3D Reactive Inkjet Printing of Polydimethylsiloxane

Abstract

Material jetting is a process whereby liquid material can be deposited onto a substrate to solidify. Through a process of progressive additional layers, this deposition can then be used to produce 3D structures. However, the current material jetting catalogue is limited owing to the constraints on the viscosity of inks that can be deposited. Most inks currently being used are either solvent or photocuring based, with the latter becoming increasingly popular due to increased throughput. Full Reactive Inkjet Printing (FRIJP) is an alternative processing method currently being investigated as a route to widen the material catalogue. FRIJP is the combination, on the substrate, of two reactive components which then react together in contact on the substrate. In this work a two-part polydimethylsiloxane (PDMS) ink has been developed, printed individually, and cured. The successful printing of PDMS has been used to fabricate complex 3D geometry for the first time using FRIJP. Through the use of a prepared substrate feature resolutions up to 48 ± 2 µm (X, Y) were possible. Curing analysis has been conducted. It was found that not only does the reaction occur to a similar degree to conventional processes, but that there is no variation in the cured sample when printed at elevated substrate temperatures.

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Cite this: J. Mater. Chem. C, 2017,
5,9733
3D reactive inkjet printing of polydimethylsiloxane†
Craig Sturgess, Christopher J. Tuck, Ian A. Ashcroft and Ricky D. Wildman *
Material jetting is a process whereby liquid material can be deposited onto a substrate to solidify.
Through a process of progressive additional layers, this deposition can then be used to produce 3D
structures. However, the current material jetting catalogue is limited owing to the constraints on the
viscosity of inks that can be deposited. Most inks currently being used are either solvent or photocuring
based, with the latter becoming increasingly popular due to increased throughput. Full Reactive Inkjet
Printing (FRIJP) is an alternative processing method currently being investigated as a route to widen the
material catalogue. FRIJP is the combination, on the substrate, of two reactive components which then
react together in contact on the substrate. In this work a two-part polydimethylsiloxane (PDMS) ink has
been developed, printed individually, and cured. The successful printing of PDMS has been used to
fabricate complex 3D geometry for the first time using FRIJP. Through the use of a prepared substrate
feature resolutions up to 48 2mm(X,Y) were possible. Curing analysis has been conducted. It was
found that not only does the reaction occur to a similar degree to conventional processes, but that
there is no variation in the cured sample when printed at elevated substrate temperatures.
Introduction
Additive Manufacturing (AM) is a method for producing complex
and bespoke designs from digital data without the need for
tooling or moulds.
1,2
The key to achieving this is fabrication
through the layer-wise addition of material. There are a number
of different technologies under the umbrella of AM, including:
powder bed fusion,
3
material extrusion,
4
photo-lithography,
5
binder jetting,
5
sheet lamination,
5
and material jetting.
6
An
advantage of material jetting over powder bed systems is the
capability to easily incorporate multiple materials within the
same structure in a fully 3D array. Examples include, combinations
of hard and soft materials, electrically conducting and insulating
polymers
7
and biomaterial components.
8
This multi-material cap-
ability enables extended functionality compared with traditional
single material AM. Coupled with the ease of co-deposition and the
high scalability of the process, this makes jetting an enabling
manufacturing process for the future.
The extension of inkjet printing technology into 3D printing
presents numerous challenges. Currently, the range of inkjet
materials suitable for AM is limited. Previous research has
focused on creating inks suitable for 2D deposition, where
the layer thickness is unimportant and material loading can be
low.
9
For 3D Printing, the volumetric throughput is related to
the layer thickness achievable given the functional loading of
the ink. Initial approaches relied on loading solvents with
nanoparticles
10
or polymer,
11–13
and evaporating the solvent to
leave the desired solid. Newer approaches using photocurable
14
and phase change materials
15
offer substantially increased
volumetric throughput in comparison, but are still limited in
their material range. A further alternative is to combine reactive
components to form the desired material during the manufacturing
process itself.
