Starch pick-up, machine factors

Note: The size press itself introduces important factors into the surface sizing operation, some of which are variables used in control of the process, and others which are “built-in” features not operationally adjustable.

1. With other factors held constant in the size press, the higher the nip pressure, the less the amount of size picked up since the size solution is then more effectively expressed from the sheet.
— if a change in the rate of size pick-up is required, no change in a single factor or variable, such as viscosity, temperature, furnish or starch pond depth, will effect the pick-up rate as much as that of a nip pressure change.
— the most immediate consideration in making any substantial adjustment in nip loading is the resultant change in nip configuration.

2. The diameter of the rolls has a great affect on the pick-up rate, since the larger the rolls, the greater the nip pressure required to express an equivalent amount of sizing solution.
— the diameter affects the roll crown requirement.
— a roll diameter that is large in comparison to the nip pressure requirement and face width is preferred, so that crowns can be held to a minimum.
— minimum crowns have several advantages because they enable the operator to employ a wider range of nip loading for a given change in roll deflection.
— where the crown requirement for the nip is small, all of the crown can be placed on one roll, resulting in minimal scouring of the roll covers.

3. Rolls may be crowned equally, but this is not a prerequisite for successful size press operation.
— from an economic standpoint, the required crowning is better placed on the soft roll as this roll must in any case be ground more frequently.
— the hard roll has less material for grinding and is more expensive.

4. As the roll cover material is made softer, there is a corresponding tendency toward increased starch pick-up.
— this is due to the wider nip line produced that effectively reduces the contact pressure in the nip.