Filling processes (particle foam)

Why should you perform a filling process simulation?

Mould filling is used for the production of particle foams. In this process, thermoplastic particles are blown into a mould and then welded using water vapour. The mould corresponds to a negative of the workpiece geometry.

Especially with technical components, the complexity of geometries is increasing. As a result, the demands on toolmaking are becoming more and more challenging, as the aim is to achieve maximum packing density of the thermoplastic particles at every point of the mould and to do so in the shortest possible development time.

The position of the injection points on the mould is fundamental here, as this is decisive for the particle flow. The cost of a test-based determination of these positions increases with the increasing complexity of the mould. This procedure is not always economically efficient, as many iterations of trial and error are necessary.

In such cases, a filling simulation can be used to define ideal injection points very quickly and to control the density distribution to generate gradients.

Generic tool with 2 varying injection point positions (left: version 1, right: version 2)

Filling process with EPP particles of version 1 (left) and version 2 (right)

What are the benefits?

The experimental definition of injection points is usually based on experience. Particularly in the case of complex geometries, large filling volumes or density gradients to be generated, a simulative approach can save a great deal of material, time and labour. The advantages are listed below.

  • Real modelling of material and particle properties as well as process influences, e.g. electrostatic charging of the particles
  • Rapid variation of the position of the injection points and quantification of the differences
  • No or only very low material costs and low energy consumption
  • Build up process knowledge with each additional simulation
  • Calculation possible regardless of the time of day or day of the week

What is the simulation capable of?

All mould geometries, filling parameters and material properties of any complexity can be modelled. A large number of simulations can be carried out within one working day.

Particularly with low bulk density polymers, attraction forces, such as those caused by electrostatic charging, can lead to problems during transport or filling. This can lead to adhesion to the walls, in the upstream feeder, or clumping of the particles and consequently to insufficient filling. Corresponding attraction forces can therefore be modelled to take this behaviour into account.

Processes with pressurised filling and cracking gap can be simulated. The change in particle size during pressurised filling and the resulting change in bulk density are taken into account.

Visualisation of critical areas when filling version 1

Velocities (left) and pressures (right) when filling version 2

The example of filling version 1 and version 2 clearly shows that there is a significant difference in particle behaviour and thus in the resulting filling result.

Version 1 shows clear underfilling of the mould at thin wall areas, overhanging geometries and inadequate reproduction of the hexagonal contour due to the filling positions arranged at the top.

Compared to version 1, version 2 can achieve homogeneous filling due to the adapted position of the injection points. In terms of speed, this is reflected above all in the reduced mutual influence of the particle flows into the mould.

The pressures in the mould are particularly relevant for filling the mould as compactly as possible. The pressure distribution shown in version 2 can be used to derive statements about the density after the introduction of water vapour. Furthermore, densities can also be controlled in this way.

By looking at individual areas throughout the filling process, local analyses can be carried out in order to optimise the process time as well as the position of the injection points.

Local consideration of the filling process based on density