16
This fully reactive inkjet printing (FRIJP) inherits the
advantages of inkjet printing, including sub-micron scale droplet
positioning, sub-femtolitre volume control,
17
and high processing
speeds,
7
whilst broadening the palette of materials available for
additive manufacturing. Such a multi-component method has been
demonstrated for the case of addition polymerisation to produce
a polyurethane polymer,
18
in the formation of nylon,
19
and the
reduction of metals, such as copper.
20
Through this method,
liquid droplets can be processed without chemical modification
or external initiating steps.
Polydimethylsiloxane (PDMS) is a silicone elastomer that is
widely used due to its low cost, biocompatibility, and optical
transparency. One principle use of PDMS is the fabrication of
microfluidic devices.
21
The transparency of PDMS in visible
light has led to developments in integrated optics;
22
where
optical waveguides have been fabricated that show mechanical
compatibility with microfluidic channels and low attenuation
losses.
23–25
PDMS has also shown interesting properties when
printing conductive tracks, for both adhesion of the ink and
flexibility of the part.
26
Recently, organosilicon self-assembled
layers have been processed through reactive inkjet printing.
27
Faculty of Engineering, Department of Mechanical, Materials and Manufacturing
Engineering, University of Nottingham, Nottingham, NG7 2RD, UK.
E-mail: Ricky.Wildman@Nottingham.ac.uk
†Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc02412f
Received 1st June 2017,
Accepted 21st August 2017
DOI: 10.1039/c7tc02412f
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However, despite these developments, a method for directly inkjet
printing controlled PDMS structures has not been demonstrated.
Despite the lack of material jetting for PDMS fabrication,
there has been some work with other additive manufacturing
methods. Through the use of a supporting bath PDMS structures
have been printed,
28
after printing the bath medium acts a support
forextendedcuringtimesandcanberemovedwithaphosphate
buffered saline solution. A multi-head extrusion system has
demonstrated the printing of dyed premixed PDMS.
29
The PDMS
is extruded through a nozzle of 200 mmforuseinvascular
representations. The non-contact dispensing of premixed PDMS
has been used for single droplet fabrication of lenses.
30
Material
jetting as a method has several advantages over material extrusion
as discussed above, but the main ones are the ability to produce
smaller features and a more scalable manufacturing method. An
advantage specific to FRIJP is the ability to mix materials on the
substrate, removing the need to mix reactive components before
printing. The mixing on the substrate allows for faster curing
components, able to fully cure at lower temperatures. These
materials are then kept in a stable form inside the printing system.
It also allows for the ability to vary the mixing ratio during printing,
facilitating functional grading of parts.
In this paper a method for the multiple component reactive
inkjet printing of PDMS structures is proposed. Two methods
were considered. First, the reactants were deposited in complete
layers, which led to the components, A and B, being printed
with a separation of around two minutes. The second method
used an improved strategy whereby both components were
printed in a single pass. This reduced the delay between
printing the PDMS components to around 0.3 s. The effect of
the surface energy of the substrate on the wetting of the first
layer of material was considered, as well as the development of a
strategy for combining reactants and producing PDMS objects.
Finally, the successful printing of an object and its subsequent
characterisation will be used to demonstrate the approach as a
method for 3D printing.
Method
Materials and printing
A commercially available two-part silicone (Polytek PlatSil
71-Silliglass)wasusedasthebasisofareactiveink.Theviscosities
of the reactants (155 mPa s and 200 mPa s) were sufficiently close
to the operating range of industrial print heads to allow for high
polymer loading in a solvent. The two parts of the PDMS ink are
referred to as A (hydride containing) and B (catalyst containing),
and the reaction is shown in Fig. 1. The formulation consists of a
1 :1 (weight) ratio of A to B, where the silicone is crosslinked in the
presence of a platinum catalyst (transported in ink B). The reaction
is not inhibited by oxygen or moisture, and as a consequence can
be carried out without a controlled atmosphere.
The advised operating range of the Dimatix Material Print
head (DMP) (Dimatix, Fujifilm) employed for the printing is
10–12 mPa s and 28–33 mN m
1
for viscosity and surface
tension respectively, but it is stated that viscosities as high as
30 mPa s and surface tensions up to 70 mN m
1
may be used.
