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Chemistry – Processing – Material Science

IN POLYMERS AND ELASTOMERS

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48 Volume 11

Pressure simulation for molding parts

Dipl.-Ing. Timo Gebauer

t.gebauer@sigmasoft.de

Executive Manager / CTO

Vanessa Schwittay

Marketing Manager & Engineering

Sigma Engineering GmbH, Aachen, Germany

Paper, DKT 2015, 29 June – 2 July 2015,

Nuremberg, Germany,

Deutsche Kautschuk-Gesellschaft e. V.,

Frankfurt, Germany

1 Introduction

The process simulation Sigmasoft Virtual Molding is already a well-established tool in the development phase of elastomer prod-ucts. It is used to prevent problems in production and to shorten the time to market of industrial applications. Already very early in the development phase runner systems can be balanced or mold temperatures optimized.

2 Simulation and reality

In spite of ongoing development and in-creasing complexity of the models it happens that the simulation results do not match the measurements at the machine. This does not only concern the simulated pressure, but also further results such as the filling behavior and many temperature related results because

of their correlation with pressure changes. Thus it becomes not only more diffi cult to decide whether the cavity fi lls as predicted, but also results as curing degree, mechanical properties of the part, material degradation, or warpage of the part may become less reli-able (fi g. 1).

If the phenomena observed at the ma-chine during serial production are not the same as the predicted values, major prob-lems can occur. Lost money, time delay, and frustrated customers are some of the conse-quences. The question now is why something like this happens and even more important how one can avoid problems like this.

3 The key factor cavity pressure

One of the key values during mold de-sign is the pressure needed to fi ll the cavity. The maximum pressure is one of the limiting factors for the machine selection. It is also

an indicator of the energy that heats up the elastomer during filling and is there-fore critical for the curing time prediction.

Pressure can be measured at various pos-itions in the system. Hydraulic pressure, pr-essure sensors in the piston or mold are the most common references for the pre-ssure. Usually only one reference is used. If the system offers more than one measured pressure the values are often contradictory.

Figure 2 shows the measured pressure at two different points in the gating sys-tem of an elastomer mold. Even though the hydraulic pressure is constant, the press-ure close to the machine (P-runner1) decr-eases at a certain point. In the first view it looks unphysical. What happened in this case is the hydraulic pressure also sees what happens in the piston. If the melt cushion becomes too small the flow beh-avior in this area can dramatically incr-ease the hydraulic pressure.

In the simulation pressure can be predict-ed at various positions as well. In addition the simulation is usually based on differe-nt models and ideal boundary conditions. Due to the selected model and assumptions for the process simulation results can vary. These variations are based on the complex interaction between material measuring errors, modeling and assumptions. There-fore results are often hard to judge.

One of the critical values for the predic-tion of pressure is the viscosity depending on shear rate and temperature. Unfort-unately there is no way to directly measure the viscosity. Depending on the system that is used, pressure or momentum are measured at specific conditions of flow.

Pressure prediction in elastomer moldsT. Gebauer, V. Schwittay

In spite of ongoing development of existing simulation models it happens that the simulation results do not always match the measurements taken at the actual ma-chine. An erroneous simulated pressure in the elastomer mold may critically affect other simulation results such as the predicted filling behavior of the form, the me-chanical properties of the component, or warpage of the finished part, as correlations between pressure changes and related temperature development exist. Errors in the predicted values could lead to serious problems in the serial production. The article gives a brief introduction to the measurement and simulation of pressure and an-swers various questions.

Fig. 1: Various results depend on the correct pressure prediction as curing degree (left) and material degradation (right)

Empty90.0087.5785.1482.7180.2977.8675.4373.0070.5768.1465.7163.2960.8658.4356.00

98,85

2.448e-010

Percent cured %Empty0.090050.083710.077370.071030.064690.052000.045660.039320.032980.026640.020300.013960.007620.00127

0.09005

0.001275

Degradation potential

90.966

65.747

51.710

91.783

90.094

60.355

47.425

91.025 YZ

XY

Z

X

Volume 11 49

Based on these values and several correcti-ons, the viscosity can be calculated. Since the setup of these systems is very sensitive the results can vary over a wide range. Especially for elastomer mixtures the variation in the viscosity measured can be more than 100%.

