M03L03 SLA.pdf

M03 L03: SLA (Part−2)
Course Instructor
Dr. Sajan Kapil
Department of Mechanical Engineering
Indian Institute of Technology, Guwahati
Guwahati, Assam
NPTEL MOOC
A Government of India Initiative
Fundamentals of Additive Manufacturing Technologies

2
M03 L01: Contents
 StereoLithography Apparatus (SLA)
o Sub-Systems
 Galvanometer
 Recoating System

3
Scanning System
Galvanometer
Shape of UV Light Laser Beam
Shape of Liquid Liquid – Vat
Build Direction
+Z Direction
Recoating System
Deep Dipping Top Feeding
3D System’s SLA
3D System’s SLA (Zephyr)
Photo-polymerization based Liquid AM
StereoLithography Apparatus (SLA)

4
1. A UV Laser beam is used as an energy source
which scan the selected region over a layer of
liquid photo-polymer to cure it.
2. The power density of the laser spot and material
properties are two major factors to govern the
curing process.
3. The Laser beam is positioned using two small
mirrors (Galvanometer) capable of deflecting the
beam in two directions. Being a very low inertia of
the mirrors the scanning speed is very high.
4. A recoating system (Deep Dipping) is used to
spread the new liquid layer on pre-build geometry
5. Z-Axis of the machine
1
2
4
3
5
1 3
4
4
2
5

5
Galvanometer
3

6
StereoLithography Apparatus (SLA): Galvanometer
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=6057

7

8

9
∠𝑜2𝑜1𝑜3 = ∠𝑐𝑜3𝑐′
= ∠𝑑𝑜3𝑑′
= 𝜃𝑥
∠𝑐𝑐′
𝑜3 = 90° ∠𝑑𝑑′
𝑜3 = 90°
∠𝑏𝑎𝑜2 = 90°
∠𝑜1𝑜2𝑎 = 90°
∠𝑜2𝑏𝑎 = ∠𝑐′𝑑′𝑜3 = ∠𝑐𝑑𝑜3 = 𝜃𝑦
Law of reflection: when a ray of light, is reflected from a
smooth surface the angle of reflection is equal to the angle of
incidence, and the incident ray, the reflected ray, and the normal
to the surface at the point of incidence all lie in the same plane.
Based on this principle one can deduce the following:

10
𝑦 = 𝑎𝑏 = ℎ tan 𝜃𝑦
𝑥 = 𝑏𝑑′ + 𝑑′𝑑
𝑎𝑜2 = 𝑜3𝑐′ = ℎ, o1𝑜2 = 𝑒
𝑥 = 𝑒 tan 𝜃𝑥 + ℎ2 + 𝑦2 tan 𝜃𝑥
𝑏𝑑′ = 𝑜2𝑜3 = 𝑒 tan 𝜃𝑥
𝑑𝑑′ = 𝑜3𝑑′ tan 𝜃𝑥 = 𝑜2𝑏 tan 𝜃𝑥 = ℎ2 + 𝑦2 tan 𝜃𝑥
𝑒 + ℎ2 + 𝑦2 tan 𝜃𝑥 , ℎ tan 𝜃𝑦
In the right angle triangle Δ𝑜2𝑎𝑏
If
In the right angle triangle Δ𝑜3𝑑′𝑑 (try to visualize the plane of
triangle)

11
Example: In a polymerization based liquid-AM process, a vat of resin has been used. A laser beam is used
to selectively scan the required area on the surface of the liquid-bed. The maximum build volume envelope
is 500 × 500 × 550 mm3. What will be distance of optical system from the surface of liquid bed.
(assume the distance between the X and Y –axis mirror to be 100 mm, the X and Y-axis mirror can oscillate
between ±22.5 )
𝑌 = ℎ tan 𝜃𝑦
ℎ =
𝑌
tan 𝜃𝑦
=
250
tan 22.5
=
250
0.55785
= 448.149
250, 250
−250, −250
−250, 250
250, −250
250, 0
0, 250
(0, 250)
𝑋 = 0

12
between ±22.5 )
𝑋 = 𝑒 + ℎ2 + 𝑦2 tan 𝜃𝑥
250, 250
−250, −250
−250, 250
250, −250
250, 0
0, 250
(250, 0) 𝑌 = 0
ℎ =
𝑋
tan 𝜃𝑥
− 𝑒
ℎ =
250
0.55785
− 100 = 348.149

13
between ±22.5 )
𝑋 = 𝑒 + ℎ2 + 𝑦2 tan 𝜃𝑥
ℎ =
𝑋
tan 𝜃𝑥
− 𝑒
2
− 𝑦2
ℎ =
250
0.55785
− 100
2
− 2502 = 242.29
250, 250
−250, −250
−250, 250
250, −250
250, 0
0, 250
(250, 250)
ℎ = 448.149 mm
𝑌 = ℎ tan 𝜃𝑦
ℎ =
𝑌
tan 𝜃𝑦
=
250
tan 22.5
=
250
0.55785
= 448.149

14
It can be observed that the x-coordinate is function of
𝜃𝑥 and 𝜃𝑦 both. If 𝜃𝑥 is constant then:
𝑥
tan 𝜃𝑥
= 𝑒 + ℎ2 + 𝑦2
𝑥
tan 𝜃𝑥
− 𝑒
2
= ℎ2 + 𝑦2
𝑥
tan 𝜃𝑥
− 𝑒
2
− 𝑦2 = ℎ2
The above equation shows that with constant change
in the 𝜃𝑦, the scanning pattern will be a hyperbola.
( where: 𝑦 = ℎ tan 𝜃𝑦)

15
figure(1)
plot(x(:,:),y(:,:),'-')
h=1000;
e=0;
i=1;
j=1;
for thetax=-22.5:0.5:22.5
for thetay=-22.5:0.5:22.5
y(i,j)=h*tand(thetay);
x(i,j)=(e+sqrt(h^2 + y(i,j)^2 ))*tand(thetax);
j=j+1;
end
j=1;
i=i+1;
end

16
b=load('b.txt');
s=size(b);
for i=1:1:s(1,1)
Thetay_o(i)=atand(b(i,2)./h);
Thetax_o(i)=atand(b(i,1)/(e+sqrt(h^2 + b(i,2).^2 )));
end
b1=b(:,1)';
b2=b(:,2)';

17
figure(2)
plot(b1,b2,'.');
hold on
plot(b1(1,1), b2(1,1)),'*');
figure(3)
plot(Thetax_o,'b','linewidth', 1.5)
hold on
plot(Thetay_o,'k','linewidth', 1.5)
h=500

18
StereoLithography Apparatus (SLA): Galvanometer: Advantages & Disadvantage
Advantages
• Low inertia of the tiny mirror moves the laser beam with a very faster speed (up
10 m/sec) on the surface of the vat-photopolymer.
• Long life of the galvanometer
Disadvantages
• Lost of energy on the mirrors
• Defocus laser beam on the platform
• Laser beam spot distortion Beam Correction

19
StereoLithography Apparatus (SLA): Galvanometer: Beam Correction
Beam deflected from a spherical lens
Locus of the focal point for
deflected beam at different angles

20
ℎ = 𝑓 × tan 𝜃
dℎ
𝑑𝑡
= 𝑓 sec2 𝜃
𝑑𝜃
𝑑𝑡
𝑣ℎ = 𝑓 sec2 𝜃 𝜔ℎ
𝑓
𝑣ℎ =
𝑓𝜔ℎ
cos2 𝜃
Therefore, to keep the constant velocity of the laser beam spot the mirror of the galvenomerter has to be changed
accordingly
It can be noted that the velocity (𝑣ℎ) of the laser beam on
the flat surface is a function of angular velocity of (𝜔ℎ) of
the mirror and angle of incident (𝜃).
𝜃

21
A single axis galvanometer with a range of ±15° is used in a SLA process. The beam deflected from the
galvanometer is passes through a flat-field lens to keep the focal point at distance of 500 mm on the flat top surface
of the liquid vat. In order to obtain a constant velocity of the laser spot as 10 m/sec, determine the range of the
galvanometer’s angular velocity.
𝑣ℎ =
𝑓 × 𝜔ℎ
cos2 𝜃
𝜔ℎ =
𝑣ℎ cos2 𝜃
𝑓
𝜔ℎ =
𝑣ℎ cos2
𝜃
𝑓
=
10
0.5
× cos2
𝜃 = 100 cos2
𝜃
𝜔ℎmax
= 100
𝜔ℎmin = 100 × cos2
15 = 100 × 0.933=93.3
𝜔ℎmin
= 93.3

22
To overcome the issues associated with the flat-field lens
the 𝑓 − 𝜃 are used along with galvanometers. The spot
position for 𝑓 − 𝜃 is roughly the product of 𝑓 & 𝜃 , unlike
flat-field lens.
ℎ = 𝑓 × 𝜃
dℎ
𝑑𝑡
= 𝑓
𝑑𝜃
𝑑𝑡
𝑣ℎ = 𝑓𝜔ℎ

