Offset lithographic printing is a cost effective method for generating many thousands of identical full color images. The process provides a blanket, where the surface of the blanket has an image. The process further provides an inking solution in contact with the blanket and the image on the blanket wherein the ink of the inking solution is transferred to the image on the blanket creating an inked image. The inked image on the blanket is then placed in contact with the desired receiver, either directly or after transfer to another surface, where the inked image then substantially transfers to the receiver. This process may be repeated for each of the desired inks in a particular image, limited by the number of stations in the printing process. The finished print is then stacked at the end of the process and allowed to dry and cure.
In typical lithographic printing processes the process generates finished sheets rapidly such that the sheets are stacked while the ink is still wet. This causes a problem of the inked surface of one sheet contacting and adhering or “offsetting” to the backside of the next sheet in the stack. To prevent this problem of offsetting it is common to dust the surface of each sheet exiting the printing process with a particulate that acts to provide a small gap of separation between the sheets. This anti-offsetting powder is typically starch. Coated and surface treated starches are also used in the industry.
Because each image in a typical lithographic print run is exactly the same, prints may then be passed through a digital printing device in order to add unique information to each sheet such as an address, a personalized coupon, or other information or image. The sheet is typically printed first by the lithographic printing process to create a shell with designated locations for the added digital information. Prior to adding the unique information, the print may be referred to as a pre-printed shell or pre-printed media. The pre-printed shell may be stored until the additional information is added, or it may be transferred to the digital printing device as soon as it is dry and cured.
One such digital printing device is an electrostatographic printer using a thermoplastic toner powder and heat and pressure fixing or fusing to generate the unique information. When the pre-printed media is passed through this printer, the anti-offsetting starch powder causes problems by contamination of the systems of the electrostatographic process, particularly in the fuser where the heat and pressure fixing of the toner powder is accomplished.
The fuser of an electrostatographic printer using a thermoplastic toner powder typically comprises a fuser member surface, typically a fuser roller or belt, in contact with the media surface providing heat to melt the thermoplastic toner. The fuser further provides an opposing pressure member surface, typically a pressure roller or belt, pressing against the back of the media providing pressure to adhere the toner melt to the media. To prevent the toner melt from contaminating the fuser member surface, an oiler may apply release oil. Further, to clean any contamination that does occur, the fuser member may provide a cleaning member. A fuser member provides heat either by means of an internal heating lamp, or by external heating such as contact with heated rollers. In a particular method of heating the fuser roller using external heating rollers, the external heating rollers are further used to collect toner contamination for cleaning by a cleaning web in contact with the aforementioned heating rollers.
Contamination by preprinted media takes the appearance of colored stripes on the oiling member and cleaning member. For preprinted media that has been appropriately dried and cured, analysis of contamination material shows that the contamination contains only trace amounts of ink or toner and is primarily composed of starch-like material and release oil. In fact, the majority of the signal that is not release oil is identical to the starch anti-offsetting powder. It is thought that the powder, having been added to the surface while the ink is still wet, has a portion of ink attached providing the colored appearance. It is also thought that the powder acts to transfer the ink to other surfaces providing intimate contact for further contamination and degradation of the surfaces. The ease with which the powder contaminates fuser parts is due to the non-thermoplastic nature of the starch anti-offsetting powder. Since the powder does not soften, it does not melt and easily transfers to the fuser. In addition, where the fuser is heated by contact with external heater rollers, the starch does not efficiently transfer to the heating rollers and the associated cleaning system. This behavior is also thought to be due to the lack of softening or melting of the toner when in contact with the high temperature heater rollers. This behavior further leads to increased contamination of the oiler member. It would be useful to provide an anti-offsetting powder that does not contribute to fuser contamination.