22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma spraying for selective metallization of 3D polymer surfaces
J. Liebeskind1 and B. Dzur2
1
Robert Bosch GmbH, Stuttgart, Germany
2
TU-Ilmenau, Ilmenau, Germany
Abstract: A new application of plasma spraying is introduced as a technology for
selective, mask-less metallization of 3D polymer surfaces. Limits for the 3D capability of
the process are investigated to derive design considerations for a spraying setup optimized
for maximum 3D capability. These design considerations are used to implement an
experimental setup and successfully prove the technology feasibility.
Keywords: selective metallization, plasma spraying, 3D surfaces
1. Introduction
Technologies for selective metallization are applied to
create surfaces which are not completely coated with
metal, but instead with a user defined layout of metal and
non-metal areas. These technologies especially focus on
plastic parts with complex 3D surfaces that would
otherwise require multi-component designs with separate
metal structures. The metal areas can be used for local
mechanical reinforcements, electromagnetic shielding or
conductive paths to form 3D circuit boards, also known as
Moulded Interconnect Devices [1].
One commercially available technology for selective
metallization is laser direct structuring (LDS). The LDS
process uses a special polymer blend that can be made
perceptive to electroplating by means of a laser [1]. This
allows selective electroplating by selective laser treatment
of the desired layout. As a major drawback, the material
cost for the required polymer becomes significant for
large parts. Also, the electroplating process has a large
environmental footprint and is time consuming and
therefore expensive when thick metal layers are required,
e.g. to conduct high electrical currents. Therefore,
industrial applications are yet limited to small parts with
comparatively thin metal layers.
In contrast to LDS, non-selective metallization
technologies are used even for large parts and thick metal
layers. These technologies include for example: physical
vapour deposition, chemical vapour deposition and
thermal spraying. They can be adapted to selective
metallization by masking non-metal areas of the desired
layout. While this can be easily done for 2D surfaces,
complex 3D surfaces require complex 3D masks. As an
additional drawback, these masks often have only limited
durability.
It has been shown that thermal spraying can also be
used as a mask-less technology for selective metallization
[2, 3]. By using small, well focused spray torches at close
range, metallization is applied only locally in an area
down to a few millimetres in diameter. Full 3D capability
is archived by mounting the spray torch on a 6 axis
industrial robot and moving the metallization area over
the substrate (fig. 1). As a major advantage compared to
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LDS, large parts and thick metal layers can be easily
archived with this technology. There are certain
requirements to the geometry of the part regarding
curvature and recessed surfaces. These limitations can be
kept to a minimum by carefully selecting the process
parameters and system components.
spray torch
substrate
6 axis robot
Fig. 1. Thermal spraying of 3D surfaces by means of a
6 axis industrial robot.
2. Spray process considerations
To achieve uniform coatings, the spray torch must be
moved over the substrate surface at constant distance,
speed and angle. The intersection point between the spray
and the surface is thereby set as the robot’s so called tool
centre point (TCP). The TCP defines the robot’s point of
rotation. The speed is also referenced to that point. For
contactless tools like spray torches, this point is outside of
the physical tool, which is particularly important for 3D
surfaces. For a path on a planar surface, the speed of the
TCP and the speed of the tool’s mounting point are equal.
But on a curved surface, the required mounting point
speed can be considerably higher than the TCP speed.
Figure 2 shows a rounded edge with the radius r between
two perpendicular surfaces. To keep the distance h
between a torch of length L and the surface constant,
different paths of the torch’s mounting plate are required
for the inner (concave) and outer (convex) edge. The
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mounting point speed v mount required for a TCP speed
v TCP on a convex surface calculates to:
𝐿+ℎ
𝑣𝑚𝑚𝑚𝑚𝑚 = 𝑣𝑇𝑇𝑇 �
+ 1�
𝑟
It can be easily seen that even for slow TCP speeds, the
required speed of the robot can be significant for large
spray torches, large spraying distances or small radii. The
mounting point speed required on a concave surface
calculates to:
𝐿+ℎ
− 1�
𝑣𝑚𝑚𝑚𝑚𝑚 = 𝑣𝑇𝑇𝑇 �
𝑟
payloads like large spray torches or spray torches with a
centre of mass far from the mounting point.
