1. 1
11th
International Conference on Micro Manufacturing
Orange County, California, USA, March 2016
Paper#65
Use of additive manufacturing in creating sub-millimeter
micro-architectured structures via investment casting
Neng Li1,2,*
, Yi-chen Lin1,*
, Benjamin Dolan3
, Lawrence Kulinsky1
1
Mechanical and Aerospace Engineering Department, University of California, Irvine
2
Beijing Institute of Aeronautical Materials, Beijing, China
3
RapidTech, University of California, Irvine
*
Both authors contributed equally to this manuscript
Abstract
Present work describes extension of the investment casting to production of metal lattices with sub-millimeter
diameter trusses. The stereolithography is used to create a sacrificial scaffold that is embedded in the investment
matrix with subsequent sublimation to create a system of 750-micron diameter pores that are filled with ZAMAK
alloy during casting. The metal lattices are created with two alternative approaches – spin casting and gravity
casting (with vacuum assist). Compression testing of lattice structures demonstrates that two casting technologies
produce metal lattices with comparable mechanical strengths.
Keywords: Metal lattice structures, additive manufacturing, stereolithography, investment casting,
micromanufacturing.
1. Introduction
There is a significant interest in maximizing
strength of materials while minimizing the weight of
the corresponding structures for a variety of
applications from civil engineering to aero-space and
automotive industries [1,2]. In order to manufacture
structures with superior strength-to-weight ratios,
recent developments emphasize the use of
micro-architectural materials (also called
meta-materials) where rather than using a solid block
of material a lattice or foam structure is created [3,
4]. Historically lattice truss structures are found at
large length scales (such as bridges and buildings).
The improvements in strength-to-weight ratios are
based on the principle that the trusses within the
structures predominately experience axial stresses
(tension or compression) when loaded, thus capable
to withstand higher loads than in the bending modes
[5]. After scaling these structures to millimeter
lengths and smaller, substantial improvements in
strength and stiffness have been predicted [6].
It has been challenging so far to manufacture
sub-millimeter metal lattice. Recent attempts in this
area are connected to the emergence of additive
manufacturing (AM). AM methods are capable of
producing lattice structures with controllable pore
shapes and sizes where truss diameter can be in
sub-millimeter region, but most of the demonstrated
approaches involve significant time and expense to
produce these structures. For example, some
sub-millimeter lattices have been produced with
Direct Metal Laser Sintering (DMLS) – quite
expensive process even for small parts [7]. Another
approach was to use stereolithography (SLA) for
manufacturing resin-based sacrificial lattice
structure, using seedless electroplating on the surface
of these lattices with subsequent high temperature
exposure that causes sacrificial lattice to vaporize
leaving hollow metal lattice structure [8]. The latter
technique is also expensive and time consuming.

2. 2
Present work demonstrates our approach to
development of an inexpensive process for
producing sub-millimeter metal lattices. The study
focuses on fabrication sequence that involves
stereolithography to produce the sacrificial lattice
structure, encasing the lattice within the investment
matrix, sublimating the sacrificial lattice, and then
using either gravity casting or spin casting of
ZAMAK3 alloy to produce the finished metal lattice.
The lattices produced via gravity casting and spin
casting were subjected to compression tests using the
Instron 3367 test frame. Visual observations and
final element modelling are also performed.
2. Experimental Procedure
2.1 Fabrication
The process flow is shown in Figure 1. The
cellular lattice structure investigated in this paper is
shown in Fig. 2. The diameter of struts is 750 µm.
The Body Centered Cubic (BCC) structure was
selected for the lattice unit cell. The cell topology is
given in Fig. 2 (b). The lattice consists of 2 layers of
9 units each as seen in Figure 2(a). The overall
dimensions of a sample are 20mm long, 20mm wide,
16mm tall, while the thickness of the top and bottom
platforms are 2 mm.
A three-dimensional (3D) CAD model is
generated with the Solidworks software (Dassault
Systemes SolidWorks Corp.). The STL file is
exported and sliced into a series of 2D layers in a
custom software of digital stereolithography (SLA)
system B9Creator1.2 (B9Creation, Corp., USA).
B9cherry resin is exposed in layer-by-layer sequence
as platform is raised 50 microns per step. The lattices
manufactured via SLA is shown in Figure 3.
