PLASTIC EXTRUSION DESIGN
Design Considerations
We have helped design and produce many custom plastic profile extrusions over the years for a variety of markets. There are a few design considerations that we commonly review listed below.
Balanced Wall Thickness
A balanced wall thickness on an extruded plastic profile is likely the single most important consideration in profile design. Why is this? Plastic materials are extruded in a semi-molten state, and the material is pushed though the steel extrusion die under pressure created by the extruder. In a balanced wall thickness condition, equal pressures are created in the die, and even flow through the streamlining and land portions of the extrusion tool are created. When the wall thickness of the extruded profile is not balanced, the pressures in the tool are uneven, and the molten plastic seeks the path of least resistance. This creates a natural tendency to flow to the thickest, most central portions of the profile first. These thicker sections will cool slower than the thinner portions of the profile. This requires additional design modifications to the tool itself, and in most cases additional cooling, or sizing, will be required. If a design can be changed slightly to accommodate a balanced wall, the tooling and development will be less costly, and improved tolerance control can be achieved.
Hollow Shapes
Hollows are not necessarily bad in terms of extrusion design. There are many design considerations where hollow profiles are required to make a functional product as part of a system. Added rigidity is a common reason to incorporate a hollow into the basic extrusion shape. To make a hollow, or multiple hollows on an extruded plastic part, internal mandrels are required within the extrusion die, which must be supported by some type of web to connect it to the balance of the die. This web separates the melt stream of the thermoplastic material, and die length must be extended to allow the melt-stream to “knit” together again before exiting the extrusion die. More extensive sizing and calibration is also required for a hollow product compared to a standard extrusion. Even with the complexities hollow shapes creates, they are preferable to an unbalanced wall thickness if the profile design will accommodate the hollow section.
Hollows or Other Details Within Hollows
Hollow profiles add complexity to an extrusion shape, but creating situations where other features are added within the hollow (screw bosses, internal holding legs, etc.) increases tooling and manufacturing complexity exponentially. The issue is getting inside the hollow section to properly cool and control the feature within the hollow. By way of example, a square hollow profile allows sizing, or calibration, to pull all around the profile under vacuum to form the shape, cool the part, and maintain profile integrity during the process. However, if internal legs are then designed into the hollow to add a feature such as an attachment slot (see diagram), there is no way to get inside the hollow to hold the internal feature(s) in place during the cooling process.
Corner Radii
Many if not most plastic compounds are notch sensitive, meaning a sharp corner creates a weak point in the profile where cracking and/or failure can occur. Sharp corners on a profile extrusion not only reduce the impact resistance of the profile, but will create a weak point where cracking will most likely be experienced due to stress concentration. Sharp corners should be avoided to the fullest extent possible in profile extrusion design. The sharpest controllable outside corner radius in plastic materials is 0.015” (0.381 mm). In reality this is a pretty sharp corner and should be acceptable in most profile designs. Ideally, a good design guide would be to have the radius be equal to the wall thickness of the product. This contributes to smoother flow of material during extrusion, and less stress at the corners of the profile.
Intersecting Legs on Exposed Surfaces
When two legs intersect on a profile shape, the intersection itself, along with radii in the corners create a thicker section on the profile. As mentioned in a previous design tip, thicker sections of a plastic product cool slower than the thinner sections, and in this case, create what is termed as a sink mark. In some situations, this would not create an issue, if it occurs on an exposed surface a visible line will be apparent opposite the intersecting leg. Some tips on handling this situation would be to add either a convex or concave feature on the exposed surface to compensate for the sink mark. Other options could include either a raised or indented pattern on the cross section (see examples), or better yet using “fold-up” technology where appropriate to eliminate the issues entirely by folding the extrusion over during cooling. This provides for a much more aesthetically pleasing surface finish, and even provides the option to emboss a texture or pattern into the exposed surface before folding over to form the final shape of the product.
