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CHAPTER 5

RAPID MANUFACTURING

The stress and displacement of the bone and bone with implant
assembly is less when the patient-specific implant fixed at the position 2
compared to that of the implant fixed at the position 1, obtained from FEA
results. Hence, the patient-specific implant modeled for position 2 in the CAD
is used to fabricate the implants by RM techniques such as Electron Beam
Melting (EBM) and Direct Metal Laser Sintering (DMLS) process using
Ti-6Al-4V and EOS SS GP1 respectively.

5.1 FABRICATION OF PATIENT-SPECIFIC IMPLANT

5.1.1 Electron Beam Melting (EBM)

In the EBM process, fully dense metal parts are built up layer-by-
layer of metal powder melted by a powerful electron beam. Each layer is
melted to the exact geometry defined by the 3D CAD model. The EBM
technology allows for high energy to be used providing high melting capacity
and high productivity. Parts are built in vacuum at elevated temperatures
resulting in stress-relieved parts with material properties better than cast and
comparable to wrought material. For each layer of powder the electron beam
first scans the powder bed to maintain a certain elevated temperature, specific
for different alloys. Thereafter the electron beam melts the contours of the
part and finally the bulk.
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The schematic working of Arcam EBM S12 machine is shown in

Figure 5.1. The equipment consists of an electron beam head with a tungsten
filament, a powder container, spreader, and a build table. The tungsten
filament reacts with excited electrons causing a beam of electrons to pass out
from the head. Two magnetic fields are present, of which the first one
organizes the electron beam in the desired shape and the second deflects the
beam to the target position. The kinetic energy of the electrons is transferred
to thermal energy, fusing the metal particles together. The electron beam
scans the metal bed in accordance to the slice data generated from the input
CAD file and solidification occurs by cooling. Once a layer of powder is
melted and solidified, the next layer of powder is spread and the process is
repeated till the part is completed. The process takes place in a bed of the
metal powder which supports the overhanging features of the part during
fabrication.

Figure 5.1 Schematic working of Arcam EBM S12 machine

(courtesy: M/S, ARCAM)
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Parts were designed in CAD and .STL files transferred to the EBM
machine for fabrication. Part orientation partially determines the amount of
time required to build the model. Placing the shortest dimension in the
Z - direction reduces the number of layers, thereby shortening build time, but
this compromises the strength of the part in the direction perpendicular to the
build direction. Part orientation also affects the surface finish and strength of
the part. The layer thickness of 0.1mm was maintained for fabrication.
Ti6Al4V powder supplied by ARCAM AB with a powder particle size of
40 - 80 µm was used. Once all the above parameters were set, the tool path
was generated. The process continues until every layer is deposited and
patterned using the electron beam.

Patient-specific implant made of Ti-6Al-4V, fabricated using EBM

technique is shown in the Figure 5.2. Ti6Al4V is the most widely used
titanium alloy and has numerous applications in the medical industry,
aerospace, automotive and marine equipment industry. Some design features
are high durability in high strength applications from cryogenic to moderate
temperatures, excellent strength-to-weight ratio, excellent corrosion
resistance, excellent fracture toughness and fatigue resistance, high ductility,
excellent biomedical compatibility and good machinability.

Figure 5.2 Patient-specific implant made of Ti-6Al-4V

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Technical specifications of Arcam EBM S12 are illustrated in the

Table 5.1.

Table 5.1 Technical specifications of Arcam EBM S12

Make ARCAM EBM S12 (Certification CE)

Building tank volume 250 x 250 x 200 mm (W x D x H)
Maximum build size 200 x 200 x 180 mm
Accuracy +/– 0.4 mm
Melting speed Up to 60 cm3/hr (material-dependent)
Layer thickness 0.05 - 0.2 mm (material-dependent)
EB scan speed > 1000 m/s
EB positioning accuracy ± 0.05 mm
Beam power 3500 W
Power supply 3 x 400 V, 32 A, 7KW
Size and weight 1850 x 900 x 2200 mm (W x D x H), 1420 kg
Process computer PC, XP Professional
CAD interface Standard STL
Network Ethernet 10/100

