Paper Technology International 2024 - Journal - Page 44
PAPERTECHNOLOGYINTERNATIONAL
Figure 2: Single and double curved honeycomb moulded part.
These were used to carry out simulation-based preliminary
investigations and variant studies and to derive design information
for process development and mechanical engineering.
An important issue was the assessment of the in昀氀uence of
imperfections in the semi-昀椀nished product on the process stability
of the forming processes. Among other things, it was shown that
when the honeycomb cores are over-expanded, uneven bonding
is compensated for by partial, local material failure. The resulting
feedback effect on the working element can be used for active
process control. Another important aspect was the geometric
design and control of the gripper elements, which have to grip the
honeycomb core securely over the narrow web edges. The length of
the gripper contact surfaces in the web direction should not be less
than half the cell length in the event of over-expansion.
As a particular challenge, a prediction tool for determining
the 3D forming limits as a function of the degree of pre-compaction
of the hexagonal honeycomb cores was developed (Fig. 3) and
experimentally compared, and a simpli昀椀ed geometric prediction
approach was tested. A key 昀椀nding is that even with ideally
frictionless mould surfaces, the curvature is not symmetrical around
the central axis. This can be explained by the volume-speci昀椀c
material accumulation and thus stiffening on the pressure side.
A 昀氀exibilization device for hexagonal honeycomb cores
made of paper and aluminum and a joining process for multi-curved
lightweight moulded parts are now available for potential users. A
key reason for the successful implementation of the project was the
excellent cooperation and expertise of the project partners, which
will be continued in future joint projects.
Prediction of Packaging Paper Formability in Deep-Drawing and
Hydroforming Processes
The geometry of packaging is one of the most important
design factors for creating the highest possible identi昀椀cation effect
with the product or brand. Extensive research in recent years has
made it possible to greatly expand the material limits of paper in
3D forming processes such as thermoforming or hydroforming, so
that packaging with high degrees of forming can be produced in
very good quality, making it a promising alternative to thermoformed
plastic trays.
However, the fact that the suitability of a paper for 3D
forming is not standardized and cannot be predicted using
conventional material parameters continued to be a problem for
industrial distribution, making it signi昀椀cantly more dif昀椀cult for paper
processing and packaging companies to select a certain material.
Motivated by this, PTS launched the cooperative research
project “UniVorsUm” together with Steinbeis University and TU
Darmstadt (PtU) in 2020. This collaboration bundled Germany-wide
expertise in the 昀椀eld of 3D paper forming with regard to material
development and process development for both deep drawing and
hydroforming of paper as well.
The aim of UniVorsUm was to develop a standardization
and prediction of formability for papers and related materials in
3D forming processes with both rigid and 昀氀exible blank holders by
de昀椀ning the essential material properties. For this purpose, a new
test strategy and associated characteristic values were developed
昀椀rstly in order to be able to assign effects and defects to the shape
changes or material damage (tolerable to a small extent, undesirable
to a large extent) responsible in the 3D forming process.
In addition to well-known standard tests such as the
tensile test, special measuring methods developed at
the research institutes were also used to characterize
the materials. These methods are able to reproduce the
often-complex stresses during the forming process more
accurately. These include, among others: In-plane and
out-of-plane shear tests (shear in the plane and across
the thickness of the material, see Fig. 4), the Curvature
Resistance Test (CRT, bending test without shear
component, see Fig. 5), mixed-mode tests (tension/
compression superimposed with compression in the
thickness direction) but also the cupping test known from
the metal sector. The analysis of the stresses acting
on the material during forming was also supported by
analyses using FEM (see Fig. 6).
Figure 3: Simulation of the forming of a 昀氀exibilized
honeycomb core.
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