The
importance of wall balance:
One of the keys to achieving a top quality profile
extrusion is to maintain uniform or near-uniform wall thickness.
When walls
are unbalanced, the flow of material will vary between thick
and thin sections.
Sections with less material will cool faster than thicker sections.
As a result, the profile may need to undergo an added cooling
phase in order to keep it from warping or bowing.
Production is slowed down and costs increase.
In contrast,
balanced walls offer these advantages:
-
Smooth
and efficient production
-
Improved
control of tolerances
-
Increased
options in configuration
-
More
material and surface finish alternatives
-
Greater
control of production costs
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Desigining
hollow sections :
In profile
design, the use of hollow sections is generally discouraged
- unless it's a trade-off
between an unbalanced wall or including the hollow section.
The cost
of the die for a hollow section increases because the part usually
requires air pressure, internal mandrels and/or vacuum sizing
to maintain the shape of the part while it cools.
Although the die cost is greater, the total set-up costs are
less than for a design with an unbalanced wall.
If a hollow
is necessary, a few design precautions will help minimize potential
problems.
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- Try
to avoid hollow sections with internal legs or projections.
There is just no easy way to maintain close tolerances
on these projections.
- If
such a leg is absolutely essential, it should project
into the hollow no deeper than the thickness of the
wall. Example: If the wall is .050-inch thick, the projection
should be a maximum of .100-inch from the outside of
the wall to the inside end of the projection.
- Avoid
designing a small hollow inside a larger. This is difficult
and costly to produce, and close tolerances are not
achievable.
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How
to handle sink marks:
Generally,
sink marks are positioned opposite adjoining legs, or projections,
to compensate for shrinkage.
For improved appearance, raising or indenting the surface at
the sink marks can disguise them.
Another
option is to add a surface design that hides the sink marks.
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Specifying
corners and radii:
During extrusion,
plastic materials bridge the sharp corners of the die, forming
radii.
The sharpest controllable outside corner is 1/64-inch. This
is a relatively sharp corner, acceptable for most product applications.
We also recommend that the inside radius be a minimum of 1/64-inch.
This will eliminate a notch-sensitive situation
that could cause a break at a sharp corner, especially important
with rigid materials. If
possible utilize larger corners.
They contribute to a smoother flow of material during extrusion.
In addition, warpage is minimized, and there is less stress
at the corners.
These
suggested radii for inside and outside corners
will assist you in designing parts for extrusion. |
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