What mistakes are commonly made with a mixer's fill level?
The fill level is often treated as a fixed machine parameter, even though it is a central process parameter. Too little or too much product in the mixer can significantly affect mixing quality, mixing time, particle integrity, segregation, liquid distribution, discharge and cleaning effort. The appropriate fill level must therefore be determined jointly for the product, mixer geometry, tooling, rotational speed and process objective.
Excessive overfilling
A common mistake is to fill the mixer close to its full geometric volume in order to increase batch output or reduce cycle time. This often removes the free space needed for product movement. The mixed material can no longer expand, reshuffle or circulate sufficiently. The consequences are longer mixing times, poorly moved zones, fluctuating homogeneity and, in extreme cases, local compaction or overloading of the drive.
With sensitive products, overfilling can also increase the pressure on lower product layers. Flakes, agglomerates, granulates and other fracture-sensitive components can then abrade or break more heavily. With sticky, moist or fat-containing formulations, overfilling can additionally promote build-up and uneven liquid distribution. Higher fill levels can lengthen the mixing time; at the same time, the fill level and the charging pattern influence the risk of segregation.
Insufficient filling
A fill level that is too low is often chosen because it initially creates generous room for movement. However, if the fill height is too low, the mixing tools no longer reach the entire product volume. Zones can then arise that are only weakly moved, while other portions of the product are worked repeatedly and comparatively intensively.
In free-fall and tumble mixers, too little product can lead to greater fall heights and higher impact energy. In tool mixers, it can result in increased relative velocity, abrasion, particle breakage or dust formation. This is particularly critical for cereals, brittle granulates, coated particles and sensitive agglomerates. Underfilling can reduce mixing performance and increase batch-to-batch variability.
Using weight instead of volume
A common planning mistake is sizing the batch solely in kilograms. However, the fill level is a volume-based quantity. Changes in bulk density, moisture, temperature, air content, particle size or formulation can mean that the same mass occupies a significantly different volume.
This is particularly problematic for light, voluminous products such as flakes, instant powders, fibres or agglomerated powders. A batch that appears correct by weight can actually overfill the mixer. Conversely, a dense product of the same mass can lie well below the required working range. The fill level should therefore be determined using the actual bulk density of the product mixture under the real process conditions.
Changing the fill level in isolation
The fill level is sometimes changed without adjusting rotational speed, mixing time, tool configuration, dosing profile or liquid addition. This often leads to unexplained fluctuations. Depending on the mixing principle, a higher fill level requires an adapted mixing time or rotational speed; a lower fill level can require a lower tool speed to limit breakage and abrasion.
The addition of liquid components must also be matched to the fill level. At a high fill level, the accessible product surface per unit time is lower; spray mist does not reach all particles equally well. At a low fill level, the liquid can concentrate on a few areas or adhere to the wall and tooling. Fill level, spray rate, droplet size, nozzle geometry and product movement must therefore be developed as a combined process window.
Ignoring the manufacturer's working range
Every mixer has a specified working range. This is based on the mixing chamber geometry, tool shape, installation depth, discharge element and mixing principle. A fill level outside this range can, even with the same product, lead to dead zones, inadequate reshuffling, high mechanical loading or unstable discharge.
The manufacturer's working range is a useful starting point, not a substitute for product validation. A fill level optimised for practical operation can lie within the permissible range, but it can rarely be transferred, as a blanket rule, to a different mixer type or a different size.
Overlooking segregation before and after mixing
A good sample taken directly from the mixer does not prove that the product remains homogeneous through to filling. Segregation can already arise during charging, through different fall heights, bulk densities, particle sizes or the order of addition. It can equally occur during discharge, in chutes, conveyors, silos and during packaging.
An unsuitable fill level can amplify these effects because it changes the movement pattern of the bulk material. Percolation is particularly relevant for mixtures of large and small, light and heavy, or differently shaped components: fine particles can migrate downward through the gaps between larger particles. Differences in particle size, density, shape and cohesiveness are key drivers of segregation.
Mixing too long to compensate for fill-level errors
A common reaction to poor mixing samples is: “Then we'll mix for longer.” However, this can rarely fully correct an unsuitable fill level. Longer mixing times often lead to additional abrasion, breakage and dust with sensitive products. With mixtures prone to segregation, over-mixing can even worsen the homogeneity again.
Instead of extending the mixing time indefinitely, it should be examined whether the mixer is overfilled or underfilled, whether the bulk density has changed, whether the tooling reaches the entire batch, and whether charging or discharge already segregates the product before or after mixing.
No trials with the original product
The biggest mistake is to set a fill level from experience values, a different formulation, or solely on the basis of the nominal volume. Even comparable products can behave quite differently owing to bulk density, moisture, fat content, particle shape, particle size or flow behaviour.
