The Basics of Gravure Coatings
The Gravure Coater has become one of the most popular types of coaters in use today due to its ability to apply less coating with more precision than other methods.There are three types of Gravure coaters: direct gravure, reverse gravure, and differential offset gravure. All three apply a uniform amount of coating across and down the web. In addition, each has its own distinct advantages and disadvantages.
The coatweights produced by the gravure roll varies with the type of coater used. The coatweight is dependent upon the coating solids, coating surface tension, gravure pattern, gravure cell depth, coater configuration and operating speed.
Direct Gravure
Coatweight Characteristics:
- Coatweight is determined by the percent of solids and the volume of the gravure cylinder.
- If you were to graph the coatweight vs. gravure speed curve, you would virtually have a straight line.
Reverse Gravure
Coatweight Characteristics:
- The coatweight is controlled by the percent of solids, gravure volume and the speed of the gravure cylinder.
- The graph of coatweight produced by this coater resembles a bell curve.
- If you increase the solids, the peak of the curve is higher. If you decrease the solids just the opposite is true. With lower solids, the reaction to the speed change is minimized.
- The volume does not affect the process as greatly as changing the solids.
Differential offset garvure
Coatweight Characteristics:
- The coatweight is affected by the percent of solids, gravure volume and the speed of the gravure cylinder.
- The coatweight curve is a positively sloped line that levels off at some point.
- The degree of slope is determined by the percent of solids (the higher the solids, the steeper the slope.)
- The slope is the result of increasing the speed of the gravure cylinder; the faster the roll turns, the higher the resulting coatweight.
- The line will eventually start to drop off if the speed is increased to the point where the gravure cylinder is not given enough time to fill the empty cells.
Additional Factore
No matter which Gravure Coating head you use, there are a number of additional factors that affect the system's performance.
- The precision of the gravure roll process is dependent upon:
- support equipment
- doctoring mechanisms (enclosed, plowing, wiping, oscillating, blue steel, stainless steel, composite, plastic)
- engraving process (mechanical or laser)
- gravure roll covering (chrome plated or ceramic)
- coating application (pump or pan)
- type of backing roll system.
- The support equipment can also affect the running conditions of each coater. For instance, the coating application systems can determine the line speed, coating appearance, slinging, solids dispersion, foaming, amount of coating needed to operate the system, control of solvent flashoff, and other variables.
- The tolerance of the coating rolls will affect the final product specifications. Factors include: TIR, roll straightness, rubber hardness, use of a backing roll or hold down assembly, RA finish of the rubber roll, the type of rubber used, the durometer of rubber, type of bearings, cleanliness, maintenance and many other specs.
BC Publications is a technical newsletter designed to provide operational and maintenance information to improve the performance of your Black Clawson Converting equipment.