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Continuous Timber Fibre Placement
Towards the Design and Robotic Fabrication
of High-Resolution Timber Structures
Mohamed Dawod1(B
), Arjen Deetman1, Zuardin Akbar1, Jannis Heise2,
Stefan B¨ohm2, Heike Klussmann3, and Philipp Eversmann1
1Institute for Architecture,
Chair of Experimental and Digital Design and Construction, University of Kassel,
Universit¨atsplatz 9, 34109 Kassel, Germany
dawod@asl.uni-kassel.de
2Institute of Production Technology and Logistics,
Chair for Cutting and Joining Manufacturing Processes, University of Kassel,
Kurt-Wolters-Str. 3, 34125 Kassel, Germany
3Institute for Architecture, Research Platform Building Art Invention,
University of Kassel, Henschelstraße 2, 34127 Kassel, Germany
Abstract. Advances in Additive Manufacturing (AM) techniques have
expanded the possibilities to fabricate unique shapes, offering various
advantages over traditional manufacturing techniques concerning mate-
rial efficiency, product customisation and process control. AM using
organic materials such as wood has been introduced by the combina-
tion with polymers to produce 3D printing filaments. These filaments
use ground wood and therefore eliminate long fibres of naturally grown
timber, losing its inherent material qualities such as anisotropy and struc-
tural performance. This research investigates strategies for a novel AM
process using continuous solid wood to fabricate high-resolution material-
efficient timber structures based on topology optimization. We exam-
ined this novel AM process in three work packages: material production,
robotic fibre placement process and a design method through topol-
ogy optimisation. The developed robotic fabrication process enables the
deployment and extrusion of a novel material: a continuous solid wood
filament made of willow withies. This process allows for a high degree of
geometric freedom to assemble timber to create homogeneous structures
at high resolution, providing the aesthetics and structural advantages of
wood on a micro scale and therefore giving entirely new possibilities for
timber construction.
Continuous Timbre Fibre Placement is part of the research projects TETHOK – Textile
Tectonics for Wood Construction, funded by the Programmlinie Zukunft of the Univer-
sity of Kassel and FLIGNUM – Solid-Wood Monofilament, funded by the Fachagentur
Nachwachsende Rohstoffe e.V. We would like to thank the students that participated
in the studio project “Robotic Wood Printing” at the University of Kassel, especially
Ole Weyhe for the development of the material flattening tool.
c
Springer Nature Switzerland AG 2020
C. Gengnagel et al. (Eds.): DMSB 2019, Impact: Design With All Senses, pp. 460–473, 2020.
https://doi.org/10.1007/978-3-030-29829-6_36
Continuous Timber Fibre Placement 461
Keywords: Additive Manufacturing (AM) ·
Continuous Fibre Manufacturing (CFM) ·Robotic fabrication ·
Wood structure ·High-resolution structure ·Topology optimization
1 Introduction
Advances in Additive Manufacturing (AM) techniques have expanded the pos-
sibilities to fabricate unique shapes by applying thin layers of material, offering
several advantages over traditional manufacturing techniques concerning mate-
rial efficiency, product customisation and process control. AM using organic
materials such as wood has been introduced by the combination with polymers
to produce 3D printing filaments for Fused Deposition Modelling (FDM) [1].
Commercially available products are made of a mixture of thermoplastic and
ground wood. In these products, the long fibres of the naturally grown timber
no longer exist, losing the inherent material qualities of timber such as anisotropy
and structural performance. Unlike 3D-printing, AM takes many forms beyond
the common belief that those terms are similar. 3D printing itself covers a wide
range of techniques of manufacturing like powder based, solid-based and liquid-
based methods [2,3]. Another form of AM is to apply fibres automatically to
create extremely lightweight and high-performance composite structures. Well-
known techniques are “Automated Fibres Placement” (AFP) and “Automated
Tape Laying” (ATL), which are widely used in the aerospace industry. They
allow numerically controlled machines to lay one or several layers of compos-
ite carbon fibres on a mould in a specific direction and density to achieve a
desired structural performance with an overall material and production con-
trol and efficiency [4,5]. Other methods like Continuous Fibre Manufacturing
(CFM) allow fibres to be used as a reinforcement in multi-axis 3D printing
processes, by combining it with processes of Fused Filament Fabrication (FFF)
or photo-polymerization technology, or both with thermoplastics, allowing the
creation of highly reinforced lightweight structures [6,7]. While these methods
predominately work with synthetic composite materials of carbon or glass fibres
impregnated with thermoset or thermoplastic resin, the application of such pro-
cesses with solid organic fibres like wood has not been explored yet. This raises
the question if it possible to use bio-based fibres such as wood in its solid-state
for AM processes. If so, is it possible to benefit from its tensile strength to
create high-resolution load-bearing structures? Therefore, this research investi-
gates strategies for a novel AM process, using continuous solid-wood filaments
to fabricate high-resolution and material-efficient timber structures based on the
method of topology optimization.