3.1 Measurement and correction problems

Figure 3 shows the result of three dif-ferent measurements for one mixture (one batch), measured at three different labora-tories. The diagram is plotted in logarithm scale. The viscosity measured at laboratory 3 is five times higher than the viscosity measured at laboratory 2. If these visco-sities are used in a simulation, the predict-ed pressure will be completely different for the three measurements.

One reason for the differences is the sensitivity of high pressure viscometers. Depending on how the elastomer is placed

in the machine and depending on the setup of the system the measured results can vary a lot.

Other effects are the various corrections which can be used to calculate the true vis-cosity (Weissenberg-Rabinowitsch, Moon-ey, Bagley). These corrections usually need a more complex setup or different experi-ments. Since every experiment has to be paid, not all of the corrections are done in every characterization.

3.2 Simulation of pressure and viscosity considering shear heating

In addition to the already complex prob-lem of known corrections, there is no cor-rection taking into account the temperature increase during the measurement. The ma-terial is pushed through a capillary with a high pressure. The higher pressure and veloc-ity are the higher the shear heating will be. Depending on the sensitivity of the viscosity

for temperature changes the real viscosity will be a lot higher than the measured vis-cosity. Figure 4 shows a virtual experiment. In this experiment a material with a certain viscosity (original viscosity) is used to sim-ulate a high pressure capillary viscometer. The simulated pressure is used to calculate the viscosity as it is done in the real experi-ment. The second curve (viscosity isotherm) shows the calculated viscosity based on this experiment. The material used to generate the viscosity was modifi ed in the simulation in a way that the temperature could not in-crease. One can see that both curves match pretty well. This proof of concept shows that the simulation of the capillary is very accurate and gives the correct pressure prediction.

A second simulation was performed. In this simulation the material was allowed to change temperature due to shear heating. Again the viscosity was calculat-ed. Figure 5 shows the original viscosity

Pres

sure

in b

ar

Hydraulic pressureP-runner1P-runner2

1 E+01

1 E+02

1 E+03

1 E+04

1 E+05

1 E+00 1 E+01 1 E+02 1 E+03 1 E+04 1 E+05

Visc

osit

y in

Pa·

s

Shear rate in s-1

Lab 1Lab 2Lab 3

1 E+00

1 E+01

1 E+02

1 E+03

1 E+04

1 E+05

1 E+06

1 E-01 1 E+00 1 E+01 1 E+02 1 E+03 1 E+04 1 E+05

Visc

osit

y in

Pa·

s

Shear rate in s-1

Viscosity isothermOriginal viscosity

Fig. 2: Hydraulic pressure and two pressure curves close to the gate (P-runner2) and close to the machine (P-runner1)

Fig. 3: Viscosity measured at three different laboratories

Fig. 4: Original viscosity and calculated viscosity for an isothermal material behavior

1 E+00

1 E+01

1 E+02

1 E+03

1 E+04

1 E+05

1 E+06

1 E-01 1 E+00 1 E+01 1 E+02 1 E+03 1 E+04 1 E+05

Visc

osit

y in

Pa·

s

Shear rate in s-1

Calculated viscosityOriginal viscosity

Fig. 5: Original viscosity and calculated viscosity for a normal material behavior

50 RFP 1/2016 – Volume 11

Report

(green curve) and the calculated viscosity (red curve) for this material. In the region of low shear rates the two curves match very well.

In these areas the shear heating is rather low so the material does not heat up too much. In the region of higher shear rates the distance between the two curves becomes bigger. At a shear rate of 1,000/s the difference between the two curves is

approximately a factor of 2. This simple

experiment shows the dramatic influence of the temperature increase during the viscosity measurement. To predict the pressure loss in an elastomer mold it is necessary to also correct this behavior.

One way to get the real viscosities is a simulation. Sigmasoft Virtual Molding can be used to find the real viscosity taking the temperature increase into account.

4 Conclusion

This short introduction in measuring and simulating pressure shows that there are vari-ous questions that have to be answered in detail before the simulation result will match the measured pressure. It is not always nec-essary to answer all of these questions but it is important to be aware of the infl uence of certain effects. This will make sure that critical values can be avoided in production.

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