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|2. Batch cycle
|3. Loading of ingredients / dosing
|6. How to improve cycle time
|7. How to improve a mixer
A dry-mixer cycle time is made of the following components, starting from an empty stage, ready to load :
To these steps, it may sometimes be necessary to account for some time for the PLC to process the change of steps, from 1 to 5 s depending on the PLC and the programming.
In case the ingredients are loaded manually, the duration of this step is simply the time required for the operator to tip the ingredients. The tipping of speed of the operator must be assessed during design, based on experience, or measured for an existing operation.
If only one operator is available for this task, it may also be necessary to calculate the time required for preparing the bags to tip and make some pre-weighing if necessary. If it takes too long, this may have an impact on the batch cycle time if the mixer is ready for loading before the operator has finalized the load preparation.
The designer should not consider that the operator will be productive 100% of the time but should account for some distraction, breaks...etc... reducing the actual time the operator is available to perform the tasks he is assigned.
When an automatic dosing process is used, it is necessary to define, or understand for an existing system, how the dosing sequence is performed, as it will have a direct impact on the duration required for dosing the ingredients. Depending on the system, the ingredients can be dosed all at the same time, the dosing duration being the longest dosing time of an individual mix component, or in sequence, in this case the total duration will be the sum of the individual dosing steps.
The mixing step is the core of the
process, the added value of the process. It can only be
performed when all the ingredients have been introduced to the
mixer, although some pre-mixing step is sometimes used. It is
possible to improve the cycle time of a mixer by questioning the
sequence of addition of ingredients to the mixer, the batch size,
the mixing speed, and the mixing end point by
performing an homogeneity analysis (many times the mixer is
overmixing versus the actual product requirement).
The discharge of the mixer is
the last step of the cycle, once completed, the mixer is free for
loading and a new cycle can restart. The discharge time depends on
the flowability of the product, the size of the discharge valves or
doors, the speed of the mixer during discharge, and the acceptable
quantity of powder residual in the mixer.
It is possible to improve cycle time by :
|Actions for decreasing cycle time
||Possible process levers
|Decreasing the time needed for individual steps||Dose quicker : increase dosing
speed or the part of dosing done in coarse speed. Accept less
Reduce mixing time : increase mixer speed, optmize mixing time through homogeneity validation
Reduce discharge time : accept few kg remaining in the mixer before restarting new batch
|Changing the cycle sequence, especially dosing of ingredients, so that some of the steps are done in hidden time||Use a gain in weight system with hoppers above the mixer in
order to be able to dose the ingredients while the mixer is
still mixing. Once the mixer has discharged, the ingredients
are ready to be loaded by gravity to the mixer, gaining almost
all the dosing time.
Note that this strategy may only be possible on some cases : possible to install the gain in weight system, accuracy achievable in line with product requirements.
The mixer capacity can be increased by decreasing the cycle time,
by increasing the batch size, or
by a combination of both. A study should be carried out to determine
the best way, which may require a new mixing validation if key
mixing sequence parameters are changed (strong batch size changes,
mixing speed change, loading sequence changes...).