JEC COMPOSITES MAGAZINE - Issue #141 - July/August 2021 - 25

Flow field dynamics in pultrusion injection boxes
Fig. 5: Comparison of haul-off forces under variation
of haul-off speeds and horizontal opening angles for
different fibre volume contents [19]
likely to be the same as in the first test series,
but this time a much lower viscosity is needed
to reach the tipping point due to a much higher
roving density. Results of both test series support
the general viability of designing injection
boxes for a wide range of materials, but also
illustrate process limitations based on different
kinetic energy levels of the system, which can
be triggered by high viscosities as well as high
fibre density. Additionally, higher fibre volume
contents do not necessarily lead to worse
impregnation results, when working with a
two dimensionally tapered die. In case the
assumptions hold up, injection boxes for larger
and especially thicker profile cross-sections are
viable. Anyhow, additional and more rigorous
investigation is needed to validate assumptions
and identify tipping points leading to inferior
impregnation results.
Conclusion and outlook
Injection boxes offer unique advantages in pultrusion
to increase productivity and decrease
safety hazards simultaneously. Investigations
for analyzing the flow in injection boxes with
respect to haul-off forces are presented. A transparent
injection box that enables an in-depth
investigation of the flow field in injection boxes
independent from a successfully operating
pultrusion process. The process dynamics of
the flow field are investigated.
The presented results display two different configurations
of the same effect. Local pressure
variances represent a reasonable explanation
to tip the process towards a lower energy state
that consists of larger, but less gaps between
rovings. In this case, coating of each roving is
less likely and the impregnation thickness increases,
which leads to a higher likeliness of insufficient
impregnation. These results indicate
that the two dimensionally tapered injection
boxes represent a viable design option, but
geometry-based pressure build-up needs to be
understood more comprehensively in order to
achieve full impregnation for scaled geometries.
Future work will focus on quantitative understanding
of pressure build-up under variation
of cavity geometry in order to design injection
boxes for more complex cross-sections, such as
wave-forms or hollow profiles. Initially, these
designs will be evaluated with transparent dies
and subsequently in a real pultrusion setup with
reactive materials. Material selection will be
based on input from industry, so that inquiries
are welcome.
More information:
https://www.ikv-aachen.de/en/research/
areas-of-research/composites/
Acknowledgements
The research project (21090N)
of the Forschungsvereinigung
Kunststoffverarbeitung is sponsored as part
of the " industrielle Gemeinschaftsforschung
und -entwicklung (IGF) " by the German
Bundesministerium für Wirtschaft und
Energie (BMWi) due to an enactment of the
German Bundestag through the AiF.
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N°141 July - August 2021 / jec composites magazine
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