This paper is structured as the following: In the Sect. 2“Methods”, we
describe a novel AM process using a solid wood filament in three work phases:
(1) Material production process to create a continuous wood fibre filament. (2)
Creation of a robotic fibre placement tool and process, as well as investigation
of adhesive systems. (3) Computational design process to design high-resolution
structural elements, while ensuring material efficiency by utilising a tailored
462 M. Dawod et al.
topology optimisation model. In Sect. 3“Results”, we evaluate the developed
material in terms of shape precision, strength and the adhesive used. The design
and fabrication process were assessed through the making of a high-resolution
beam as a demonstrator. In Sect. 4, we conclude by discussing the overall per-
formance of the process and the advantages and limitations raised in each phase
and the possibilities this process holds.
1.1 Motivation
The research described in this paper is part of an ongoing interdisciplinary
research cluster consisting of six professorships in the fields of architecture, arts,
fabrication technology, simulation technology, wood structures and material sci-
ence – to develop a continuous filament made of solid wood through automated
processing of willow withes – a rapidly renewable resource in Germany and
Europe.
1.2 Contribution
The main contribution of this paper is a novel AM process using a continuous
solid wood filament to create structural load-bearing elements, providing the
aesthetics and structural advantages of wood on a micro scale and therefore
giving entirely new possibilities for timber construction.
2 Methods
2.1 Material Production
The raw material used for creating our solid wood filament is split willow with-
ies. These strips are produced by splitting off the outer surface of one-year-old
Fig. 1. The raw material used. Left original willow withes branches after harvesting
[8]. Right split willow withes strips.
Continuous Timber Fibre Placement 463
withes using a cutter machine. They are extremely flexible, but very heteroge-
neous, as they vary in length (1.20–1.40m) and the cross section is inconsistent
(4.9–6.5 mm in width and 0.7–1.4 mm in thickness) (see Fig. 1). Therefore, we
developed several tools for homogenising the strips to ensure a constant filament
property. This process goes through two steps (1) by trimming the profiles to
a specific width and height. (2) joining the material through a combination of
glueing and spatial joining to create a continuous filament.
For homogenising the willow, we tested and adopted several tools to control
the thickness of the material like using an automated wood planning tool or
cutter milling tools with high-speed motors. Due to the flexibility of the milling
tools to be customised, we designed an adjustable trimming device which con-
sists of four compact routers with milling heads. It trims first the top and bottom
to achieve a specific thickness using two routers; then another two routers were
used to cut the sides to the desired width (see Fig. 2). Furthermore, we created
a mechanical process to join individual strips together to obtain an endless fila-
ment automatically. Besides using a simple end to end overlap joint, we are also
investigating other micro joining solutions to achieve a stronger overlap bonding
and strength.
Overlap Joining
I - The Raw Material
II- Surface Trimming
cutter milling tool
cutter milling tool
III- Side Trimming
Joining
Trimming
Processed Wood
Extruder
Feeder one
Feeder Two
Extruder
adhesive
Fig. 2. A diagram showing the material production process. Left the trimming process,
to customize the material profile in width and thickness. Right the joinery method used
to create an endless material.
2.2 Robotic Fabrication Process
Our timber fibre placement process has several challenges compared to common
3D printing methods. Using continuous fibres in an AM process requires the
material to be cut and reapplied during the process. Nonetheless, choosing the
464 M. Dawod et al.
correct adhesive considered to be the most crucial part of this process. Unlike
other processes where the glue is either mixed or impregnated with the material,
the glue in our case must be applied directly on the material outer surface.
