Depending on the application various tool types are commercially available, for
instance the woodruffer cutter that can create undercuts. High-aspect ratio features,
dominant in one dimension, are best achieved with drill bits rather than end mills.
The problem is that thelength of cut, which is defined by the length of the cutting
edge, restricts the fabrication depth. Custom tools are available with longer necks
for deep cuts; they can be tricky to operate at high-feed speeds though. Tool
manufacturers provide recommendations for milling that relate the cutting diameter
to spindle speed to feed rate. The purpose of these tables is to provide optimal
parameters for milling without tool breaking. It is important to emphasize that most
of the data is calculated for cuts shallower than two tool diameters and high chip
load. Chip load is the thickness of the removed material by each cutting edge, and it
is calculated by dividing the feed rate (meters per minute) by the spindle speed
(RPM) times the number of flutes. The flutes are the spiral cutting edges starting at
the tip of the tool and protruding towards the shank (Fig.3.6c). Let us consider that
the diameter of a two-fleet tool is large (3 mm) in comparison to the chip load
(5μm) then at extremely low spindle speeds (1000 RPM) the maximum feed rate
should not exceed (1000 RPM5e6m 2 ¼0.1 m/min). This would be very
inefficient, as the milling would take extremely long time. Increasing the feed rate
without compensating for the spindle speed could cause dragging of the tool against
the substrate. Therefore, spindle speed needs to be corrected accordingly. Higher
spindle speeds than those recommended by the tool manufacturer result in friction
that wears off the cutting edges. Thus, the optimal milling conditions are defined by
knowing the characteristic chip load for a tool with a given diameter and maximal
spindle speed.
Optionally, coolant can be added during the fabrication process to improve heat
dissipation and prevent melting of the plastic substrate. The cooling agent must be
selected such not to interact with the substrate. Increasing the number of passes and
reducing the depth per pass diminishes the amount of build-up and hence can
prevent melting. Another option is to use air stream as coolant, which is easily
achievable by splitting the air supply to the spindle and directing it at the substrate
surface.
2.6 Laser Ablation
Laser ablation is another rapid fabrication technique for direct translation of
computer-aided laser design (CAD) on to polymer substrates. Laser ablation can
also be used for patterning surfaces of photopolymers and /or antiadhesive layers by
forming micro- or nanoimprints (Fig. 3.7). These patterns can then be
functionalised with proteins, oligonucleotides or whole cells. Power, stability and
low operational cost of CO 2 lasers make them suitable for cutting and engraving
PMMA. The wavelength of CO 2 laser lies in the far infrared spectrum (10.6μm)
and due to the constraint of light diffraction this limits the focused laser beam
diameter [ 14 ]. Evidence exist that shorter wavelengths, i.e., ultraviolet laser
96 N. Dimov