Last updated: 2026-09-04 · 6 min read

Sintered Metal or Molded Plastic? — How Powder Metallurgy and Injection Molding Split the Same Small Part

Powder metallurgy and injection molding both make large volumes of small, near-net-shape parts economically, and they look interchangeable until you ask what the part must do. The real fork is metal versus plastic — strength and heat versus weight and color. A sintered component carries stiffness, wear resistance and temperature no thermoplastic matches; a molded part consolidates snaps, hinges and clips into one shot with molded-in color.

Key Facts

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  • Powder metallurgy presses fine metal powder in a die, then sinters it in a high-temperature furnace where particles bond into a solid metal part — porous but strong, capable of oil impregnation for self-lubricating bushings.
  • Injection molding injects molten thermoplastic into a hardened steel cavity, cools and ejects a finished part — light, corrosion-proof, color-moldable, with enough geometric freedom to replace a multi-piece metal assembly in one shot.
  • Both share volume economics — high tooling precision, excellent repeatability, minimal waste — but diverge sharply on material capability, geometry limits and finishing needs.

    • Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.

    How Do the Two Processes Compare?

    Five dimensions, each rooted in the metal-versus-plastic divide:

    Dimension Powder Metallurgy Injection Molding
    Base material Iron, bronze, steel powders Thermoplastics and filled compounds
    Strength and heat Metal-level stiffness, wear, temperature Lower absolute strength, heat-limited
    Geometry Press-direction limited, modest complexity High complexity, snaps, hinges, thin wall
    Color and finish Metallic; plating or coating after Molded-in color, texture, cosmetic faces
    Weight Heavy; metal density Light; often a metal-replacement win

    The rows all descend from one fact: one process leaves metal, the other plastic, so the choice is made by the part’s duty before the process. Sintered metal wins wherever the component is a load path, a bearing, a wear surface or heat-exposed: stiffness, fatigue resistance and temperature ceiling are intrinsic metal properties. The controlled porosity powder metallurgy leaves is even an asset when a bushing must hold oil and self-lubricate — a feature injection molding cannot reproduce.

    Pressing and sintering constraints limit geometry: powder is compacted along one or two axes, so deep side features, sharp undercuts and thin walls are hard or impossible without secondary work. Injection molding inverts those strengths — a closed cavity lets one shot produce complex, multi-directional features — snaps, hinges, ribs, bosses — with fine detail and molded-in color. It is the go-to route for replacing assembled metal with one light, corrosion-proof part.
    The trade: a molded part will not match sintered metal on strength, stiffness or heat.

    How Do You Choose Between Them?

    Four rules, in order:

    1. Start from the load, heat and wear the part must carry. A genuine metal duty — a stiff structural member, a loaded bushing, a hot or abraded surface — points to sintered PM, because those are intrinsic metal properties a thermoplastic cannot attain no matter how filled. A part whose demands are strength-to-weight, corrosion resistance, insulation or one-shot geometry consolidation points to injection molding. Write the duty requirements first and let them pick the material class; the process follows.
    2. Score the part on geometry and integration. If the part consolidates several components or has features off the press axis, molding is usually cheaper and lighter. If it is a simple, mostly axisymmetric pressed shape needing metal properties — a gear blank, a link, a bearing — PM forms it cleanly. Count the parts each process can replace and the tooling it needs.
    3. Compare total landed cost at your real volume. Both are volume games with rising tooling and falling unit cost, but they cost differently: a PM compacting die and sintering fixture versus a hardened injection mold. At high counts both look attractive, so compare total landed cost per good part including finishing — PM often needs plating or oil treatment, molding rarely does beyond color or texture. Model each route as it will ship.
    4. Prototype the duty before committing the tool. Because the metal-versus-plastic call is a performance bet, validate on real samples: make a few sintered and a few molded parts and test the governing property — pull, torque, wear, heat, drop — the way the assembly will use it. A part that should work as plastic replacement sometimes fails a stiffness or creep check, and one chosen for metal sometimes could have been lighter molded all along.

    Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

    The process choice follows the material choice, and the material choice follows the duty. Argue process before physics and you relitigate the same constraints twice.

  • 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

    Is powder metallurgy the same as metal injection molding?

    Related but distinct, and the difference is geometric. Conventional PM presses dry metal powder in a die and sinters it — fast and economical but limited mostly to shapes a punch can compact along one or two directions. Metal injection molding mixes fine metal powder with a plastic binder, injects the feedstock to get complex 3D geometry, then debinds and sinters.

    MIM buys far more shape freedom than pressed PM but at higher cost and with shrinkage to manage. When people ask “metal or plastic molded,” MIM sits on the metal side and can take the intricate geometry pressed PM cannot, while thermoplastic injection molding stays on the plastic side with its own lightness and color benefits.

    When should I replace a metal part with molded plastic?

    When the metal was carrying loads well within what a suitable plastic or filled compound can hold, and you can bank the molding benefits — lower weight, no corrosion, molded-in color, and consolidation of several pressed or fastened parts into one shot. Corrosion resistance and part consolidation are the classic wins: a cluster of plated steel brackets often becomes one glass-filled molded housing, lighter and cheaper at volume.

    The replacement fails when the duty is genuinely metal territory — sustained high temperature, a stiff primary load path, an oil-impregnated bearing, or an electromagnetic property. Define the metal part’s actual service stresses and temperatures, then prove a molded candidate meets them on tested samples.

    Which is cheaper at volume?

    Neither wins universally; the answer is total landed cost per good part at your quantity. Molded plastic generally runs cheaper per part for a given simple shape because resin is inexpensive, the cycle is fast, and the part often needs no finishing. Sintered metal carries higher material and furnace costs and frequently adds plating, oil or sizing.

    But a metal part may eliminate a thicker, ribbed, glass-filled plastic design or replace several components, narrowing the gap. The honest comparison prices each full route to a shipped, in-spec part at your real count — tooling, material, cycle and finishing together.

    Takeaways

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  • The material class follows the duty — metal for load, heat and wear; plastic for geometry, weight and corrosion — and the process follows the material.
  • Powder metallurgy is press-direction limited; injection molding consolidates multi-part assemblies into one shot with features in every direction.
  • Compare total landed cost per shipped part at your volume, not headline tool price, and prototype the boundary case before committing either tool.

    • Choosing between sintered metal and molded plastic is not a process debate — it is a duty debate.
    Define what the part must carry, survive and look like, and the material class picks itself; the process is just the most economical way to form it.

    Sources

    1. MPIF — Metal Powder Industries Federation
    2. Society of Plastics Engineers — Technical Papers
    3. Plastics Technology — Processing Tips
    4. HASCO — Mold Base Standards
    5. Empa — Swiss Federal Laboratories for Materials Science

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet