Supercalender process conditions, variables and grinding

Below are some general notes on supercalendering, covering process variables and conditions that affect supercalending as well as grinding recommendations for supercalender rolls.

Valmet has some exciting polymer and composite roll coverings used in demanding soft nip and supercalender positions. These include the Valmet Calender Roll Cover CJ, CL, CP and Eco, which all provide excellent sheet quality and service life. More can be learned about them in the Related Links of this article.

Process variables affecting production of SC paper

Surface temperature

Higher temperature pasticizes fiber on the surface, leaving the middle layers elastic. High temperature affects sheet moisture, so incoming moisture must be higher than reel moisture. Less pressure is required when using high temperatures.

• For every 10 °C (50 °F) increase in temperature, gloss increases by 2 points.
• Fore every 8.8 °C (47.8 °F) increase in temperature, PPS roughness decreases by 0.1 µm.

Moisturizing and steaming

In gradient calendering, moisture is added just prior to each stack. This helps keep moisture from the machine section low, which also helps with blackening and optical properties. Steaming affects sheet glazing and gloss development.

• For every 1 point increase in gloss, blackening index increases by 1 point.
• For every 3.5 ml/min reduction in Bendtsen porosity, blackening index increases by 1 point.
• For every 1.5 g/m² of steam applied to the sheet, Hunter gloss increases by 4 points.

Linear load

Reduces roughness and increases gloss, reduces bulk and opacity, increases blackening index.

• For every 28.6 kN/m (163 pli) of load, Hunter gloss increases by 1 point.
• For every 33 kN/m (190 pli) of load, PPS-10 roughness decreases 0.01 µm.

Number of nips

The more nips, the more effect calendering has on smoothness and gloss. The achievable smoothness level depends on the furnish - GWD is easier to calender than TMP. Gloss is more affected by temperature. Smoothness is more affected by paper structure.

• For every nip point, sheet roughness decreases by 0.04 µm.

Speed

Affects dwell time in nips. Lower speeds increase dwell time and therefore improve paper surface properties. Higher speeds will roughen paper at a constant linear load.

• For every speed increase of 360 m/min (1170 fpm), gloss decreases by 1 point.
• For every speed increase of 500 m/min (1640 fpm), sheet roughness increases by 0.1 µm.

Roll material

Polymer covers can withstand higher speeds, temperatures and linear loads more than cotton filled rolls. The roll hardness (modulus of elasticity) determines compressibility of the web in the nip. Rolls must be hard enough to withstand long runtimes in the machine and also have the ability to be doctored. Roll hardness impacts the nip width; it is recommended to use rolls with hardness values of 90-94 ShD.

Picking

Picking on supercalender rolls can be eliminated by blowing air in the fly roll pockets to remove the humid air.

Process conditions that affect supercalender polymer roll life:

• Wrong crown: Mill personnel must check the roll crowns after the rolls get up to operating temperature.
• Stacks running unheated rolls (cold ends): Rolls can be larger in the middle vs. the ends of the rolls.
• Stacks running heated rolls (end failures): Paper does not carry the heat away from the edges, therefore the polymer roll edge temperature increases. This increases the diameter and thus the pressure applied to the roll ends - and the roll fails. Mill personnel must machine a relief zone just outside of the sheet run. Micro taper of 0.036 mm / 100 mm (0.0004 inches / inch).
• Intentional heating: This is the use of inductive heaters or air showers to control cool or hot zones of the rolls.
• Abrasive wear: Coatings can be abrasive. Polymer rolls wear more in the center of the sheet than at the ends. They can form relatively high spots outside the sheet run, which contact the mating roll surface and cause localized high niploads.
• Bad nip profile: Polymer rolls must be ground to very tight tolerances 0.018 mm (0.0007 inches).
• Profile, marking and objects through the nip: These create significantly higher nip loads, many over the limits of polymer covers.
• Thin covers: Worn rolls will have less of a cushion in the nip and will act stiffer. The internal stresses will be greater, and less forgiving to profile errors.
• Cover selection: Materials are a mix of polymers, fillers and reinforcements. The maximum temperature of a roll is determined by its glass transition temperature (thermoset, amorphous) or melt temperature (thermoplastic, semicrystalline) of the matrix.

Calender roll grinding recommendations:

• Steel rolls: 0.007 to 0.015 inches diameter reduction, more if banding is present.
• Polymer rolls: 0.015 to 0.030 inches diamter reduction, more if banding is present.

For more information on optimizing your supercalender, contact your Valmet representative.