CRINK
………is a word coined to describe the appearance of the surface fibre of folding box board, when it has been over dried and caused to shrink prematurely. This surface fibre then becomes compressed by the effect of the inner sheet shrinking, which causes the surface to take on the appearance of surface creping at a micro-level.
COMPACT SOLUTION – PREHEAT
One option is to preheat over the first two cylinders after the presses if there is room.
The specifically developed shortwave lamp used in the Compact IR heater produces energy that penetrates the sheet to a depth of between 350 and 550 microns, depending on sheet density and formation and only heats the fibre and not the water, so that as the fibre through the sheet heats up and transfers the energy to the water that provides the vapour pressure differential between the moisture in the middle of the sheet and the surface to cause the moisture to move to the surface. This enables a better transfer of heat back into the sheet to provide the latent heat of evaporation. Or put another way, in this instance the Compact IR is only used to bring the moisture to the surface and not provide evaporation.
Because the energy is applied after the presses there is no loss of bulk as there would be if the presses were to run at 65°C (150°F). The movement of moisture from the middle of the sheet to the surface creates a better ‘Z’ direction profile and helps to cause a more even shrinkage profile through the sheet thickness and improves the energy transfer from the cylinder surface to the sheet, thereby helping to prevent surface fibre premature shrinkage and improving cylinder thermal efficiency.
With post press solids details we can provide exact expectations from the preheat, but roughly on a 50% solids sheet, providing a sheet temperature rise of 25°C (45°F) would give about 9% increase moisture removal from the same cylinders.
COMPACT SOLUTION – SIZE PRESS PROFILE CONTROL
A further way is to control the profile at the size press. The common way for papermakers to control profile at the size press is to over dry, going down to 2% and lower and while this provides some success on the lighter weight papers, it is not as successful on board. With board, where the bulk is greater and the moisture is more difficult to get to, the surface fibres degrade due to being over dried and consequently internal shrinkage occurs. There is still a poor moisture profile in the middle of the sheet, where the last vestiges of moisture lie. This in effect provides uneven size hold out, even when using a metering bar size press.
We recommend Compact IR over the last top and bottom drying cylinders before the size press to provide moisture profile control and allow the average into the size press to rise to between 3% and 5% with a maximum peak to peak of 0.3%. (Not 2 Sigma) Typically the IR input capacity would be sized to remove 3.5% peak to peak spreads.
In this position the Compact IR will penetrate the sheet to the centre from both sides and cause the centre moisture to move to the sheet surface to evaporate and cool it, before the surface fibre has been over dried, thereby presenting a sheet to the size press with a flat CD and ‘Z’ direction profile at a precise average. This average could then be controlled to provide repeatable and ideal pickup conditions to suit the surface strength requirements of the sheet without Crink. As the damper sheet surface has better and provides more even absorption for the size that an overheated one, it will improve the surface quality. There might even be a saving on starch.
Typically a system to remove 3.5% moisture at the size press on solid boxboard, would use about 40 kW/ton and give a production tonnage increase of between 8% and 12% depending upon grade and profile at an average of 4% before correction.
Correcting the CD and ‘Z’ direction profile at the size press would eliminate most of the poor profile going to the machine calenders and would give a more layflat sheet so that less curl correction would be necessary after the size press, so thermal efficiency would be improved and Calliper spread would be reduced.
When profile control is applied before the size press there may be a need to apply some Compact IR at the calenders to make up for a lack of drying after the size press on some of the lighter weights boards and to accurately control the final moisture average and moisture related calliper profile immediately before the calenders.
COMPACT SOLUTION – WHERE THERE IS NO SIZE PRESS
PRE CALENDER PROFILE CONTROL
Where there is no size press then the above solution equally applies and Crink will be much reduced. Further, there will be significant improvement in calliper spread, Bendtsen / Sheffield smoothness and the layflat properties of the finished sheet.
The elimination of CRINK has to be a major priority whatever its cause, if quality is to be improved.
