Thursday, 20 March 2014

Doctor Blades

One of the major components in the printing set up at Burleigh is the 'doctor blade'. Currently, a very thin steel blade is attached to the heavy, metal housing. This the sits along the top edge of the press, with pressure being applied along the back edge of the copper roller. The thick ink is applied (cold) along the length of the heated roller which, as it rotates, pools the warmed ink into the cavity between roller and blade. As the doctor blade is locked into place, it then does a second job of wiping the surface of the inked roller clean, leaving the ink deposited only in the positive marks.

A steel blade set up to print a copper roller (chrome faced) Here, we are checking the angle of the blade so that we can mimic it on the second press.

Although this set up (using a steel blade) is working, we fear that it is contributing to the increasing damage that is frequently appearing on the chromed surface of these engraved rollers. Also, the steel would cause havoc with our newly laser engraves rubber sleeves, so over the course of the project, we have been testing alternatives.
A selection of the blades which we have tried. On the top, is the steel which is currently used. Below are a selection of commercial alternatives (other metals with various angled edges and coatings), plastics, rubbers and urethanes.


















      































We began by trying a variety of shore hardnesses of urethane 'squeegee' blades, as used in screenprinting applications. The harder the shore, the cleaner the rubber would wipe. However, the thickness of these was problematic as it altered the angle that the blade met the roller, causing it to jump around during printing.
Squeegee blade on rubber roller. The blade is sitting too high.

Resulting prints from initial trial with squeegee blade
After sourcing thinner lengths or hard shore urethane (this time with various profiles), we returned to Burleigh to conduct more trials. The results were promising, however the press which we used is the less favoured one within the factory. We realised that the scraper bar housing on this press leaves less room for adjustments to the angle and pressure of the blade. Once again, this blade is too high up the roller

Triple shore urethane on rubber roller. (2nd press)
On our most recent trip, we were able to use the 'favourite' press, with our improved roller (with metal core) and triple shore urethane blades. By setting the angle and pressure as we had learned from previous trips, we were able to pull successful prints.

Resulting print from urethane blade on favoured press.


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