Routing & Trimming
Polypropylene is what many
people mean when they say "plastic" because it is the base of many
products and capable of being fabricated by many processes. From fiber to film,
from injection molding to thermoformed sheet polypropylene is as versatile as
it is varied. It can be formulated to result in a wide range of melt points,
weights, stiffness and machineability. Formulations can provide a substance
somewhere between traditional rubber and conventional plastics. Other possibilities
may be filled or reinforced grades which offer good stiffness and stability.
One property of polypropylene,
chemical or solvent resistance, makes it ideal for tanks, vessels and bottles
used in the chemical industry. PP is also used for clean room furniture and
fixtures. Some other auto interior and trim parts, shrouds, covers, storage
bins are all PP products which may be trimmed or routed in the fabrication process.
Most PP products are machined
on CNC routers. Hand held electric or air routers do not normally give satisfactory
results. In most instances, PP is a difficult material to work with because
of its gummy nature. It is always susceptible to reweldment or wrap around of
waste material on the cutting tool. It can be challenging to obtain a proper
finish on the end product. Feed rates are critical for productivity and cutting
tool selection is critical for best results. Anyone who has cut or trimmed anything
but the most dense and stable PP can attest to the above. Machining PP is a
continuous improvement process often initiated by a basic trial and error process.
There are a few principals
to employ when machining polypropylene. Every attempt should be made to cut
large chips. This can be accomplished by use of slow helix tools shown in figures
1 and 2. Slow helix tools tend to take a larger chip than conventional helix
tools and are available in single or double flutes in both upcut and downcut
spirals. Here is where some trial cuts should be made to determine whether single
or double flutes and up or down spiral works better in a specific application.
A single edge O flute, shown in figure 3, may also be the best answer for a
particular job. Slow helix tools are also available with a bearing pressed on
the end of the cutting edge for guided trimming operations if a CNC router is
not available. Because of the gummy nature of PP and the inherent heat generated
by cutting action, high-speed steel tools are not recommended. Solid carbide
bits will outperform high-speed steel, carbide tipped or diamond tools and are
the only type recommended for cutting PP.
High feed rates should
be employed along with lower spindle speeds. This will tend to abate reweldment
behind the cut and waste wrap around. Feed rates should be increased until such
time as the finish is unacceptable. Spindle speed should then be reduced until
the finish is once again acceptable. The process can then be repeated until
the optimal result is achieved. This process should be repeated, then catalogued,
for each unique set up.
One may want to consider
a two-pass process to optimize both feed rate and piece part finish. If a tool
changer is available, the second pass can be taken with a finishing tool such
as shown in figure 4. In all instances, when the depth of cut exceeds the cutting
edge diameter of the tool by more than a factor of three, multiple passes should
be taken. When such is the case, the second pass should be taken with the same
tool as the first cut.
Fixturing also becomes
very important to achieve acceptable productivity. It is often recommended that
PP set ups use gasket tape to improve hold down. Many times, however, the tape
is just placed on the spoilboard surface. When the vacuum is turned on, the
foam has no place to go but a flat compress. The result is the tape loses its
memory and allows the part being cut to vibrate. Any inconsistency or warpage
in the part will also be exaggerated by the flattened gasket tape. Either situation
will facilitate tool breakage and less than achievable finish. This problem
can be mitigated by making a channel in the spoilboard before applying the gasket
tape. Typically the channel should be one half the thickness of the gasket tape.
Grooving the spoilboard will enable the part to achieve a better vacuum and
will prolong both the life of the tool and the gasket tape. See figure 5 for
Polypropylene can be cut
effectively in a CNC router environment. It is, however, a more complex environment
than we have previously discussed with routing and trimming either PET or ABS.