Thermo101 Draftangles

A TECHNICAL ARTICLE 2006 VOLUME 25, #1

Draft Angles

Some time ago, we discussed
shrinkage and warpage. At that
time, we pointed out that plastic, like
most other materials, increases in
volume when heated and decreases
in volume when cooled. And we
said that to form the desired shape,
the hot plastic is pushed against a
cool mold surface. It follows that as
the plastic cools, it shrinks. But the
mold doesn’t change in dimension.
If the mold is male or positive, or if
even a portion of the mold is male or
positive, the plastic will shrink onto
the mold surface. And if the mold is
not properly designed, we will have
a devil of a time getting the part off
it. Thus we face the subject of draft
angles.

Draft Angles – Defined

The best definition of a draft angle
is the angle the mold wall makes
with the vertical. If the mold wall is
vertical, the draft angle is zero. Recall
that most thermoforming molds are
single-surfaced. That is, the sheet
is pulled into or over a single mold
surface. For draw-down into a
female or negative mold, the sheet
is constrained on its outer surface
by the mold. As a result, when the
sheet cools, it tends to shrink away
from the mold surface. As a result,
it is entirely feasible to thermoform
into a female mold having zero draft
angles. Most part designers prefer a
slight draft angle, say 0° to 2°, “just
in case.” The average is generally
1/2° to 1°.

On the other hand, when the sheet
is drawn over a male or positive mold,
it is constrained on its inner surface
by the mold. As a result, when the
sheet cools, it tends to shrink onto
the mold surface. To release the part
from the mold, it is necessary to
provide a draft angle on the vertical
mold surfaces. The amount of draft
depends strongly on the volumetric
change in the polymer. If the polymer
is amorphous – PS, PVC, PC – the
draft angle may be no more than
2° to 3°. If the polymer is crystalline
– PE, PE – the draft angle may be in
excess of 5°. The average is generally
4° but the designer must be alert to
effects of temperature variation and
recrystallization rates.

A textured surface requires
an increase in draft angle. It is
recommended that the draft angle be
increased at least 1° for every 0.2 mils
[0.0002 in or 5 microns] in texture
depth. Keep in mind that increasing
applied pressure, sheet temperature,
and mold temperature will result in
greater penetration of the sheet into
the texture.

What About Parts
With Male and Female
Components?

Multiple-compartment trays and
pallets1 can pose series drafting
issues. Consider a female cavity
bordered by two male segments. The
sheet will attempt to shrink away
from the female mold surface but
onto the male segments. Excessive
draft on the male segments may
allow the sheet to release from the
female mold surface before the sheet
has replicated the mold surface. On
the other hand, inadequate draft
on the male segments may allow
the sheet to satisfactorily form the
female mold surface, but the sheet
may “lock” onto the male segments.
The problem is exacerbated2 when
molding compartment trays where
the male portions are interrupted.
Essentially interrupted walls in
the molded part. In addition to the
shrinkage issues, interrupted male
segments may also be sources of
internal webbing3.

How Serious is the Draft
Problem?

The draft angle can lead to serious
dimensional changes in the formed
part. Consider a simple example, a
10-inch male mold. The vertical wall
is 1 inch wide at the top. Consider
a draft angle of 5°. The width at
the bottom of the vertical wall is
determined as follows:

The increased width on one side is
10 x tan 5° = 0.875 in. The total width
at the bottom is then 1 + 2 x 0.875 =
2.75 in.

This is a substantial dimensional
change in the thickness of the vertical
wall.

When is the Draft Angle Not a
Draft Angle at All?

When it is used for something
else. The classic example is the drink
cup. The sidewalls are tapered as
much as 20° for stacking purposes,
not shrinkage. In multi-compart-
ment parts, care must be taken in
the design to accommodate both the
draft angle required for shrinkage
and the necessary stacking taper.
Stacking lugs, stand-offs, or rings
are often designed into complex
parts, simply because it is not always
possible to predict the exact local
shrinkage. ¦

Keywords: draft angle, taper,
shrinkage

THERMOFORMING

101

1 These parts are sometimes called androgynous,
meaning that they have both
female and male characteristics.

2 Exacerbate: To aggravate.

3 Webbing will be discussed in a later
lesson.

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