SAMPE Journal - September/October 2016 - 51

Feature Article

Figure 5. The extrusion process
and its characteristics.

main disadvantage of PU in pultrusion, which is sticking
of the material to the die causing a deformed crosssection of the die and the profile. To avoid this, costly
internal release agents (IRA) have to be added to the PU
matrix10-14.
The process-integrated inline joining of the reactive
thermoset matrix with a thermoplastic top layer requires
the following key characteristics of the matrix material:
- To approach the (usually higher) extrusion speed
with the pultrusion line, a low start viscosity for a fast
impregnation and a fast curing are required.
- In order to reach a good joining quality between
thermoset matrix system and thermoplastic top layer, a
good adhesion of the reactive matrix system is required.
PU performs particularly well in the above mentioned
characteristics, which makes it the material of choice.
Furthermore, due to its polarity, it is well suited for
joining it with well-established thermoplastics like polar
polyamide.
The Extrusion Process
Process Characteristics
The extrusion process (Figure 5) starts with the
thermoplastic granulate which is dragged into the
rotating screw and subsequently melted by shear
warming and conductive heat transfer. At the end of the
extruder, the thermoplastic melt continuously passes
through a die which forms the profile. In the following
calibration unit, the melt profile is physically frozen in its
shape. The profile is further chilled in a cooling zone and
finally hauled off at low forces, cut and stored.
Taking a look at the temperature and pressure
trajectories (Figure 5, including the respective
pultrusion characteristics for comparison), one notes
the different extrusion characteristics compared to
pultrusion. The granulate temperature starts at ambient
temperature and is gradually raised in the extruder to
melt processing temperature. Depending on material
type and die geometry, die temperatures differ between
SAMPE Journal, Volume 52, No. 5, September/October 2016

usually 180°C to 260°C which is up to 180 K over usual
pultrusion temperatures. In the calibration unit, the profile
temperature is fast lowered below melt temperature and
subsequently approximates ambient temperature15-16.
Concerning the pressure level, there is also a big gap
of up to 300 bar between pultrusion and extrusion. While
pultrusion works mostly by drag flow without a pressure
gradient, extrusion relies on a melt pressure of up to 300
bar to overcome the flow obstacles of the high-viscous
melt.
Resulting Challenges Posed by Temperature and Pressure
The above mentioned different temperature and
pressure levels and developments result in three major
challenges for the combination of the two processes.
These are:
- Risk of thermal degradation of the PU matrix: The
higher processing temperature of a thermoplastic melt
can lead to thermal degradation of the PU matrix during
coating.
- Risk of mechanical deformation of the PU matrix:
The high pressure of the thermoplastic melt might lead to
deformations of the thermoset composite profile, when
the two meet at a point where the thermoset matrix is
still liquid or gel.
- Risk of unsteady flow phenomena during continuous
joining: The different flow types of the single processes
(drag flow in pultrusion, pressure driven flow in extrusion)
might lead to unsteady flow conditions in the joining
areas, leading to poor joining quality and waviness of
the surface.
Besides the above mentioned process related
challenges, material related challenges apply. As
mentioned previously, for a good joining quality,
chemically compatible materials (polarity, functional
groups) have to be used.

Table of Contents for the Digital Edition of SAMPE Journal - September/October 2016