Laser product marking in motion - how does it work?

In one-off production through to small series, marking is usually carried out in cycles. In contrast, in large-scale production or continuous production such as extrusion, products are usually marked as they move past. This procedure is called "marking on the fly" - MOTF for short. The movements of the product and the deflection unit for the laser beam are combined in a special way - for flawless and precisely positioned marking.

Stationary marking vs. MOTF

The REA JET FL and CL - i.e. fiber and_{CO2 laser marking systems}- are suitable for marking both stationary and moving products. When marking stationary products, the polygons ⁽¹⁾ are processed as defined in the label layout ⁽²⁾. For marking moving products, these are modified by the so-called time-current tracking vector. The REA systems can work in both operating modes without further intervention - by using different installation settings for different print jobs.

Standstill marking operating mode

Deactivating the product movement in the installation settings configures the device for marking at standstill. The label has to be smaller than the marking field of the corresponding lens in both dimensions and can then be freely positioned within the field using the x-offset, y-offset and label rotation parameters. It should be noted that the marking fields are different sizes for different focusing lenses. For example, a label that is intended for use with an f=160mm lens may be too large for an f=100mm lens.

MOTF operating mode

By activating product movement in the installation settings, the device can be set up for marking on moving products. Additional parameters such as clock source ⁽³⁾, sensor distance ⁽⁴⁾, etc. have to be specified for this.

When marking moving products, the vector trains of the label to be marked are modified by the (possibly time-varying) tracking vectors. This turns the mirror movement shown in Figure 1 into the pattern shown in Figure 2, with a product movement from right to left.

This deformation of the vector trains is compensated for by the real product movement so that the desired marking is produced on the product. A precise definition of the product movement is essential to ensure that the correction by the tracking vector is correct at all times with MOTF. This includes both the direction and amount of the product speed. The direction of the product movement is specified in the form of the head rotation⁽⁵⁾. The amount of speed is specified directly when the internal cycle is selected and by specifying the rotary encoder clock rate when an external cycle is selected.

An error in this tracking vector can have different, sometimes serious, episodes depending on the product speed, marking time and type of object to be marked. While plain text is only slightly distorted at low speeds, this can lead to complete illegibility at higher speeds or larger deviations. Machine-readable codes become unreadable very fast due to errors in the tracking vector.

The accuracy required when specifying the tracking vector often exceeds the capabilities of the human eye and mechanical adjustment. Fine adjustment via the device software with a view to the marking result is therefore essential. The following two illustrations show the procedure proposed for this, in which a rectangle is marked in a test run. The entire length of the marking field should be used for marking this rectangle in order to observe the maximum effects.

Figure 3 shows the result of laser marking a square when the head is rotated incorrectly. Due to an incorrect specification of the tracking vector, the marking perpendicular to the direction of movement deviates from the desired result (closed square). In the left part of the figure, the head rotation is specified too low, on the right too high.

Figure 4 shows the result of the laser marking of the square with an incorrect rotary encoder clock rate (product speed). Due to the mismatch of the tracking vector, the marking in the direction of movement deviates from the desired result (closed square). In the left part of the illustration, the speed is too high, on the right too low.

Both effects are usually observed when the tracking vector is mismatched. The superimposed result shows both effects individually:

The tracking of the product moving in a straight line is carried out by an adapted rotary movement of the mirrors of the deflection unit. The additional rotational movement of the mirrors required for this depends on the distance of the product to the mirror. A deviation from the nominal focus distance (depending on the lens used) therefore has the same effects as an incorrect product speed (incorrect rotary encoder clock rate or incorrect product speed with an internal clock source).

Glossary
(1) Polygonal tracks: A polygon course is the trace of a path that is composed of a finite number of straight line segments.
(2) Label: Also called label - the content to be marked with information about the position
(3) Clock source: A sensor that detects the arrival of a product to be marked
(4) Sensor distance: Distance of the clock source to the marking field from the direction of the incoming product
(5) Head rotation: Deviation of the laser axis at any angle to the direction of movement, depending on the mounting position

Author: Christian Dini, laser expert at REA Elektronik GmbH