JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022 - 88

TECHNOLOGY additive manufacturing
between the plastic and the cured binder depends largely on the number
of free functional polar groups of the interacting molecules, although it is
almost impossible to objectively evaluate the role of the chemical mechanism
of adhesion among all other adhesion interactions.
Nevertheless, on the surface of the cured binder, there are free hydroxyl
(-OH) groups that have not started curing and form a chain. In a polyamide
molecule, carboxyl (-COOH) and amino (-NH2
) groups act as functional
reactive polar groups. Thus, the formation of hydrogen bonds at the
adhesive-substrate interface becomes possible.
In a PETG molecule, the functional groups are hydroxyl (-OH) and carboxyl
(-COOH), which are also capable of forming hydrogen bonds.
However, it is hard to estimate the exact number of functional end groups
at the polymer ends since it is strongly dependent on the molecular weight,
synthesis conditions, and the number of precursors involved in the polymerisation
reaction. According to the chemical interaction, we also distinguish
polyamide, because if there are unreacted epoxy groups at the interface,
the presence of a primary amine (-NH2
) at the end of the polyamide
chain can serve as an additional hardener for a given system: it forms an
already strong covalent bond provided that the impregnated fibre and plastic
interact under high temperatures.
At any rate, due to the adsorption forces, the presence of polar groups in
the PETG and PA monomer molecules has a positive effect on the adhesive
strength to the epoxy-based binder. From this point of view, the number
of carbonyl (-C=O) groups on the surface of the PETG molecule is
greater than that of PA, which gives us reason to expect reliable adsorption
interactions during printing. In addition to the adhesive strength, the final
properties of the composite material can be influenced by cohesion. The
final product contains composite plastic and infill plastic in addition to fibre.
It can be either the same plastic or two materials with different properties.
If the optimal laying parameters are correctly selected for each pair of
materials, the entire load applied to the product rests on the fibre. But still
in some cases, depending on the nature of the applied load, failure of the
sample may occur due to poor cohesion. In CFC 3D printing, we use the
following pairs: CFC PA (PA12)-Smooth PA (Polymaker) and PETG-
PETG (by REC). Using the same material allows high strength characteristics
to be achieved in the composite material. The properties of some of
the plastics used in CFC technology are shown in Table 2.
Main parameters of CFC
3D printing with PETG and CFC PA
The pair of plastics used in CFC printing is called a profile. Below, we will
look at the two basic profiles available in Anisoprint's Aura slicer:
- PETG (Clear) - PETG (Clear) by REC
- PA12 (CFC) - PA12+10% carbon fibre (Smooth PA) by Polymaker
Reinforcing fibre used for CFC 3D printing: carbon fibre (1.5k monofilaments)
and basalt fibre impregnated with epoxy-containing binder.
A lot of parameters influence the printing process. The settings of each
profile are carefully selected to achieve the best properties in the resulting
composites. Physical and mechanical characteristics of 3D printed composite
materials also largely depend not only on the materials used, but
also on the printing parameters themselves. There are about 300 adjustable
printing parameters, but nevertheless it is possible to distinguish several
parameters that can have the greatest influence on the laying process
and properties of the final composite:
88 jec composites magazine / N°
146 June - July 2022
Tab.2: The properties of some of the plastics used in CFC technology
PETG
Testing
method
Young's modulus
(MPa) (X-Y)
Tensile strength
(MPa) (X-Y)
Elongation at break
(%) (X-Y)
Bending modulus
(MPa) (X-Y)
Bending strength
(MPa) (X-Y)
ASTM
D638
ASTM
D638
ASTM
D638
ASTM
D790
ASTM
D790
PA12
(CFC)*
1580
56.8
15.86
1580
69
(Clear)**
1120
36.5
2.41
2060
76
PA12+10%carb. f.
(Smooth)***
3304
71.6
3.57
3535
110
*Nozzle temperature = 260 °C, printing speed = 50 mm/s, chamber temperature = 40 °C
Specimens were annealed at 80 °C for 60 min and dried for 48h prior to testing.
**Nozzle temperature = 235 °C, printing speed = 50 mm/s, chamber temperature = 40 °C
***Nozzle temperature = 280 °C, printing speed = 45 mm/s, bed temperature = 50 °C
Specimens were annealed at 100 °C for 6h and conditioned at room temperature for 24h prior to testing.
- Printing temperature;
- Fibre printing speed;
- Layer height and width;
- Plastic flow multiplier.
Naturally, these are not the only important parameters affecting the final
properties of the composite, later we show the influence of these parameters
on CFC 3D printing. Changing each of them directly affects the
properties of the resulting composite.
The key difference between the CFC technology and the FDM process
is that the plastic remains in the melt state longer than usual and interacts
with the fibre before it is laid out on the working surface, so the temperature
parameter is particularly important in CFC processing to prevent
polymer degradation. The beginning of the degradation process can be
traced by a change in melt colour, a sharp decrease in viscosity, and uneven
layup. The processing temperature for each of the plastics presented
differs. The recommended processing temperature for PA12 (CFC PA),
declared by Polymaker, its manufacturer, is 250-260 °C. The processing
temperature for PETG (REC) is in the range of 230-240 °C and these
temperatures can vary depending on the desired final product properties
and processing method. It should be remembered that the nature of both
plastics is different and PETG, being mainly an amorphous polymer in
the processing, does not have clearly defined temperature boundaries of
phase transitions in contrast to semi-crystalline PA 12. Also, during the
transition of the polymer into the melt, depending on the degree of crystallinity,
the density will change and, accordingly, viscosity, which greatly
affects the behaviour of the polymer and the nature of its transition into
the viscous-fluid state in the processing. During the printing process, the
fibre deposition rate varies from 2 to 20 mm/s depending on the product
configuration. The fibre deposition speed affects how much time the
melt remains in the chamber of the composite extruder and the strength
between the printed layers. Usually, when the printing speed is increased,
the interlayer adhesion becomes weak because the melt does not have
time to interact with the lower layer to form a strong 'weld' and at the
moment of exposure the melt, depending on viscosity, may not properly
wet the fibre itself. It is possible to increase the deposition rate along
with increasing the processing temperature, but there is a risk of polymer
degradation and, consequently, deterioration of physical and mechani

JEC COMPOSITES MAGAZINE - Issue #146 - June/July 2022

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