Advanced Manufacturing
Choices
MAE 156-265
Spring 2012, Dr. Marc Madou
Class 10
MICRO-MOLDING
TECHNIQUES- LIGA
The Graduate, starring Dustin Hoffman, is released by Embassy Pictures in 1967.
A memorable poolside scene from the movie:
Mr. McGuire:
Come with me for a minute. I want to talk to you.
I just want to say one word to you. Just one word.
Ben:
Yes, sir.
Mr. McGuire:
Are you listening ?
Ben:
Yes sir, I am.
Mr. McGuire:
PLASTICS.
Ben:
Exactly how do you mean ?
Mr. McGuire:
There is a great future in plastics. Think about it. Will you think
about it ?
Ben:
Yes I will.
Table of Contents
• Overview
• Plastic Molding
– Liquid Resin Molding
Techniques
– Injection Molding
(IM)
– Compression Molding
• LIGA
• Soft Lithography
Overview
Liquid Resin Molding
Techniques
– Since mold inserts are typically limited to
rather soft metals like nickel, or brittle
materials such as silicon and glass, micromolding based on low viscosity liquid
resins (instead of high viscosity polymer
melts) is a very attractive approach.
During liquid resin molding, the lowviscosity reactive polymer components are
mixed shortly before injection into the
mold cavity, and polymerization takes
place during the molding process.
– Under liquid resin molding techniques, we
distinguish between reaction injection
molding (RIM) and transfer molding.
RIM
Liquid Resin Molding
Techniques
• In the RIM setup shown
on the previous slide, two
or more highly reactive
liquid resins impinge and
mix inside a mixer and are
injected into the mold.
• In RIM, the
polymerization is usually
mixing activated, so it can
quickly react at room
temperature or slightly
above and convert from
liquid to solid.
Liquid Resin Molding
Techniques
– A transfer molding setup, for a thermally
activated system, is simpler and less
expensive than a RIM system. Here, the
reactive species have been pre-mixed at room
temperature, and the mixture is poured into
the transfer pot.
– A plunger squeezes the mixture through the
sprue into the mold cavity. To cure the
mixture, the mold is heated or radiated,
depending on the resin system. With the
lowest viscosity resins, vacuum-assisted
transfer molding is used, which is even less
expensive than mechanical transfer molding.
TM
Liquid Resin Molding
Techniques
• Two complications associated with liquid
resin molding techniques are that an internal
mold release agent must be used and polymer
shrinkage during polymerization.
• Release agents are added to the bulk polymer
and tend to reduce the thermo-mechanical
properties of the molded polymer. External
mold release agents, coating the surfaces of
the mold insert are unsuitable, since they are
difficult to introduce into all the micro
features of the mold insert.
• Since the resin must be fully cured at
somewhat elevated temperatures, the cycle
time is typically more than 10 minutes.
Liquid Resin Molding
Techniques
Injection Molding (IM)
• Injection molding (IM) is
based on heating a
thermoplastic material till
it melts, thermostatting the
mold parts, injecting the
melt with a controlled
injection pressure into the
mold cavity, and cooling
the manufactured goods.
• Injection molding is
probably the most widely
used technique in
macroscopic production of
polymer parts.
Injection Molding (IM)
• In conventional injection molding of
micro parts the mold remains closed, and
the walls are kept at a uniform
temperature above the glass transition
temperature (Tg) of the polymer, so that
the injected molten plastic mass does not
harden prematurely.
• Conventional thermoplastic injection
molding is the technique used, for
example, to make compact disc (CDs),
involving features about 0.1-μm in depth
and minimum lateral dimensions of 0.6
μm; that is, an aspect ratio of 0.16.
Compact discs are made by injection
molding of polycarbonate in a cavity
formed between a mirror surface block
and a stamper (i.e., mold or master).
Injection Molding (IM)
• Conventionally, electroforming
from a glass master generates
nickel stampers. The principal
attributes required of a stamper
substrate are toughness, thermal
shock resistance, thermal
conductivity, and hardness. For
IM in particular, toughness,
thermal shock resistance, and
thermal conductivity are critical.
Typical Conditions
for Injection Molding
Mold temperature
85°C
Polycarbonate temperature
330°C
Clamping force
60 tons
Injection time
1s
Cooling time
2s
Injection Molding (IM)
Injection Molding (IM)
• The metal negative disc (the
father) could be used to
replicate the CDs, but would
wear out too soon. By plating
the metal negative disc (the
father) several metal positive
disc are produced and called
(the mother).
