Plastics News Europe - September 2019 - 19

additive manufacturing

Thermosets are polymeric materials
that irreversibly solidify due to
cross-linking between molecules. The
solidification process of thermosets is
dominated by an exothermic chemical reaction, the curing reaction.
Heat, mixing, or both can activate the
curing reaction. As curing progresses
and the molecules begin to cross-link,
the viscosity of a thermoset increases
due to the emerging three-dimensional molecular network and declining mobility of the molecules. The
competing mechanisms of flow and
curing reaction must be considered
during processing of thermosetting
resins. Due to the exothermic reaction, the temperature of the thermosetting resin rises and its viscosity decreases, which facilitates fibre
impregnation. At the same time, the
high temperatures result in an accelerated cross-linking reaction, which

september 2019

ent

Matrix materials

Direction of travel
m
Fila

Heat
source

Compaction roller

Continuous carbon fibre
Build plate
Schematic of the DED process
Qualitive influence of fibre length
Glass fibre in polypropylene via injection moulding

1.0
0.8
Performance level

into short and long fibre-reinforced
composites. The ratio of fibre length
to fibre diameter, or aspect ratio, is an
important property of discontinuous
fibre composites. Long fibres are
those whose critical length is significantly longer than the critical length
required to transfer the load from the
matrix to the fibre. Long fibres are
considered to exceed an aspect ratio
of 100, whereas short fibres have an
aspect ratio of less than 100. Figure 1
shows the change of stiffness,
strength and toughness as a function
of fibre length. It can be seen that the
addition of short fibres already has a
significant effect on the stiffness of
the composite, whereas the strength
and especially the toughness really
only increases with the length of the
fibres and reach their full potential
with continuous fibres.
Continuous fibre-reinforced composites are used in structural applications where increased stiffness,
strength and toughness are required.
By stacking single plies of continuous
fibres, composites are often made into
laminates. The fibres in each ply have
a defined orientation and the arrangement of the plies is tailored to enhance
the strength of the composite in the
primary load directions. This often
leads to the use of more fibre than is
needed for a given application.
Fibre-reinforced composite materials are produced in different ways,
dependent on the type of matrix material chosen. Polymers can be classified as thermoplastics or thermosets based on their molecular
structure and material properties.
Thermosetting and thermoplastic
matrices need different process control due to their fundamentally different flow and solidification behaviour and other characteristics.

Toughness

0.6

Strength

Stiffness

0.4
0.2
0.0
0.1

1.0

Short fibre

Long fibre

100

Fibre length, mm
Figure 1 Effect of fibre length on the normalised stiffness, strength
and impact performance of a fibre-reinforced composite

causes the viscosity to increase faster.
During this transition, the viscosity increases until the resin ceases to flow.
At this gel point, all the molecules are
interconnected.
A major advantage of thermosets
in composite manufacturing is the
very low initial viscosity of the matrix,
which leads to an easy and fast impregnation of the fibres. In general,
cured thermosets are harder, more
rigid and brittle, and their mechanical
properties are not heat sensitive.
However, since the solidification process is a chemical curing reaction,
thermosets have longer processing
times compared to thermoplastics.
In a thermoplastic polymer, the
molecules are not cross-linked with
each other. Instead, the individual
molecules are held in place by weaker
intermolecular forces such as van der
Waals forces and hydrogen bonds. In
polymer processes, thermoplastics solidify as they are cooled from the
molten state and the long molecules
are no longer allowed to move freely.
When reheated, these materials regain the ability to "flow" and the
molecules are able to slide past each
other with ease. Thermoplastic polymers in the molten state have a fairly

random molecular structure. They
harden, or vitrify, as they are cooled.
This solidification and melting process is reversible.
Compared to thermoset based
composites, fibre-reinforced thermoplastics show advantages such as
higher impact strength, shorter cycle
times, and recyclability. They can be
welded onto other components
(mostly other thermoplastics, but also
metal in some cases). However, the
high viscosity of a thermoplastic resin
makes it more difficult to impregnate
the fibres and the surface finish can
be poor.

Polymer DED process
Currently, the most reliable method
available to produce fibre-reinforced
composite laminate materials is
through autoclave moulding. The
pressure and temperature in the autoclave compress the layers, creating
dense and void-free materials. To
date, this has been the only process
able to eradicate such defects as
voids, resin-rich areas, and delamination during production of the material - all of which have a detrimental

➡ Continued on page 20

Plastics News Europe - September 2019

Table of Contents for the Digital Edition of Plastics News Europe - September 2019