What Micro Injection Molding Means, in Numbers

Industry practice calls micro molding any job with shot weights between 1 and 0.1 grams. Tolerances run from 10 to 100 microns. That is the working definition from Accumold, a company that does little else.

The Arburg demo at Fakuma shows the extreme end. Each micro-filter weighs 0.0022 grams. The whole four-part shot weighs less than one standard resin pellet.

LSR parts go smaller still. One molder runs 1.7 by 0.9 millimeter silicone parts at 0.0005 grams each. Thirty-two cavities fill from a total shot of 0.125 grams.

Scale changes everything downstream. Melt that sits too long in the barrel degrades. Shots this small leave no room for process slop. For the full exhibit details, see Arburg’s Fakuma 2026 micro-molding demonstration.

Why a Standard Press Fails at This Scale

A conventional screw cannot meter a 0.007-gram shot. One published micro-molding case pairs that part weight with a 3.11-gram runner. The runner weighs over fifty times the part, and roughly 98 percent of each pellet is waste.

Press makers say to use no more than 70 percent of rated shot capacity. The screw stroke should stay at or above half the screw diameter, or 15 to 20 percent of rated capacity. Drop below that and injection turns inconsistent. Micromolders typically run at 10 to 50 percent of machine capacity. The sizing math behind this is laid out in proper injection-unit sizing for small-shot molds.

Residence time is the quieter killer. Tiny shots mean the melt waits in the barrel between cycles, and heat-sensitive resins degrade while waiting. Arburg runs its micro process first-in, first-out for exactly this reason. Short, consistent residence time is the only way to get it right.

That is why micro cells run all-electric. Servo drives repeat clamp position and shot size to a few thousandths of a millimeter. A few tenths of a milligram of extra melt changes the reject rate. Servo-electric micro cells with FIFO melt handling are standard practice now; the engineering challenges of micro-injection molding covers the full picture.

Tolerance Lives in the Tool, Not the Press

In micro injection molding, the press positions the mold and the mold makes the dimension. Micro molders quote ±0.0005 inches with Cpk of 1.3 or better, which runs about six defects per hundred thousand. Some hold 0.005-millimeter concentricity on a 25-millimeter hub.

Cavity count stays low on purpose. One to four cavities is the norm; more cavities multiply variation faster than they cut piece price. Builders cut these tools to Class 101 standards, built for over a million shots. One program pushed Cpk to 1.7, which is eleven parts per million.

Ask a supplier how they hold tolerance, not whether they can. Answers should name steel grades, grinding equipment, and measurement tools. Vague answers mean the tolerance lives in the quote, not the tool.

Venting Decides Quality at Micro Scale

Trapped air ruins micro parts faster than anything else. Vent lands on these molds run 0.0005 to 0.0020 inches deep, and the vent goes at the end of fill. Per-resin starting depths differ sharply: POM vents around 0.015 to 0.025 millimeters, while PPS and LCP need 0.005 to 0.012.

Small vents clog. Good micro molds add vented flash traps and vented ejector pins, which clear themselves each cycle. The same venting logic applies at larger scale; see our venting and cooling fundamentals.

Vent depth is not a guess. The calculator below gives per-resin starting depths for the resins micro molders actually run, including POM, PPS, LCP, and PEEK. Use it when you review a supplier’s DFM.

Vent depth calculator by Topworks Plastic Mold

Gate and Runner Choices for Sub-Gram Parts

Gate choice in micro injection molding follows the resin family. Thermoplastic micro parts usually gate through a hot runner with a valve gate. LSR micro parts run a valve-gated cold runner with small sub-runners. The valve gate keeps drool and stringing off a part you can barely see.

Pin-point gates and tunnel gates handle self-degating where the part allows it. Pin-point gates live in three-plate tools and gate centrally or at multiple points. Tunnel gates shear the part off as the mold opens. Either way, the gate vestige has to land where the buyer accepts it.

Hot runners cost real money on a micro tool, but they erase most runner waste. Compare the 3.11-gram runner against the 0.007-gram part and the math makes itself. Cold runners survive where resins are heat-sensitive or colors change constantly.

Materials and What They Do to Tool Steel

Micro molding runs engineering resins. POM, LCP, PEEK, PBT, PVDF, and LSR cover most programs. Filled grades are common — PC with glass fiber, carbon fiber, or PTFE for stiffness and wear resistance in the part. Those same fillers wear the mold.

A glass-filled part grinds soft steel the way sandpaper grinds wood. H13 holds up where P20 would not. Our mold steel grades guide lays out the five common grades by volume, surface requirement, and resin.

Corrosive resins add another constraint. PVC and flame-retardant grades attack standard steel, which is why stainless grades like S136 exist. Name the resin before you name the steel.

Where the Money Goes in Micro Molding

Demand is not the problem. Accumold broke ground this month on a $33.25 million expansion in Ankeny, Iowa. The project adds 53,000 square feet and about 150 jobs, with completion set for February 2028. The new building nearly doubles the company’s production capacity. Its current headcount is 425.

The money in a micro program concentrates in mold precision and process engineering, with metrology proving both. In micro injection molding, machine time is the smallest line. You are not buying press hours. You are buying microns, measured and proven.

Quotes should separate tooling and piece price, with metrology as its own line. If a quote lumps everything into one line, you cannot see where the microns are paid for. See how the pieces fit in our injection molding cost sheet.

How to Source Micro Molds from China

Start with the shot, not the price. Ask the supplier what injection unit will run your part and what fraction of its capacity your shot uses. If they plan your 0.05-gram part on a standard 60-gram press, walk away.

Ask how they vent the tool and at what depth per resin. Ask why the cavity count is what it is. Ask what Cpk they will sign for, and on which dimensions. Then ask to see the measurement equipment.

Good micro suppliers answer with numbers and equipment lists. Weak ones answer with confidence. Our injection mold design principles guide covers the DFM side, from walls and gates to draft and tolerances that survive production.

One question separates the two kinds of suppliers. Ask it before you ask the price. How many microns deep are your vents?