MAXIMISING CONVEYOR-BELT STABILITY FOR HIGH SPEED, DIGITAL PRINTING

Digital-inkjet printing is the fastest growing print process in the graphics and packaging industries. It is flexible and has the advantage of allowing print-on-demand, reducing lead times for many print products. It does not require a template and allows variable content to be printed onto a range of different surfaces. In addition, technological advances within digital printing have brought about cost-effective short runs, flexibility in versioning and design, and higher resolution with smaller ink-drop size.

In terms of the printing process, inks, printhead design and ink-substrate interactions are all important contributing factors. In tandem, the printing process has to be correctly executed or enabled in a printer with good machine controls, software and machine design. Machine design needs to provide a suitable operating environment for the entire printing process. Aside from protecting the printheads from interferences, it should enable stable transportation of the substrate during the printing process at the given speed range. 

To illustrate this point, IPCO commissioned a study to compare the conveyor-belt stability of steel and plastic for digital printing. The aim was to measure the vibrations (up and down movement) of both materials on a testing printing conveyor of speeds from 30–300m/min. 

The trials were carried out by a research group at the Fraunhofer Institute for Production Technology IPT in Aachen, Germany. The plastic belt tested was made of a typical stiff material used for digital printers and tensioned at 3,000 N, as recommended by the supplier. The IPCO steel belt was 1,600 SM steel grade and was tested under three different levels of tension – 4,600, 5,500 and 6,400 N. While secondary issues relating to conveyor-belt tension did not fall within the remit of the project, elongation can be a potential issue with plastic belts, whereas steel belts are virtually stretch free.

The results showed that at 30m/min, the highest amplitude range recorded for the plastic belt was ~450µm, while figures for the steel belt – at the same speed – ranged from ~130µm to as low as ~85µm. 

At 180m/min, the highest plastic belt amplitude range recorded was ~300µm, while the steel belt – at the same speed – ranged from ~90µm down to ~50µm.

At 300m/min, the highest amplitude range recorded for the plastic belt was ~200µm, while the steel belt demonstrated figures ranging from ~60 µm to as low as ~30 µm.

“Digital-inkjet printing is the fastest growing print process” 

In summary, the steel belt was shown to be 3–6 times more stable (less vibration) than the plastic belt, depending on speed and tension. For the steel belt, it is evident that higher tension meant lower vibrations. But for most printing applications, tensioning of 4,600–5,500 N is sufficient.

From the study carried out by Fraunhofer IPT, IPCO could conclude that the vibration of a steel conveyor printing belt was shown to be as much as six times less than that of a plastic conveyor printing belt. Its key features such as stability, flatness, straightness, durability and ease of cleaning, provide high productivity, high precision and high print quality with minimal vibration. 

This superior conveyor-belt stability performance means that the position of the print substrate is assured at all times and press speeds can be maximised without compromising quality.

In terms of steel belts, typical grades used in digital printing are IPCO’s 1,600 SM and 1,650 SM. Both are made of martensitic stainless steel that exhibits excellent mechanical properties, very good wear resistance and repairability. These steel grades can even withstand heat up to 300º and can therefore be used in certain drying steps. The typical thickness range of steel belts for printing, is 0.4–1.2mm, with widths available from 400mm up to three metres. 

Steel belts for printing may either be solid or perforated, depending on the substrate printed. Thicker substrates such as corrugated board, ceramic tiles or board, require a vacuum to keep them fixed and flat during the inkjet-printing process. Perforated belts allow the suction forces from the vacuum table to fix the substrate to the belt. 

Switching over to steel-belt technology is not simply a matter of replacing the plastic belt. The conveying system as a whole has to be designed to cope with the steel belt tensioning requirements. Steel belts also require effective tracking if they are going to run as straight and true as they are flat and stable. 

The IPCO HST system is designed for high speed applications where precision accuracy is required. Offering tracking accuracy to <0.01mm, the HST is suitable for conveyors running at speeds of up to 25m/s (1,500m/min). 

“HPT is suitable for use on presses with belt speeds up to 300m/min”

Electronic controls continuously measure the belt position and automatically adjust for belt-edge variations. This enables instantaneous adjustments to ensure tracking accuracy at high speeds. MQTT compatibility enables communication with end user, process-control systems. A sensor package consisting of two optical sensors manages the collection of the information required to track the belt with precision at high speeds.

The HST complements the IPCO HPT system, specifically developed for the digital-printing industry. Offering belt tracking accurate to +/- 0.1mm, the HPT is suitable for use on presses with belt speeds up to 300m/min. 

IPCO’s Special Engineering team works with customers on a project basis to ensure that the end solution meets the specific design constraints of the system and that performance and production benefits are optimised for a specific process.