31
Two jettable inks were produced by creating 60 wt% solutions
of PDMS A and B using octyl acetate (OA) as a viscosity modifier
(Sigma Aldrich; O5500). This solvent was chosen as it was
miscible with the PDMS formulations and offered a suitable
vapour pressure for printing. The solutions were miscible and
stable during formulation and printing. To further tune the
viscosity, print head heating was used.
Ink viscosity was determined using a concentric cylinder
rheometer (Malvern Kinexus Pro) with a shear rate sweep between
1s
1
and 1000 s
1
and a temperature ramp between 20 1Cand
70 1C. The ink formulation was judged to be printable when the
viscosity was under the 30 mPa s threshold; further heating was
found to achieve an ink viscosity below 20 mPa s at 60 1C. An
important factor in inkjet printing, and also on how ink droplets
form on the substrate, is the ink interfacial tension which was
determined through the pendant drop method with a drop shape
analyser (KRUSS DSA 100). It was found, however, that while the
surfacetensionwasinsidetheprintablerange,itwaslowerthan
the ideal range. The values for the viscosity and surface tension,
for the PDMS components, solvent, and final inks are presented
in Table 1.
From the work of Reis
32
it is understood that droplet volumes
can be affected by a number of factors: ink viscosity, ink surface
tension, and nozzle dimensions. This meant that despite the
similar parameters associated with the A and B inks, differences
were to be expected in droplet volume. This variation would then
affect mixing ratio and the final curing. To accommodate this,
firstly 100 000 droplets were printed into a low weight metal pan
and weighed, and then the obtained mass was used to change the
droplet ratio during printing. Droplet masses were found to be
10.4 ng and 11.92 ng for ink A and B respectively.
Fig. 1 Crosslink reaction of PDMS in the presence of a platinum catalyst, the silicone hydride bond Si–H is replaced with an additional Si–C bond.
Labelled are the compounds in each component of the PDMS formulation.
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FRIJP process
Inkjet printing was carried out using an in-house printing
system. Two DMP cartridges each with 16 nozzles were driven
with an arbitrary function generator AFG3252C (Tektronix,
USA) and amplified to 30 V. The heads were positioned and
fired using a PixDro LP50 (Roth&Rau, Netherlands) printer. The
DMP cartridges have a closed loop thermal control system,
which was used to maintain the head temperature at 60 1C.
Strategy 1: layer. Complete films were printed one reactant
at a time using a single nozzle as shown schematically in Fig. 2.
The droplet spacing was set to 35 mm, with a total of 114 passes
for each layer (4 mm
2
). In total 228 passes were required to
print a complete reactive layer and these were carried out at a
translation speed of 120 mm s
1
.
Strategy 2: line. Each line was printed with both components
in a single pass. It took around 0.3 s for both components to be
deposited at the same location (from ejection of drop A). The
drop spacing in this printing was determined to be 40 mm in the
Ydirection for both components. In the Xdirection, the drop
spacing for ink A was also 40 mm. In order to equalise the mass
mixing ratio and to accommodate the limitations of the system
that did not allow a precision below 1 mm, proportions of the
drops of ink B were spaced at 45 mm and 46 mm to allow for an
average drop spacing of 45.85 mm.
Substrates
Three material substrates were employed, standard glass slides,
PTFE, and glass chemically modified with 1% 1H,1H,2H,
2H-perfluoroctyltriethoxysilane (PFOTS) (Sigma Aldrich 667420)
in anhydrous toluene.
To create the PFOTS-glass the pre-cleaned glass slides were
first treated with 70% nitric acid (Sigma Aldrich 438073). They
were then rinsed with anhydrous toluene (Sigma Aldrich
568821). After drying they were chemically coated with the
PFOTS solution overnight. They were once again rinsed with
anhydrous toluene before being baked in an oven at 100 1C for
1 hour.
The contact angle of the prepared PDMS ink was evaluated
against the substrates using the sessile drop method with a
drop shape analyser (KRUSS DSA 100). It was found that both
the PTFE and PFOTS produced a higher contact angle than that
of glass. A final substrate, of the cured PDMS was analysed
which showed increased wetting over the PTFE and PFOTS. The
results of the contact angle, shown in Table 2, suggest that the
best feature resolution would be possible using the PFOTS
substrate.