5.1.2 Direct Metal Laser Sintering (DMLS)

DMLS is an 'additive' technology that works by fusing together

very fine layers of metal powder using a focused laser beam. The schematic
working of EOSINT M270 DMLS system is shown in the Figure 5.3. The
supports are necessary because the powder alone is not sufficient to hold in
place the liquid phase created when the laser is scanning the powder. The
supports and components are built with a layer thickness of 40 µm. Each layer
is scanned with the laser fusing the powder to the previous layer below it, and
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forming the new build layer. The base is then lowered one layer, a fresh layer
of powder is deposited, and the next layer is scanned. A powerful 200W
Yb-fibre laser is precisely controlled in the X and Y co-ordinates allowing for
exceptional tolerances to be held (< ± 0.1mm).

Figure 5.3 Schematic working of EOSINT M270 DMLS system

(Courtesy: M/S, EOS)

Patient-specific implant of EOS Stainless Steel GP1, fabricated

using DMLS technique is shown in the Figure 5.4. EOS Stainless Steel GP1 is
a Stainless Steel (SS) powder which has been optimised especially for
EOSINT M270 systems. It is a pre-alloyed SS in fine powder form. This is
characterised by having good corrosion resistance and mechanical properties,
especially excellent ductility in laser processed state and is widely used in a
variety of engineering applications. This material is ideal for many part-
building applications (Direct Part) such as functional metal prototypes, small
series products, individualised products or spare parts. Using standard
parameters the mechanical properties are fairly uniform in all directions. Parts
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made from EOS SS GP1 can be machined, spark-eroded, welded, micro shot-
peened, polished and coated if required. Technical specifications of EOSINT
M270 DMLS are illustrated in the Table 5.2.

Table 5.2 Technical specification of EOSINT M270

Effective building volume 250 x 250 x 215 mm

Building speed (material-dependent) 2- 20 mm3/s
Layer thickness(material-dependent) 20 - 100 µm
Laser type Yb-fibre laser, 200 W
Precision optics F-theta-lens high speed scanner
Scan speed Up to 7.0 m/s
Variable focus diameter 100 - 500 µm
Power consumption Max. 5.5 KW
Nitrogen generator standard
Compressed air supply 7,000 hPa; 20m3/h
System 2000 x 1050 x 1940 mm
Weight Approx. 1,130 Kg
Software EOS RP Tools; Magics RP (Materialise)
CAD interface STL

Figure 5.4 Patient-specific implant made of EOS SS GP1

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5.2 MEASUREMENTS OF PATIENT-SPECIFIC IMPLANTS

The comparison of dimensional accuracy of the RP model with

CAD model is carried out by 3D non-contact type ScanWorks V5 scanner. The
surface roughness of the model is measured by TESA Rugo Surf 90G equipment.
The hardness of the model is measured using KRYSTALTECH-KAS hardness
tester.

5.2.1 Dimensional accuracy

It is necessary to measure the dimensional accuracy of the implant

since the pre-fabricated implants are available in a standard range of sizes and
shapes which do not conform to the geometric shape of the patient’s bone and
are selected to near suitable during surgery. This needs more surgical
interventions, time and may cause micro movements with subsequent bone
resorption and loss of fixation and discomfort to patient. Uniform stress
distribution on the bone-implant interface surface can be achieved by
patient-specific implant which will reduce the uneven bone remodeling and
premature loosening of the implant.

