Blog Archives

Part 1: Conditions of the Reverse Roll Coater

Meeting the Requirements of the Reverse Roll Coater

The use of a coating device known as the "reverse roll coater" is widespread in the coating of various thin films onto a variety of substrates. The metering and applicating mechanisms employed by this type of coater make it adaptable to this broad usage.

To qualify as a true reverse roll coater, two conditions must be met.

The first is the requirement for a reverse metering nip (gap). This occurs when the coating must pass between two rolls whose surfaces are travelng in opposite directions. This is illustrated in...
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Part 2: Two Basic Approaches to Reverse Roll Coating

Nip Fed Reverse Roll Coating vs. Pan Fed Reverse Roll Coating

Reverse roll coating appears in two basic forms - the nip fed and the pan fed. There are various configurations of each that have been deployed over the years, but the current popularity of two basic approaches really stands out today. The first of these is commonly described as the 3-roll nip fed reverse roll coating. The metering and applicator rolls have already been described in Part 1 of this Series. The third roll...
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Part 3: Factors that Influence Defects in Metered Roll Coating

Controlling Ribbing and Cascading in Metered Roll Coating

Part 1 and Part 2 of this series we discussed how reverse roll coating 1) can be used to apply a broad range of coating viscosities, 2) can apply these coatings over a broad range of thicknesses, and 3) can function over an equally broad speed range. Attempts to define limits are only generalizations, because there is an interdependency among all these...
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Part 4: Position of the Wetting Line Affects Roll Coating Thickness

How the Wetting Line Affects Ribbing and Cascading in Roll Coating Thickness

In Part 3 of this series we discussed how to control defects in metered roll coating to prevent ribbing and cascading. In Part 4 we discuss how the position of this wetting line affects the thickness of the metered coating and the ribbing and cascading phenomena. The "wetting line" as illustrated in Figure 5 can be described as the point at which the coating pulls away from the metering roll as it...
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Part 5: Controlling the Position of the Wetting Line in Reverse Roll Coating

Metering Roll Ratio vs. Capillary Number to Increase/Decrease the Wetting Line

In Part 4 of this series we discussed how the position of this wetting line affects the ribbing and cascading phenomena in reverse roll coating. The most important factor in controlling the position of the wetting line is the metering roll ratio. The second most important factor influencing the position of the dynamic wetting line is the capillary number of the coating. The capillary number is defined as:

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Part 6: Affects of Non Newtonian Flow and Viscoelasticity on Reverse Roll Coating

Non Newtonian Flow vs. Viscoelasticity

In Part 5 we discussed how the metering roll ratio and the capillary number control the position of the wetting line in reverse roll coating. Other complications that are likely to be encountered are the rheological considerations of non Newtonian (typically shear thinning) flow and viscoelasticity. Viscoelasticity occurs when a liquid possesses the ability to extend and relax in response to the application and subsequent removal of a shearing force. It is a liquid possessing a...
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Part 7: Using Hydraulic Force to control the Capillary Number and Metering Roll Ratio

In Part 6 we discussed the affects of non newtonian flow and viscoelasticity on reverse roll coating. One other factor affected by both the capillary number and the metering roll ratio is the roll separating force exerted by the coating as it passes through the converging geometry of the metering gap. See Figure 7. This hydraulic force is proportional to the capillary number so it increases with both viscosity and applicator roll speed. At some point, this force can cause the rolls to...
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Part 8: Mechanical Design of the Reverse Roll Coater

Reverse Roll Coaters must be of Uncompromised Precision

In Part 7 of this series on reverse roll coating we discuss the use of hydraulic force to control Metering Roll Ratio and Capillary Number. The mechanical design of the Reverse Roll Coater must be uncompromised when it comes to precision. Rolls must be made of high strength steel or cast iron of sufficient diameter and wall thickness to provide substantial resistance to the gap separating forces encountered...
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