Chemical analysis
Chemical analysis of the PDMS samples was conducted with
two aims. The work involving the Raman spectroscopy was used
to study spatial variation in the samples, while FTIR analysis
was used to quantify bulk residual hydride density. Raman and
FTIR peaks used to determine residual cross link component
were: silicone hydride 913 cm
1
(Si–H bend), and siloxane at
488.6 cm
1
(Si–O–Si vibration).
34
The last bond, siloxane, was
used to calibrate the intensity of the captured waveform allowing
the comparison of multiple samples. Along with the printed
samples, PDMS calibration samples were cast directly onto
CaF
2
glass slides. Three weight concentrations 1 : 1, 2 : 1, and
3 : 1, A : B were selected to produce PDMS samples with an
Table 1 Ink, solvent, and solution properties. Through the use of an unreactive diluent and print head heating; viscosities in the printable range were
achieved (o30 mPa s)
Fluid Viscosity (mPa s) 20 1C Viscosity (mPa s) 60 1C Interfacial tension (mN m
1
) Droplet mass (ng)
PDMS A 155
a
95.6 0.6 —
PDMS B 200
a
74.8 2.1 —
Octyl acetate 1.85
33
— 27.8
33
Ink A: OA (60% loading) 38.2 1.3 19.7 0.25 23.6 0.25 10.4 0.24
Ink B: OA (60% loading) 34.0 0.1 17.7 0.12 23.3 0.13 11.92 0.18
a
Material MSDS.
Fig. 2 Process of FRIJP showing line print in the Ydirection, with number
of lines increasing in the Xdirection. Each ink is printed in series, first ink A
(yellow), then ink B (blue) only when both inks are printed can the curing
occur (green). Layers are counted in total reacted (green) layers printed.
Table 2 Contact angle of PDMS ink A on different substrates using the
sessile drop technique (KRUSS DSA100)
Substrate Contact angle PDMS Ink A
Glass o21
PDMS
a
20–151
PTFE 251
PFOTS
b
551
a
Two component PDMS spin coated onto a clean glass slide.
b
A
fluorosilane (perfluorooctyltriethoxysilane) coated glass slide (PFOTS).
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estimated, 0%, 33%, and 50% excess hydride respectively. Cast
samples were mixed following manufacturer instructions and
allowed to cure for 24 hours at room temperature.
Raman spectral analysis was used to investigate the spatially
varying cure, using a Horiba–Jobin–Yvon LabRAM Raman
microscope (HORIBA, Japan), with an excitation laser wave-
length of 532 nm and a 600 lines mm
1
grating. The detector
was a Synapse CCD detector. The Raman shift was calibrated
using a Si(100) reference sample. Scans were conducted in the
print direction (Y), transverse to the print direction (X) and
through the printed depth (Z). The complete scan dimensions
were 750 mm750 mm100 mm(X,Y, and Z).
The FTIR-ATR was conducted on a Frontier (Perkin Elmer,
USA) system. The ATR accessory was exclusively used for all
sample analysis. Scans were initially conducted from 4000 to
650 cm
1
, at a resolution of 4 cm
1
. Later scans reduced the
range to 2500 cm
1
, thereby increasing the scan speed without
losing information. The resulting spectra were taken from the
average of four scans corrected by a baseline. The analysis was
conducted in both Perkin Elmer’s Spectrum software and using
MATLAB to perform curve fitting for determination of the
peak area.
Profile analysis
The macroscale profiles of the printed patterns were captured
using an Alicona Infinite Focus G5 (Alicona, UK) surface
measurement device via focus variation. Typical samples prepared
in this work were smooth and transparent, so could not be directly
measured. Instead, a physical replica of the sample was created
using AccuTrans replica PDMS. The PDMS works well because it
has a colour variation that aids the software in resolving features.
35
Results and discussion
Process overview
The goal of this work was to determine a method, or range of
methods, to achieve a final component that had comparable
resolution, freedom of design, and complexity to a single
component ink. The subsequent method of FRIJP described
in this paper is demonstrated for the production of droplet
grids, thin fluid films, and complex 3D patterns. The various
methods of analysis conducted on the samples produced using
the layer and line deposition methods can be seen in Table 3.
