Last updated: 2026-09-04 · 5 min read

Injection Molding vs SLS: When Do Sintered Nylon Parts Stop Making Sense?

SLS builds nylon parts from powder with no tool: a laser traces each layer, the bed drops, fresh powder is spread, and hours later parts emerge in their own support. Injection molding forces melt into a machined cavity after the tool is built. The two processes are neighbors at different points on the volume curve, and the selection question is finding where your program crosses between them — on economics, material, and design freedom simultaneously.

Key Facts

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  • SLS carries no tooling investment and delivers parts in days — ideal for prototypes and low volume. But its per-part price is nearly flat, so economics stop improving as volumes grow.
  • Injection molding pays first (the tool), then delivers unit costs that fall with volume, plus dense consolidated material and surfaces that arrive as the tool’s finish.
  • The transition failure is the direct copy: a part designed for SLS — zero draft, no gates, sharp walls — dropped into a mold produces sink, drag marks and ejection trouble that read as tool problems but are design-origin problems.

    • Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

    How Do the Two Processes Actually Compare?

    Five decision factors, each pointing at the crossover:

    Factor SLS Sintered Nylon Injection Molding
    Upfront cost No tooling Tooling investment first
    Per-part cost Nearly flat with quantity Falls steeply with volume
    Lead time Days from file to parts Weeks for tooling, then seconds per part
    Material state Porous unless sealed; layer signature Dense, consolidated, flow-oriented
    Surface as built Grainy; finishing adds steps The tool’s polish or texture, as built

    The cost rows define the crossover: SLS prices each part from machine time and powder, so quantity ten costs ten times the same number. Molding amortizes a fixed tool across every unit — the curves cross at a quantity depending on part size and tool complexity, typically between dozens and a few hundred units. Programs that know their crossover before quoting make this decision with arithmetic instead of anxiety.

    The lead-time row is why SLS exists in product development: a revision on Friday and test parts next week is speed injection molding cannot mirror. The material row is where duty separates the fields: sintered nylon’s porosity is structural — it lets moisture and fluids into the bulk, lowers effective strength, and carries layer-bond behavior that molded parts’ flow orientation gives differently.
    The surface row closes most appearance conversations: an SLS part’s grainy surface needs sealing or coating to approach what a molded part simply is.

    How Do You Choose — and When Do You Switch?

    Four steps, in order:

    1. Let the quantity curve decide, with the real tool number. The crossover arithmetic works only with honest inputs: a real tool quotation, the quoted per-part price at production rates, and the SLS supplier’s per-part price for the actual part. Plot total cost against quantity and find the crossing. The quantity moves with three levers: tool complexity (a high-cavity tool pushes the crossing outward), part size (big parts are powder-hungry in SLS and tool-expensive in molding), and expected design revisions (every revision on steel costs engineering changes; SLS revisions cost a file edit). The output is a stated transition quantity — written down, not felt — becoming the program’s schedule anchor.
    2. Check the property list the duty actually needs — porosity first. The comparison is not “nylon versus nylon” but “dense versus sintered.” Porosity: sintered parts breathe — moisture and fluids move through the open structure, so parts sealing fluids, holding vacuum, or living in humid service either get sealed or move to molding. Mechanical behavior: SLS material carries its thermal history — layer bonds and scan patterns — so elongation, impact and fatigue differ from dense molded stock. The verification habit that ends the argument: qualification tests run on parts made by the production process, because the material story genuinely changes with the process.
    3. Weigh the design differences, then redesign for the switch. SLS rewards: no draft (powder provides release), no gates or parting lines, internal channels and lattices no mold could fill. Molding rewards: uniform walls with drafted faces, radiused transitions, gates planned into non-critical zones, undercuts priced honestly. The professional pattern is a scheduled redesign: once the SLS phase has frozen the functional geometry, a molding-oriented pass redraws the non-functional geometry for the process that will make it — draft added, walls unified, gate location negotiated. The SLS-era part becomes the reference for function while the molded part becomes the reference for production.
    4. Plan the transition path deliberately — including bridge quantities. The messy weeks in every SLS-to-molding program are the ones nobody scheduled. The clean pattern names the phases: prototype (SLS, design iterations), bridge (continued SLS or a simple bridge tool for interim quantities), production (molded parts, process-qualified). Bridge tooling deserves its own arithmetic: an aluminum or pre-hard single-cavity tool can sit below SLS’s per-part price at moderate quantities and delivers molded material before the production tool exists. The handover discipline: design freeze names the drawing revision; molded parts’ qualification re-runs duty-relevant tests; supply cutover is dated, with SLS continuing past first molded delivery as safety margin.

    Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. The related guide covers adjacent topics.

    The selection question is not “which process is better” but “where does your program cross between them” — on economics, material, and design freedom simultaneously. The transition failure is always the direct copy.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Are SLS parts as strong as molded nylon parts?

    Differently strong, and the differences follow the material structures. Molded nylon is a dense, consolidated solid; sintered nylon is particles fused by a moving laser with small but real porosity. Tensile strength of well-built SLS nylon can land near molded values, while elongation and impact typically sit below, and fatigue — where porosity acts as a crack-start population — shows the clearest gap.

    Build orientation adds spread to all of it. Moisture completes the picture: nylon absorbs water wherever air gets in, and porosity is more air. Static loads at modest temperatures pass SLS comfortably; fatigue duty, impact near limits, or any fluid-sealing role push toward dense molded material.

    Why do SLS parts feel grainy and absorb moisture?

    Both behaviors trace to the same root: the parts are born porous. Each laser track fuses powder into a small melt pool; the surface is frozen powder particles at the boundary of fusion — a matte, granular skin. The same open structure lets water vapor move into the bulk, and nylon’s chemistry absorbs it — dimensions drift, properties shift.

    The fixes are a finishing ladder: media tumbling rounds the grain; resin infiltration or coating closes the pores, addressing both moisture and surface at once. The specification discipline: decide early whether the part’s duty needs a sealed surface, because sealing chosen late forces a finishing process into a schedule without one.

    When does bridge tooling beat both pure SLS and the production tool?

    Bridge tooling earns its place wherever molded material is needed before the production tool can deliver, at quantities the SLS price line punishes. Design is frozen, duty tests demand dense material, launch quantities arrive in weeks, and the production tool needs months.

    The arithmetic: compare bridge-tool cost plus per-part price against the SLS cost of the same interim quantity. Honesty checks: is the design truly frozen (a bridge tool absorbing a revision is expensive), do bridge quantities clear the arithmetic, and will the production tool’s material differences invalidate bridge parts’ data? Programs that keep bridge tooling in their vocabulary stop paying the false choice between waiting and overpaying SLS.

    Key Takeaways

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  • SLS and injection molding are neighbors at different points on the volume curve — SLS for prototypes and low volume, molding for production where amortized tool cost pays off.
  • The decision runs three curves simultaneously: economics (crossover quantity), material (dense vs porous), and design freedom (tooling constraints vs powder-bed liberty).
  • The transition failure is the direct copy — schedule a redesign pass once functional geometry is frozen, and plan bridge quantities deliberately so the switch is not discovered at launch.

    • The honest decision method runs all three curves at once — money, material, and design freedom — and finds the quantity and the duty where SLS stops making sense and the tool starts paying for itself.
    Programs that know their crossover before quoting make the transition with arithmetic; programs that improvise it discover the switch in their launch week.

    Sources

    1. Plastics Technology — Nylon Injection Molding
    2. Society of Plastics Engineers (SPE) — SLS vs Molding Comparison
    3. EOS — SLS Nylon Material Properties
    4. Plastics Today — Bridge Tooling Strategies
    5. AMPIM — Rapid Manufacturing Methods Compared

    Sources: Plastics Technology · Manufacturing.net · Society of Plastics Engineers (SPE) · ThomasNet