1. Definition

What is the cycle time of a mixer ?

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

How to calculate a batch process cycle time ?

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

Those different steps are typically added to calculate the cycle time, although one should be careful to the steps done in hidden time (for example, if the dosing is done in a hopper above the mixer, it can be done, at least partially, in hidden time)

Figure 1 : example of the cycle time break down for a powder mixer

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.

One important point to note is that, if the mixing is very fast, which can be the case with double shaft paddle mixers working in fluidized state, or with centrifugal mixers, the mixing time may not be a bottleneck and thus part of the cycle time. For example, if the mixing is 1 min and its discharge 1 min but the dosing of ingredients in a hopper above the mixer takes 5 minutes, the dosing will actually define the cycle time of the production line.

The rest of the page below helps you to answer to the question : how to reduce a batch cycle time ?

3. Loading of ingredients / dosing

How to load and dose materials quicker ?

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.

To speed up the dosing, the size of the dosing equipment (screw conveyor, vibrating tube, rotary valve...) must be increased, and / or the dosing sequence must be done longer in high / coarse speed and the fine dosing reduced, but this may impact the accuracy.

4. Mixing

How to mix quicker ?

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

How to discharge quicker ?

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

How to improve the capacity of the mixing process ?

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...). You may wish to contact a powder handling / mixing consultant to carry this analysis for you.

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

Common issues in batch powder dosing and how to fix them, Thomas Lamotte, Processing magazine, March 2018