Substrate surface treatment
The ability of PDMS to spread on most material surfaces is
desirable when using moulding techniques, but for inkjet
printing it directly reduces the feature resolution. Three mate-
rial substrates; glass, PTFE, and PFOTS-glass were used in this
project to increase the printed resolution of features on the
substrate. Contact angle measurements were conducted, but
the most robust way of gauging substrate suitability was by
printing a grid. Table 2 shows the results from contact angle
measurements, and Fig. 3 shows the results of printing a
disconnected grid.
The printed grids in Fig. 3 show that the cleaned glass
produced the largest droplet size, which was greater than
150 mm. The PFOTS-glass and PTFE had similar spot sizes, at
48 2mm, and 64 2mm respectively. The smaller and more
circular drops on the PFOTS-glass were likely due to the
decreased surface roughness. The ability to clean the PFOTS-
glass, and the increased hardness over the PTFE meant that it
was the selected substrate for continued work.
Table 3 Analysis conducted on the line and layer PDMS samples
Printing scheme Analysis type Analysis aim
Layer Optical microscopy Determine how the liquid pinning can be improved through the microstructuring method.
Line Optical microscopy Determine the drop spacing suitable for FRIJP of PDMS using the line based method.
Line and layer Profile analysis Determine distribution of material across the printed sample.
Layer Raman microscopy To determine if there are concentration gradients in printed samples.
Line FTIR-ATR To determine the degree of cure in the FRIJP method.
Fig. 3 (a) The result of printing a single drop of ink A and B onto an untreated glass slide. (b and c) Showing printed grids composing both ink A and ink B
onto (b) on a prepared PTFE coated slide and (c) PFOTS-glass.
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Stable films with layer printing
To inkjet print complex geometries, there needs to be a degree
of stability in the printed areas on the substrate, i.e., the contact
line should not move, keeping any structures in place whilst
curing proceeds. This stability is a balance of three forces, the
ink–substrate interaction (typical called pinning), the ink–air
interfacial tension, and the force of gravity
36,37
Printing stable
lines with single inks typically involves selecting a suitable fluid
volume to be deposited in an area through modification of the
drop spacing. Typically, the inkjet printing of single inks
benefits from relatively fast curing times, which reduces the
need for prolonged ink–substrate stability. However, multiple
inks printed using the layer strategy have an increased time
before curing can occur, this is due to the initial requirement of
depositing both reactive components. Once both inks are
printed the curing process may also be slower due to the mixing
that is involved. These factors mean that FRIJP inks need to
have a greater stability, over a longer period, to produce the
same geometry. It was found that with the PDMS ink in this
work, the ink–substrate interaction was weak. This weak
pinning, coupled with the long printing times, resulted in
uncontrolled coalescence, as illustrated in Fig. 4.
To reduce the effect of uncontrolled liquid redistribution
noted above, enhanced pinning between liquid–solid PDMS
was investigated. The process used, termed microstructuring,
involved the creation of pinning sites which act to increase the
stability of any connected lines or films by creating an already
established set of disconnected pinning sites. This method has
been explored previously in regards to increasing solvent ink
loading on a substrate.
38
Fig. 5 shows this process over a total of
four reactive layers (eight printed layers). Initial spacing was set such
that the first layer of the printed grid did not coalesce, only after the
second layer of ink A was printed did coalescence occur. Once both
components were printed the successive printing of layers resulted
in a controlled coalescence. Finally, after four reactive layers the
spreading was complete and complete coverage was achieved
between the drops in the initially printed grid. This was accom-
plished with no undesired spreading outside the designated area.
Fig. 6 shows how this method is capable of producing
continuous cured PDMS films with a high corner definition.
Ultimately, the microstructuring method, along with substrate
surface modification, allows for increased feature resolution
and feature control.
Fig. 4 Microscope image of depinning and coalescence (labelled) present
when the microstructuring pattern is not used, printed on to polished PTFE
substrate heated to 80 1C. The intended pattern was a square, however
liquid ink receded and bulged before curing could occur.