• Each (mother) is used to create
stampers.The stamper is a
nickel plated disc used in the
mold cavity for the replication
of CDs. (CDs are injection
molded)
Injection Molding (IM)
• Pros and cons of injection
molding are summarized in
this Table.
• Example materials in IM
include polycarbonate (PC),
polymethylmethacrylate
(PMMA), polystyrene (PS),
polyvinylchloride (PVC),
polypropylene (PP),
polyethylene (PE), and
polyacrylnitrilbutadienstyro
l (ABS).
Pros
Cons
Good for small structures with low aspect
ratio, e.g., CD and DVD
Only low-molecular-weight polymers (may
reduce mechanical and thermal strength)
Good for large, high-aspect-ratio, and 3D
features
More expensive equipment
Excellent dimensional control
Cyclic process only
Short cycle time (as low as 10 s)
High stress on master
High productivity
High residual stresses on molded parts
Closed mold process enables packing
pressure application
Compression Molding (also Relief
Imprinting or Hot Embossing)
• The basic principle of compression
molding (also hot embossing and
relief imprinting) is that a polymer
substrate is first heated above its
glass transition temperature, Tg (or
softening temperature).
• A mold (or master) is then pressed
against the substrate, fully
transferring the pattern onto it
(embossing).
• After a certain time of contact
between the mold and the
substrate, the system is cooled
down below Tg, followed by
separation of the mold and the
substrate (de-embossing).
Compression Molding (also Relief
Imprinting or Hot Embossing)
• Importantly, the hot
embossing process can be
achieved in either a cyclic
process or a continuous
process.
• Compression molding
provides several advantages
as compared with injection
molding, such as relatively
low costs for embossing
tools, simple process, and
high replication accuracy
for small features.
Compression Molding (also Relief
Imprinting or Hot Embossing)
Pros
Cons
Low polymer flow
More difficult for structures with high
aspect ratio (near Tg processing)
High-molecular-weight polymers (with
better mechanical and thermal properties)
Less dimensional control (open mold
process)
Simple process
Planar features only
Continuous or cyclic (see Figure 17.7)
High residual stresses on molded parts
Good for small structures
Difficult for large parts and multiple feature
depth (too high a pressure and temperature
are required)
JENOPTIK HEX 04 hot embossing system
manufactured by Jenoptik Mikrotechnik
GmbH (http://www.jenoptik.com).
LIGA
• LIGA: lithography,
electroplating and molding
• 1st electroforming:
–
–
–
X-ray exposure (irradiation)
Developing
electroforming for final metal
product or for mold insert
Dr. Ehrfeld, IMM
LIGA
• Plastic molding and 2nd
electroforming/casting slip
–
–
plastic final structures or lost
mold
metal or ceramic final parts
Dr. Ehrfeld, IMM
LIGA
• Mask :
–
–
low Z membrane
high Z absorber
• Alignment of substrate with
mask is difficult since no visible
light can pass through the mask
membrane
• Sample is moved vertically
through the irradiation band
with a precision scanner
Exposure station and
masks
LIGA
• Optimum wavelength for
optimum pattern transfer is 0.2
to 0.3 nm :
–
–
Diffraction increases as
wavelength increases
Secondary electron emission
increases as the wavelength
decreases
• Variation in critical dimensions
at this wavelegth between the
ends of a 500 µm high structure
is estimated at 0.2 µm
Optimum X-ray
radiation
LIGA
Optimum X-ray
radiation
LIGA
• Problems for plating in small plastic
cavities:
–
–
–
–
Wetting agents must be added
otherwise even 50 µm posts do not
plate
pH changes at the bottom of cavity
No agitation possible
In principle, smaller features should
plate faster but if the aspect ratio is
too high they actually plate slower
Electrodeposition
LIGA
Molding and demolding
•
Reaction injection molding (RIM):
–
•
Injection molding:
–
•
Mold is kept above the glass transition
tempearture and molten plastic is
injected (e.g. CD’s)
Compression molding (also hot
embossing):
–
•
Mixed reagents pumped into the mold
A molding tool is pressed into the plastic
material at temperatures above the glass
transition temperature
Demolding requires extra smooth walls
and internal mold release agents
LIGA
• Surface micromachining made motors
with torques in the picoNm range
possible, with LIGA 10-6 to 10-7 Nm are
possible (more z-axis i.e. more torque)
• Combination of traditional and LIGA
machining
LIGA
LIGA
• Small Ni turbine
• Gas flow meter needs a cap.
• Integrated fiber measures
rotation speed
• Major competition from:
–
–
Deep RIE (subtractive)
Deep UV (additive).
Soft Lithography
• See practicum