23
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=10766

24
If a laser beam deflected through a galvanometer by an angle of 𝜃 strikes the vat – photopolymer bed. In
that case the shape of the cross section of the beam will change to ellipse from circle. The major axis of
the ellipse will be 𝑑/ cos 𝜃 and minor axis will be 𝑑. The area of the beam can be given as:
𝐴𝜃 =
𝜋
4
𝑑
cos 𝜃
𝑑 =
𝜋
4
𝑑2
cos 𝜃
=
𝐴𝑂
cos 𝜃
𝜃
R R’
𝜔
O a
x
R
𝜔
O
𝐹 − 𝜃 lens 𝐹 − 𝜃 lens
𝑑 × sec 𝜃
𝑑
𝑑
𝜃
𝜃

25
In a Vat-photo-polymerization based process, a laser beam got deflected by an angle of 𝜃 after striking
the galvanometer on incident on the vat-photo-polymer bed. If the power of the laser is 𝑃𝐿, what will be
the power intensity in the beam spot.
𝐼 =
𝑃𝑙
𝐴𝑂
cos 𝜃
𝐼 =
𝑃𝑙
𝐴𝑜
cos 𝜃

26
Also as per beer-lambert’s cosine law, energy absorbed by a surface is proportional to the cosine of the
angle of between the striking beam and normal.
𝐸𝑎 = 𝐸𝑜 × cos 𝜃
𝜃
R R’
𝜔
O a
x
R
𝜔
O
𝐹 − 𝜃 lens 𝐹 − 𝜃 lens
𝑑 × sec 𝜃
𝑑
𝑑
𝜃
𝜃

27
𝜃
R R’
𝜔
𝑉
𝑜 = 𝑅 × 𝜔
O a
x
𝑥 = 𝑅 tan 𝜃
𝑑𝑥
𝑑𝑡
=
𝑅
cos2 𝜃
𝑑𝜃
𝑑𝑡
𝑉
𝑎 =
𝑅𝜔
cos2 𝜃
𝑉
𝑎 =
𝑉
𝑜
cos2 𝜃

28
Telecentric lenses facilitate normal incidence of laser beam onto the target plane. However, a major
disadvantage of telecentric lens is that the scannable area is limited by size of lens. Hence, these lenses cannot
be used to produce normal incidence over a large print area desirable in 3D printing applications. Therefore
the size of the all AM system using galvanometer are limited in size.

29
Solution-02: Height increase

30

31
Recoating
3

32
curing process.
1
2
4
3
5
1 3
4
4
2
5
4

33
StereoLithography Apparatus (SLA): Recoating
Recoating

34
StereoLithography Apparatus (SLA): Recoating
• Recoating is the process of establishing a new layer of fresh resin over the previously cured layer.
• A successful recoating step is one that is capable of establishing a fresh layer of liquid resin of thickness
exactly equal to the desired thickness, 𝑡𝑙, within a reasonably short time.
• An incorrect layer thickness will adversely affect the accuracy of the product.
• Owing to effects related to the viscosity and surface tension properties of the liquid resin, it is not easy to
meet these requirements. As a result, one needs to adopt quite a complex recoating cycle involving
following stages:
1. Leveling
2. Deep dipping
3. Elevating
4. Sweeping
5. Obtaining Build Position
6. Z-wait

35
StereoLithography Apparatus (SLA): Recoating: Leveling
• If resin did not shrink upon polymerization, this step would only be
necessary at the start, to insure that the resin was at the proper z-
level.
• However, typical resin undergo about 5% to 7% total volumetric
shrinkage.
• A level compensation system (contact/non-contact) is implemented
in SLA.
• The level compensation system maintained a tolerance in the z-level.
• If the resin lies within this tolerance limits, the levelling operation is
completed.

36
StereoLithography Apparatus (SLA): Recoating: Deep dipping
• The elevator fully immerses the previously completed part
under computer control to allow the resin to flow over the part
• Owing to the high viscosity of the resin, the resin will take
quite some time before it assumes a level surface over the
entire part.
• Hence, there will be a depression in the resin surface above the
part.
• The depression can be quite significant over regions with
trapped resin volumes. Such volumes occur when there are
pockets of liquid resin within the part interior that do not have
connecting passages to the rest of the liquid in the vat.

37
StereoLithography Apparatus (SLA): Recoating: Deep Dipping
The time, 𝑇𝑑, taken for the depression to be thus 'closed' has been
studied by means of viscous flow analyses and experimental studies
𝑇𝑑 ∝
𝑅𝑐 × 𝜇
h𝑑
2
where 𝑅𝑐 is the so-called 'critical circle radius' for that cross-section, 𝜇
is the viscosity of the resin and ℎ𝑑 is the depth of the depression.
A critical circle is the maximum distance on any cross-section’s
geometry that resin must travel to flow off the part.
As the value of 𝜇 and 𝑅𝑐 are fixed the only possibility of reducing time taken for filling the depression is by
increasing the dipping height (ℎ𝑑). Hence called as deep dipping.

