Feature Stories Archive
for Plastic Routing:
article is the last of a two-part series designed to help
fabricators improve their machining of plastics.
make the transition from the routing of wood or aluminum to
the machining of plastics, there are a number of preliminary
procedures and considerations that can help ease the
conversion and ensure a smooth transition.
factors in the routing of wood and aluminum can become some
of the most significant aspects of plastics machining. Good
planning and preparation can help ease these factors and the
costs associated with the startup of a new machining
is the second of a two-part series that discusses the need
to have active preparation when making the transition from
wood routing to plastic routing. Part 1 discussed the CNC
router and its associated hardware. Part 2 will discuss
tooling and material selection as well as programming
the customer's requirements, fabricators may have leeway in
specifying exactly what material they will have to rout.
This is an important option and should be taken advantage of
whenever feasible in order to reduce machining headaches
down the road.
sheet manufacturers produce variations of their common sheet
goods that are marketed as having improved machining
characteristics. By all means, try these. They typically
produce better chips, do not melt or scar as much and can
sometimes reduce the problems associated with chips wrapping
on the cutters during plunging operations.
grades of the same type of sheet can exhibit tremendously
different routing characteristics. UV protectants can
inhibit good chip formation, fillers can cause premature
cutter wear and different sheet colors can affect the
quality of the edge finish. Some grades may even be listed
as co-polymers. These co-polymers may be listed as a single
sheet type but frequently contain a second plastic in the
formulation. It is better to test cut new grades before
estimating tooling costs and routing times because of these
characteristic to verify with sheet manufacturers is if the
material exhibits different machinability going in the
extrusion direction or across it. This can be thought of as
cutting with the grain or cross-grain. Most plastics do not
react differently in regards to extrusion direction, but the
few that do have caused some fabricators many
selection should be considered early during the preparation
process. While it will be nearly impossible to gage tool
life (and therefore costs) if it is a new process and/or
material, tooling selection may greatly impact cycle times,
fixturing costs, programming needs and material waste.
Across the major tooling manufacturer's in the United States
today, there are more than 90 style of dedicated plastic
tooling with 700 to 800 tools to choose from. Narrowing your
choices down to one or two tools or styles of tools from a
particular manufacturer can help you determine material
removal rates, chip extraction direction and the best
diameters for your applications. This can factor in heavily
when later trying to design fixtures and determine optimum
first consideration during programming is cut direction. For
almost every plastic used by fabricators, conventional
cutting will yield a superior edge versus climb cutting. The
exceptions are typically composites or combined materials.
While conventional cutting is very often the norm in routing
(as opposed to climb cutting during milling), nesting
software that seeks to increase raw material yield and cycle
time by reducing the number of cuts per plastic sheet can
force tooling into a climb cut configuration. If this style
of software is used by fabricators, test cuts should be made
to determine whether or not the climb cut edge finish is
acceptable. If it is not, raw material usage and cycle times
need to be re-evaluated.
programming consideration is plunging. Many soft plastics
can exhibit severe chip wrap on the cutter after repeated
plunges. Many hard plastics can craze or "spider web"
depending on tool geometry, plunge speeds, spindle speeds
and fixture support.
reliable method for solving chip wrap is ramped cutter
entry. This prevents the chips from forming in a continuous
curl and will work 100 percent of the time. Ramped entry
requires special attention to the location of entry holes
and the location of nearby cut paths. Other methods of
solution involve pre-drilling of entry areas and specially
designed router bit point geometries.
soft plastics, poor plunge finishes on hard plastics can be
solved through a variety of methods. Ramped entry will once
again yield consistent finishes but is not the only method.
Craze-resistant grades of many popular plastics are
available and can withstand the plunge pressures of standard
router bits. Solid fixturing with support directly
underneath the entry point can also improve the consistency
of plunging operations. Special point geometry on cutting
tools is typically the last resort in these types of
materials. The best thing to remember when cutting holes in
hard and soft plastics is that router bits are for making
traversing cuts and drills are for making repetitive holes.
The geometry of drills is far superior to that of router
bits for plunging and they should be used whenever
programming consideration should be to part fixtures. High
quality, rigid fixturing methods are essential to
maintaining premium edge finishes and there are programming
methods that can enhance the adhesion power of various
axis routing, a common method of increasing holding power is
to leave the paper masking on sheet goods and to rout
through the plastic, but not the bottom masking sheet. This
ensures that the vacuum is not lost and provides extra
vertical and lateral grip on small pieces. Routing in this
method requires extremely flat spoilboards and may require
frequent surfacing of the spoilboards to maintain their
surface dimensions depending on humidity and temperature
routing programs can benefit from higher spindle speeds that
reduce the size of the chips being formed and consequently
can reduce the force being applied against the fixtures.
Multi-flute tools can also be used to achieve this benefit,
but they are susceptible to increased instances of melt and
accelerated cutter wear.
The order of
operations in the CNC router program should also be examined
for areas of improvements. Small cuts and pockets should be
made first to utilize the vacuum area under scrap material.
By making large part cut-outs last, scrap material can be
fixtured the same as finished material and assist in part
hold down. Multiple depth cuts, skin cutting or tabbing can
also be utilized during programming to improve part
hold-down, edge finishes, and feed rates.
planning in the areas discussed in this article and the last
article &emdash; Materials, Tooling, Programming, Runout,
Collets, Vacuum, Dust Collection, Coolant &emdash; can help
prevent some of the common missteps that are associated with
startups of new jobs or processes. Addressing these issues
can ensure that the tools, equipment, and programs are ready
for use when the job begins and that the run costs are
inline with the estimated costs. w
Van Niser is
director of Plastic Application Engineering at Onsrud
Cutter. Readers are invited to send questions to Van Niser
at Onsrud Cutter, 800 Liberty Drive, Libertyville, Il 60048;
to go to the PMF feature archives.
Plastics Machining & Fabricating
F: (847) 362-5028