A minimum TCP speed is required to reduce the heat
flow to the substrate and avoid thermal damage in the
form of melting, decomposition or deformation. This
means that a setup is favourable with a low spray
temperature to reduce the minimum TCP speed and
increase the 3D capability of the process. Metallization at
low spray temperatures requires precise adjustment of
parameters to optimize deposition efficiency. Plasma
spraying is excellent for an accurate control of process
conditions [4, 5]. This makes it a good choice for low
temperature metallization. Also, because of the low
electrical input power required, a passive (convective)
cooling concept can be used instead of an active (water)
cooling concept. This allows a small and lightweight
spray torch which reduces the constraining contour of the
torch, further reduces the required robot speed and
reduces load based robotic acceleration restrictions.
3. Experimental plasma spray setup
In order to investigate the capabilities of low power
plasma spraying for selective metallization of polymer
surfaces, a setup consisting of a Reinhausen PB3 plasma
torch mounted on a Kawasaki RS005L industrial robot
was used. The Reinhausen PB3 plasma torch uses a
pulsed DC atmospheric nitrogen plasma with 1 kW
electrical input power. It features a passively cooled
copper nozzle with radial powder injection. With a mass
of 300 g it is lightweight enough to be a negligible load
for the robot. The upper limit for a robot’s speed is not
constant within the operating range of any given 6 axis
robot, but is dependent on the exact position within its 6
degrees of freedom. A typical value for the Kawasaki
RS005L about half way between its base and the
maximum operating range is v mount = 1000 mm/s. Copper
powder was used at a spray distance h = 10 mm and a
TCP velocity v TCP = 200 mm/s. With these parameters,
the copper powder gets hot enough to get reasonable
coating quality and the substrate keeps cool enough to not
take any damage. It was demonstrated that convex as well
as concave polyphenylene sulfide (PPS) surfaces with a
radius down to 5 mm can be selectively coated with the
same quality as a planar surface (fig. 3).
Fig. 2. Required path of the spray torch’s mounting
point to maintain constant distance and angle to an a)
convex and b) concave surface.
While this is lower than the speed required for a convex
surface with the same radius, another limit can be seen
from the robot’s path in figure 3 b. While the robot’s
change of direction is continuous for convex surfaces, it is
discontinuous for concave surfaces. This is a problem
especially for robots limited in acceleration by heavy
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Fig. 3. Convex and concave surfaces with selective
copper metallization from mask-less plasma spraying.
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4. Conclusions
The feasibility of thermal spraying for selective
metallization of 3D polymer surfaces was evaluated.
Several criteria for the selection of a spray setup were
worked out and plasma spraying was identified to best
meet these criteria. A plasma spray setup was successfully
demonstrated to be able to coat convex as well as concave
surfaces with a radius down to 5 mm. Its excellent 3D
performance scales very well over several orders of
magnitude from small to very large parts. This makes the
technology very distinctive and different from other
metallization technologies and interesting for potential
industrial applications.
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5. References
[1] R. Schramm, Structuring and Metallization. ThreeDimensional Molded Interconnect Devices, Carl Hanser
Verlag (2009)
[2] P. Fauchais et al., Quo vadis thermal spraying?.
Journal of Thermal Spray Technology, 10, 1 (2001)
[3] P. Fauchais et al., Developments in direct current
plasma spraying. Surface and Coatings Technology, 201,
5 (2006)
[4] T. Zhang et al., The influence of process parameters
on the degradation of thermally sprayed polymer coatings.
Surface and Coatings Technology, 96, 2–3 (2007)
[5] P. Fauchais, Understanding plasma spraying. Journal
of Physics D: Applied Physics, 37, 9 (2004)
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