Vacuum Roll Characteristics
Vacuum rolls and belts have a number of characteristics that are potentially useful to converters. What benefits are derived depends on the application, which will be addressed later. A summary list of characteristics follows.
- Single Side Contact – A very popular feature of vacuum rolls is the ability to control tension in a web while touching only one side.
- No nip is used, so a potential source of injuries is eliminated.
- Vacuum rolls can transport a wet web, coated but not yet dried or cured, without touching the coated side.
- Once the coating is dried or cured, it can be kept free from damage or contamination because it is not touched by the vacuum roll.
- Large Tension Differentials – Vacuum rolls and belts can sustain larger differences between Tension In and Tension Out than other technologies.
- A vacuum roll between two sequential process steps in a converting line can enable each step to be carried out at its optimal tension level, even when there is a big difference between the two levels.
- Extremely Low Tensions – Tensions can be minimal since the holding force from the pressure difference does not depend on tensions.
- Low line tensions can be achieved.
- High precision web metering can be implemented.
- Less slippage means more accurate speed control can be provided.
- Web accumulation can be done.
- The web can be held in place when a web break occurs.
- Small Wrap Angle – Compared to “S” wraps, the wrap angle required for a vacuum roll is fairly small for a given tension differential. For belts, wrap angle can be very small. Line layouts can be improved, for better use of space.
- Boundary Air Removal – At higher speeds, boundary air dragged into the area of a roll wrapped by the web can significantly lower coefficients of friction and thereby compromise tension control.
- Vacuum rolls help remove boundary air allowing higher speed operation.