they were not square to how the fabric was sewn. If the blanket is crooked in this respect, this will cause major tension and slur problems. This should be a quality check before mounting a blanket. A good rule of thumb is this: If it's not fitting on the cylinder properly, there is likely a good reason for it. Check!
There are of course more properties of a blanket that the makers will take into account, but these seven characteristics are the ones that will matter the most in the press room.