Trade press conference K 2004on June 22 and 23, 2004, in Ludwigshafen

Better orientation through integrative simulation Knowledge of process-related properties gives better insight into

structural behaviour of moulded parts

Report by Dr. Stefan Glaser,

Engineering Plastics Application Development

BASF Aktiengesellschaft67056 LudwigshafenPhone: +49 621 60 – 0Germanyhttp://www.basf.de

Communication Plasticswww.basf.de/plastics

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Dr. Sabine PhilippTel.: +49 (6 21) 60-4 33 48Fax: +49 (6 21) 60-4 94 97E-Mail: Sabine.Philipp@basf-ag.de

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One thing that until now has prevented engineers arriving quickly at an

optimal design of moulded part – highly stressed ones in particular –

has been the lack of information about process-related material proper-

ties, ie, properties – for example anisotropy due to fibre orientation

- that the part acquires during the moulding process.

BASF is now using a new integrative design approach that takes into

account fibre orientation in the moulding, resulting in a more accurate

prediction of the mechanical and thermal behaviour of the part (fig-

ure 1).

Anisotropy makes optimal part design difficult

To reiterate, the moulding process influences the orientation of the rein-

forcing fibres and therefore the mechanical properties of the part - stiff-

ness, tensile strength, resistance to heat distortion etc. This is of partic-

ular significance for parts that are subjected to high service loads and

temperatures, or are prone to warpage (figure 2).

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The fibre orientation within the moulded part is non-uniform, resulting in

differing material properties at different places.

Anisotropy due to fibre orientation is particularly problematic when

designing complex parts with the help of computer simulation. Since the

local fibre orientation is usually not known, the material is generally as-

sumed – wrongly – to behave isotropically. To compensate for error, the

material’s characteristic stiffness values have to be reduced using a

correct factor. Unfortunately, this factor, which is obtained by comparing

computed and measured values of the material’s elastic moduli, is only

valid in certain circumstances (figure 3).

The three steps of integrative simulation

This is where integrative simulation comes into play. In a first step, the

fibre orientation within the part is determined by a mould-fill simulation

using MOLDLFLOW, the most widely used commercial mould-fill simu-

lation software. The computation takes into account the properties of

the moulding compound – melt viscosity, fibre content etc. – as well as

the process parameters such as injection speed and holding pressure.

Secondly, the information gained about the fibre orientation in the moul-

ded state is then used in a non-linear anisotropic material model de-

veloped by BASF. The specifically devised software module is called

FIBER. With the help of this model (and module), the mechanical prop-

erties of the resin/fibre composite are calculated from the various fibre

orientations and the separate mechanical properties of the resin matrix

and fibres. It is thus possible to take into account the process-related

material properties during the computation.

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In the third and final step, a structural analysis of the part is carried out

with either LS-DYNA or ABAQUS, two common commercial finite-ele-

ment software packages, to which the BASF’s material model extension

has been added.

New “FIBER” software module links fill simulation and structural analysis of part

The software module – called FIBER – which processes the fibre-orient-

ation data for use in the structural analysis was developed by BASF.

FIBER transfers the fibre orientations determined from the mould-fill

simulation to the finite element mesh of the part’s structural model and

works out the local material parameters. Because the transfer is purely

geometrical, the data can be applied to a variety of meshes. User-

defined functions allow non-linearity and complex failure modes to be

included in the description of the material – something that was not pos-

sible until now. Fundamental to the whole process is efficient manage-

ment of the vast quantity of input data required.

FIBER thus forms a link between mould-fill simulation and structural

analysis of the part (figure 4).

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Precise prediction right up to failure

To demonstrate the power of integrative simulation, torsion tests were

carried out on a structural beam – the LU carrier – made from an Ultr-

amid® (nylon)/metal composite and the results compared with those of

the simulation (figure 5).

The usual computation, which is based on non-linear yet isotropic ma-

terial behaviour and a standard correction factor of 0.75, is unable to

predict the beam’s stiffness accurately. The error is due to an inad-

equate description of the material. By contrast, the integrative approach

with FIBER takes into account the beam’s anisotropic stiffness charac-

teristics, resulting in a more accurate prediction of the beam’s flexural

response. Simulated and experimental results agree very closely right

up to the failure of the part (figure 6).

That the beam turns out to be stiffer than predicted by the conventional

method is due to the anisotropic fibre orientation.

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As the mould-fill simulation shows, the restricted melt flow in the ribs

causes the fibres to align themselves along the axis of the ribs (fig-

ure 7).

In the torsion test, the highest tensile loads happen to occur along these

ribs. Thus the beam is strongest precisely where the greatest loading

occurs (figure 8).

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