My invention relates to a rotary screen printing machine comprising a cylindrical thin walled stencil provided with an internal squeegee for pressing the printing paste through the stencil, the squeegee consisting of a thin resilient metal strip which is secured in a mounting, such a device is known from applicant's copending U.S. Patent application Ser. No. 383,155 now U.S. Pat. No. 3,933,093 issued Jan. 20, 1976.
The use of a metal squeegee marks a great advantage in this technique. The uniformity of the printing work was improved thereby and it was furthermore possible to vary the angle between the squeegee strip and the inner wall of the stencil over a greater interval. Also a noticeable saving of weight was achieved which in practice is of importance when the squeegee is mounted and dismounted.
In use of this new squeegee type a sympton is brought to light which so far has not been observed. This sympton consists of a vibration of the squeegee strip which is produced in a situation of so called marginal layer spreading between the squeegee strip and the stencil. This situation arises when the pressure in the dye paste is insufficient to lift the squeegee strip far from the stencil. The aforementioned marginal layer spreading is produced when the distance between the squeegee strip and the stencil is in the same order of magnitude as the unevennesses consisting of the roughness of the stencil and the squeegee blade.
In case of marginal layer spreading the sympton occurs that the force of friction increases when the relative velocity between the squeegee and the stencil decreases. The squeegee will in that case be slightly pulled along in the direction of the rotating stencil. The force of retention of the squeegee support increases and as a consequence of its elasticity, the squeegee blade moves back again, while the relative velocity between the squeegee blade and the stencil increases and the friction decreases. As soon as this return movement is terminated the relative velocity decreases again and the friction increases. As a consequence a vibration is produced which causes undesired variations in the printing intensity.