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Mixer / Batch cycle time

How to calculate a mixer cycle time ?

How to reduce / optimize a mixer cycle time ?

Question or remark ? Please contact us at powder.process@protonmail.com


Section summary
1. Definition
2. Batch cycle time components
3. Loading of ingredients / dosing
4. Mixing
5. Discharge
6. How to improve cycle time
7. How to improve a mixer capacity

1. Definition

The mixer cycle time should not be confused with the mixing time. Although the mixing time is included in the cycle time, the cycle time covers actually all the process steps to be completed to load materials to the mixer, mix and discharge. It is the time required to complete the batch, it does not apply in continuous processes.

2. Batch cycle time components

A dry-mixer cycle time is made of the following components, starting from an empty stage, ready to load :

- Loading of ingredients / dosing

- Mixing

- Discharge

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.

3. Loading of ingredients / dosing

3.1 Manual loading

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.

3.2 Automatic dosing

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.

In some complex systems, there may be a combination of the 2 dosing strategies explained above.

4. Mixing

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).

5. Discharge

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.

6. How to improve cycle time

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 dosing precision.
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.

7. How to improve a mixer capacity

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...).

Note that the notions above can also widely be applied to wet mixers !

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