Use of router bits during
the production of thermoformed plastic parts is one of the oldest applications
of router tooling in plastics. Almost since the inception of pressure forming
and vacuum forming, router tooling has been used to remove flashing, separate
parts, and create holes or cutouts. The earliest plastic application router
bits were designed to facilitate hand operations and reduce operator fatigue
in repetitive operations such as part trimming and cutouts. With the advent
of 5-axis CNC routers, the tooling has evolved into specialized bits to reach
deep into cuts and prevent part movement on large, single piece components.
This article will highlight the differences between the two styles (manual and
CNC) of tools and provide information on their selection, application, and use.
Most thermoformed parts
are made of softer materials that react well to O-flute style tooling. In air
router applications, the most common choice has been single edge and double
edge straight tools. Single edge tools typically feed the easiest and double
edge tools will usually give a better finish and allow faster feed rates in
the hands of an experienced operator.
Typically High Speed Steel
(HSS) is the material of choice in air routers due to its ability to withstand
impacts and its grinding characteristics that allow a sharper edge than Solid
Carbide (SC) after machining. These properties lend themselves to use in environments
where tool breakage and operator fatigue are common. Other features common to
hand tooling for use in thermoformed parts are the extended shank and cutting
edge lengths. The extended shanks allow for a larger colleting area and an additional
surface for internal guide bearings to ride on. This improves tool rigidity
and helps to reduce cutter breakage while improving the overall "feel"
of the routing operation. The extended cutting edge length gives extra cutting
area past the nose bearings and allows the operator to present the router bit
to the material without having to remove any safety or guide components from
the router head.
With the increased use
of CNC 5-axis machines to trim parts, the viability of SC has seen a large increase
when compared to its use in air routers. The improved repeatability and constant
feed rates of these machines versus hand routing has allowed the increased stability
and rigidity of SC to manifest itself as faster feed rates and improved edge
finishes without the vibration associated with HSS. As with HSS, SC tooling
for trimming is typically sold with extended cut lengths and shank lengths.
The longer shanks are used for reach when trimming complex mold parts that may
require the router head to maintain a longer than normal distance from the part
due to interference. The longer cut lengths are used to ease the programming
of difficult contours and cutouts. Because of factors relating to router head
orientation and interference, it is not atypical to present a cutting edge to
a trim surface at something other than a perpendicular angle. By having a longer
cut length to cut this larger effective area, a shorter, simpler program can
The use of CNC machines
has also led to additional geometries beyond straight O-flues being successfully
applied to the trimming of thermoformed parts. SC low helix spirals have seen
very good success in both single edge and double edge configurations. Typically
these spirals are of an O-flute or modified O-flute style and will be used with
an upcut helix when the parts are thicker than .090" and have a solid fixturing.
If part movement or chatter is present, a downcut can be used to increase the
effectiveness of the router fixture and improve part edge finishes.
Solid, effective fixturing
is one of the fundamental keys to successful trimming operations. More often
than not, an improvement in hold down capability can do more to improve a trimming
operation than adjusting feed parameters, picking the optimum tools, and tweaking
program parameters combined.
The material that the CNC
fixture is formed out of is the best place to start when examining the capability
of a routing operation. While plywood is one of the best materials to use, its
cost for construction time can be difficult to justify. A poured urethane foam
base can also be expensive, but its ease of use is usually a benefit to shops
without a dedicated mold and fixture department. The drawback of poured fixtures
is their susceptibility to dimensional instability and sometimes poor rigidity
depending on their construction.
The key to an effective
fixture is the ability to distribute its holding power over a large area as
well as specifically reaching the edges where routing operations are to take
place. The single biggest failure of fixtures is their inability to supply adequate
vacuum to where it is needed. Good forethought in the design phase of a fixture
will ensure adequate room to run internal piping and vacuum supply to the edges
of the areas to be routed. When the configuration prevents internal vacuum supply
from reaching an area, the next best method is to use deep vacuum supply channels
on the top surface of the fixture to distribute vacuum throughout the cutting
In instances where the
part/fixture interface is an irregular fit (inconsistent parts, poor fixture
design, or poor alignment), good gasketing is essential for retaining vacuum
pressure at the cutting interface. A good gasketing material will be either
a low durometer, flexible, dedicated gasket material or a closed-cell crush
resistant foam tape. Weather stripping is not an acceptable gasketing material
due to its low bond strength adhesive in lateral stress applications and its
inability to recover from repeated stress and compression cycles.
