An ideal-typical setup follows the principle: as few product transfers as possible, the shortest possible free fall distances, and a controlled mass flow from raw material to packaging. Every additional conveying stage, every silo, every chute and every free fall can promote segregation, abrasion, dust formation or cross-contamination. This is particularly critical for mixtures with large differences in particle size, bulk density, shape or flow behaviour.

The structure described below outlines a practical reference setup for batch and continuous powder processes. The actual design must be adapted to the product, hygiene requirements, throughput, formulation and explosion protection.

Ideal-typical process layout

Raw material supply → screening → buffering and dosing/weighing → defined product feed → mixer → gentle discharge → short, enclosed transfer → filling or downstream process

The process begins with raw material receipt and storage in suitable containers, small bins, big bags, sacks or silos. Raw material supply should already support the dosing accuracy required later on. This includes stable bulk density, limited moisture, particle size that is as uniform as possible, and product-appropriate emptying of the containers. For cohesive or bridging powders, suitable discharge aids are required, for example agitators, flexible walls, vibration aids or specially designed discharge elements. These aids must be set so that they stabilise dosing without destroying particles or prematurely segregating the mixture.

A check screen is often advisable before dosing. It removes foreign bodies, packaging residues, hard lumps or unwanted oversize particles. The screen should be positioned as early as possible in the process and before precise dosing. For sensitive granulates or cereals, the screening machine must be designed to be particularly gentle so that it does not itself cause abrasion and particle breakage. Magnetic or metal separators can be used in addition where metallic foreign bodies are to be expected from raw materials or conveying equipment.

After screening, each raw material passes into a small intermediate container designed for the product, or directly into the dosing station. Long storage in large intermediate silos after screening should be avoided where possible, because material can segregate again there during filling, vibration and discharge. Where intermediate buffering is unavoidable, the vessel should be designed for mass flow, that is, movement of the entire product cross-section as uniformly as possible. Mass flow can limit segregation during discharge, but it does not guarantee a fully homogeneous emptying if the mixture has already separated laterally or by particle size during filling.

Dosing is carried out gravimetrically where formulation accuracy and reproducibility are essential. For batch processes, gravimetric weighing into a weighing vessel or a mixer mounted on load cells is suitable. Main components are often introduced with a coarse dosing phase followed by a fine dosing phase. Fine dosing reduces overshoot and increases weighing accuracy. Sufficient time for the weighing signal to settle must be allowed after each component; flexible connections, vibration decoupling and dust-tight venting prevent external forces or pressure fluctuations from distorting the weighing signal.

For continuous processes, loss-in-weight feeders, or LIW feeders, are particularly suitable. The dosing vessel and the discharge element are mounted on load cells; the control system regulates the discharge speed from the measured weight loss per unit time. This controls the actual mass flow rather than just the speed of a dosing screw. LIW feeders are particularly helpful where bulk density, fill level or flow behaviour can fluctuate. However, their refill concept is critical: during refilling, gravimetric measurement is usually interrupted temporarily and bridged by a model-based volumetric dosing mode. The refill process, vessel size, refill speed and the subsequent stabilisation must therefore be matched to the required mass flow and the permissible dosing deviation.

For minor components such as flavourings, active ingredients, vitamins, colourants or trace elements, the dosing task is particularly demanding. They should be dosed as close to the mixer as possible, so that only a short conveying distance follows the addition. For very small quantities, a premix with a suitable carrier material may be required. The carrier component should have particle properties as similar as possible to the main components, so that the premix does not segregate again during feeding and in the mixer.

The combination of the dosed components upstream of the mixer must be calm and controlled. A common collecting hopper must not result in a high, uncontrolled product fall or in pre-segregation. Short, as near-vertical as possible, dust-tight transfers with a large cross-section are favourable. Long inclined chutes, baffle plates, high free fall distances and fast-running conveying screws should be avoided, or used only after product trials. They can generate abrasion, separate large and small particles, or cause the product flow to pulsate.

The mixer must suit the mixed material and the mixing task. It should reliably reach the entire batch volume without causing excessive shear, particle breakage or local over-mixing. What matters is a defined working range for the fill level, a suitable tool geometry, a reproducible rotational speed, and a mixing time limited to the earliest point at which sufficient homogeneity is reached. For liquid additions, spray nozzles must be positioned in a sufficiently agitated product zone. Droplet size, spray rate, viscosity and mixing intensity must be matched so that no local over-wetting, lump formation or build-up occurs.

After mixing, product discharge is often the most critical stage for segregation. Ideally, the mixture is transferred directly from the mixer to the downstream process or filling without intermediate storage. The outlet should be large enough to allow a uniform mass flow with little pulsation. Controlled mass flow in the discharge vessel or mixer reduces the risk that initially only certain fractions are discharged. Large heap cones, long fall distances, steeply inclined chutes, conveying vibration and pneumatic conveying can degrade a good mixture again. Transfer chutes should therefore be designed short, vertical, smooth-walled and, as far as possible, free of abrupt cross-section changes.

Where an intermediate vessel after the mixer is unavoidable, it should be as small as possible and serve only as a short-term buffer. Its design must allow uniform discharge. Funnel flow, that is, core flow with stagnant zones at the walls, can intensify segregation and retain old product residues. Mass flow is usually the better starting point, but it must be confirmed by tests with the actual mixture. Complete emptying can also be problematic if fractions discharge separately at the last portion of product; the vessel and filling process should therefore be checked for homogeneity over the entire discharge sequence.

Control and quality

The control system should record all weighed quantities, dosing times, target and actual mass flows, mixing time, rotational speed, liquid quantities and, where applicable, temperature. For continuous plants, ratio control of all dosing streams relative to the main component is central. For batch processes, formulation management, plausibility checks, release after weighing, and complete batch documentation are advisable.

Quality assurance does not only check the sample in the mixer. It must also assess homogeneity at the beginning, in the middle and at the end of discharge. Relevant characteristic values include the coefficient of variation of a marker component, particle-size distribution, bulk density, moisture, dust content, breakage content and, where applicable, the uniformity of a liquid addition. Only when these values remain stable across dosing, mixing, discharge and filling is the process genuinely low in segregation.