Print head maintenance

Abstract

A maintenance unit for an inkjet system with a print head assembly having at least one print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the at least one print head towards a substrate includes a wiper to wipe along the surface of the at least one print head; a force actuator to apply a force to the wiper in a direction perpendicular to the surface of the at least one print head; a force measuring unit configured to determine a wiping force with which the wiper is pressed against the surface of the at least one print head; and a controller configured to control the force applied by the force actuator in dependency of an output of the force measuring unit in order to press the wiper against the surface of a print head with a predetermined wiping force.

1 . A maintenance unit for an inkjet system with a print head assembly, said print head assembly comprising at least one print head, the at least one print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the at least one print head towards a substrate, the maintenance unit comprising:
 a wiper to wipe along the surface of the at least one print head;   a force actuator to apply a force to the wiper in a direction perpendicular to the surface of the at least one print head;   a force measuring unit configured to determine a wiping force with which the wiper is pressed against the surface of the at least one print head; and   a controller configured to control the force applied by the force actuator in dependency of an output of the force measuring unit in order to press the wiper against the surface of a print head with a predetermined wiping force.   
     
     
         2 . The maintenance unit for an inkjet system according to  claim 1 , wherein the maintenance unit further comprises:
 a position sensor to measure the position of the wiper relative to the surface of the at least one print head; and   a set point generator for generating a set point corresponding to a desired position of the wiper relative to the surface of the at least one print head seen in a direction perpendicular to the surface of the at least one print head,   wherein the controller is arranged to drive the force actuator in dependency of an output of the position sensor and the set point, wherein, in order to wipe along the surface of the at least one print head, the set point generator is configured to output a set point corresponding to a position in which the wiper is at least partially inside the at least one print head, and wherein the controller is configured to limit the maximum applicable force of the force actuator to the predetermined wiping force.   
     
     
         3 . The maintenance unit according to  claim 1 , further comprising a wiper moving device for moving the wiper,
 wherein the controller is configured to drive the wiper moving device such that the wiper is moved along the surface of the at least one print head.   
     
     
         4 . The maintenance unit according to  claim 1 , further comprising a frame and a guide to guide movement of the wiper with respect to the frame in a direction parallel to the wiping force. 
     
     
         5 . The maintenance unit according to  claim 3 , wherein the wiper moving device is configured to operate on the frame to move the wiper. 
     
     
         6 . The maintenance unit according to  claim 1 , wherein the force actuator is an electromagnetic actuator. 
     
     
         7 . The maintenance unit according to  claim 6 , wherein the electromagnetic actuator is a Lorentz actuator. 
     
     
         8 . The maintenance unit according to  claim 3 , wherein the guide is configured to guide movement of the wiper without applying significant forces to the wiper, or wherein the guide guides movement of the wiper while applying a constant force to the wiper. 
     
     
         9 . The maintenance unit according to  claim 3 , wherein the wiper moving device is configured to move the wiper in two degrees of freedom in a plane parallel to the surface of the at least one print head. 
     
     
         10 . The maintenance unit according to  claim 1 , wherein multiple wipers are arranged on a common wiper support frame. 
     
     
         11 . The maintenance unit according to  claim 10 , wherein respective wiper moving devices are provided in between the respective wiper and the support frame, such that movement of each wiper can individually be controlled by the controller. 
     
     
         12 . The maintenance unit according to  claim 10 , wherein the wiper support frame is moveable relative to the print head assembly in one direction only, and wherein the wiper moving devices on the wiper support frame are configured to move the respective wiper in a direction different from said one direction of the wiper support frame such that the wiper is moveable in a two dimensional plane parallel to the surface of the at least one print head. 
     
     
         13 . The maintenance unit according to  claim 1 , comprising a heating device to heat the wiper in order to melt ink fluid that has accumulated on the wiper, thereby removing the ink fluid from the wiper. 
     
     
         14 . An inkjet system comprising:
 a print head assembly with at least one print head, the at least one print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the at least one print head towards a substrate; and   the maintenance unit according to  claim 1  to perform maintenance on the at least one print head.   
     
     
         15 . The inkjet system according to  claim 14 , wherein the wiper of the maintenance unit is moveable between an operational position in which the wiper is able to perform a wiping action with respect to the at least one print head, and a non-operational position in which the wiper is arranged at a distance from the print head assembly, such that the maintenance unit is not interfering with normal printing activities. 
     
     
         16 . The inkjet system according to  claim 15 , wherein a printing direction is defined which corresponds to a direction in which substrates pass the print head assembly for printing purposes, and wherein the maintenance unit is moveable in a horizontal direction perpendicular to the printing direction. 
     
     
         17 . A method to perform maintenance on a print head of a print head assembly, said print head being an integral unit configured to eject droplets of ink fluid from nozzles arranged in a surface of the print head towards a substrate, said method comprising the following steps:
 providing a wiper which is moveable along the surface of the print head to remove ink from said surface;   moving the wiper along the surface of the print head while pressing the wiper against the surface of the print head with a force actuator;   determining a wiping force with which the wiper is pressed against the surface of the print head by the force actuator; and   driving the force actuator based on the determined wiping force in order to press the wiper with a predetermined wiping force against the surface of the print head.   
     
     
         18 . The method according to  claim 17 , further comprising the following steps:
 urging the wiper to an unreachable position inside the print head with a force actuator while moving the wiper along the surface of the print head; and   while urging the wiper to said position, keeping the maximum by the force actuator applied force below a predetermined value.   
     
     
         19 . The method according to  claim 17 , wherein ink fluid is purged out of the nozzles prior to moving the wiper along the surface of the print head.