By 6CProto Engineering Team · Updated August 14, 2026
Extrusion pushes heated material through a die to create continuous profiles with a constant cross-section, while injection molding fills a closed mold to create discrete three-dimensional parts. If your part is a long profile, tube, or strip, extrusion fits; if it has complex geometry, molded features, or varying cross-sections, injection molding is the better choice. Both processes are mature and reliable, but they are not interchangeable, so the decision is best made geometry first, then volume and cost.
Step 1: Is the Cross-Section Constant?
Extrusion is a continuous process: material is melted and forced through a shaped die, then cooled and cut to length. It suits anything that can be described by a constant cross-section along one axis: hollow profiles, solid bars, gaskets, edge trims, cable ducts, and flexible seals.
Holes and features that run the full length are free in extrusion because they are part of the die. If a design changes cross-section at any point, needs a flange at one end, a mounting boss, or molded-in details, extrusion alone cannot make it, and injection molding becomes the natural fit.
Draw the part and trace one axis through it. If the shape is the same at every point along that axis, extrusion is a candidate. If the section changes, molding or a hybrid design is required.
Step 2: Where Do the Details Live?
Injection molding produces discrete parts with three-dimensional detail, including ribs, bosses, threads, and undercuts, at the cost of a more expensive mold. Housings, connectors, brackets with bosses, and parts with complex wall geometry are natural molding candidates.
Some parts are neither pure profiles nor pure moldings. A tube with molded end fittings is a common hybrid: the profile is extruded, and the fittings are molded or machined onto it. Recognizing these hybrids early avoids forcing the whole part into one process.
Hybrid designs need coordination. The extruded profile and the molded fittings must be toleranced together, and the assembly method, whether adhesive, snap-fit, or mechanical fastening, should be decided before tooling.
Step 3: Compare Tooling and Economics
Extrusion dies are relatively inexpensive compared to injection molds because the tool is a simple shaped opening. Complex profiles with thin walls, multiple cavities, or tight tolerances cost more, but the tool is still usually a fraction of a mold.
Injection molds cost more because they are complex steel tools with cavities, cores, cooling, and ejection systems. Mold cost rises with part size, cavity count, and features such as slides and lifters.
Request a quote for the die and the parts separately so the economics are clear. Die life and maintenance matter too: a well-made die can run for a long time, but wear changes dimensions over time, so ask how the supplier monitors die condition and when re-cutting or refurbishment is recommended.
A single supplier that offers extrusion and molding can also cut the profile, punch holes, and assemble end fittings, which reduces handoffs and the tolerance risk at each joint. Confirm which secondary operations are done in-house before comparing quotes, because outsourced steps add lead time and coordination cost that should appear in the comparison.
| Comparison | Custom Extrusion | Injection Molding |
|---|---|---|
| Tooling cost | Low to moderate | Moderate to high |
| Geometry | Constant cross-section | Free-form 3D |
| Production | Continuous, high volume | Cyclical, discrete parts |
| Typical parts | Tubes, channels, seals, profiles | Housings, connectors, molded assemblies |
| Tolerances | Dependent on profile and material | Dependent on material and mold |
| Secondary work | Cutting, punching, assembly | Trimming, finishing, assembly |
Materials: Thermoplastic, TPE, and Aluminum Profiles
Both processes handle thermoplastics such as PVC, ABS, PE, PP, nylon, and TPE. Extrusion works especially well with flexible materials like TPE and silicone for seals and tubing. Injection molding covers the same polymer families plus filled compounds.
Aluminum extrusion is a separate category: aluminum is pushed through a die to form structural profiles. That process is distinct from plastic extrusion and has its own design rules, tolerances, and finishing options such as anodizing.
Material choice should consider the operating environment. A flexible seal that must resist ozone and weathering points toward TPE or silicone extrusion, while a rigid structural rail points toward aluminum. Confirm the material's temperature range and chemical resistance for the application, and check compliance requirements for food, medical, or electrical applications, since not all TPE or PVC grades are suitable for every application.
