In MIM, the gate and runner system does two jobs at once: it delivers powder–binder feedstock to every cavity at roughly the same time and pressure, and it sets the fill pattern that decides how uniform the green part's density is. That second job matters more in MIM than in plastic molding, because the part shrinks about 15–20% linearly during sintering, and any cavity-to-cavity or section-to-section density variation converts directly into dimensional variation on the finished part. A tool that fills unevenly cannot sinter evenly — feed-system design is where as-sintered tolerance control starts, before the furnace is ever loaded.
This page covers the feed system only: where to put the gate, how big it should be, which runner cross-sections and layouts balance best, and how to read molding defects back to a gating cause. For the wider geometry rules — walls, drafts, undercuts — see the guide to optimizing MIM design; for where molding sits between feedstock and sintering, see the MIM process steps.
Why Gating Matters More in MIM Than in Plastic Molding
A plastic part mostly stays the size it was molded at. A MIM part does not: after debinding, sintering densifies the compact from green density up to 95–99% of theoretical density, and the accompanying linear shrinkage — roughly 15–20% for common feedstocks, with published values spanning about 12–20% — multiplies whatever density gradients the molding step left behind. If one end of a part packs denser than the other because of how feedstock flowed in, that end shrinks less, and the part bows or drifts dimensionally with no visible molding defect at all.
This is why the standard as-sintered tolerance of ±0.3–0.5% of nominal dimension (about ±0.025 mm on features under 3 mm) is really a statement about fill uniformity as much as furnace control. Balanced filling, uniform packing, and consistent gate freeze-off across cavities are what keep every part in the batch starting sintering from the same place. Get the feed system wrong and the symptoms surface after sintering as scatter, warpage, or cavity-to-cavity dimensional differences — expensive to trace back because the evidence is one furnace cycle removed from the cause.
Gate Placement: Thick-to-Thin Flow, Weld Lines, Venting
The commonly recommended starting point is to gate into or near the thickest section, so material flows from thick to thin. Thick sections need packing longest; if the gate feeds a thin region first, the thin section freezes and seals while the thick one is still contracting, which shows up later as sink, low local density, and uneven shrinkage. Flow length matters too: a gate near the center of mass shortens the flow path and the pressure required to complete the fill.
- Gate at or near the thickest section — let the flow run thick to thin, not the reverse.
- Keep weld lines off load paths: where two flow fronts meet, orient the gate so the meld line lands in a non-critical region; this is application-specific, not a universal rule.
- Vent the last-fill areas: trapped air and decomposing binder gas cause short shots and burn marks near the end of fill, so gate placement and vent placement have to be solved together.
- Keep gates off cosmetic faces: the gate leaves a witness mark, and on a visible surface it becomes a finishing cost. Gate where a small vestige is functionally acceptable.
- Respect ejection and degating: the gate must be reachable for removal on the green part or by post-sinter finishing.
Gate removal deserves its own decision in MIM. Green parts are fragile — brittle, binder-held compacts — so robotic or knife degating right at the press has to be gentle; the alternative is degating after sintering, where the vestige is ground or abrasive-finished off a much harder part. Where the gate sits determines which of those routes is even available, which is why placement is reviewed against the finishing plan, not just the fill pattern.
Gate Sizing and Gate Type Trade-Offs
Two sizing starting points are widely cited for molding generally: gate thickness at about 50–75% of the adjacent wall thickness, and gate width at about 2–3 times the gate thickness. Treat these as trial-shot starting points, not as standards — MIM feedstocks vary in solids loading and rheology, and the final gate size should come from short-shot evidence on the actual tool. Upsizing is not free: a larger gate fills more easily but leaves a bigger vestige, stays open longer during packing (shifting green density), and on abrasive MIM feedstock the whole gate land is a wear item anyway.
| Gate type | Vestige and degating | Typical use in MIM |
|---|---|---|
| Side / edge gate | Visible witness; degated on green part or after sintering | General workhorse; easy to resize during balancing |
| Tunnel / submarine gate | Self-degating on ejection; small vestige | High-cavity tools where touch labor must be minimized |
| Pin-point gate | Smallest vestige; highest pressure loss | Small parts with cosmetic surfaces; needs higher injection pressure |
| Direct sprue gate | Large witness mark at sprue | Single-cavity tools or very large thick-walled sections |
The side-gate versus pin-point-gate choice is the classic trade-off. Side (edge) gates are robust, easy to adjust during balancing, and forgiving to cut — but every part needs a degating step. Tunnel (submarine) gates degate automatically on ejection, which suits high-cavity MIM tools, at the cost of higher pressure demand through the smaller gate section and faster wear against powder-loaded feedstock. Hardened steel at the gate and runner lands is normal practice in MIM tooling precisely because the feedstock is abrasive.
Runner Cross-Sections, Length, and Multi-Cavity Balancing
For cross-section, the full-round runner has the lowest flow resistance per unit of area and is preferred when the runner can be split across both mold halves. When the runner must sit entirely in one half, a trapezoidal section with a rounded bottom is the standard compromise. Half-round and flat runners are less efficient and generally not recommended when either alternative fits the tool design.
| Runner section | Flow efficiency | When to use |
|---|---|---|
| Full round | Best — lowest resistance per area | Preferred; runner split across both mold halves |
| Trapezoidal, round bottom | Good compromise | Runner must sit entirely in one mold half |
| Half round | Poorer — more resistance and cold surface | Not recommended when a full-round or trapezoidal option fits |
| Flat / rectangular | Worst of the common options | Avoid for MIM feedstocks unless the tool design forces it |
Keep runners as short as the cavity layout allows. Long runners raise pressure demand and heat loss, and in high-cavity MIM tools the extra injection pressure can push the tool toward flash if clamping force or tool support is marginal. A general plastics-molding heuristic holds that every 90° turn in a runner may call for about a 20% reduction in diameter to offset losses — treat that as a rough heuristic to be checked with fill analysis, not a rule that replaces it.
