Blankets for offset printing are constructed to meet high demands. Printers want a blanket that not only transfers a quality image from the plate to the blanket, but can function at high speeds with different papers. They want a blanket that can resist change from different chemicals such as solvent, fountain solution and ink. The want it to bend nicely around the cylinder and into the gap and at the same time maintain the stiffness necessary to hold together under extreme conditions at high speeds. And they want it to maintain its height for a long time and be resistant to piling. Offset blankets are constructed to meet all these demands. Let's look at the two main components of an offset printing blanket.
The Blanket Face
This part of the blanket is more commonly described as the surface layer - the part that comes in contact with the plate and paper. The blanket face is constructed in such a way that it can transfer image as sharp as possible. To do this, blankets are constructed with a smooth surface. However if they are manufactured too smooth, it could create more resistance for the ink to release onto the paper. Manufacturers try to achieve a balance. Good communication is required as they need to know how your particular paper type is affected by the blanket in use.
The rubber compound itself on the compressible layer must also have a low reactivity to offset press chemicals. Solvent, ink and fountain solution are continually pushed onto the blanket. The rubber must prevent these chemicals from penetrating. Not only that, it must not react by swelling, shrinking or cracking. Some inks, such as UV inks will cause the rubber to swell. You must order the right blankets according to the chemicals that you use.
|Layer 1: Face 2. Fabric 3. Compressible 4. Fabric|
The fabric layer consists of strong quality thread woven at right angles. Each direction is called a warp or weft. The warp is considered the strongest and so the blanket is constructed in such a way that the warp wraps in the circumferential direction around the cylinder. This direction receives the most stress when it is tightened against it. Comparably, very little stress is put laterally on the blanket.
The compressible blanket contains a layer that is sponge like in design. This layer can have a closed or open cell design. The open cell design allows air to flow throughout the blanket and has its applications. However the more popular design is the closed cell since it retains its compressible state longer and has an increased resistance to smashing over a longer period of time.
Conventional blankets, though still used have become less popular in offset printing. This is for several reasons:
- Increased latitude in paper thickness.
- Less dot gain.
- Better smash resistance.
- Ability to recover thickness quickly after every impression.
Lastly, the carcass consists of a layer that sits against the face of the cylinder. It must be built in the same way as the previous fabric layer, but must be able to resist solvents and what other chemicals seep behind the blanket. Neither water nor ink should be able to break down this layer.