Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 140

The basic idea behind replication technologies is the combination between the more expensive
silicon-based technology and the low-cost replication in polymers. The few drawbacks of replication
technologies are:
Since the master is to be removed from the molded structures, freestanding structures with
undercuts cannot be fabricated. A combination with polymeric surface micromachining could be
a solution for this problem.
Only few micromachining technologies can meet the required smoothness of the master mold.
Due to contamination and fast diffusion in microscale, release agents used in macroscale cannot be
used for the release process in microscale.
The mold can be fabricated in silicon with the established micromachining techniques. Conven-
tional machining techniques such as drilling, cutting, milling, and turning can be used for structures
down to several tens of microns. Bulk silicon micromachining can be used for structures with high
aspect ratios. Metal mold can be electroplated with the help of a structured thick resists such as SU-8
and PMMA. For instance, the fabrication of nickel mold from structured PMMA was established and
called LIGA (Lithographie-Galvanoformung-Abformung, German acronym for lithography, electro-
plating, and molding). In the following, three replication techniques are discussed in detail: injection
molding, hot embossing, and soft lithography.
4.2.2.1 Injection molding
Injection molding is carried out at temperatures above the glass transition temperatures of amorphous
thermoplastics such as polymethylmethacrylate (PMMA), polycarbonate (PC), and polysulfone
(PSU). For semicrystalline thermoplastics, such as polyoxymethylene (POM) and polyamide (PA), the
molding temperature should be higher than the crystallite melting point [79] . Table 4.8 compares the
characteristics of different polymers commonly used for micromolding.
Mold inserts for injection molding can be fabricated with common microtechnologies such as bulk
micromachining and LIGA. Alternatively, bulk-micromachined parts can be used as masters for
electroplating [80] . After separation from the silicon part, the metal parts can be used as negative mold
inserts to fabricate replicas of the silicon parts [80] . Structures in micromixers may have high aspect
ratios and very small filling channels. Together with the high viscosity of melted plastics, high
pressures are required for the injection. To avoid the high pressure associated with the high viscosity,
reaction injection molding (RIM) can be used. This technique involves mixing of two or more reactive
chemicals. The mixture flows into the mold at relatively low temperature, pressure, and viscosity.
Curing occurs in the mold at relatively low temperatures and pressures. The excess prepolymers are
fed back in a recirculation loop by pumps. The entire process, from mixing to demolding, typically
takes less than one minute. The low processing viscosity allows good filling and high molding
accuracy.
Figure 4.12 shows the schematics of a typical injection molding system. The machine consists
of a screw, an injection nozzle, a heater, and a mold insert. To start with, polymer pellets are loaded
into the screw. The piston pumps the polymer into the accumulation zone, where it is melted by
a heater. If the desired polymer amount is reached, the piston moves forward and pushes the
polymeric melt into the mold cavity through the nozzle. The required pressure is typically on
the order of 500 e 2,000 bars [81,82] . After cooling, the melt solidifies, and can be taken out from
the mold.