For multi-cavity layouts, naturally balanced runners — geometrically symmetric branches that give every cavity the same flow length and restriction — are strongly preferred over unbalanced fishbone-style layouts, which are associated with cavity-to-cavity variation in fill, part weight, dimensions, and appearance. When the layout cannot be perfectly natural (family molds with different parts, or plate-size limits), artificial balancing — deliberately sizing gates or runner restrictions differently per cavity — is the correction tool, and it is validated by short-shot studies and per-cavity weight tracking rather than assumed. In family molds, adjusting the runner is often preferred over adjusting gates when the parts differ significantly in volume.
Reading Defects Back to the Feed System
| Symptom | Common feed-system cause | First correction |
|---|---|---|
| Short shots | Gate too restrictive, or air trapped at last fill | Enlarge gate or add/improve vents at the last-fill region |
| Burn marks near end of fill | Trapped gas — venting, not gate size | Add vents; review gate position relative to last-fill areas |
| Weld line in a load path | Gate location puts a meld line across a critical section | Relocate gate so fronts meet in a non-critical region |
| Flash at parting line | Injection pressure too high, often from long or tight runners | Shorten or enlarge runners; review clamping and tool support |
| Cavity-to-cavity weight scatter | Unbalanced runner layout | Move toward a naturally balanced layout, then fine-tune per cavity |
| Dimensional scatter after sintering | Green density variation inherited from fill imbalance | Rebalance feed system; track per-cavity green weight to confirm |
Not every defect is a gating defect, and saying so saves real debugging time. If all cavities fill identically but parts distort or show density gradients within a single part, look at sintering support and wall-thickness design before touching the tool. Blistering and carbon residue usually trace to debinding, not the feed system. And a dimension that drifts on every cavity together points at furnace recipe or feedstock lot — gating only explains differences, not common-mode shifts. The diagnostic habit that pays off is weighing green parts cavity by cavity: weight scatter at molding predicts dimensional scatter after sintering.
Because gate and runner geometry is frozen at tool cutting, the cheap place to solve it is on paper. A DFM review of the gating concept — flow path, vent plan, degating route, balance strategy — costs hours; the same problems cut into hardened steel cost weeks. Emitech reviews the gating and runner concept as part of the mold DFM loop, with tooling delivered in 15–20 days from approved drawing and molding run across 17 MIM injection machines; the mold cost structure behind that is covered separately on the MIM tooling cost page, since this page stays on the engineering of the feed system itself.
Frequently Asked Questions
Q: Where should the gate be placed on a MIM part?
Start from the thickest section so the flow runs thick to thin, keep weld lines out of load paths where the application allows, vent the last-fill areas, and keep the gate off cosmetic faces. Placement is application-specific — a latch housing and a surgical instrument tolerate very different witness marks — so the gate concept is reviewed against function and finishing, not chosen from a generic table.
Q: How big should a MIM gate be relative to the wall?
Commonly cited starting points are a gate thickness of about 50–75% of the adjacent wall and a width of about 2–3 times the gate thickness, then confirmed with short-shot trials on the actual tool. A larger gate is not always better: it fills more easily but leaves a bigger vestige, extends packing time, and shifts green density — all of which have to be weighed per part.
Q: How is a multi-cavity MIM mold balanced?
Prefer a naturally balanced layout — symmetric branches giving every cavity equal flow length and restriction. Where the layout cannot be fully natural, use artificial balancing: deliberately different gate or runner restrictions per cavity, validated cavity by cavity with short-shot studies and green-part weight tracking. Unbalanced fishbone layouts are workable but are associated with cavity-to-cavity weight, dimension, and appearance variation.
Q: Can MIM runners and sprues be recycled into new feedstock?
It depends on the binder system. Some feedstock suppliers allow limited fractions of clean re-ground runner material; blending re-ground runner into critical parts without supplier validation is not recommended, because repeated thermal and shear history degrades the binder and shifts molding behavior. When reclamation is not qualified, runners are simply process scrap, which is one more reason to keep runner mass modest.
Q: Which defects point to gating rather than sintering?
Use the pattern, not the individual defect. Differences between cavities — weight scatter, fill variation, dimension scatter cavity to cavity — point at the feed system. Uniform behavior across all cavities — consistent distortion, common-mode dimension drift, blistering — points at sintering support, furnace recipe, or debinding. Weighing green parts per cavity before they enter the furnace is the fastest way to tell the two apart early.
Gate and runner decisions are locked the moment tool steel is cut, so they deserve the same rigor as the part drawing itself: a flow path that runs thick to thin, vents where air actually ends up, a runner layout that is naturally balanced where the cavity count allows, and a degating route that matches the finishing plan. That review is cheap; re-cutting a hardened MIM mold is not.
If a part is heading into MIM tooling, send the drawing for a gating and DFM review with the quotation — Emitech returns quotes within 24 hours, including feed-system and moldability feedback, through the contact page.
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