Fig. 5 Substrate microstructuring, disconnected droplet printing to maximise
the effect of pinning on a substrate where single PDMS ink does not pin. (a) A
single layer of each component, ink A and B is deposited (shown in the
schematic, phase 1), the initial spacing of the ink is such that the ink A does not
coalesce. (b–d) Additional layers printed onto the grid, controlled spreading
occurs (shown in the schematic, phase 2 for spreading, and phase 3 for
bridging) and finally a continuous film is produced.
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The mechanism by which the microstructuring method
works is similar to spin coating a substrate with a compatible
polymer, but in this case the application of the microstructured
pattern can afford greater control. This means it is possible to
maintain the high feature resolution of the underlying substrate
whilst increasing the pinning forces.
Once stable films were printed using the layer method,
further layers were deposited to create thick films. In total
50 reactive layers, involving 50 layers of ink A and ink B, were
deposited. The surface profiles of these films were then
measured. The flatness of the top surface was of particular
importance, since to build 3D structures, a known and repeatable
layer thickness is required. It was expected that the long time to
mixing would reduce flatness, but that the increased substrate
temperature would mitigate this.
The results of the profile analysis are shown in Fig. 7.
Differences in height profiles can be induced by altering the
substrate temperature – for example, increasing the temperature
from 40 1Cto601C shifts the profile from dome shaped, to one
that is relatively flat. However, this uniformity is still subject to
edge effects where material recedes from the outer edges. The
layer height for these prints were; 12.1 1.8 mm, 12.4 1.4 mm,
and 11.3 1.9 mm for the sample printed at 40 1C, 60 1C, and
80 1C respectively. This height is calculated from the average and
standard deviation of the heights across the total profile divided
by the number of layers which was 50. This movement is known
to be caused by the surface tension of the liquid, as discussed by
Thompson.
39
Further heating appears to maintain the same
profile but increases the variation of material height throughout.
Printing line strategy
The microstructuring method was employed when the mixing
and subsequent curing time was too long for stability to be
achieved using the layer strategy, however, this approach was
unnecessary when using the line strategy. With the removal of
the microstructuring to achieve the desired geometry, it was
possible to experimentally determine the lowest drop spacing
that produced a stable line. To determine the suitable spacing
for this ink system, a range of separations were printed and
optically analysed. The follow separations were trialled: 30 mm,
Fig. 6 (a) Microscope (Nikon ECLIPSE LV100ND) image of a printed film,
and (b) SEM (Hitachi Analytical Benchtop SEM TM3030) image of a printed
film, sample platinum coated for 90 s, printed on a polished PTFE substrate
heated to 80 1C. High sample waviness was caused by the initial micro-
structured film process, visible in the microscope image, the SEM image
however. To confirm that the sample was a continous film the top left
corner was peeled.
Fig. 7 Profiles of the printed PDMS using the layer method with micro-
structuring and 50 reactive layers. (a) Printed at substrate temperature of
40 1C, (b) 60 1C, (c) 80 1C. Missing data is due to either high angles that are
unscannable or the replication process causing incorrect replication.
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35 mm, 40 mm, and 45 mm. These distances only controlled the
spacing of ink A, whereas ink B was the product of the drop
spacing and the mixing ratio. The corresponding spacings used
for ink B were; 34.4 mm, 40.1 mm, 45.8 mm, and 52.6 mm; which
were based on the mixing ratio from the droplet masses of A : B
1 : 0.87. The results of the drop spacing test are shown in Fig. 8.
To understand the stability of the printed layers within the
context of AM, the performance of multiple layers was then
assessed. To maximise printing speed, the lowest droplet
separation that achieved stable lines was chosen. The line
stability was judged by the line morphology variation over many
layers. For example, the 30 mm spacing showed continual spreading
on each layer. From the results of the line stability tests a primary
drop spacing of 40 mm was selected for future printing.
Substrate microscopy was used to characterise the printed
films, and during the printing process after every layer an
image was captured of the bottom left corner. Fig. 9 shows
the growth of the PDMS sample during the first five layers.
