Core Mix Phase of the change
From the moment of first change to the Board-line speed to the moment of last change to Stucco and selecting Fibre-glass it usually takes some 5-7 minutes. Considering number of changes performed during this change it is pretty efficient.
Good preparation, communication and cooperation between operators on the “wet” side of the Board-line is essential for success of the change. Some advanced planning and changes could prove to be the factor determining success or failure of the change.
Suggested sequence of changes:
- Board-line speed (should be changed gradually)
- Stucco set point (should be changed gradually and in conjunction with changes to the Board-line speed to ensure that board weight is maintained)
- Total liquids set point – should be changed gradually and in conjunction with changes to Stucco set point. Stucco Weight Belt Feeder will not change flow suddenly but rather gradually – this provides opportunity to change total liquids timely – Process Water Pump will respond to the change immediately. Change in ratio between dry feeds and total liquids will affect the “head”. Sequence and speed of changes should be determined by what is happening on the forming table.
- Change from DCP to HEM configuration (board must be heavier for this to be successful. It is good idea to change pumps before this operation (before change starting) – this is to avoid changing two pumps at the same time.
- Select “Fire Rated” board in Conductor NT – Keep in mind that selecting FR will introduce additional dry feeds to the mix. Also keep in mind that selecting FR will not select Fibre glass Cutter.
- Adjust BMA, Fluidiser, Retarder etc in accordance with other changes. It is always better not to rush with reducing fluidity of the slurry till flows are stable.
- Select Fibre glass Cutter and make sure it started. Cutters should be checked before the change by Maintenance.
- Splice the pink face paper and change face and back print. This should be performed only after all other changes are in place. Dry End operators will use this to separate two products.
- Follow the splice to the Wet End Transfer. Splice will need to be rejected. We also reject board when changing from longer board to the shorter size. To minimise gap in the Dryer it might be a good idea to finish 1312RE production on the same length you want to start 1312FR production.
- Change Dryer Chain speed. Exact timing of this is not strictly determined in our plant at this point of time. Dryer Chain speed will affect drying time of board already in the Dryer (1312RE). Dryer Chain speed should not be changed before FR board is starting to enter Dryer Loading section as the earliest and no later than FR board starts entering the Dryer.
Drying Phase of the change
Drying phase of the change is the phase which will ultimately determine success of this product change. Calcination of the board resulting in poor bond and soft edges is generally happening in late stage of the drying process – diffusion phase of the drying.
Finding a balance between drying time and drying temperature is the key for preventing calcination of the board.
Drying time is directly determined by Dryer Chain speed. Timing of the change to Dryer Chain speed is very important. If change is performed too early (the same way as change from RE to SR as an example) then 1312RE board will be exposed to the drying process for extended period of time (up to 14.7 minutes). If Dryer Chain speed is changed too late then Dryer might not be able to dry 1312FR board in reduced drying time. So far we have been changing Dryer Chain speed once 1312FR board is about to enter the Dryer. This means that 1312RE board will be exposed to extended drying time but for less than 14.7 minutes and that 1312FR board will be exposed to normal drying time for that product. Timely changes to Dryer temperature set points can ensure that extended drying time is compensated for by reduced drying temperature.
Gap in the dryer is important factor determining success of this product change. It has great impact on operation of the Dryer – particularly on temperature profile. It is very important to avoid or minimise gap in the Dryer created by rejecting board on Wet End Transfer.