Case Study No. 1
Change on 03/11/2007
Core Mix phase during this particular change of product took less than 7 minutes to complete. Face paper was spliced at 12:43 – 55 seconds after selection of Clay and Vermiculite and start of Fibre Glass Cutter. That is enough time for Clay and Vermiculite to reach the Mixer. This change is easily visible as slurry changes colour.
This particular change was pretty messy and control over the “head” on the Forming Table was lost for some time – slowing down of Board-line was reversed at one point to deal with the massive overflow of the slurry over Dam assemblies (12:39:09 to 12:39:54)
Due to problems with the “head”, splice and change of length (4.2 m to 2.4 m), total of 16 boards of 2.4 m were rejected at the Wet End Transfer. This created additional gap of 2.4 m between boards in the Dryer. Not all of these boards had to be rejected but operators involved were not aware of the impact on the drying process this excessive gap will have.
Board manufactured between 12:33 and 12:40 was rejected at the Dry End due to calcination (poor bond and soft edges).
It took 19 minutes and 31 second from first change of Board-line speed to change of Dryer Chain speed. This delay is partly due to the excessive Wet End reject delaying arrival of FR board to the Dryer.
Actions related to the Drying phase of this change took some 1 hour and 39 minutes to complete. Numerous adjustments to natural gas output were made during this change as well – mainly in order to deal with excessive difference between set point and real temperature caused by excessive gap in the Dryer.
Attempts were made to compensate extended drying time and excessive gap in the Dryer but it is fair to say these were not timely or adequate.
Considering how big gap in the Dryer we had to deal with, I have some doubts that losing of product was avoidable but more aggressive adjustments could minimise it.
Chart below presents Dryer temperature profile during this product change. It is important to note that temperatures for TIC9000 and TIC9020 have barely changed during this process (due to the lack of decisive changes to temperature set points). On the other side, changes of TI9000, TI9001, TI9020 and TI9021 were quite substantial. This was caused by combination of factors – 500 mm gap created during Core Mix phase and in particular by introducing excessive (2.4 m) gap created by rejecting boards at the Wet End Transfer. Note that temperature at the exit of the Dryer (TI9021) was in excess of 120 degrees C for quite some time.
It is fair to say that this particular product change was not executed well in either Core Mix phase nor in Drying phase. During Core Mix phase we lost control over the “head” on the Forming Table and that introduced an element of panic and rushing with other changes (reversing process of slowing down etc). Overflow over Dam assemblies affected board edges and operator rejected boards with this defect as it is pretty much standard response to this type of situation in our plant. Not all boards had to be rejected at the Wet End Transfer but operator is not to be blamed for doing it as he was not made aware of the importance and impact that gap in the Dryer causes on the Drying phase.
Changes related to the Drying phase were not enough to compensate for extended drying time. It was probably impossible to compensate for the excessive gap but it was possible to compensate for extended drying time by more aggressive changes.
One positive element of this product change was that we became aware of possible consequences for quality of the product and decided to perform additional testing. Faulty product was identified and prevented from being released to the market.