The patient-specific implants fabricated using EBM and DMLS

process is measured by non-contact type ScanWorks V5 scanner to compare
the dimensional accuracy of the fabricated model with CAD model as shown
in the Figure 5.5. The V5 scans at the rate of up to 458,000 points per second
allows users to measure areas quickly while maintaining a dense point
resolution of approximately 14 microns (0.00055 in.). The V5 sensor is light
and compact enough to facilitate measurement in even the hardest-to-reach
areas, while its dynamic range enables the sensor to produce accurate
measurements on highly reflective and dark surfaces in all lighting conditions
without the need for prior surface treatment. The technical specifications of
the scanner are shown in the Table 5.3.
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Figure 5.5 ScanWorks V5 3D non-contact type scanner systems

(courtesy: M/S, Perceptron Inc., USA)

PolyWorks Inspector is a powerful software solution that uses high

density point clouds and contact-probe datasets to control the quality of parts
and tools at every phase of manufacturing process. It is used as standard
dimensional control and engineering analysis software for prototype, first-
article, manufactured and assembled parts inspection.

Table 5.3 Technical specifications of ScanWorks V5, 3D non-contact

scanner
Make Perceptron Inc., USA
Model ScanWorks V5
Dimensions 115 mm x 100 mm x 80 mm
Mass 438 gm
Profile density 7640 points/line
Scan rate 458400 points/second
Mean point-point resolution 0.0137 mm
Measurement accuracy 0.0240 mm 2 corner test (NIST standard)
Safety Class 2M, 660 nm laser
Environment 10 ºC to 40 ºC
Protection Sensor IP64 / Enclosure IP31
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This 3D measurement system was delivered with the PolyWorks

software suite from InnovMetric software Inc., the leading 3D metrology
software that offers a wide array of point cloud engineering tools for quality
control and inspection.

5.2.2 Surface roughness of the implants

Surface roughness is an important factor to be studied to improve

the bone-implant contact. Bone-implant interface is enhanced by the higher
surface roughness (Shalabi et al (2006) and Ann Wennerberg (1998)). The
deviation of the actual surface topography from an ideal atomically smooth
and planer surface. A measure of the surface roughness is the ‘rms’ deviation
from the center line average. Surface roughness is a quantitative calculation of
the relative roughness of a linear profile or area, expressed as a single numeric
parameter (Ra). Surface roughness of implant is measured using TESA RUGO
SURF-90G equipment as shown in the Figure 5.6.

Figure 5.6 TESA RUGO SURF-90G

(Courtesy M/S, TESPA Tools (P) Ltd.,)
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5.2.3 Hardness of the implants

It is necessary to measure the hardness of the implant for long term

usage. Hardness of the material is defined as resistance of metal to plastic
deformation, usually by indentation. However, the term may also refer to
stiffness or to resistance to scratching, abrasion or cutting. It is the property of
a metal, which gives it the ability to resist being permanently, deformed
(bent, broken or have its shape changed), when a load is applied. The greater
the hardness of the metal, the greater resistance it has to deformation.
Titanium is a white metal and has the best strength to weight ratio among the
metals. Titanium is 40% lighter than Steel and 60% heavier than Aluminum.
The hardness of the implant is measured by KRYSTALTECH, KAS model
Rockwell hardness tester as shown in the Figure 5.7.

Figure 5.7 KRYSTALTECH, KAS model Rockwell hardness tester

(Courtesy: M/S, KRYSTALTECH)
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The hardness value of the patient-specific implants is measured

using 2.5 mm diameter ball type diamond probe indenter when the minimum
load of 10 Kgf and maximum load of 150 Kgf applied on the implant for
10 seconds.

5.3 RESULTS AND DISCUSSION

Measurements such as dimensional accuracy, surface roughness

and hardness of the patient-specific implants made of Ti-6Al-4V and SS GP1
using EBM and DMLS respectively are carried out to evaluate the mechanical
properties of the implant.

In this work, the following five specific tools are used to measure
and compare the model fabricated using EBM and DMLS process with CAD
model.

A global comparison was performed by calculating the

deviation between each digitised point and its corresponding
CAD reference. A colour map was displayed according to the
tolerance set as illustrated from the Figures 5.8 and 5.9.

To ensure that the ray of light is projected without

interference, the clearance of the part was measured by
comparing 3D distances between two planes.

The wall thickness was extracted by computing 3D distances

between two points. This ensures its robustness and validates
that the part met the design requirements as shown in the
Table 5.4.

