Routing & Trimming
Acrylics are a very popular
thermoplastic because they have so many desirable properties and because they
can be processed in several ways. Acrylics can be shaped by casting, extruding
or molding to result in a material ranging from soft and pliable to rigid and
brittle. These acrylics can be transparent, opaque or colored depending upon
usage and can vary from thin sheet to thick solid surface materials.
The popularity of acrylics
attests to the range of characteristics from optical clarity to rugged marble
like surfaces. In all instances, however, acrylics can have excellent surface
hardness, resistance to liquid, chemical and environmental surface damage and
good thermal as well as mechanical stability.
Acrylic sheet products
can be rigidized by thermoforming to a three-dimensional shape then spraying
a fiber reinforced plastic underlayer. The resultant product has a glossy acrylic
surface for desired finish and durability to aging and elements and a rigid
underlayer for stability and load carry capabilities.
Usage of acrylic materials
varies from aircraft canopies to lenses of all types for lighting, autos and
enclosures. Sign makers utilize acrylics for weatherability, color, finish and
backlighting capabilities. Bath bays, shower enclosures, panels and whirlpools
are mainly rigidized acrylic materials. The recent success of solid surface
materials made with acrylic based resins and mineral or inorganic fillers have
attained popularity because of exceptional stain and scratch resistance. Newer
applications for solid surface have included wall treatments, bath surrounds,
furniture and architectural treatments. New applications for acrylics have also
come from the marine industry. Work is being done on acrylic/ABS hulls to take
advantage of the surface finish and corrosion resistance of the material. Smaller
parts of many boats are now made with acrylic materials.
Machining acrylics can
be quite easy or very difficult depending on the machining environment. Most
cast sheet, once thermoformed, will be brittle and prone to chipping or cratering,
if not machined properly. Thinner materials (under .375") can be trimmed
quite easily with a single edge straight O flute solid carbide router bit (see
figure 1). If several holes must be cut or if internal shapes must be routed,
slow helix O flute spiral solid carbide bits are suggested. These bits are available
in either an upcut (see figure 2) or a downcut (see figure 3) design. Usage
of either will depend on fixturing, part hold down and desired chip flow.
Thicker acrylic materials
(over .250"), particularly small parts cut from sheet, can best be machined
in two passes. The first pass should be a skin cut .015" oversize and leaving
.015" of material at the base. The second pass should then cut on size
completely through the material. This system not only improves finish but eliminates
the potential problem of the cutting tool contacting the off fall, dislodging
the part causing scrap or tool breakage (see figure 4). Two tools are generally
recommended for this modified skin cut system: either a solid carbide two flute
upcut with acrylic geometry (see figure 5) or a three flute slow helix solid
carbide finishing tool (see figure 6). Usage will depend on material thickness.
Use the three-flute tool for over .500" thick acrylics.
In order to achieve optimum
finish when machining acrylics, increased emphasis must be placed on finding
the optimum feed and speed. If the feed rate is too slow or the spindle RPM
is too high, several undesirable things can occur. If the cut chips are rewelding
or scratching the piece part, if the piece part has a chalky or white finish
or has scallops the feed rate and/or spindle speed must be adjusted to achieve
a better finish and improve production. If spindle speed is 14,500 RPM, start
feed rate at 200 inches per minute and increase until finish is optimized. If
spindle speed is 18,000 RPM, start at 250 inches per minute. If spindle speed
is variable, there is more latitude in making incremental feed and speed adjustments
to optimize finish.
Machining solid surface
acrylics is often done with hand held routers at the site where such counter
tops or inlays are installed. Solid surface materials can be machined with solid
carbide for straight cuts or with carbide tipped tools for profiling or straight
cuts. Increasing applications for CNC are found in this solid surface market
with the fabricators. All acrylics, in fact, are best machined on CNC routers
where the controlled environment will lead to maximum productivity, better finish
and increased tool life.