The negative impact of uneven calendering on web properties and strength Frederic Parent FPInnovations, Pointe-Claire, QC, Canada Location: Date: Myrtle Beach, SC October 2012
Outlines Purpose of calendering How does non-uniform calendering affect web properties Strength uniformity vs web breaks Roll grinding performance measures vs calender rolls out-of-roundness Case study: how changes in grinding techniques led to round rolls and reduce variability (and web breaks)
Why are Calenders Needed? Develop surface properties Control thickness Build the reel Thickness reduction = Densification Surface replication = Smoothing
Calender Stacks - Example Paper machine calender Heated Variable crown Driven On-line or off-line 1 to 12 nips or more Nip load: 50-200 kn/m Temp.: 40 to 100 C One or more stacks
The Importance of Getting Uniform Properties Improving uniformity: Less web breaks (less weaker zones) Less quality issues Less customer complaints/claims The most affected properties by non-uniform calendering Gloss Thickness Strength 5
What Controls Web Breaks? Local strength of web Press Tension variation Press Tension Distribution Risk of web breaks! Web Strength Distribution Web Related Factors Press and Converters Related Factors Average MD Strength Strength Variation Defects (wrinkles, holes, ) Tension Tension Variation
What is Strength Uniformity? Many webs (like paper) are non-uniform material Tensile strength tends to follow Weibull distribution F(T)=1 exp[ (T / Ts) m ] m factor Weibull modulus (m factor) is strength uniformity parameter The higher the m value, the better the strength uniformity Tensile Distribution Higher uniformity m=20 Lower uniformity m=12
Web Break Rate vs. Strength Uniformity 4 Break Rate (%) 3 2 1 0 12 14 16 18 m-factor Worse Better 8
Quantification of Strength Variability Tensile strength analysis are completed with Instrom apparatus Tensile tests at 1 m interval in MD Characterize average strength and strength uniformity Calculate MD tensile, MD stretch, MD TEA and strenght uniformity factor Benchmark strength properties against other mills (in-house database)
MD Tensile Variations 10
Quantification of Properties Variability Tapio is a high accuracy and high resolution apparatus that measures web properties in MD (and CD). MD resolution: 12.8 mm Property measurement of: Basis weight (30 g/m2-800 g/m2) Thickness (up to 500 µm) Ash content (g/m2) Gloss top and bottom (%)
Thickness Variability Due to Barring 100 MD caliper Caliper (micrometers) 90 80 70 60 50 0 10 20 30 40 50 60 70 80 90 100 MD length (inch) 600 500 400 FFT of MD caliper Frequency associated to calender barring Wavelength = 2.1, Amplitude 10-15 um (0.0004-0.0006 ) Power 300 200 100 0 0 1 2 3 4 5 6 7 Frequency (cycle/inch)
Strength Uniformity vs Calendering In our research, it was found that strength uniformity is mostly affected by pulp furnish variations and by calendering operations Calender rolls roundness after grinding is an issue for many mills
Rolls Regrinding Performance Measures To Get Round Rolls Example for a modern high speed paper machine: Concentricity within 0.0002 Roundness within 0.0002 Residual barring amplitude less than 0.00005
Quantification of Roll Roundness LVDT value Round roll Angular position LVDT value Oval roll Angular position LVDT value Periodic signal Roll with barring Angular position
Round vs Oval Profiles Measured Rouleau 46 avant Rouleau 13 arriere Rmax-Rmin = 0.00021 Rmax-Rmin = 0.00186
Roll Surface With and Without Barring With barring Roll CS-543 : front Line spacing = 0.0001 in No barring SC11 Roll 5 : front Lines spacing = 0.0002 in
Case study: Significantly Lower Strength Uniformity and Higher Web Breaks 30 m-factor 20 10 0 Mill AB Competitor 1 Competitor 2
Where Do The Variations Come From? 90 épaisseur mp14 lisse MD caliper measured in lab Caliper épaisseur (um) Um 85 80 75 70 65 1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 pouces inches 1.5 FFT analysis shows that tensile variation is related to the caliper variation created by calendering Power Density 1.0 0.5 Variation in caliper and tensile every rotation of the King roll Tensile Caliper 0.0 0 10 20 30 40 Frequency (Hz)
King Roll Shell Thickness Variations Inner shell measurement Outer shell measurement The thickness of the shell varies, from 0.004 to 0.007 because of the inner shell eccentricity Variation Shell thickness d'épaisseur (millième de po) variation in inches 0.004 0.003 0.002 0.001-0.001-0.002-0.003-0.004 0 Variation d'épaisseur de la coquille (6 po avant) 0 10 20 30 40 50 60 70 80 Temps (sec) Time (sec)
Correction of Inner Shell Eccentricity 0.004 Mesures initiales 2 juin 0.003 STD grinding procedure Amplitude (po) 0.002 0.001 0-0.001 avant centre arriere -0.002-0.003-0.004 0 10 20 30 40 50 60 70 80 Temps (sec) Mesures 15 juin - final 0.004 Modified grinding procedure with application of performance measures for regrinding Amplitude (po) 0.003 0.002 0.001 0-0.001-0.002-0.003-0.004 0 10 20 30 40 50 60 70 avant centre arriere Temps (sec)
Strength Improved After Re-grinding Calender Rolls (No More Cycle) 400 375 Before After 350 325 300 275 250 225 200 1 16 31 46 61 76 91 106 121 136 151 166 181 196 211 226 241 256 271 286 301 316 331 346 361 376 391 406 421 436 451 466
Strength Uniformity Improved After Re-grinding Calender Rolls 25 20 Before After m-factor 15 10 5 0 11/02/09 11/05/09 11/08/09 11/11/09 11/14/09 11/17/09 11/20/09
Feedback From Customer: 50% Reduction in Web Breaks 8 Mill B Break Rate (%) 6 4 2 0 Before After
Summary Calender rolls out-of-roundness can negatively affect web properties Up to 20-25% variations in caliper and gloss Up to 15-20% variations in strength Significant strength variations increase web breaks Quality and performance issues Customers complaints/claims/loss Calender rolls re-grinding: Calender rolls must be reground with care to achieve roundness and concentricity tolerances required by the process
To reach us: Frederic Parent Tel: 514-782-4653 E-mail: frederic.parent@fpinnovations.ca Web site: www.fpinnovations.ca 2011 FPInnovations. All rights reserved. Copying and redistribution prohibited. FPInnovations, its marks and logos are registered trademarks of FPInnovations.