The flatness and 3D angle between two planes were computed

using PolyWorks’ Geometrical Dimensioning & Tolerance
(GD&T) engine.
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Using Inspection tools, where the part is out of tolerance relative to

the CAD model can be easily assessed. The report generation tool of the
software is used to prepare the inspection results. In addition to the data the
snap shots are used to illustrate where the part is out of tolerance by using
different colours to indicate ranges of tolerance. From the Table 5.4 and
Figures 5.8 and 5.9, it is observed that,

The model is analysed for fixed tolerance values of ± 0.500 mm.

Out of 96561 points scanned, 93621 points match with the

CAD model in the 3 level, which is 96.955 % fitment with
the CAD data.

Out of 59293 points scanned, 56655 points match with the

CAD model in the 3 level, which is 95.551% fitment with
the CAD data.

Table 5.4 Point cloud data obtained through ScanWorks V5 for

implants fabricated using Ti-6Al-4V and SS GP1

Description Arcam EBM EOS DMLS

Object(s) Ti-6Al-4V SS GP1
CSYS World World
Alignment Best-fit to surfaces Best-fit to surfaces
Ref Implant Implant
Max Distance & Max Angle 4.000 & 45 4.000 & 45
Tol+ (mm) 0.500 0.500
Tol- (mm) -0.500 -0.500
No of Points 96561 59293
Mean -0.090 -0.112
StdDev 0.518 0.887
Pts within +/-(1 * StdDev) 86246 (89.318%) 51510 (86.874%)
Pts within +/-(2 * StdDev) 90923 (94.161%) 54631 (92.137%)
Pts within +/-(3 * StdDev) 93621 (96.955%) 56655 (95.551%)
Pts within +/-(4 * StdDev) 95361 (98.757%) 58597 (98.826%)
Pts within +/-(5 * StdDev) 95912 (99.328%) 59293 (100.000%)
Pts within +/-(6 * StdDev) 96190 (99.616%) 59293 (100.000%)
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Figure 5.8 Comparison of point cloud data of EBM (Ti-6Al-4V) model

with CAD model
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Figure 5.9 Comparison of point cloud data of DMLS (SS GP1) model
with CAD model
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Comparison of mechanical properties of patient-specific implants

fabricated using Ti-6Al-4V and SS GP1 are shown in the Table 5.5

Table 5.5 Comparison of mechanical properties of patient-specific

implants made of Ti-6Al-4V and SS GP1

Patient-specific implant
S.No Mechanical properties
Ti-6Al-4V SS GP1
Dimensional accuracy
1 96.955 % 95.551%
(at 3 level)
Surface Roughness (Ra )
2 14.13 1.49
(µm)
3 Hardness (HRC) 25 16

From the Table 5.5, it is observed that the dimensional accuracy,

surface roughness and hardness of the implant fabricated using Ti-6Al-4V is
higher than the implant fabricated using SS GP 1. It is also observed that the
dimensional accuracy of the implant made of Ti-6Al-4V is achieved by
96.955 % at 3 level which is higher than that of SS GP1. This will enhance
comfort to the patient and reduces the surgical time and surgical procedures
considerably.

Implant with high surface roughness is enhancing the bone-implant

contact (Peter Thomsen et al (2008)). Surface roughness value of implant
made of Ti-6Al-4V is higher than that of SS GP1, which helps to improve the
bone-implant contact and hence pre mature loosening of the implant may be
minimised.
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Higher hardness shown in the implant may enhance the life and
longevity of the implant (Bunyamin Aksakal and Hanyaloglu (2008)). The
hardness value of implant made of Ti-6Al-4V is 30 % higher than that of SS
GP1, which may improve the implant stability and long term usage and
minimise the failure of implant due to forces acting (BW and gait phase),
wear and tear.

From the measurements, it is observed that the mechanical

properties of the patient-specific implant made of Ti-6Al-4V are better than
the implant made of SS GP1.