US 2014/0184682 A1
Print gap compensation
Inca Digital Printers Limited
Variations in the print gap, the spacing between nozzles and substrate, can lead to a distortion of the image. If printing is being carried out bi-directionally then there is a possibility of double images, where the printing in each direction does not line up.
The figure shows a printhead jetting drops to a substrate. In the first example the substrate 86a is spaced away from the printhead, which is moving to the right relative to the substrate. Drops ejected at point x1 reach the substrate at x2. In the second example the substrate 86b is closer to the printhead due to distortion of the substrate or bed. The flight path of the drops is shorter, so for the drops to land at x2 the release point needs to be delayed from x1 to x3.
Inca Digital manufactures flat-bed printers which use a large table on which the substrate is mounted. The table passes up and down underneath the full width printhead. A small number of multiple passes over the substrate are made to reach the full print resolution and to mask jetting errors. The problem of image distortion is greater if the table size is large. Even if the table is made to very tight manufacturing tolerances, distortions due to thermal expansion during operation may occur and be significant. For instance, a large UV curing lamp system is used and generates a significant amount of heat.
The solution is to determine variations in the table flatness under operating conditions, and then adjust the timing of the drop data to compensate for variations in the flight path. A test pattern consisting of short lines or dots is printed in two passes. The pattern is repeated over the table, although it has been found that just 5 positions are sufficient for a 3m long table. The spacing between the marks on the forwards and backwards passes depends on the print speed, jet velocity and the gap between the nozzles and the substrate. The pattern is printed twice, at two different print heights, with a small offset between each pattern so they do not overlap. Each test image is analysed using a scanner that may be mounted with the printhead.
Although the exact spacing of the printhead to substrate is not known, the difference between the two positions can be accurately determined. Assuming the jet velocity is constant, simultaneous equations can be used to find the jet velocity and the actual print gap for the printhead. This enables corrections to be determined to remove the distortion, delaying or advancing the release of drops.