Printing Units Research Articles

In press conditions and their effect on runnability and convertability in digital package printing

A major advantage of digital printing is that printing can be performed on demand, but like all printing techniques it suffers from specific runnability problems. This study reports on an investigation of problems during printing and how to improve the runnability, with the aim of pointing out the most critical factors. Test printing has been performed on three different paperboard types in digital print units, based on both dry and liquid toner electrophotographic technology. The results show that the three main problems during printing were feeding in sheet-fed print units, cutting in web-fed print units, and increasing curl after printing. Feeding problems in sheet-fed print units were mainly explained by curl and varying format size of the paperboard, but also by exceeding thicknesses limits for the press. Curl after printing principally occurred due to high temperature loads caused by, for example, fusing and conditioning. Copyright © 2005 John Wiley & Sons, Ltd.

Finishing of Digital Prints – A Failure Mapping

The new possibilities with personalized and on-demand printing are strongly dependent on successful finishing. Today, however, the finishing is often seen as the week link in digital printing. The area is in great need of improvements to attain higher quality and cost effectiveness. A research study, based on a market analysis, was performed in close co-operation with the graphical industry. The study includes tests of different folding techniques and its function in a Print-on-Demand production process. The objective is to analyze the printed papers fold quality and functionality in the finishing process.Three different digital printing units, using the xerographic technique, were tested. Uncoated and coated paper qualities of stock weights above and below 150g/m2, developed for digital printing, were used in the study. The folding tests were performed 1-2 hours after printing. A single fold was applied to each printed test image, using two different folding machines and several folding strategies. The tests were performed in a climate–controlled laboratory, where the settings 20°C and 30%, 50% and 70%RH were used.Results show substantial differences between different folding techniques. The use of creasing was of vital importance in all cases. The results also indicate a diverse function of the tested digital printing units, most probably explained by dehydration of the papers in the printing process.

How to Measure Dimensional Stability

Dimensional changes in multicolour offset printing may lead to runability problems because of water uptake in the paper structure depending on the time elapsed between printing units, web tension and the paper grade. Traditionally this paper property has been characterized as “hygro-expansion” calculated from the elongation of specimen strips exposed to different levels of relative humidity at equilibrium. These “static” values obtained at equilibrium conditions may not correlate at all with experienced runability problems as these problems occur only for dimensional changes appearing while the paper runs inside the press between the printing nips. This paper describes “dynamic hygro-expansion”, where moisture is picked up from water vapour (%RH) and “dynamic hydro-expansion”, where moisture is printed onto the paper surface similar to an offset printing press. Results reported from different newsprint grades as well as coated papers reveal the advantage of describing in-plane dimensional problems as part of a dynamic process.

Multimotor electronic line shafts for rotary offset printing presses: a revolution in printing machine techniques

In rotary offset printing presses, mechanical line shafts are being replaced by electronic shafts which consist of synchronized position and speed-controlled AC motors driving the complete printing units or single printing cylinders. The dynamic stiffness of the multimotor system to satisfy the requirements of high printing accuracy can be achieved by high-dynamic, cascaded current, speed and position control of the the drives, which can be combined with disturbance observers in critical cases, or by direct drive technology.

A Computer Technique for Illustrating Three Variables in a Pictogram

A technique is described for displaying three—dimensional data by using the overprinting capability of digital computer printing units. The procedure is illustratd by mapping the distribution of species—abundance characteristics and physical features on several study areas based on data from systematically placed field sampling points.