Extrusion pushes molten polymer continuously through a shaped die, so it makes anything with a constant cross section: tube, profile, trim, sheet, gasket and channel. The die is cheap compared with an injection tool and the material cost per foot is low, but the process has a minimum practical run that surprises buyers more often than the tooling price does.
How the die and the calibration work together
The die opening is not the shape of the finished profile. Polymer swells as it leaves the die and then shrinks as it cools, so the die is cut to a shape that becomes your profile after the calibration and cooling stages. That is why the first run involves adjustment, and why the trial is a normal part of the programme rather than a sign that something went wrong.
What tolerances are realistic
Extrusion tolerances are looser than moulding tolerances and are usually expressed as a percentage of dimension rather than an absolute figure. Wall thickness varies around the profile, corners round off, and hollow sections can distort. Design the mating parts so they tolerate that, or plan a secondary operation on the features that must be exact.
The minimum run nobody mentions
Starting an extrusion line wastes material until the profile stabilises, and stopping it wastes more. That scrap is charged, so there is a practical minimum quantity below which the setup dominates completely. Ask for the minimum in pounds or feet at the quotation stage, because it decides whether the process is available to you at all.
Sheet, profile and everything between
Sheet extrusion produces flat stock for thermoforming and fabrication and is a commodity business with its own economics. Profile extrusion produces the shaped sections used in trim, seals and framing. Co extrusion runs two materials at once, which is how a rigid profile gets a soft sealing lip in one operation and no assembly.
Questions people ask about custom plastic extrusion
How much does an extrusion die cost?
Far less than an injection tool for a comparable part, because it is a shaped plate rather than a cavity set with moving parts. Complex hollow profiles and co extrusion dies cost more.
What is the minimum order?
Set by startup scrap rather than by policy, and usually expressed in pounds of material. Ask early, because for some profiles it is larger than a first order would be.
How tight can extrusion tolerances be?
Looser than moulding, and usually proportional to the dimension. Where a feature must be precise, machine or punch it after extrusion rather than asking the die to hold it.
Can two materials be extruded together?
Yes. Co extrusion combines a rigid body with a flexible lip or a coloured stripe in one pass, which removes an assembly operation entirely.