Tooling Corner: Die design for extrusion

By: 
November 04, 2002



Figure 1. These examples show the application of die design guidelines.

Editor?s note: Chris Rauwendaal is a designer, consultant, and seminar instructor
who has written extensively on process engineering and extrusion topics.


Die design for extrusion can be rather complicated, since the size and shape
of the extruded product varies from that of the die flow channel. Multiple mechanisms
affect the size and shape changes in the extruded product, and these can be
controlled by die design.

Basic Considerations

The objective of an extrusion die is to distribute the polymer melt in the flow
channel such that the material exits from the die with a uniform velocity. The
actual distribution is determined by the flow properties of the polymer, the
flow channel geometry, the flow rate through the die, and the temperatures of
the die and the polymer melt. If the flow channel geometry is optimized for
one polymer under one set of conditions, a simple change in flow rate or in
temperature can make the geometry less than optimum.

Except for circular dies, it is essentially impossible to create a single flow
channel geometry that can be used for a wide range of polymers and operating
conditions. For this reason adjustment capabilities are often incorporated into
the die. These allow adjustment of the flow while the extruder is running. The
flow distribution can be changed mechanically, thermally, or both ways. Mechanical
adjustment devices include choker bars, restrictor bars, and valves.

Thermal adjustment involves changing the die temperature locally to adjust the
flow locally. Mechanical adjustment capabilities complicate the design of the
die but enhance its flexibility and controllability.

Some general rules are useful in die design:

  • No dead spots in the flow channel.

  • Steady increase in velocity along the flow channel.

  • Easy assembly and disassembly.

  • Land length about 10x land clearance.

  • No abrupt changes in flow channel geometry.

  • Small approach angles.

In die design, problems often occur because the product designer has little
or no appreciation for the impact of product design details on the ease or difficulty
of extrusion. In many cases, small design changes can drastically improve or
degrade the extrudability of the product. Some basic guidelines in profile design
minimize extrusion problems:

  • Use generous internal and external radiuses on all corners; the smallest possible
    radius is about .5 mm.

  • Maintain uniform wall thickness (important!).

  • Make walls no thicker than 4 mm.

  • Make interior walls thinner than exterior walls (for cooling).

  • Minimize the use of hollow sections.

Figure 1 illustrates applications of these guidelines to several different profiles.


Figure 2. This is an example of a partition in the flow channel of the die.

Flow balancing. Mechanical adjustment of the die flow channel can be done in
two basic ways. The length of the channel (land length) can be adjusted to make
sure the average flow velocity is uniform. The other method is to adjust the
height of the channel.

Balancing by land length. The land is the final portion of the die flow channel
just before the exit of the die where the channel cross section is constant.
Flow balancing by adjusting the land length has to be done such that the average
flow velocity in one section of the die is the same as in another section. This
is generally necessary when the product wall thickness (channel height) is not
uniform. For channels with a rectangular shape the land length ratio should
be equal to the height ratio raised to the power 1+n, where n is the power law
index of the polymer. For most polymers the power law index varies between .3
and .5.

This means that when the height of one channel is 5 mm and 3 mm for another
channel, and the power law index is .5, the land length ratio should be (5/3)1.5
= 2.15. The 5-mm-high channel should have a land length that is 2.15 times longer
than the 3-mm-high channel. One of the problems with balancing by land length
is that cross flow can occur. This can be avoided by incorporating a partition
between the thick and thin portions of the channel. This is illustrated in Figure
2.


Figure 3. These two designs illustrate unbalanced and balanced channel height design.

Balancing by channel height. Balancing by land length does not always yield
satisfactory results. Another method is to balance by channel height, as shown
in Figure 3. This figure shows a U-shaped profile with circular sections. The
circular sections are thicker than the walls. Without balancing, the flow through
the circular section is substantially greater than the slit section.

If the flow channel is designed as in Figure 3a, flow in the thin sections would
be much slower than the thick sections; resulting in considerable distortion
of the extruded product. Figure 3b shows the same basic shape but with circular
pins mounted in the circular sections of the die such that the wall thickness
is uniform throughout. The flow velocities from die 3b will be more uniform
than those from die 3a, and, as a result, little distortion would occur in the
product extruded from die 3b.

Size and shape changes. The shape and the size of the extruded product are different
from that of the die flow channel. The extrudate can expand as it exits the
die; this is often called ?die swell?, even though the term ?extrudate
swell? is more appropriate. Extrudate swell occurs because of elastic
recovery of the plastic; however, swelling can also be affected by air entrapment
and foaming.

The extrudate decreases in size as a result of draw down and cooling. Draw down
occurs because the velocity at the take-up is higher than at the die exit. Draw
down is necessary to have a certain level of tension in the line to keep the
extrudate from sagging. In non-circular products draw down changes the shape
of the product because the plastic melt in corners flows more slowly than in
other regions of the die. As a result, material disappears from the corners
by draw down. This is why a square flow channel produces a bulged product (see
Figure 4). The shape change is also affected by the elastic recovery of the
plastic.


Figure 4. These illustrations show how a corrected die can produce a desired extrudate shape.

Cooling reduces the size of the product because the plastic density increases
as the temperature decreases; this is particularly true for semi-crystalline
plastics such as LDPE, PP, and HDPE. Shape changes can occur as a result of
nonuniform cooling. When the extrudate enters a water bath the outside layers
cool rapidly and solidify. As a result, a rigid, solid skin forms that grows
in thickness as the extrudate continues to cool along the water bath. On the
other hand, the inside material is still at high temperature, and as this material
cools it can form shrink voids if the outside layer is too rigid to deform.
It is also possible that the outside layer is pulled to the center if the thermal
contraction force is high enough to deform the outside layer. An example of
a shrink void is shown in Figure 5.


Figure 5. Rapid cooling can cause a shrink void in a part, as shown in the illustrations above.

Size and shape changes can also occur in the extruded product by relaxation.
This is the reduction of internal stresses due to changes in molecular configuration
over time. When the extrudate is stretched extensional stresses are introduced.
The stresses can relax over time. The relaxation of internal stresses can lead
to a reduction in length and an increase in cross sectional area. If the internal
stresses are not uniform, the relaxation can lead to warping of the extruded
product.

From these considerations it is clear that the process that produces size and
shape changes in an extruded product is rather complicated with several mechanisms
at work at the same time. As a result, it can be quite difficult to predict
what die geometry produces the desired product shape and dimensions. This is
why die design is one of the most challenging aspects of extrusion engineering.


CONTACT INFORMATION

Rauwendaal Extrusion Engineering

Los Altos Hills, CA

Chris Rauwendaal

(650) 948-6266; [email protected]

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