The Plasticast (Ransam & Randolph Corp.,
USA) Investment is mixed with water in 100:38
volume ratio. In order to avoid premature
solidification, the mixing should be done within 3
minutes. The investment mix is then placed in a
vacuum chamber for 2 minutes for degassing. Pour
the degassed investment mix down the side of the
investment flask (Ransam & Randolph Corp, USA,
diameter 44.5mm, height 44.5mm for spin casting;
diameter 76.2mm, height 101.6mm for gravity
casting) where the lattice with the attached wax
filaments is placed. Vacuum the invested flask about
1.5 min and fill the flask to the top edge of the
investment flask. It is critical not to agitate the flask
during the process of solidification at room
temperature for 2 hours. After hardening, remove the
sprue base and start to burn out the wax pattern. The
wax burnout schedule is given in Table 1.
The lattices produced via stereolitography are
used as sacrificial structures to generate a negative
pattern of the channels filled with a metal during
investment casting process. Present study compares
lattices produced with two different investment
casting techniques: a gravity casting and a spin
casting. In this paper, the alloy selected for casting
was ZAMAK3 zinc alloy widely used due to its low
melting point (385℃) and good fluidity in a molten
state.
2.1.1. Gravity casting
In order to increase the hydrostatic head of the
poured metal, the cylindrical funnel is used (see
Figure 4(a)). The mold is heated according to the
schedule in Table 1 and cooled to 480℃before
Figure 2. Lattice design: (a) Lattice;
(b) Structure of the unit cell
(a) (b)
Figure 3. Lattices manufactured via SLA
Figure 1. Process flow

3. 3
casting. While significantly above the melting
temperature of the alloy used, experimentation
indicated that mold temperature of 480℃ resulted in
the highest success rate. The mold is then set on a
vacuum investment table. A vacuum of -100 KPa is
then applied to the bottom of the mold to assist in
drawing the material into the mold and reducing the
likelihood of voids. The metal
is poured into the funnel and the negative pressure is
applied for 1 min after which the pressure is released
and the metal is allowed to solidify for 15 minutes at
the room temperature. Subsequently the mold is put
into cold water to clear out the investment and
separate the resulting metal lattice.
2.1.2. Spin casting
The spin casting is prepared by rotating the arm
at least 3 full rotations and locking the arm in place
with a pin. Once again, the mold heated to 480℃ is
then removed from the furnace and placed in the
carrier at the end of the spin arm. The crucible
(movable ceramic container into which the metal is
poured) is moved to touch the mold. Metal is poured
into the crucible and heated with a blow torch to stay
liquid. The pin is released and the centrifugal force
caused by the rotation of the spin arm pushes the
liquid metal into the mold and fills its cavities.
Because of the smaller size of the mold and due to
convective cooling during the spinning the
solidification of the metal happens much faster
during the spin casting than for gravity casting.
Within 5 minutes after the casting the mold can
be placed in the cold water and the metal lattice can
be separated from the investment. The process of
spin casting is shown in Figure 4(b). Metal lattice
structures formed with the gravity casting and spin
casting can be seen in Figure 5.
Figure 5. Investment casting results for:
(a) Gravity casting; (b) Spin casting
2.2 Mechanical characterization
To prepare the samples for mechanical testing,
the metal tubes and fibers that were conduits for the
molten metal into and out of the lattice structures are
sawed off and the top and bottom surfaces of the
lattice structures are planarized and polished with an
end mill on the milling machine (model 2831237,
Bridgeport Corp., USA). The compression tests were
carried out at room temperature using INSTRON
3367 testing machine (INSTRON Corp., USA) at
room temperature. Loads and displacements were
recorded. The loading rate was set up to 5mm/min.
3. Results and Discussion
3.1 Finite Element Modeling
We have used finite element modeling to
estimate the elastic modulus of the lattices and yield
strength of cellular lattices. The models aim to
Flask size: up to 6.3 cm (diameter) × 6.3
cm(height)Using for spin casting
Flask size: up to 10.2 cm (diameter) × 15.2
cm (height) Using for gravity casting
Step 1:Dry out Ambient to 150℃ and hold 1 hour Ambient to 150℃ and hold 3 hours
Step 2:Dewax Raise over 1 hours to 370℃ and hold
1 hours
Raise over 1.5 hours to 370℃ and hold
1.5 hours
Step 3:Wax
burnout
Raise over 2 hours to 730℃ and hold
2 hours
Raise over 2 hours to 730℃ and hold
2 hours
Step 4:Prepare for
casting
Reduce to casting temperature allow for stabilization.
