If the dosage amount is not the same as the dosage amount

The influence of pressure, viscosity and temperature on actual dosing quantities

Pressure-time dispensing systems are a proven standard in dispensing technology – and for good reason. They are robust, economical, and technically perfectly adequate for many applications. As long as material properties and environmental conditions remain stable, they deliver reproducible results. However, dispensing is not a purely time-controlled process, but a physical one. And physical processes react to changes. As soon as processes operate within tight tolerances or material properties vary, a fundamental question arises: Does the dispensed quantity truly remain constant – or is it only constant under ideal conditions?

In a pressure-time system, the dispensed quantity results from the interplay of applied pressure, opening time, and the current flow behavior of the medium. It is therefore directly dependent on the prevailing process conditions.

The key influencing factor is viscosity. This is not a fixed constant but changes due to temperature variations, batch differences, aging, or mechanical shear stress. Even slight temperature changes can significantly affect the flow behavior of highly viscous or filled media. Additionally, systemic influences such as pressure losses or pressure fluctuations along the media path come into play. In pressure-based systems, these factors directly affect the dispensed quantity. In applications with larger tolerances, this often remains uncritical. However, in precision-relevant processes, even small deviations can lead to measurable changes in the dispensed quantity and thus to quality deviations.

The decoupling of physical influencing factors is a key advantage of volumetric dosing methods.

An established technical implementation of volumetric dosing is the progressive cavity screw principle. Here, a rotor and a stator form consecutive conveying chambers, which are continuously moved from the media inlet to the outlet by the eccentric movement of the rotor. The medium is transported by continuous displacement. The dosing quantity results from the geometrically defined conveying volume per revolution and the rotational speed of the rotor. This produces a virtually pulsation-free, continuous volume flow. The principle is particularly suitable for highly viscous, filled, or structure-sensitive media where uniform and reproducible dosing is required.

Whether a dosing system operates with long-term process stability depends significantly on the application's boundary conditions. In processes with larger permissible tolerance ranges and stable material properties, pressure-time systems remain an economically viable solution. However, if dosing tolerances are reduced, material properties vary during the process, or high repeatability is required, the assessment changes. The decisive factor is then no longer solely the system's robustness, but rather the physical principle by which the dosing quantity is generated.

An additional technical aspect arises in automated applications: With variable web speeds, the volume flow rate must be proportionally controlled to ensure a constant quantity per path. Without this coupling, acceleration and deceleration phases directly alter the quantity applied per path.

With the application valves of the RotoStream series Implement precisely this necessary coupling: The speed-proportional control synchronizes the volume flow in real time. This ensures that the dosing quantity per millimeter of web length remains absolutely constant – regardless of the complexity of the component geometry or the dynamics of your handling system.

"The switch from pressure-time systems to volumetric dosing systems is particularly justified when processes are no longer determined solely by economic efficiency, but by precision and reproducibility."