The fill level should therefore be systematically tested with representative original product. A test series with several fill levels and adapted rotational speed and mixing time is advisable. Homogeneity, mixing time, fines and breakage content, bulk density, dust content, liquid distribution, segregation after discharge, residual discharge and cycle time should all be evaluated. The fill level influences both mixing and segregation processes and should therefore be treated as a critical process parameter.
How amixon® handles variable fill levels in vertical mixers
With sensitive cereals, flakes, extruded products, clusters and coated particles, the fill level is a critical process parameter in many mixing systems. In amixon® vertical mixers, by contrast, mixing quality can be maintained over a wide fill-level range, provided the product, tool design and operating parameters suit the task. The vertical SinConvex® mixing screw generates three-dimensional product movement: it conveys the mixed material upward near the wall, while at the centre the product flows back down under gravity.
Circulation follows the product volume
The conveying capacity of a vertical mixing screw can be described approximately as follows:
Iv=π/4⋅(D2−d2)⋅φ⋅S⋅n⋅ζ
Here, Iv is the circulation volume flow of the mixing screw, D is the outer diameter and d is the inner diameter of the screw. φ denotes the fill level, S the pitch of the screw, n the rotational frequency and ζ a velocity coefficient.
The equation shows that as the fill level increases, the circulated volume flow rises proportionally. At the same time, the quantity of product to be mixed also increases proportionally. Relative to the batch present, the specific circulation performance therefore remains essentially constant under geometrically comparable conditions. This explains why the vertical mixing motion can remain similarly effective over a wide fill-level range. amixon® describes this relationship, for geometrically similar apparatus, as the basis for a size-independent specific mixing performance.
This does not mean that the fill level no longer plays a role. It continues to influence mixing time, product column height, discharge, liquid addition, pressure loading on sensitive particles, available headspace, discharge behaviour and the possible use of additional tools. However, with vertical SinConvex® circulation, it is not a limiting factor in the same way as with mixing principles whose effect depends strongly on a specific fall height, a free product surface, or a locally formed vortex zone.
Wide fill-level flexibility
According to the manufacturer, amixon® vertical twin-shaft mixers of the HM series are described for fill levels of approximately 10 to 100 percent of the usable volume. The mixer can be operated with variable rotational speed and is designed to homogenise dry, moist and viscous products with different particle sizes, bulk densities and flow properties.
For vertical single-shaft mixers of the VM series, effective three-dimensional reshuffling begins, according to the manufacturer, at a fill level of around 10 percent. Cone mixers of the AM series also cover wide working ranges, depending on the product and configuration; amixon® quotes fill levels of approximately 5 to 100 percent. For powdery bulk materials, however, sufficient headspace is required. The maximum fill level actually usable therefore depends on flow behaviour, product volume, venting, liquid addition and mixer geometry.
For sensitive cereals, this flexibility offers practical advantages. Smaller formulation batches can be processed in the same mixer without necessarily having to switch to a smaller apparatus. For seasonal variants, product development, frequent formulation changes or fluctuating production volumes, this can reduce changeover effort and additional mixing capacity. Particle movement remains three-dimensional; mixing intensity is matched, via tool design, rotational speed and mixing time, to fracture resistance, coating and the desired homogeneity.
Product protection and process limits
For fracture-sensitive cereals, a low tool speed is often advisable in order to limit impact, friction and abrasion energy. The vertical conveying motion can reshuffle the product comparatively gently. However, flake breakage, abrasion or coating damage cannot be ruled out entirely as a blanket rule. In addition to mixing, charging, liquid addition, discharge, fall heights and downstream conveying equipment can also affect product integrity.
The use of cutting rotors or other intensive additional tools is generally not required for sensitive cereals. Where liquids such as flavourings, oils, syrups or coatings are added, the spray technology must be matched to the fill level and product movement. Droplet size, spray rate, nozzle position, product temperature and viscosity influence whether uniform wetting is achieved without local over-wetting and build-up.
Fill levels near the upper limit must be checked, particularly for dusty or light products, for sufficient venting space, safe charging and free product movement. Likewise, very low fill levels must ensure that the mixing screw and product circulation reliably reach the entire batch. amixon® recommends determining the minimum fill level of a planned plant through mixing trials with the original product.
Trial-based design
The specific design should be carried out with the original cereals. Fill level, rotational speed, mixing time, tool design, charging, liquid addition and discharge are examined together. Mixing homogeneity, breakage and fines content, coating distribution, dust formation, bulk density, flow behaviour, residual discharge, and possible segregation after the mixing process should all be evaluated.
The trials define not just a single optimal fill level, but a robust, permissible working range. This allows a vertical mixer to be used flexibly for different batch sizes without losing sight of product quality, particle integrity and process reproducibility.