Adhesives Investigation. Using thermoset or thermoplastic adhesion typi-
cally used in an AFP/ATL process was not an option due to their relatively
high melting or heat activation point. Therefore, we investigated other safer
alternatives, which could reduce possible damage to the willow. The selection
of the most suitable adhesives was defined based on the differing and small
joints, fast achievement of the initial strength and simple processing with the
robotic arm. Moreover, the price of the adhesive was taken into account to
ensure a cost-effective printing process. Depending on the later use of the solid-
wood filament properties like water and heat resistance can be necessary. Due to
these requirements, a preselection of adhesives was implemented. We examined
Cyanoacrylates and UV radiation curing systems as chemically reacting adhe-
sives and contact adhesives as a physically bonding adhesive. In our initial tests,
the bonding surfaces were mechanically pre-treated through grinding and clean-
ing. From the results discussed in Sect. 3, the contact adhesive demonstrates the
highest potential according to the above criteria.
Compaction Roller
Cutting Blade
Guiding system Tool Center Point TCP
Extruder Head θ
Stepper Motor
Movement Direction
Fiber Placement Tool
Wood Filament
Force Direction
Fig. 3. A diagram showing the main components used in developing the wood filament
placement tool. The tool features an extruder, a cutting blade, a guiding system and a
silicon compaction roller. The extruder controls the material deposition. The cutting
blade is used to cut the filament at a certain length. The guiding system is a channel
used to guide the filament to the lay-down zone after being cut. The silicon compaction
roller is used to activate the glue through applying pressure on the fibres against the
workpiece using the robotic arm. The whole system is oriented at an angle less than
45◦to avoid the breakage of the filament during impact with the compaction roller and
the application area.
Continuous Timber Fibre Placement 465
Robotic Tool Development. We developed a robotic fabrication process
which applies the continuous wood-filament through a spool and an extrusion
method similar to AFP/ATL techniques. We tested multiple methods to coat
the filament with the contact adhesive like attaching a resin container to the
tool or pre-coating the material before the application time. Pre-coating the fil-
ament was optimal as it allows the adhesive to dry out on the bonding surface,
which makes it faster to bond and ready for instant application just by applying
local high pressure. Based on that, we developed a robotic end effector which
features an extruder, a cutting blade, a guiding system and a silicon compaction
roller. The extruder controls the material deposition by a stepper motor and an
Arduino board to control the speed relative to the robot movement. The cutting
blade is used to cut the filament at a certain length and is controlled by a pneu-
matic actuator with an electromagnetic valve. These two devices are connected
and regulated through the I/O of the industrial robot. The guiding system is
a channel used to guide the filament to the lay-down zone after being cut. The
silicon compaction roller is used to activate the glue through applying pressure
on the fibres against the workpiece using the robotic arm. The whole system is
oriented at an angle less than 45◦to avoid the breakage of the filament during
impact with the compaction roller and the application area. (See Fig. 3).
Robotic Path Planning. Robotic movement of a single extrusion head is
defined by following a trajectory which represents a single wood strip. In the
case of curved paths, the flexibility provided by the loaded wood filament needs
to be considered. The robotic trajectories are planned as follows: the robot moves
to the starting point of the wood filament that will be placed. From that point,
the extrusion and movement along the path starts at the same time. Having the
cutting point at 50 mm distance to the tool centre point (TCP), we stop the
movement and extrusion 50 mm before the endpoint to cut the material. After
cutting we continue the movement only without extrusion to the end of the
wood filament. In the end, we reload the material by extruding it 50mm until it
reaches the TCP again. All the steps are repeated for the placement of the next
wood filament. Since the cutting device is placed inside our end-effector, we can
only place wood filaments with a minimum length of 50 mm.
2.3 Design Using Topology Optimisation
Our robotic fabrication process enables the design of architectural components
at a higher resolution compared to current engineered timber products such as
Cross-laminated timber (CLT) [9], allowing the design at a scale of the material
itself. This necessitates the application of computational design processes which
can deal with a large number of elements and can optimise material usage. As we
are aiming for making structural elements, we developed a topology optimisation
model to optimize the material distribution for a given ground-structure with
an objective to maximize the stiffness of the structure. The optimisation process
was carried out as in the 88 line MATLAB code of Andreassen et al. [10]. We
466 M. Dawod et al.
f = 1
x
z
300
50
1
1
Fig. 4. The boundary conditions, loads and mesh layout of the Messerschmitt-B¨olkow-
Blohm (MBB) beam that is used to test the Topology Optimization model. The beam
with the shown mesh has 60,350 finite elements, 15,351 nodes and 30,699 free degrees
of freedom.
integrated two major changes to get a representation of the used solid wood
filaments. First, the design domain was discretized by truss elements instead
of continuum elements as shown in Fig. 4. Second, instead of using the filter
radius for sensitivity filtering, we filtered the sensitivities in one direction. For
filtering the sensitivities of each finite element, only the adjacent elements were
considered that are part of the set of where the considered finite element belongs
to. For this research one set of elements formed a straight line that represents a
single solid wood filament (Fig. 5).