The first layer is directly printed onto the PFOTS-glass, and due
to the fast curing rate it does not recede. Successive layers print
onto the PDMS film and build up the layers to create the 3D
object.
As a demonstration of capability to print a fully 3D object in
PDMS, a stepped ziggurat design was printed (Fig. 10). The
design contained five steps, each printed with 30 reactive
layers. Profile analysis shows the layer heights were less than
those created with the layer printing strategy at; 8.3 1.9 mm,
7.3 1.4 mm, 6.3 2.1 mm, 7.5 1.5 mm, and 6.8 1.5 mm for
the first to last layer respectively. This reduced layer height is
expected as the line based method used an increased drop
spacing. In example, this increased spacing over a fixed 4 mm
square area would result in 19 000 drops printed for the line
strategy, versus 26 000 for the layer one.
A consequence of the stepped ziggurat design was that it
could be used to determine any effect of printed line length on
the profile. For example, it can be seen in Fig. 11 in the Y(print)
direction, increased scan length resulted in a more uniform
layer height. The variation in this single sample is 17 mmforthe
first layer, increasing to 34 mm for the penultimate, shorter scan
layer. The printing strategy in the layer based method is controlled
Fig. 8 Determination of the minimum drop spacing for stable line formation of printed PDMS. Ten layers of five lines were printed at different drop spacings.
Fig. 9 FRIJP Line strategy printing directly onto the PFOTS-glass. The substrate temperature is 80 1C.
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so that each line printed takes the same amount of time, i.e. smaller
printed patterns do not reduce the line scan time. Therefore, the
only cause for this increased variation is due to a temperature
gradient, where the sample on top is cooler than the bottom. The
substrate was set to 80 1C and from previous results, reducing the
substrate temperature led to increased variation.
The profile of the printed ziggurat, moving in the Xdirection
across the stepped regions, shows that the first two steps
produce a highly uniform and level line, but with further layers
an inclination appears. This sloping is likely related to the increased
variation in the other direction, indicative of increasing time to cure.
Curing in printed PDMS
The Raman analysis was conducted on samples printed with
the layer based printing method. This was done to determine if
the FRIJP process is capable of mixing the PDMS system on the
substrate during printing. The FTIR-ATR system was used to
determine degree of cure after determining that there were no
concentration gradients in the printed PDMS.
The process of FRIJP relies heavily on the ability of the two
components to mix in situ, as there is little mixing from the
drop impact.
40
Therefore, in the case of FRIJP PDMS the sole
mechanism for mixing is the diffusion of the two components.
The miscibility of the inks aids this diffusion, and in the case of
PDMS no effects from improper mixing were observed, e.g.
interfaces, liquid PDMS, gelling. Confocal Raman was conducted
to study if, and how, the substrate temperature can affect the
mixing in RIJP. The analysis had two objectives. First it aimed to
determine the residual crosslink component across the sample.
Second, it set out to determine whether the PDMS is a homo-
genous polymer or if there are compositional variations.
Fig. 10 Stepped ziggurat, 4 4 mm PDMS consisting of 5 steps, each
printed with 30 reactive layers.
Fig. 11 Profile analysis of the stepped ziggurat printed design conducted on the Alicona. Line scans are taken as an average in a number of lines and
shown as (a) line scans immediately before a step in X, (b) line scans in Y, (c) area profile, with highlights where line scans were taken from.
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For the analysis the residual hydride concentration is compared
to the initial concentration. As mixing and reaction occurs this
hydride will decrease, depending on the formulation once correctly
mixed, there may or may not be excess hydride by design. To
determine what the correct residual is, three calibration
samples were also analysed with the Raman. The calibration
samples were made following the manufacturers guide. In
Fig. 12, the residual hydride component is compared to the
initial concentration of Si–H in the PDMS A component (The
representative spectra as shown in Supplementary 1, ESI†). This
demonstrated that although there wasn’t exact correspondence,
the printed samples tended to show similar PDMS A intensities
to those seen in 1 : 1 cast samples, suggesting that printing
results in material similar to that produced by more traditional
manufacturing methods.
