Extrusion Design Guidelines

Plastic extrusion is a linear process. This is an advantage in that it allows one tool to produce parts of any cut length. Design features such as end caps, fold-overs or other structures which cut across the axis of an extruded part generally cannot be accomplished in-process, and must be considered a secondary operation.

Hollow shapes generally require specialized water/vacuum calibration equipment to manufacture. Sandee vacuum tank capabilities allow simple round and square tubes to be produced in diameters up to approximately 4 inches. When designing hollow parts of greater size or complexity, Sandee’s engineering department is available to help design the most effective way to extrude the profile.

Co-extrusion is the production of an extruded part in more than one color or material. This is accomplished by feeding several extrusion machines into a single co-extrusion die. Using this process, a single part can be produced in up to three component colors. Certain compatible dissimilar materials are co-extruded in this same fashion.

Durometer is a measure of material hardness. PVC (vinyl), in particular, offers an especially wide spectrum of durometer availability. Utilizing co-extrusion in a single profile in vinyl may be designed to incorporate structures that are rigid and flexible such as a “living hinge.”

There are, of course, increased costs associated with co-extrusion, as set-up and machine cost increases as a function of the number of machines required.

Some common extrusion designs have a special place in the hearts of Sandee design engineers and extrusion operators (not unlike the special place a certain government agency has in the hearts of most working people come April 15 of each year). We’re talking hairpins, menu boards and major ulcers here. Certain shapes are just very difficult to manufacture. Long, closely parallel legs or wide shapes with lots of tolerance-sensitive channels. If possible, try to design these profiles in smaller component pieces—or give Sandee engineering a call. We’ll be happy to explore other options with you.

When designing a part for extrusion, try to keep wall thickness uniform. Uneven walls or structures with thickness greater than nominal can cause a profile to torque and bow when it cools. The maximum extrudable wall thickness for most materials is 0.250 inches; the minimum is 0.020 inches.

Surface characteristics vary widely with materials being extruded. High-impact styrene, for example, generally produces a matte finish; ABS is semi-glossy; and acrylics and polycarbonates offer high gloss. Die lines are an unavoidable aspect of the extrusion process, and will appear to a greater or lesser degree on extruded parts of any material. These lines can sometimes be minimized in select areas by specialized polishing techniques. Areas requiring such attention should be highlighted on design drawings. Sink marks are formed wherever walls intersect, and are also an unavoidable part of the extrusion process. Again, they can be mitigated to some extent in areas of special concern.

The extrudable profile is calculated by measuring its developed width. A flat sheet 4 inches wide has a developed width of 4 inches. A 4-inch flat sheet with a 2-inch leg bisecting it at 90 degrees vertically has a developed width of 6 inches. A 4-inch flat sheet with 2-inch legs top and bottom has a developed width of 8 inches, and so on. The maximum Sandee developed width for extruded profiles is 36 inches.