Thermo101 Cuttingedge
A TECHNICAL ARTICLE 2006 VOLUME 25, #3
The Cutting Edge
For those of you who came in late,
we have been examining the various
aspects of part design. In this lesson, we
focus on the edge or periphery of the
part. The first thing we need to realize
is that the part weve just thermoformed
is still attached to the plastic that held
it in the clamp frame while it was
being formed. This is true whether the
entire assembly, formed part and edge
material, is removed to a separate fixture
or whether the formed part is punched
from the trim material immediately after
forming. Weve discussed trimming
in earlier tutorials. In this tutorial, we
discuss the characteristics of the edge
itself.
Registration
Trimming devices need to trim the part
where the designer wanted it trimmed.
This means that the trim line and the
trim device must register. The accuracy
of registration is a design issue. In heavy
gauge forming, it is impractical to ask a
trim device to trim within thousandths
of the design trim line everywhere along
the trim line. Heavy-gauge parts may
be fixtured between the time they are
formed and the time they are trimmed.
Fixturing allows for some residual stress
relaxation and often improves the trim
registry. In thin-gauge forming, the trim
device should be able to trim very close
to the design trim line. Because many
thin gauge parts are axisymmetric,
meaning that the trim line is round,
registration focuses on the degree of
ovality of the formed trim line prior
to the trimming step. Thin-gauge parts
are often trimmed within minutes of
being formed. Certain polymers such
as polypropylene continue to crystallize
after forming. As a result, the design trim
line and the final part edge peripheral
location may be quite different.
Heavy-Gauge Cut Edge
The nature of the final cut edge
depends strongly on the trimming device.
In many robotic trimming steps, the edge
is rough-cut initially. This edge finish
may be adequate if the cut edge of the
part is completely hidden in the final
assembly. Polycarbonate skylights that
are edged in aluminum are examples.
Often the product requires a smoother
edge. For robotic trimming to achieve
the desired edge, the rough-cut edge
is routed a second time while the part
remains on the trim fixture.
In some applications, the edge must be
as smooth as the overall plastic surface.
Here are some ways of achieving a very
smooth, even polished edge.
Fine grit sanding followed by Crocus
cloth or 1200-grit polishing
The above method, followed by
pumice polishing
For certain plastics, a light wipe
with a mild solvent will smooth
trim cuts. Care must be taken to
minimize the amount of solvent that
is absorbed into the polymer.
Flame-polishing is popular with
transparent amorphous plastics
such as acrylics and polycarbonates.
Flame-polishing is not recommended
with plastics such as PVC.
Laser cutting. The laser is a high-
intensity beam that cuts plastic by
melting and vaporizing it. The cut
line is usually very smooth.
Thin-Gauge Cut Edge
Thin-gauge trimming is substantially
simpler than heavy-gauge trimming.
Nevertheless, the trim edge charac-
teristics can be quite important to the
customer. There are three major issues
with the cut edges of thin-gauge parts:
Trim dust and fibers, known as angel
hair and fuzz.
Microcracks that can grow into the
formed part as it is flexed
Jagged edges that can cut or abrade
the user
Edge and surface contamination are
often the results of problems in the
trimming step. But not always. It is very
difficult to trim polystyrene without
generating very tenacious trim dust. It
is often difficult to trim polypropylene
or PET without generating fibers and
fuzz. Adding antistatic agents to PS,
either as an additive that is compounded
into the polymer or as a topical coat
to the sheet prior to forming, helps the
trim dust problem. If fuzz and fibers are
objectionable to the customer, they are
often minimized by passing the container
edges through a hot air knife. The heat
shrivels the fibers to microscopic size.
Microcracks and jagged edges can
also be healed by heating the edges
with hot air. One approach is to collect
and nest a stacked, counted number of
parts and pass the stack though a hot air
tunnel prior to packaging or boxing for
shipment. ¦
Keywords: registration, flame
polishing, laser cutting, trim dust,
microcracks
THERMOFORMING
101