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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.
Wall Balance Advantages
- Smooth and efficient production
- Improved control of tolerances
- Increased options in configuration
- More material and surface finish alternatives
- Greater control of production costs
- Less vary between thick and thin sections
Designing 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.
- 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.
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.
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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