Method and device for producing a tear line in a planar workpiece along a predetermined contour by material removal by means of a laser

1. A method for producing a tear line on a planar workpiece by material removal, said planar workpiece having a visible side and a reverse side opposite said visible side, comprising:

generating a pulsed laser beam from a laser generator, said pulsed laser beam having pulses of an energy determined by a pulse amplitude and a pulse length;

introducing said pulses sequentially, in machining cycles repeated more than once, along a predetermined contour for the tear line at a respective removal location from the reverse side of said workpiece, wherein material removal is performed for each removal location to a respective predetermined residual wall thickness, wherein said wall thickness may be zero;

terminating, upon reaching a deactivation criterion correlating with the respective predetermined residual wall thickness, the introduction of the laser pulses in a space-resolved manner with respect to the removal locations, generating a measurement signal curve for each machining cycle, which is formed by measurement signals via the removal locations, which are each caused by detection of a transmitted part of the pulse amplitude of one of the laser pulses by means of at least one sensor of an array of sensors for each removal location;

generating and storing a reference signal curve as a step temporally preceding said machine cycles, said reference signal curve being formed by reference signals causing the pulse amplitude via the removal locations with no planar workpiece being arranged between the laser generator and the array of sensors, the pulse amplitude of the laser pulses being kept constant in the steps of generating and storing said reference signal curve temporally preceding said machining cycles as well as in the machining cycles, and the achievement of a respective predetermined percentage of the reference signal or of a predetermined distance from the reference signal by the measurement signal being used as the space-resolved deactivation criterion for each removal location.

2. The method according to claim 1 , wherein said laser pulses have a shorter pulse length during a first one of the machining cycles than during subsequent ones of the machining cycles, so that the energy of the laser pulses is so low that no material removal results.

3. The method according to claim 2 , wherein a measurement signal equal to the reference signal is already generated for some of the removal locations in the first machining cycle, and no further laser pulses are introduced at these removal locations in subsequent machining cycles.

4. The method according to claim 2 , wherein a measurement signal which is greater than the respective predetermined percentage of the respective reference signal or whose distance from the reference signal is smaller than the predetermined distance from the reference signal is already generated for some of the removal locations in the first machining cycle, and no further laser pulses are introduced at these ones of the removal locations in subsequent machining cycles.

5. The method according to claim 2 , wherein a measurement signal which is smaller than the respective predetermined percentage of the respective reference signal or whose distance from the reference signal is smaller than the predetermined distance from the reference signal is generated for some of the removal locations in one of the machining cycles, and in subsequent machining cycles laser pulses are introduced at these ones of the removal locations with a shorter pulse length than those introduced at the removal locations for which no measurement signal has been formed yet in this one machining cycle.

6. The method according to claim 1 , wherein the step temporally preceding said machining cycles is carried out only once for machining workpieces of the same type with the same contour for the tear line and is then used for producing the tear line in further workpieces of the same type.