Thermo101 Therim

A TECHNICAL ARTICLE 2006 VOLUME 25, #4

The Rim

So, we know about draft angles
and corners and wall thickness
variation and on and on. But
what about the rim? You know,
the region of the formed part that
forms the periphery of the part.
This lesson focuses on some of the
important issues dealing with the
rim. In the next lesson, we’ll look at
the characteristics of the trimmed
edge itself.

Does the Rim Have a
Function in the Part?

Other than just being the edge
of the part, let’s say. In thin-gauge
forming of axisymmetric parts
such as cups, the trimmed-out
rim is usually manipulated in a
post-molding operation known
are rim-rolling. Here, the cup is
rotated along its axis as the rim is
heated and softened. The rotating
action forces the soften rim against
a shaping ring that effectively
rolls the rim into an annulus. The
rolled rim provides great stiffness
to an otherwise flimsy thin-walled
container.

Staying with thin-gauge products
for a moment, the rim design for
lidded containers often requires
interlocks and detents that must be
quite precise. In certain instances,
the container rim may include
denesting features that allow
stacked containers to be readily
separated by the customer.

What about the rim on a heavy-
gauge part? Often the rim is the
finished edge of the part. The rim
may be very simple, such as the
trimmed end of a flat surface. Or it
may be very complex, with radii,
chamfers, and ridges. The rim
may be designed to fit
into or over another
part, Or it may be
trimmed to accept
secondary assembly
features. The part
design may require
the trim line to
be “hidden,” so that the rim is
U-shaped with appropriate radii
or chamfers.

Can We Get the
Formed Part Off the
Mold?

Before we contemplate this
question in detail, remember that
thermoformed parts shrink as they
cool. So they shrink away from the
sides of a female or negative mold
cavity and onto the sides of a male
or positive mold cavity. If we build a
simple cup mold, for example, and
design the rim so that the plastic is
formed over a ring at the mold top,
we need to provide adequate draft
to get the thing off the mold. In
other words, the rim will not have
right-angled sides. Does this affect
the design? By the way, this design
is often called a “dam” design. This
design minimizes excess plastic
from being drawn over the edge of
the mold and into the mold cavity.
Frequently a trim line concentric
to the dam will also be molded in.
This is often called a “moat.”

We discussed the hidden trim line
a minute ago. How are we going to
get the part off the mold? Flip-up
sections? Removable sections? It is
very difficult to get moving mold
sections to seat without a gap
between mating parts. As a result,
we may wind up with a “witness
line” right at the most cosmetic
portion of the part. And keep in
mind that, without plug assist,
parts really thin rapidly when
vacuum- or pressure-drawn into
parallel-walled mold sections.

What About Texture?

Whenever you draw textured
sheet, the texture flattens. In grained
sheet, the effect is called “grain
wash.” The typical rule of thumb is
that texture flattening is acceptable
if the local draw ratio is less than
about two or the local thickness is
more than half the original sheet
thickness. The real problem occurs
in the rim area where the sheet is
often drawn into sharp corner radii.
One design method is to chamfer
the rim. A second is to facet the
surface design. A third is to use
a series of steps. In each of these
cases, the objective is to trick the
eye into seeing local architecture
rather than texture.

The alternative to drawing
textured sheet is to texture the
mold. However, as any mold maker
will tell you, it is very difficult to
build uniform texture into very
sharply radiused corners.

You should never fall into the
habit of leaving rim design to the
end of product design. ¦

Keywords: rolled rim, moat,
dam, hidden trim line,
textured sheet

THERMOFORMING

101

This is the last TF 101 column written by
Throne. The new TFQ editorial staff will determine
whether the column will continue.

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