The extrusion process is quite flexible and allows for a range of profile shape designs. In order to achieve consistent results from run to run, however, consideration must be given to wall thickness, corner radii, hollows and sink marks. Tooling design is dependent on the profile shape, the attributes of the chosen material, as well as the anticipated quantity produced during any potential production run.
Likely the most important aspect of a successful extrusion design is the shape’s wall thickness. For optimum results, a uniform or balanced wall thickness provides numerous benefits including, lower costs (both tooling and part price), improved quality, better surface finish, the ability to accommodate more complex profile features, and a wider selection of materials.
Unbalanced wall thicknesses result in thin and thick profile sections which cause variations in the way a material flows through the tooling. Since the thinner sections will cool faster than their thicker counterparts, these circumstances will result in warping or bowing to the heavier side of the profile. In order to minimize these effects (not eliminate), elements in the tooling are needed to compensate for these circumstances. Materials with lower melt viscosities, making them more fluid when exiting the tooling, are much more difficult to control with unbalanced walls. The unpredictable nature of the resulting hot/cool material flows make modeling these outcomes more difficult and thus, unbalanced walls require more tooling complexity, time and cost.
Balanced or uniform wall thicknesses, on the other hand, allow for better outcomes from both a processing and a cost perspective. Such profiles allow for more design complexity to the overall shape. In addition, more material options are available to shapes that incorporate uniform wall thicknesses.
Some examples of profile modifications to achieve uniform wall thicknesses appear below.
Radii and Corners
Corner radii are important for helping material flow through the tooling and are an important consideration for helping reduce the stress that naturally occurs at corners. Poor design of the corner radii can contribute to a weak intersection that could break under certain forces.
In general, the sharpest outside corner we recommend has a 1/64” radius (0.016” or 0.4mm). This produces the effect of a sharp corner and works for most profiles. We recommend a 1/32” inside radius (0.032” or 0.8mm).
While a hollow section is preferable to an unbalanced wall, hollows require mandrels and other cooling techniques that result in increased tooling costs and production tolerances. Unless employed to contribute to a part’s rigidity, hollow sections should be avoided. In cases where the hollow is necessary, the part design should “minimize” any elements inside the hollow like a leg or “projection” for there are few methods to control these elements during cooling. In these cases, generous tolerances are necessary. And while we have seen designs that incorporate a hollow within a bigger hollow, this approach just makes the problems mentioned above more severe, leading to greater tooling cost and tolerances.
Sink marks appear on extrusion profiles where there are two adjoining legs. In these circumstances, the point at the intersection of the legs retains heat longer than the surrounding surfaces which causes nonuniform shrinkage of the plastic as it cools in this area. If the part’s surface appearance is important, these sink marks can be masked by incorporating an indentation or rib in these areas (see figure below). In cases where the surface on which the sink mark is anticipated is critical to be smooth and uniform, techniques can be utilized to minimize the sink mark’s effect; these techniques do, however, add cost and time to the profile’s development.