Coating Fluidity
Flow characteristics describe how a sizing or coating suspension reacts to the shear forces present between a metering roll and the paper surface. Understanding film press rheology is essential for ensuring that the liquid transfers evenly without creating streaks. This property is governed by the solids content, the temperature and the specific polymers used in the formulation.
Application Speed
The viscosity of the coating under high shear conditions determines the maximum speed at which a machine can operate without defects. If the fluid becomes too thick, it may skip or leave an uneven pattern on the board surface. Modern film press rheology involves the use of thickeners that maintain stability while allowing for thin layers at high velocities.
These additives prevent the suspension from splashing or leaking at the edges of the application zone. Maintaining the correct flow profile ensures that the coating weight remains within the specified tolerance.
Surface Absorption
The way a coating interacts with the base sheet depends on the balance between its viscous and elastic properties. Controlled film press rheology prevents the liquid from soaking too deeply into the fibres, which preserves the barrier properties and the brightness of the coating. If the suspension is too mobile, it penetrates the pores and fails to form a continuous film on the top of the sheet.
This interaction is a primary factor in determining the printability and ink holdout of the finished packaging material.
