How should a printing blanket be correctly mounted?
Apart from the blanket mounting instructions included in the press manufacturer's manual, an in-house operating instruction should be written with the following information: type and size of printing blanket; quantity and size of underpacking sheets/foils and torque values, which should be tested and documented so that before and during blanket mounting, they only have to be checked.
Other items for inclusion in the incoming inspection/operating instruction could be:
Checks of the blanket thickness, size and barring. Visual inspection of the blanket surface and reverse side. Naturally, this also applies to a similar inspection of underpacking sheets or foils. Thus the need to demount and alter the packing at the last minute can be avoided.
Measuring the blanket packing outside the press precludes surprises.
The blanket should always be wider than the underpacking sheets or foils to reduce the ingress of cleaning agents and inks between the cylinder and the underpacking and between the underpacking and the blanket.
No dirt particles should be allowed to get in or under the blanket packing. This means cleaning the cylinder surface, clamping channel, clamping tool and mounting area.
On printing presses with printing blankets lying next to each other:
When several printing blankets are on a cylinder, the straight blanket edge must always be mounted to the A or B side. This reduces gap formation and consequential paper creasing.
Overlapping blanket mounting on blankets lying next to each other. After tensioning the first blanket, the overlap has to be butted with a “bone folder”. The tension between the blankets also reduces gap formation.
If screws are used to fix the blanket bars, the ones in the middle of the bar should be tightened first. This is the point at which the bars exhibit the least deformation.
Tension the blanket to the torque specified by the press manufacturer (also look at: "What tension/torque is required for a printing blanket?")
Do not forget to "full" the blanket after mounting. This minimises both gauge loss and dot ghosting.
If your blanket clamping system adjusts the tension simultaneously at both ends, the blanket must be “fulled” in both directions. This reduces both gauge loss and dot ghosting.
Measuring the blanket packing after “fulling” and tensional adjustment minimises surprises.
Recording of the blanket mounting data: Date, blanket type, thickness of underpacking sheets/foils, packing height, operator, reason for demounting
The measurement data collected should be recorded and evaluated. These data will be very helpful when judging a printing blanket.
Check and record blanket packing heights. The measurement data collected should be recorded and evaluated. These data will be very helpful when judging a printing blanket.
What tension/torque is required for a printing blanket?
Definition: The torque of any turnable wheel, shaft or screw is defined as the measure of a force's tendency to produce torsion and rotation about an axis, equal to the vector product of the radius vector from the axis of rotation to the point of application of the force and the force vector. Values are always given in Nm.
Printing blankets are tensioned to a torque of 1.5 to 130 Nm, depending on the blanket clamping system. The elongation of the blanket is between 1% and 2%.
The low torque values (e.g. 1.5 Nm) apply to clamping screws on bars while the high torques (e.g. 130 Nm) are applicable to clamping spindles in large-format offset presses.
A torque wrench should be used to tension the blankets, according to the torque values specified by the press manufacturer. Non-adjustable torque wrenches that only measure fixed torque are recommended.
Tensioning the blanket with a torque wrench serves the purpose of guaranteeing reproducible blanket tension on all printing stations and by all press operators.
Calibrate the torque wrench regularly; keep the tensioning elements clean.