Misting in the transfer nip of a film coater.

摘要

Paper coating consists of depositing a thin film onto the paper in order to improve its printing and optical properties. Although many processes are available, nowadays many efforts are being made in developing new techniques that allow operating at high speeds reducing runnability problems, and therefore shut downs.;The present work is dedicated to the development of the understanding of the behavior of the fluid flow in the transfer nip focusing on the film splitting process as well as one major runnability issue related to it, misting. For that purpose, a laboratory film coater was developed for the hydrodynamic characterization of Newtonian fluids and suspensions. It consists basically of a deformable roll (transfer roll), a rigid roll (backing roll) and a delivery system including a doctor blade to control the amount of fluid to be transferred. The roll speed, the applied load between the rolls and the flow rate can be easily varied with the film coater.;The pressure distribution of the fluids into the nip was determined by means of a miniaturized dynamic pressure transducer mounted on the backing roll. This transducer is connected to a data acquisition card via a slip ring, so that the pressure data are continuously recorded. Several pressure profiles are obtained for every roll revolution.;The flow downstream the nip was visualized by means of a high-speed video system, which consists of a high-speed CMOS camera having a full resolution of 1024 x 1024 square pixels and a diode copper vapor laser as illumination system. The maximum record speed is 462 frames/s and a total of 512 images can be stored at full resolution.;The shear rheological properties of the tested fluids was determined with the AR-2000 rheometer from TA Instruments in a Couette configuration of 30 mm and 28 mm for the inside and outside diameter, respectively. The extensional properties of the suspensions were evaluated by means of an orifice flowmeter based on the pressure entry technique, in which the extensional viscosity is a function of the flow rate and the pressure drop data of the tested fluid passing through a small orifice.;This investigation has shown that misting can be generated by two different mechanisms: film splitting and a spray effect due to air entrainment into the nip. Spray was found to occur with a soft deformable gap at high speed and a low load is applied between the rolls. The air entrainment can be avoided if the applied load is increased; however, the flow can become periodic. (Abstract shortened by UMI.).