<H1><B><center>Offset Printing Blanket Thickness & Other Properties</H1></B></center>

Offset printing blankets have specific characteristics that make them suitable for mounting on press.  Properties such as thickness, caliper and squareness are just a few.  

Why it's hard to make offset printing blankets.

Before delving into this, we must understand that producing a rubber blanket with just the right properties is not a simple task.  The demands from any offset printing press are physical and chemical.  

The press must be able to run a variety of papers, running conditions and even ink. It must be durable enough to run at high speeds and yet sensitive enough to transfer a dot perfectly. Sometimes these demands require opposite compositions in the blanket.  Manufacturers must balance the demands of the printing press to make it work. 

Here is a diagram of an offset printing blanket showing all the components that make up it's thickness.

I've spoken before about how a blanket is construction, but here I will break down a list of all the properties as well as checks for quality control before mounting a blanket on your offset printing press.

1. Squareness

It happens sometimes.  The blanket itself was not cut square.  This makes mounting the blanket on the offset printing press a nightmare.  One side of the cylinder will have a snug fit while the other side is loose.  This requires the printing press operator to over tighten, compromising the integrity of one side.

Another issue is when it is cut square, but not according to how the fabric is sewn.  It will mount on the press just fine, but there will be slurring and tension problems. Best to check this before mounting on the press.

2. Thickness

Also commonly called "Caliper". This is an easy check. It can be done with a simple outside micrometer.  Here is the one we use.  It is cheap and works well.  Spring loaded ones also work fine as well.

Outside micrometer

I also recommend checking it after being put on your offset printing press. The reason is that thickness can change when put under tension.  The caliper when mounted is after all the most important property at the end of the day, right?

The best tool for this is one that is becoming more popular in recent years. It is commonly called a coating thickness gauge.  It is mainly used by the automotive painting industry. It can tell the thickness of a paint coating to the thousandth of an inch.  It measures from any surface to the metal surface below - perfect for an offset printing blanket.   Here is the exact one we use and what it looks like.

For blanket thickness on press.

The less expensive ones tend to give inconsistent readings. Beware!

3. Solvent Resistance

Offset printers use strong solvents for cleaning and washing up printing units.  The ink and fountain solution themselves are chemicals.  Printing blankets must have the resilience to maintain all of their properties despite exposure to these harsh chemicals. Without this resilience, cracking, swelling or image distortion can result.

As new chemicals are introduced to the market for cleaning blankets, manufacturers must adjust and develop new rubber compounds to adapt.

4. Compressibility and Reboundability    

Yes, those are real words used in the manufacturing process. Here is the simple definition: The ability to reduce thickness when under load by a substrate.

Today, most blankets have this specially designed compressible layer to reduce thickness, yet at the same time not bulge under pressure. 

This must be accomplished without compromising dot gain.  Offset printers rely on having consistent image transfer.  A greater "squish" will result in more dot gain which is unacceptable, or must be accounted for before coming to the press.

5. Release

This is the ability of a substrate to come off the printing blanket while running.  Blanket makers make compositions that will release, but it cannot all be accounted for.  So the press design will also compensate for this.

For example, web presses will have the upper cylinder slightly positioned over the bottom one to help facilitate a consistent release. Notice this in the image below.

6. Hardness

Offset printing blanket manufacturers will measure hardness with an instrument called a Shore A scale.  This can be a misleading measurement since the real hardness of a blanket is of the rubber layer on top. The inner layers cannot give a proper hardness reading.  Therefore a reading is usually done of the rubber layer without the inner carcass. When checking this durometer, it is best to get this from the manufacturer since they will be able to supply this.

7. Smash Resistance

When running a paper that is narrower than the blanket (in almost all cases) the edge of the paper will literally "cut" into the blanket.  The blanket therefore needs to have resistance built into it to stop this.  

The edge of the paper will also produce a buildup that can smash the blanket.  Pressmen will usually scrub this off with some water since the blanket will smash over time.

Buildup of paper on an offset printing blanket.

Conclusion

The list is by no means comprehensive but gives the main properties needed to be built into an offset printing blanket that directly effect the press operator.  Understanding them will help to know what properties can be expected from an offset printing blanket.