(a) (b)
Table 1. Investment Mold Heating Schedule
(a) (b
)
Spring
Crucible
Flask
Figure 4. Investment casting:
(a) Gravity casting; (b) Spin casting

4. 4
describe the behaviour of the structures under
compressive loads. All the simulations are performed
by the commercial finite element software
Solidworks (Dassault Systemes SolidWorks Corp.).
The mesh of the lattice includes 119536 nodes and
73784 elements.
3.2 Boundary Conditions
To coincide with the experimental uniaxial
compression test, the translational degrees of
freedom on the bottom face of the lattice are fixed
while the rotational ones are free. For the upper face
of the lattice, all the translational degrees of freedom
are fixed except in the loading direction. The contact
between the specimens and loading instruments in
the normal direction is set as a hard contact, which
preserves the planarity of the top and bottom planes
of the lattice. The displacement of the top platform is
set as a control factor. The force and displacement
are predicted by the simulation and a stress strain
curve can be plotted.
3.3 Deformation of the lattice
Figure 6. Simulated lattice structure under
compression indicating distributions of: (top) Von
Mises Stress;(bottom) Strain
The simulated lattice structure under the strain
of 7.8% is presented in Figure 6. Initially,
compression results in an elastic deformation of the
lattice. The strain contours show that, almost all the
compressive load is carried by the struts nodes. By
increasing the applied force, the struts start to buckle
and a fracture initiates at the nodal regions. The
highest stress is experienced by the nodes at the
corners of the lattice structure. Figure 6 illustrates
the Von Mises stress distribution in the lattice at
7.8% strain. The maximum value of residual stress
could reach to 89 MPa, while the minimum value is
9.2 MPa. The figure shows that the stress distribution
is similar for each unit cell and that highest stress
and thus the plastic deformation (buckling) will
occur in the trusses at the locations closest to the
nodes, while the lowest stress level is in the trusses
between the nodes.
3.4 Mechanical characterization results
Figure 7 shows the manufactured cellular lattice
structure under compression during mechanical
testing. Figure 8 presents the results for mechanical
testing of the representative samples produced via
gravity casting and spin casting as well as simulation
results. It can be seen that the samples produced via
spin casting and via gravitational casting behave
nearly identical under compression. It is also evident
that simulation overestimates the strength of the
structure, likely due to the fact that it does not
account for internal defects of the structures created
Figure 7. Compression test on Instron 3367
Figure 8. Load-Displacement curves for
microfabricated metal lattices

5. 5
during fabrication process.
4. Conclusion
A micromanufacturing process flow to create metal
lattices with 750 micron diameter trusses involves
creation of sacrificial lattice structure via
stereolithography, embedding it in the investment
matrix, performing a heat treatment to create a
designed network of sub-millimeter channels after
submimation of the sacrificial scaffold, and casting
zinc ZAMAK alloy with two alternative methods –
spin casting and gravity casting. Results of
compression tests performed on Instron test station
demonstrated that both, gravity casting and spin
casting produce lattice structures with comparable
strengths.
5. References
[1]. Beyond Engineering: How Society Shapes
Technology (Oxford University Press, Oxford,
1999).
[2]. H.G. Allen, Analysis and Design of Structural
Sandwich Panels (Pergamon Press, Oxford, 1969).
[3]. Deshpande VS, Fleck NA. Collapse of truss core
sandwich beams in 3-point bending. Int J Solids
Struct 2001; 38(36–37):6275–305.
[4]. Deshpande VS, Fleck NA, Ashby MF. Effective
properties of the octet-truss lattice material. J Mech
Phys Solids 2001; 49(8):1747–69.
[5]. J. Borrego, Space Grid Structures: Skeletal
Frameworks and Stressed-Skin Systems (MIT Press,
Cambridge MA, 1968).
[6]. V.S. Deshpande, M.F. Ashby, N.A. Fleck, Foam
topology bending versus stretching dominated
architectures, Acta Materialia, 2001, 49, p.1035
[7]. B. Gorny, T. Niendorf, J. Lackmann, M. Thoene,
T. Troester, H.J. Maier Mater. Sci. Eng. A, 528
(2011), pp. 7962–7967
[8]. T. A. Schaedler, A. J. Jacobsen, A. Torrents, A.
E. Sorensen, J. Lian, J. R. Greer, L. Valdevit, W. B.
Carter, Ultralight Metallic Microlattice, Science, 334
(6058) 962-965 (2011)
Acknowledgements
The authors are grateful to Mr. Steve Weinstock for
his generous support with sample preparation and
mechanical testing.