ENGRAVED-ROLLER PRINTING; COLOUR VARIABLE PRINTING

Two-colour engraved roller printing machine

Principles of operation of engraved roller printing:
A traditional two-dye engraved roller printing machine is shown diagrammatically in Figure. Each engraved cylinder (F) of roller printing machine, mounted on a steel mandrel (L), is forced during printing fabric as it travels around a pressure bowl (A) of printing machine with resilient covering (B). The engraved roller printing machine must be of robust construction because pressures of several tonnes are applied and each mandrel is driven by a single large crown wheel. As the rollers rotate, a furnishing roller (G) transfers print paste (colour) from a colour box (H) to the engraved cylinder, filling the engraving and smearing the whole surface. This surface colour is almost immediately removed by the steel blade known as the colour doctor (J). This doctor must be precisely ground, sharpened and set, at the optimum angle and tension, to leave the surface perfectly clean. Engraved areas retain the colour in parallel grooves and the doctor blade is ‘carried’ on the crests between the grooves.

The fabric is then forced into the engraving and most of the printing paste is transferred. The cushion between the pressure bowl and the engraved roller clearly plays a critical role in the uniform transfer of colour across the fabric width and along its length. The cushion is obtained by rolling about ten layers of a lapping fabric with linen warp and woollen weft around the pressure bowl. Any irregularity, including any ridge at the end of the lapping that may develop if it is not perfectly matched with the first end, can cause a visible fault in a critical print. The cushion progressively hardens with use and requires changing from time to time. Because the lapping must be protected from colour printed beyond the fabric edges or forced through the fabric, an endless printing blanket (C) must be used and washed and dried continuously before returning to the point of printing. In addition, a back-grey (D) is used to absorb colour and give greater resilience, unless the blanket provides enough resilience and is able to hold the excess colour satisfactorily. In the printing of lightweight or knitted fabrics the woven back-grey is often ‘combined’ with the fabric to be printed, using small amounts of a suitable adhesive. This helps to maintain dimensional stability under the tension applied during printing.

After transferring its colour, the engraved roller is cleaned by a brass blade known as the lint doctor (K). This removes any loose fibres that may have stuck to the surface of the roller and could become trapped under the colour doctor, where they could cause a colour streak to be printed. The lint doctor on the second (and any subsequent) roller may be seen to remove a significant amount of the previously printed colour that the roller takes up from the fabric. Without the lint doctor, this colour would be carried into colour number two and cause rapid contamination, although some contamination is unavoidable because the lint doctor will push a little colour into the empty engraving.

The printed fabric is now separated from the back-grey and blanket and carried on to the drying section of the engraved roller printing machine, avoiding any contact of the print face until some drying has been achieved. In the UK drying cylinders were often used in the past, but hot-air dryers are more universally appropriate.

On a multi colour machine the printing rollers, with colour boxes and other auxiliary equipment, are arranged around a larger pressure bowl with minimum separation of the rollers. Setting and maintaining the correct registration (that is, fitting each colour of the design relative to the others) requires an arrangement for separately rotating each roller a small distance while the drive to all the rollers is engaged. The original box-wheel device was elegantly simple but introduced a serious hazard to the printer’s hands as it required the insertion of a rod, or tommy key, into a hole in a wheel rotating near the meshing drive gears. A safer device has been described by Gleadow, who has also given a more detailed account of engraved roller printing.

It is necessary to correct the pattern fit from time to time during printing, because each roller will have a different effective circumference if there are significant differences in the area engraved or the pressure applied. Thus the fabric will be stretched or allowed to relax to a minute degree between rollers and the fit of the design will gradually be lost. As already noted, combining with a back-grey can improve stability.

Good pattern fitting also requires a perfectly rigid fit of the engraved cylinder on its mandrel. To this end, the cylinders and mandrels are tapered and a forcing jack is required to mount and to remove the mandrel. In addition a key, or ‘tab’, on the inside of the cylinder engages in a slot along the mandrel.