An important consideration of the FRIJP process is that due
to transport in the film it is possible for the mixing ratio across
the sample to change. Confocal Raman was used to determine
if the ratio between A and B is constant throughout the printed
sample. There was no evidence for spatial dependency in any of
the samples printed (with the exception of the 40 1CXdirection
scan). In the case of samples prepared at the lower temperatures,
scanned in the Xdirection, there is significance in the residual
Si–H and location on the print. This residual concentration in the
low temperature sample is likely related to the domed profile also
observed. It was found that there is no evidence for depth
dependent concentration, which has been found in AM inkjet
printed components cured with UV radiation.
41
This demonstration
of mixing & reacting in situ suggests that it is possible to produce
homogenously crosslinked PDMS samples. However, care must be
taken that the individual components are deposited in the correct
ratio and that limited redistribution of ink occurs before curing.
A further analysis was conducted on the samples printed
using the second, line based, printing strategy. This printing
strategy was also conducted with the corrected droplet ratios.
The results of this analysis are shown in Fig. 13, they show that
residual Si–H intensity in the printed samples corresponds to
the 1 : 1 cast calibration. This shows that in all cases the printed
samples were the same, or better, than the cast ones.
Conclusions
In this work the Reactive Inkjet Printing of Polydimethylsiloxane
has been demonstrated. For the first-time multiple component,
reactive jetting has been used to produce PDMS structures with
shape definition in a crosslinked polymer network. A stable
FRIJP ink was developed from a two-part silicone system. The
viscosity of each component was such that a 60 wt% loading of
polymer was achievable whilst maintain a printable ink.
In FRIJP, as the reaction is separated into two inks, the time
to cure not only includes the reaction, but also the time for
deposition and mixing. To increase the control of the profile
Fig. 12 Quantified residual crosslink component in each printed sample in
relation to uncured part A also included are cast samples created at 1 : 1 2 : 1 3 : 1
A : B ratio, data is shown as mean standard deviation (XY:n=75,Z:n=50).
Representative Raman spectra for the PDMS is shown in Supplementary 1 (ESI†).
Fig. 13 Residual cross link component Si–H in the calibration and printed
samples collected with FTIR-ATR. The bond at 912 cm
1
was used to
determine the residual. The errors are from the curve fitting GOF of both
the calibration and residual combined. Three sets of samples were taken
from three separate prints.
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each one of these steps had to be shortened. The major concern
was the time to mixing, which was dependent on the printed
geometry, which was around 2 minutes (for 4 mm square
sample) in the layer based approach. To reduce this both inks
were printed in a single pass, the time to mixing was then
independent of printed geometry and only dependent on head
printing speed. The time to deposition was calculated to
around 0.3 s. The reviewed literature stated no convective
mixing occurred, leaving only diffusion based mixing for the
PDMS system. The only method to increase this was then
through substrate heating to elevate the liquid temperature.
Through the use of substrate heating it was found that the
lowest degree of profile variation was found in the 80 1C print.
This substrate heating also had an observable effect on the
curing, with decreased residual Si–H components observed
through Raman. When the line scan strategy was employed,
to reduce the mixing time, substrate heating was maintained.
The reduction on time to mixing had a direct effect on the
printed profile, with the line scans showing much greater layer
uniformity than the layer strategy. The sample curing was also
improved with the change of strategy, where the samples
showed residual Si–H component comparable to the conven-
tionally manufactured 1 : 1 references.
The result of these improvements is a highly cross-linked
PDMS network directly printed onto a glass substrate which
was modified to increase the feature resolution. The printed
geometry had an acceptable topology for a FRIJP part. A
complex structure has been demonstrated, showing the cap-
abilities of the FRIJP process to produce a single material
sample without the aid of support. As it has been shown that
PDMS samples can be produced through FRIJP the possibility
to replace traditional casting techniques exists.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Engineering and Physical
Sciences Research Council [grant number EP/1033335/2], EPSRC
Centre for Innovative Manufacturing in Additive Manufacturing.
The authors are grateful to Dr Andrew Davies for technical
assistance and to the Nanoscale and Microscale Research Centre
(NMRC) for providing access to Raman microscopy facilities.
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