Tolerances and Finish: Profile vs. Molded Features
Extrusion tolerances depend on profile size, wall thickness, material, and whether the feature is open or closed. Typical dimensional tolerances for simple plastic profiles are modest, and profiles often require secondary machining for precise features.
Injection molding can hold tighter tolerances on discrete features, typically ±0.05–0.13 mm for well-controlled parts, because each feature is formed in a machined cavity. Surface finish depends on the mold finish, and texture can be specified on the tool.
Both processes can achieve cosmetic finishes, but molded parts can reproduce texture and logos directly from the tool, while extruded profiles usually need secondary printing, painting, or applique for graphics. If branding is important, plan the decoration early: text molded into the tool is permanent and cheap per part, while printed labels add a per-part cost and an inspection step.
Volume: Continuous Runs vs. Cycled Molds
Extrusion is inherently high-volume because it runs continuously, but low-volume custom profiles are also feasible with simpler dies. The economics improve sharply once a die is set up and the line is running.
Injection molding also favors volume, but the mold investment means minimum volumes are often higher before unit cost becomes reasonable. Bridge tooling and low-volume molds exist for smaller runs, but the crossover depends on part cost and mold cost.
For very small runs, consider machining or 3D printing a representative sample before cutting a die or mold. A short sample run from the actual die is even better, because it reveals the real surface finish and tolerance of the process. Many suppliers offer a trial run before committing to full production.
Consider downstream handling before choosing extrusion. Long profiles need cutting to length, packaging, and possibly drilling or slotting, so include those operations in the cost comparison. Packaging is easy to underestimate for long or delicate profiles, especially for overseas shipping, so request a delivered-cost quote rather than an ex-works price.
Common Misconceptions
- Extrusion is always cheaper. Tooling is cheaper and unit cost falls at volume, but extrusion only makes sense if the geometry is a constant cross-section.
- Molding is required for anything plastic. Many products that look molded are extruded profiles with machined or molded end details.
- One die means one product. A single die with different cut lengths and end treatments can serve several SKUs, which is often cheaper than molding each variant separately.
- Tolerances are uniform across the profile. Open and closed features, wall thickness, and material all affect achievable tolerances, so confirm them per profile.
6CProto Expert Views
6CProto engineering perspective: Match the process to the geometry, not the other way around. If a part is a profile, extrusion is the economical route; if it needs molded-in features or varying cross-sections, molding is worth the tooling. Review the profile cross-section and the molded features with a DFM check, and confirm tolerances and secondary operations before choosing.
Conclusion
Extrusion and injection molding serve different geometry families. Use extrusion for continuous profiles at low tooling cost; use injection molding for discrete, three-dimensional parts. Consider aluminum extrusion separately for structural profiles, and validate the design early to avoid costly tooling mistakes.
Review the full product, including end fittings, holes, and assembly, before choosing. The part that is extruded may still need molded or machined components, and the total system cost is what matters. Document the profile drawing with tolerances on critical dimensions, and confirm the material and finish with the supplier before tooling.
FAQs
Can extrusion produce holes or features along the length?
Yes. Holes, slots, and channels that run the full length are part of the die and cost little. Intermittent features require secondary machining.
Is extrusion cheaper than injection molding?
Tooling is cheaper, and continuous production lowers unit cost at volume. But extrusion only makes sense if the part geometry is a constant cross-section.
Can I get a prototype before committing to an extrusion die?
Often yes. A short prototype run or a test die can validate the profile, and machined samples can represent the cross-section before the die is finalized.
What is the minimum quantity for custom extrusion?
It varies by profile and material. Some dies support short runs, while complex profiles need larger quantities to amortize the die. Ask the supplier for their practical minimum for your specific profile.
Can extrusion and injection molding be combined in one product?
Yes. Extruded profiles with molded end caps or fittings are common, and the two processes complement each other when the design needs both a continuous section and discrete details.
Sources
- 6CProto Custom Extrusion Services
- 6CProto Injection Molding Services
- ISO 2768-1:1989 – General tolerances
- ISO 9001:2015 – Quality management systems