Last updated: 2026-09-02 · 5 min read

What Is Micro-Molding — and When Are Your Features Too Small to Fill?

Micro-molding is injection molding scaled to where conventional equipment and habits stop working — shot weights measured in milligrams or sub-gram quantities, features finer than a human hair’s width in places, and flow paths so short that surface tension competes with injection pressure — and a feature is too small to fill when the resin cannot and pack it before it freezes. The limit is not one number: it is set by wall thickness, flow length, material rheology and how much pressure the melt still carries at the end of the path.

Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.

Key Facts

  • Micro-molding is a regime, not just small parts: below roughly a few hundred milligrams of shot, the melt’s residence time, cushion control and injection resolution change character, which is why purpose-built machines with screw-and-plunger or all-electric micro units exist.
  • Freeze decides everything: a micro feature fills only if the melt front reaches it while the material is still fluid enough to be packed — thin sections, long approach paths and fast-freezing resins each push the limit closer.
  • Verification is half the problem: features too small for conventional gauges demand optical metrology, CT scanning or sacrificial sectioning — a measurement plan specified at DFM, because “too small to fill” and “too small to verify” fail the program equally.

    • Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. The related guide covers adjacent topics.

    What Micro-Molding Actually Means

    Micro-molding covers two overlapping territories. The first is micro parts: complete components that weigh milligrams — gears, discs, tips — where the entire shot is a fraction of a standard screw’s cushion and the machine must control a melt volume the size of a pinhead, shot after shot, without drift.
    The second is micro features on conventionally sized parts: micro-channels, thin rib networks, fine texture, tiny through-holes — where the part is ordinary but one or two details live at the fill limit.
    The two share the same physics and demand different logistics; a program that needs a sub-milligram gear and one that needs micro-channels on a palm-sized housing will both meet the freeze problem, but their tooling, machine and metrology answers differ, which is why the DFM review separates them explicitly instead of filing both under “small.”

    Where the Fill Limit Sits

    Feature What Decides the Limit Design Response
    Thin walls Flow-length-to-thickness ratio: the longer the path, the thinner the wall can afford to be — and low-viscosity grades extend the ratio Shorten the flow path, gate closer, or pick a more fluid grade
    Micro-channels and grooves Melt must enter, fill and pack before freeze; air entrapment doubles the difficulty Vents at the channel ends, or vacuum assist for blind features
    Tiny through-holes Core pin strength and melt wedging decide survival of the pin as much as filling Staged pin design, or drill after molding when ratios exceed steel’s tolerance
    Fine ribs and bosses Feeding: a micro rib cannot be packed from a thick neighbor without sink or voids on the cosmetic side Local thickening at the junction, or accept and specify the fill-only condition
    Micro texture Replication needs the melt to the texture before the skin freezes Higher melt and mold temperature on that face, verified by replication measurement

    What the Program Needs Beyond an Answer

  • A machine with micro credentials: injection resolution and shot control at the scale of the part, plus the shot volume matched so the melt never sits long enough to degrade — residence time is a micro-molding enemy.
  • Toolmaking at the same scale: electrode wear, wire-cut precision and steel polish behave differently at micro dimensions, and the toolmaker’s evidence at this scale — trial shots, replication checks — is the qualification, not the print.
  • A material plan for milligram shots: regrind is out of the question at this scale, lot control is tighter, and dried resin stays dried — moisture shows up in micro parts before it shows anywhere else.
  • Metrology specified before the first shot: optical measurement, CT or sectioning, with the drawing tolerancing only what can actually be measured — a tolerance nobody can verify is a dispute scheduled for later.
  • A process window built the honest way: DOE on the micro features themselves, because a window validated on the main body of the part says nothing about the ten-micron detail at the end of the flow path.

    • Frequently Asked Questions

    How small is too small for injection molding?

    There is no single cutoff — features once thought impossible are routine in dedicated micro-molding today.
    The practical limit is the freeze calculation: wall thickness against flow length against material rheology, with pressure margin at the end of the path. When the calculation says the melt arrives frozen, the geometry is too small as drawn — the fix is geometry, gating or material, not optimism.

    Can micro features be added to a normal-sized part?

    Often, yes — micro texture, channels and fine ribs on an ordinary part are a fill-and-replicate exercise handled with local gating, vents and hot faces, without micro-molding machinery. The program needs the metrology plan that matches, because the measurement difficulty arrives with the feature regardless of the part’s size.

    How do you know a micro feature actually filled?

    By measurement, not by looking: optical microscopy for surface replication, CT for internal channels, and sectioning for closed features that nothing else can see. The verification method is named at DFM and demonstrated at the trial with actual shots — a filled-looking sample is not evidence until the instrument says so.

    Key Takeaways

    1. Micro-molding spans sub-gram parts and micro features on normal parts — both governed by the same freeze rule: the melt must and pack the detail before it solidifies.
    2. The fill limit is a calculated line, not a folklore number: flow length, wall thickness, rheology and venting decide it, and the design responds by shortening paths, gating closer or choosing a more fluid grade.
    3. Metrology is part of the design: a micro feature that cannot be measured cannot be qualified, so the measurement plan is fixed at DFM and proven at the trial.

    Send aCatMould the detail at the edge of the drawing — the channel, the rib, the disc — and the DFM response gives the freeze calculation for it, the machine and tooling route that matches its scale, and the metrology plan that will prove the feature filled, packed and measured to the tolerance the print dares to ask.

    Sources
    Plastics Technology — micro-molding machines and practice
    MoldMaking Technology — micro toolmaking and replication
    SPE — micro-scale molding guidance

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