SAMPE Journal - March/April 2022 - 42

FEATURE / ADHESIVE-FREE THERMOPLASTIC SANDWICH PANELS
Figure 9. Thermoplastic sandwich structure produced nonisothermally.
pre-impregnated
face sheet flows in the aramid paper honeycomb during the
curing process. Thus, a development of meniscus can be observed. In this case,
the bonding degree is defined by the size of the meniscus.
During the isothermal manufacturing process of thermoplastic sandwich
panel, the heat, needed for the melting of the core-skin-interface, is transferred
from the hot molding press through the skin. As shown in the Figure 11, the
core is partially melted and pressed. It corresponds to the core melting model,
which is described in theoretical part9
. With increased temperature of the
molding press, the domination of the melted part was observed. The amount
of the contact surface determines the bonding degree between the skins and
the core.
In the non-isothermal process, the thermoplastic matrix of the skin is heated
over the glass transition or melting temperature & becomes viscous. As shown in
the Figure 12, the cold core penetrates in the softened face sheet. Additional to
the penetration, the partial melting of the core can be noticed. In this case, the
bonding degree is defined by both values, penetration depth & contact length of
melted core. Furthermore, the contact boundary between the penetrated core &
the face sheet is partially disappeared with increased temperature of the infrared
radiator. It can be referred to the healing phenomenon, in which the diffusion of
molecular chains takes a place6,7
.
DRUM PEEL TEST
The microscopy analysis demonstrates the difference in the bonding formation
during the isothermal and non-isothermal processes. In order to define the
impact of the different formation on the bonding degree, the drum peel test
was performed.
The drum peel test (Figure 13) is used to determine the bonding quality
between a face sheet and a honeycomb core of a sandwich compound. The
peel resistance is defined by peeling one face sheet around a circular drum11,12
The drum peel trials were performed for two variants, isothermal and non.
isothermal
(Figure 14). All tested specimens show great results and reach the
minimum peel resistance value (1 N/mm). While the peel resistance of the
isothermal variant is 2 N/mm, the non-isothermal panel reached 6 N/mm.
As shown in the Figure 15, the non-isothermal variant has three times
greater peel resistance as isothermal one. It can be explained by the different
bonding formation during compression molding. As described in the
microscopy analysis, the degree of bonding in the non-isothermal variant is
determined by penetration depth and contact length. In isothermal variant, it
is only defined by contact length.
CONCLUSIONS
The application of the thermoplastic mono-material sandwich panels has
many advantages as recyclability, reduction of processing cycle times and
42 | SAMPE JOURNAL | MARCH/APRIL 2022
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Figure 12. Development of interface contact
during non-isothermal process.
Figure 11. Development of interface contact
during isothermal process.
Figure 10. Formation of meniscus in thermoset
sandwich structure.
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SAMPE Journal - March/April 2022

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