2.4 Demonstrator
As a demonstrator, we designed a beam using the topology optimisation process
with a size of (500–160 mm) and truss grid resolution of 10 mm. This resulted
a mesh of 3,266 finite elements, 867 finite element nodes, 1,706 free degrees of
freedom. The topology optimization model gives as result a dense set of lines
that represent the finite elements and the relative densities of the finite elements
(0–1). We applied a simple post-processing filter with a threshold value. For this
demonstrator we used a threshold value of 0.5, which means that only elements
Continuous Timber Fibre Placement 467
Fig. 5. The results obtained from the topology optimization model for the MBB beam
with four different element orientation angles. Lef t the result is shown with intermediate
densities. Right the results are shown were the post processing filter is applied. The
filtering lengths are set to respectively 8, 16, 32 and 64. The volume fraction was set
to 0.50
were used with a relative density of at least 0.5. From this set of lines, we
applied a rationalisation process to create the tool-paths for robotic fabrication,
by grouping and connecting the distinct type of segments forming continuous
straight lines. Then these paths were sorted based on their orientation to create
an optimal printing sequence. Afterwards, we explored two placement techniques;
(1) (Layered) we placed every unique set of lines on a separate level (see Fig. 6
Left). (2) (Interwoven) we placed a line from every unique set in a sequence,
generating an interweaving effect (see Fig. 6Right). By applying a placement
Fig. 6. A diagram showing the different placement techniques. Left showing the layered
technique where every unique set of lines with the same orientation are stacked on top
of each other. Right showing the interwoven technique where a line from every unique
set of lines with the orientation is placed in a sequence.
468 M. Dawod et al.
Fig. 7. The fabrication setup consists of an industrial robot, a robotic end effector (see
detailed description Sect. 2.2), a linear axis, material feeding and the fabrication pad.
Fig. 8. The produced material. Left, the trimmed wood material (on the right) com-
pared to the original one (on the left). Right, an analysis showing the curvature of the
material before (on the top) and after (in the bottom).
method, we multiplied the resulted layers four times vertically, this made the
overall object thickness 16 mm using a 1 mm thickness filament.
For our extrusion method, we used an industrial robot placed on an additional
linear axis to create a working area of approximately 2 by 0.5 m (See Fig. 7).
We used a processed willow filament (See material production process in the
Sect. 2.1) with a thickness of 1 mm and width of 5 mm. This filament was coated
manually on both sides with contact glue.
Continuous Timber Fibre Placement 469
3 Results
3.1 Material - Continuous Solid Wood Filament
Through our material production process, we were able to create an endless
semi-standard wooden filament using willow withes as raw material. As the wil-
low withes are naturally curved in different directions along its axis, we could
only homogenize the strips to a certain extent (see Fig. 8). Depending on the
chosen machining process, the produced filament reaches different grades of
straightness with differing value fluctuation concerning the material width or
thickness. As the non-homogenized basic material shows mainly cross sections
of 4.9–6.5 ×0.7–1.4 mm [width/thickness] and tends to distort and deflect in the
processing procedure, it is mandatory to adapt known wood-technical processes
in accordance with these characteristics (see Fig. 9a, b). For this purpose, we
examined an automated planning, milling and sawing with regard to process-
ability, reproducibility and achievable material properties. As an example, it is
currently possible to process 100 strips of a length 140 cm, 5 mm in width and a
thickness of 1 mm in around 45 min on average with a dual-side milling machine
in a close tolerance range. The initial test of maximum tensile stress of the
processed material showed a reduced data spread in comparison with the basic
material. Furthermore, for a large part of the samples an improvement of the
tensile stress regarding the cross section is observable. Especially samples with
planed surfaces indicate to a large extend improved properties. An enhancement
of the tensile strength for homogenized willow withes may surmise from a reduc-
tion of stress peaks due to the curved shape of the basic material and the clearing
of smaller defects going along with the material abrasion (see Fig. 9c).
