Vacuum Roll Coating
Vacuum rolls (sometimes called vacuum drums or suction rolls) and vacuum belts have been growing in popularity in converting applications. The essential functions of these devices are tension control and/or web metering. But why use a vacuum roll instead of “S” rolls or nip rolls? At what points in a web processing line should vacuum devices be considered?
To answer these questions, this overview begins with a description of vacuum rolls and belts. Then useful characteristics of these vacuum devices are listed, and typical applications explained. Finally, a summary of design considerations is included.
As shown in Figures 1 and 2, a vacuum roll is typically a driven hollow steel roll with a large number of holes. A vacuum is applied inside the roll to pull the web against the roll surface through the required wrap angle. Deckles may be used inside the roll to adjust the vacuum zone to match web width.
Porous drums or porous inserts in a steel roll have also been used. Special concerns for this approach are that typically a higher vacuum is needed and there is a tendency for the porous material to plug up with dirt over time.
A vacuum belt is quite similar. In this case, a belt with a large number of holes is used. It runs in a loop over two rolls, one of which is driven. A vacuum belt allows tension control even when very little or no wrap angle is available. Internal deckling may be included to adjust for web width.