Introduction to eight different Spreader methods [Animation Demonstration]

This article focuses on eight different coating methods, including gravure coating, reverse roll coating, knife roll coating, metered rod coating, slot die coating, dipping, curtain coating, and air knife coating.

NOTE: Technical Coating International coats and laminates primarily using the direct (or positive) gravure printing process shown below.

gravure coating

The gravure coating process relies on an engraved roll running in a coating bath that fills the roll's engraved dots or lines with coating material. The excess coating on the drawdown blade is scraped off by the drawdown blade, and then the coating is deposited on the substrate as it passes between the engraved roller and the pressure roller.

reverse roller coating

During this process, paint is metered onto the applicator roll by precisely setting the gap between the upper metering roll and the applicator roll below it. The coating is "wiped" from the applicator roll as it passes over the bottom back-up roll. The diagram illustrates a 3-roll reverse roll coating process, although 4-roll versions are common.

                                                    Gravure Coating Reverse Roll Coating

Knife Roll Coating "Gap Coating"

The process relies on the coating being applied to the substrate and then passed through a "gap" between a "knife" and a back-up roll. Scrape off excess as the coating and substrate pass. This method can be used for high viscosity coatings and very high coating weights such as plastisol and rubber coatings. There are countless variations of the fairly simple process, robust, diligent and somewhat imprecise.

Metering Rod (Meyer Rod) Coating

During this coating process, excess coating is deposited on the substrate as it passes over the bath roll. A wire wound metering rod, sometimes called a Meyer rod, allows the desired amount of coating to remain on the substrate. The quantity is determined by the diameter of the wire used on the rod. This method is very amenable to non-precision engineering of other components of the coater.

Slot die (slot, extrusion) coating

In the slot die process, the coating is extruded through the slot and onto the substrate either by gravity or under pressure. If the coating is 100% solids, the process is called "extrusion", in which case the line speed is usually much faster than the extrusion speed. This makes the coating much thinner than the width of the groove.

                                                                Metered Rod Coating                                                         Slot Die Coating

Dipping (Dipping) Coating

In this simple process, the substrate is dipped into a coating bath, which is usually of low viscosity to allow the coating to return to the bath as the substrate emerges. This method is often used on porous substrates.

Curtain Coating

In the curtain coating process, a bath with grooves in its base allows a continuous curtain of coating to fall into the gap between two conveyors. Objects to be coated, such as doors, pass along the conveyor at a controlled speed and thus receive the coating on their upper surfaces. Smelly and crude versions of these machines were once common.

                                                                       Dip Coating Curtain Coating

Air knife coating

A simple process where the coating is applied to the substrate and excess paint is "blown off" by a powerful jet of air knife. This method is commonly used in water-based paints and is particularly noisy.