Hand air routers work very
well in trimming thermoformed parts when the router is new or well maintained
and the setup of the router is done correctly. If the system is not done correctly
the resulting trimming operation will have built in wastes of both time and
Air routers, like with
most machines, wear out over time with normal use. The hard part is determining
when to perform maintenance and repair. Digging into the manuals for the air
routers should result in a maintenance frequency recommended by the manufacturer
which should be the first point of reference. If a manual is not available,
or a maintenance schedule is not found, here are some tips for repair and maintenance
for air routers.
The item most overlooked
and first to need repair in an air router is the front nose piece bearing. The
estimated useful life of this bearing is 100 hours. For most operations this
could be one to two weeks of use. Depending on the materials cut the life may
be longer or shorter. The bearings in the nose piece are normally shielded bearings
and not sealed. Often the chips and dust from the trimming operations are find
their way into the bearings and cause premature wear. The router motors should
be inspected after 500 hours or sooner and repaired after 1000 hours to ensure
One of the more overlooked
essentials in proper setup of a hand air router is the air pressure at the point
of use. The easiest way to ensure the proper air pressure and air volume is
to install a gauge at the point of use. With a gauge in line at the point of
use, the router should be placed under load and the gauge readings verified
to a recommended steady pressure range of 90 to 100PSI. Under normal use, the
pressure can be watched to ensure the router is operating within the recommended
pressure range at all times. Air pressure and volume is to an air router as
voltage and amperage is to an electric router. If the pressure drops to below
the 90PSI when under load it could be a sign that the air feed line is too small
or the overall volume of air is under capacity. Solutions could be installing
larger feed lines and/or adding an air tank close by to act as a supply capacitor
to the system. The impact of low pressure or low air volume is lower speed (RPM)
and reduced horsepower resulting on more difficult cutting and extra tool wear.
Lower speeds can slow production resulting in longer times and higher costs
to produce the same parts.
Once the proper tools have
been selected and a solid fixture built, the last factor to consider when routing
thermoformed parts on a CNC 5-axis router is the router itself. Proper programming
and feed and speed selection will determine the viability and profitability
of an operation.
The largest advantage of
a CNC router over hand routers is the increased feed rates attainable and the
improved consistency of the finished part. With multi-flute plastic specific
tooling and solid fixturing, feedrates of 250-350ipm are attainable in 5 axis
trimming and cutout operations. Lower feed rates are typically required when
general purpose router tooling and poor fixturing are utilized. Most often these
problems manifest themselves as inconsistent chatter marks in poorly fixtured
areas such as corners or extended, overhanging flashing.
Programming for optimum
finishes and speed should take into account not only feeds and speeds, but also
cutter entry locations and angle of presentation of the cutting edge to the
workpiece. When router bits enter a softer working material, they will often
"walk" or enter in an eccentric spiral configuration. This is the
result of the point geometry being ground to produce flat cutting bottoms and
the lack of a center pointed drill tip to keep the cutters entry concentric.
The best method to eliminate evidence of a spiral entry is to plunge the cutter
in an area of scrap and then rout to the cutaway area perimeter. This applies
both to flash trimming and cutout or interpolated hole work.
An additional concern when
programming for 5-axis work is the orientation of the cutting edge to the work
surface. Router bits are designed the be run perpendicular to the surface being
cut and any misalignment can lead to increased instances of cutter breakage
or chatter. While 5-axis tooling is designed to handle these increased stress
loads and the cutting edge lengths are intentionally elongated to compensate
for the increased effective depth of cut, the best method is still to provide
extra time during the programming phase to keep the router head oriented perpendicular
to the cut plane.
Using routers for trimming
of thermoformed plastic parts has been, and will continue to be, one of the
best methods of separating finished parts from extra forming material. As CNC
5-axis machines become more popular, the attention paid to cutter selection,
router and fixture rigidity, and programming will have to increase in order
to maintain profitability. The best method is to analyze the process early in
the development cycle and plan for necessary factors instead of attempting to
improve poor fixtures or programs after their creation.