Parent Category: Industries

Simulation of the Parison Pressing Step (NNPB process)

The simulation of the parison pressing step, which means both glass feeding and glass forming processes is feasible with CFD software NOGRID points. Glass forming operations include large free surface deformations, conjugate heat transfer and complex contact phenomena. In this short report, glass container forming processes are modeled to provide insight into the impacts of the various stages of forming on final container quality. The model uses NOGRID points and includes the effects of viscoelasticity, surface tension and time-varying heat transfer.

Lightweight glass containers produced by the narrow neck press and blow process are subject to ware quality defects due to contact between the glass and the plunger. These defects are a response to the deformation of the glass and unsuitable tooling materials. To overcome these problems, current research is developing a greater understanding of the response of the glass and interaction with the tooling during the parison forming process.

Scheme of parison press processFigure 1: Scheme of the parison press process

The geometry is axis-symmetric, but the reality is 3D and to understand also 3D effects (such as temperature inhomogeneities) we model the problem in full 3D. In order to apply shear stress boundary conditions, a friction law, where the shear stress is proportional to the relative tangential velocity at the boundary, is used. When the slip coefficient is close to zero, a small velocity difference results in a high stress and the boundary condition is similar to a sticking wall. If the slip coefficient is very high, also a high velocity difference results in a small stress at the boundary and the result is, that the fluid slips at the wall.

CFD simulation parison pressing

Figure 2: Simulation of parison pressing with NOGRID points: Velocity profile during plunger movement

If the fluid comes into contact with walls, there are four different methods to handle what to do. The touch always-keyword defines a contact, where the entities are always in contact with the fluid. Here is nothing special to do. Touch liquid (used in this simulation) means, if a fluid comes into contact with this entity, the fluid must penetrate the entity before the contact is flagged as true. Touch geometrical means, the contact is flagged as true if the distance between the free surface and the wall is lower than a prescribed value. If the geometrical distance between a NOGRID points point and a line or surface is lower than a certain fractional amount of the smoothing length the contact is flagged as true. A further contact condition is the solid-statement. Here the contact problem is solved by the govern forces. If the govern forces at the NOGRID points point are oriented in the direction to the contact line or the contact surface, the contact is flagged as true, otherwise the contact is flagged as false and NOGRID points tries to move the considered point a bit away from the contact line or contact surface.

CFD simulation parison pressing

Figure 3: Simulation of parison pressing with NOGRID points: Temperature profile during plunger movement

The computation time is about 4h (fine full 3D model) and 240,000 points (for all parts) are used. One can reduce the computation time by using a half or a quarter model. The mass conservation during the complete pressing step is always satisfying. The numerical mass fluctuations at each time step are below 1% of the initial gob mass and also the difference between the initial gob mass and the final mass is below 1%.

As shown in figure 4 the point density is variable in space and moves with the plunger. This is a core feature of our software and helps both to reduce computation time and to increase the quality of the results as well.

CFD simulation parison pressing - temperature profile and point cloud

Figure 4: Simulation with NOGRID points: Temperature profile and point cloud during plunger movement