Inspired by Star Wars and Harry Potter 😊
It has been a very strange year for all in 2020. But also, an interesting start in 2021 to have some time to reflect on post-warp.
We all know how to handle warp, this within the limitations of the material combinations used for a certain corrugated composition and within the corrugator parameters we can control.
If we have a corrugator for running heavy BC for fruit and vegetable, it will be more challenging to make a flat low grammage E flute….
This was a choice made at a time the corrugator was specified to have massive preheaters and maybe prolonged hotplate sections to have enough heat to run at higher speed. Perhaps also glue rolls with an engraving that spits a lot of starch in an uncontrolled way.
If we have a corrugator with parallelism issues also S -warp or irregular CD warp will make our corrugating lives difficult. (You can find in this blog an older article about S-warp)
We are aware of all of this but sometimes people (or dark forces) make our lives even more difficult by sending us green rolls, rolls with humidity profile issues or thickness issues (tension). These can convert into all different interesting shapes except perfect flat board.
And then one day we tackled all the above challenges, and we are running perfect board. At least that is what comes out of the corrugator, right after corrugating.
On good days, the board stays flat until delivery at the end customer, but on other days the flat board seems to be influenced by external forces 😊 and turns into a warped board nevertheless it was flat right after corrugating. This is also known as post-warp, or in Jedi language: “darth vader-shaped”. The force awakens!
To understand this effect, one needs to gather more data about the absolute moisture content of the board coming out of the corrugator and the influences of the climate around the corrugator in the transportation zone in stock or work areas, or even in the warehouse of the customer who may be miles away from the corrugator. It is always interesting to know the absolute moisture of your board coming out directly after corrugating. Is it dry or is it wet or to wet? Is it 4% or7%, or is it 9.5%?
Too dry and you get cracking issues in converting or/and the shape of the sheets can change if stocked in a very wet area.
More commonly it will be too wet at production and the shape of the stack will adapt to the humidity of the environment where it is transported or stored, hence the sheets may change in shape. Moisture will get out of the sheets and condensate at the edges of the stacks which are colder than the inside of the stack, sometimes creating honeycomb at the edges of the sheets making it impossible to print a nice solid.
Too wet board will lose part of its strength properties and will be difficult to feed into further converting and in addition the shape can and will change!
The board that comes out at 70%C with an absolute humidity of 9.5% will be shocked by the climate that most probably will try to abruptly dry out the board and bring it to an equilibrium of around 7%. One can clearly see this happening very quickly with the top sheets warping up. Depending on the circumstances this can change the complete shape of the stack to an extent that it cannot be converted anymore.
Out of experience it is good to keep the absolute moisture of the board as close as possible to the absolute moisture of the top liner. If in single wall the topliner’s absolute humidity is 7% and the corrugated board’s humidity is around 7%, then the dimensions will stay stable and the top liner will create a lesser amount of wash boarding and honeycomb.
When the separate layers of the single wall have a big moisture difference i.e. liner 6%, fluting 9% and inner liner 8%, a lot of work will be done on the corrugating settings, glue-amounts and preheating to get a flat sheet out of the corrugator. Internal tensions will be created in the sheet as the 9% material will dry out to maybe 7.5%, the liner will get more wet to 7.5 % and the inner liner will dry out to 7.5% . This means that all three layers will change in shape. Let us hope that all these fighting forces end up in a zero force and produce a flat outcome. Studies revealed that if the difference between the different sheets is 2 % or more it is very difficult to make flat board.
To add complication, we deal with the climatical conditions in transport areas, stocking areas and converting areas… lord Voldemort of the corrugating process changing the shape of the sheets drastically.
Harry potters magic wand
The magic wand to avoid the above problem is to control your plant environment as much as possible and avoid big humidity shocks moving board from one area to another. Keep doors closed in winter times. Avoid air displacement remoistening of area’s that are too dry.
Wand 1: moisturizing unit
Follow and monitor the final absolute humidity of the board coming out of your corrugator.
Wand 2: device to measure absolute moisture by weighing and drying board in a small oven .
Wand 3: Temperature and moisture logger.
Monitor the climate in the corrugator area, transport zones and converting zones by using simple temperature and moisture loggers that provide measurements over time.
These cheap devices are provided with software and allows you to make graphs of your environment.
This drawing is an analysis made in a corrugating plant by placing the datalogger in several areas. The temperature/humidity graph (top) matches the weather graph (bottom) in zone 2. This is the moment when the stacks just came out of the corrugator. You can see a big increase in relative humidity in a first stage but then the stacks cool down and moisturize the environment roughly from close to 40% to above 52 %. During that day, the stacks came out flat and stayed relatively flat because we made sure that the absolute humidity of the stacks was close to 7%.
Some days earlier the same job was run but the outside conditions were completely different, it was freezing cold (zone 1). (Source KMI= Royal Meteorological Institution, showing outside climate for that week). Unfortunately, the data logger was not in place at this time, but I guess it would have measured an extreme dry environment as the building would be heated up to fight the outside cold. As an example, my home office is 22°C when it is freezing outside, and the relative humidity drops to around 30%. Big temperature and humidity differences have a big influence.
Most probably the stacks being produced that day where closer to 8.5% absolute humidity. The board being too wet and the climate being too dry created a lot of post-warp that day.
Sheets produced during climate zone 1, checked after 12 hours
Sheets produced during climate zone 2-3 checked after 12 hours
Day two showing same stacks having post-warp
The Force Awakens (2015)
May the force be with you…… so you can measure monitor and take adequate conclusions based on measured values and statistical data.
Even Harry potter could not do any magic without his magic wand, absolute and relative humidity measuring devices we call them in corrugating language.
At the same time some magic spells from the big book of magic were needed and in corrugating language this is called some knowledge about absolute, relative moisture, dimensional stability of paper .
May the force be with you.
But you know, happiness can be found even in the darkest of times, if one only remembers to turn on the light.” ― Albus Dumbledore.