0
50
100
150
200
250
Basic
Material
Planed
Surface
Milled
Surface
Milled
Edges
MPa (c) Max-Tensile Stress - Profile Reference
0
1
2
3
4
5
6
7
8
Basic Material Milled Edge 3.1 Milled Edge 2.1
mm (b) Edges Trimming
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
Basic Material Planed Surface Milled Surface
mm (a) Surface Trimming
Fig. 9. The test results of the homogenisation process of the material and the maximum
tensile stress test. Graph (a, b) shows the material thickness and width variation before
and after processing, respectively, (a) represents the material after processing using a
wood slicer and a cutter milling tool, respectively and (b) the material after trimming
the side to reach a width of 3.1 and 2.1 mm. Graph (c) shows the maximum tensile
stress test in four cases (original material, planed surface, milled surface and a milled
edge at 3.1 mm thickness).
470 M. Dawod et al.
3.2 Adhesives
During the robotic application, these various adhesive systems show differing
challenges and thereto related bonding results. Cyanoacrylates enable a rela-
tively fast joint of the willow in 4–7 s, however the precise application on the
small surface is difficult due to the low viscosity and a clogging of the appli-
cation nozzle is observable. Furthermore, the sensitivity to water might be a
problem for later use. This disadvantage does not occur while the usage of radi-
ation curing systems at which activation of the applied adhesive takes place
by irradiation with ultra-violet (UV) light. The challenge with this method is to
ensure full exposure of the non-transparent and partially difficult to access adher-
ents. Special UV curing adhesives, which enable a pre-activation are promising
alternatives, however, they come along with increasing material expenses. At the
current point, contact adhesives seem to demonstrate the highest potential for
continuous wood fibre placement. Depending on the workpiece geometry, defined
areas or the whole wooden filament can be pre-coated with the adhesive. After
the necessary drying, joining through precisely applied pressure is quickly achiev-
able. This fast processing within the main process makes the contact adhesive
attractive for the research project. Yet there are also challenges like the strict
adherence of contact bonding time, precise adjustment of force being applied
and visual quality of the joint (Fig. 10).
Fig. 10. This image of the fabrication process shows the interweaving pattern resulting
from the strips placing sequence.
Continuous Timber Fibre Placement 471
3.3 Demonstrator
Our robotic fabrication system was able to successfully place the wooden fila-
ments with a high degree of freedom and relatively high precision. Challenges
for the placement precision remain the tolerances because of the strips inherited
wavy nature. Our robotic fabrication process shows high efficiency in terms of
speed ∼100 mm/s (up to potentially 250mm/s on robot full speed) in compari-
son to 3d printing methods as of Fused Filament Fabrication with wood filaments
∼40 mm/s. This is due to the coated material needing only localized pressure to
activate the glue and enabling an instant structural connection between layers.
The fabrication time for the beam was 64 min. We coated the material at intervals
to overcome the maximum application time of 60 min of our adhesive. In terms
of the design, the topologically optimised beam demonstrates the different levels
of stiffness in localised areas through the number of intersections, superposition
and varying lengths. The intertwined placement showed better performance in
terms of stiffness against the layered one. Nevertheless, a design-related irregular
distribution of the material and therefore the high density of strips caused an
uneven height incrementation of material in certain parts of the prototype. This
results in an inconsistent height profile of the structure (Fig.11).
Fig. 11. The high-resolution topology optimised beam.
472 M. Dawod et al.
4 Conclusion and Outlook
Our robotic fabrication process enables the deployment and extrusion of a novel
material: a continuous solid wood filament. The method allows for a high degree
of geometric freedom to assemble wood at high resolution to create material effi-
cient structures. The homogenised filament shows a better performance than the
raw willow withies concerning the maximum tensile stress, due to the elimination
of local material defects. Contact adhesives demonstrate the highest potential for
the process in terms of fast application, strength, cost and durability. The topol-
ogy optimised beam shows different levels of stiffness in localised areas through
the number of intersections, superposition and varying lengths. Moreover, the
nature of the glueing method affects the overall process and defines the type of
application. We are planning structural experiments on the demonstrated beam
to validate and measure its performance and the mechanical behaviour of our
material. We are also planning to investigate bio-based adhesives, which can be
applied and reactivated in more extended periods. As the topology optimisa-
tion is currently used in the 2D model, we are planning to scale it to a higher
resolution 3D model. In addition, we want to investigate feedback processes
through the integration of force sensors, allowing adaptive tool path refinement
and increase the fabrication speed by adding multi-filament printing slots to the
fibre placement tool. Furthermore, we want to integrate more robotic fabrica-
tion logic into the topology optimisation process and extend it with strength
and stability constraints.
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