38
The amount of deep dip is about 7.5mm for SLA-250 and 18 mm for SLA-500. How fast SLA-500’s deep
dipping process is as compare to SLA-250 ?

39
Find out the ratio of the deep-dipping time required for the two cross-sections shown in the following figures.
d
d
3
2
d

40
StereoLithography Apparatus (SLA): Recoating: Elevating
• The third stage starts once the depression has closed reasonably.
The platform is elevated until the top layer of the part is above the
resin surface.
• As a result, there will be a layer of thickness exceeding 𝑡𝑙 resting
on part's upper surface
• This step is required so that during next step of sweeping only the
material above the part should be disturbed not rest of the resin.

41
StereoLithography Apparatus (SLA): Recoating: Elevating
• At this point the recoating blade traverse one end of the vat to
another end and sweeps the access material.
• For majority of part geometry the optimum sweep period is
about 5 second.
• However, it appears that recoating behavior is quite sensitive to
the presence of trapped volumes of resin in the part. For
instance, for parts without trapped volumes, changing the blade
velocity in the range 5 to 120mrn/s does not significantly affect
the final layer thickness. However, this is not the case when
there are trapped resin volumes.

42
StereoLithography Apparatus (SLA): Recoating: Sweeping
• If we ignore the viscous behavior of the resin, we would expect the resulting layer thickness, L, to be equal
to the preset gap, g.
• Owing to finite surface tension effects, some resin has adhered to the trailing edge of the blade
• Due to viscous drag there will be a bulge close to the leading edge. (leading edge bulge)
• A consequence of this is that the actual layer thickness, 𝑡𝑙, following the sweeping process will be smaller
than the gap, 𝐿𝑔.

43
StereoLithography Apparatus (SLA): Recoating: Sweeping
The resin flow rate under the blade would be equal to:
𝑉𝑏𝐿𝑔
2
At a location well beyond the trailing edge of the blade, the velocity distribution within the resin layer of
thickness L can be expected to be uniform. And hence the flow rate (/deposition rate) can be: .: 𝑉𝑏 × 𝑡𝑙
Equating the two flow rates: 𝑡𝑙 =
𝐿𝑔
2
𝑄 = න
0
𝑙𝑔
𝑣𝑦𝑑𝑦
𝑣𝑦 =
𝑣𝑏
𝑙𝑔
× 𝑦
𝑄 = න
0
𝑙𝑔
𝑣𝑏
𝑙𝑔
× 𝑦 𝑑𝑦
𝑄 =
𝑣𝑏
𝑙𝑔
𝑦2
2 0
𝑙𝑔
𝑄 =
𝑣𝑏𝑙𝑔
2

44
StereoLithography Apparatus (SLA): Recoating: Obtaining Build Position
• Next, the elevator is lowered again such that the part is in the
right position for scanning the next layer.
• The resin surface at the beginning of this stage is still
disturbed.
• In particular, owing to finite surface tension effects, there is
usually a visible crease at the solid to liquid interface around
the perimeter of the part.
• Experiments have shown that this crease fades away
exponentially over time.

45
StereoLithography Apparatus (SLA): Recoating: Z-Wait
• A waiting period (called 'Z-Wait') is introduced for the resin
surface to blend and reach the configuration shown in figure.
• The Z-wait period is usually determined through a compromise
between surface non-uniformity and build time.
• One needs to choose a longer Z-wait when a large 𝑡𝑙 is chosen.
These observations might give the impression of unduly long Z-
Wait.
• However, fortunately, with current technology, it is possible to
complete computation and adjustment of resin level per layer
within one second. The problem is still noteworthy because
practical parts contain thousands of layers.

46
StereoLithography Apparatus (SLA): Recoating: Zephyr
The ZephyrTM system has a vacuum blade that picks up resin from the side of the vat and applies a thin
layer of resin as it sweeps across the part. This speeds up the build process by reducing time required
between layers and greatly reduces problems involved when building parts with trapped volumes.

47
curing process.
1
2
4
3
5
1 3
4
4
2
5
4

48
StereoLithography Apparatus (SLA): Z-Axis
Leadscrew/Ball Screws Rack & Pinon Parallel Kinematics: Scissor Lift

Thank You
Dr. Sajan Kapil
Department of Mechanical Engineering
Indian Institute of Technology, Guwahati
Guwahati, Assam
Research Int.: Manufacturing Automation, 3D Printing, CAD/CAM
Email: sajan.kapil@iitg.ac.in

M03L03 SLA.pdf

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