Last updated: 2026-09-05 · 5 min read

The gate is the last millimeter of the runner and the first millimeter of your part — the point where molten plastic enters the cavity. Its type and position decide weld line placement, packing effectiveness, cosmetic witness marks and even how the part is degated after molding. There is no universal best gate: each type trades vestige, cost, fill control and automation in a different ratio, and the correct choice falls out of four questions — where the appearance surfaces are, what the part geometry demands, how the part gets separated from the runner, and what volume justifies. That is why gate location appears on the DFM agenda with a reason attached, rather than as a default.

Key facts

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  • The gate sets flow origin: weld lines, packing path and sink behavior all radiate from it.
  • Every gate leaves a vestige; the decision is where it lands and how big it reads, not whether it exists.
  • Manual-degate gates (edge, tab, fan) versus auto-degate gates (submarine, pin-point, hot runner) drive different runner handling and labor.
  • Gate sizing is resin- and wall-specific; undersized gates shear the melt, oversized gates freeze too late.
  • Gate choice belongs in the DFM before the deposit — moving a gate after steel is cut is a re-cut, not a tweak.

    • Gate types — Injection mold gate — the narrowed opening where melt enters the cavity. Common types: edge gate (standard, leaves vestige), pin-point gate (auto-trims, small mark), submarine gate (below parting line), valve gate (flush, cosmetic). Gate type affects vestige, packing and flow balance. The related guide covers adjacent topics.

    What a gate actually does

    Between the injection nozzle and the cavity sits the delivery system — sprue, runners, then gates. The gate is deliberately the narrowest point, and that restriction is doing four jobs at once: it controls the speed and pattern of cavity fill, it lets packing pressure transmit into the cavity before freezing shut, it determines where the flow front welds meet, and it defines the scar left on the part when the runner separates. Change the gate and you change all four outcomes simultaneously, which is why the choice is an engineering decision and not a detail.

    Gate — the controlled constriction through which melt enters a cavity. Its type, size and position are specified together in the mold design; the mark left after degating is the gate vestige, and its acceptable location and size should be agreed on the drawing, the way any cosmetic requirement is.

    The common gate types at a glance

    Gate type How it works Best for Trade-off
    Edge gate Rectangular channel entering the part edge; manually or automatically trimmed. Flat and box parts, first-time molds, resins sensitive to shear. Vestige on the edge; manual degating adds a labor step.
    Tab gate Melt fills a small tab off the part edge first, then flows into the cavity. Optical and appearance parts — keeps jetting and gate stress out of the visible surface. Extra tab material to trim; slightly larger vestige zone.
    Submarine (tunnel) gate Angled tunnel through the parting line; the part shears itself free of the runner at ejection. High-volume automated production; two-plate molds needing automatic degating. Wear-prone tunnel edges; vestige on the part face near the tunnel exit.
    Pin-point gate Tiny gate fed from a runner plate above the cavity, typical of three-plate molds. Small parts, multi-cavity layouts, gates on top faces where edge gates cannot . Three-plate mold cost; small visible vestige on the gated face.
    Direct (sprue) gate The sprue feeds straight into the cavity with no runner. Single-cavity deep or round parts; simplest flow with minimum pressure loss. Largest vestige; manual sprue removal; big gate freeze time.
    Fan gate Wide, tapering fan spreads melt across the full part width. Wide flat parts needing uniform flow front and low orientation. Long trim line; manual finishing work after molding.
    Hot runner valve gate Heated manifold delivers melt directly; a valve pin opens and closes the gate mechanically. High-volume appearance parts, large parts, programs where runner scrap must disappear. Manifold investment and maintenance; valve gate leaves the smallest vestige class.

    How the choice gets made: four questions in order

    1. Where are the appearance surfaces? The vestige must land where the spec allows it. If no visible face can accept a mark, the options narrow to hidden-face gating, tab gates or valve gates — before any cost conversation starts.
    2. What does the geometry need from the flow? Flat wide parts want a uniform flow front; thick parts want packing transmission; long thin parts want low pressure loss. The fill requirement filters gate types that physically deliver it.
    3. How will the runner separate? Manual trimming, automatic shear at ejection, or no runner at all. At high volumes the degating labor line item becomes decisive — this is where submarine and hot runner gates earn their complexity.
    4. What volume justifies the answer? A prototype batch and a two-million-shot program ask different questions of the same gate. Volume decides how much mold complexity is amortized, and the honest quote shows that math rather than hiding it.

    Applied in this order, the gate decision becomes a documented chain of reasons — which is exactly the format the DFM review should present it in: proposed gate, position on your geometry, and the reasoning, open to objection before the deposit moves.

    Gate vestige: what to agree on the drawing

    Every gate leaves a mark; pretending otherwise is how the T1 argument starts. The productive move is to specify the vestige the same way other cosmetic requirements get specified:

  • Location zone. Name the face or edge where the vestige may live — “non-visible face, within the assembly overlap” is a usable instruction; “somewhere discreet” is not.
  • Acceptable height and halo. How proud the vestige may stand, and how much witness ring around it reads as acceptable. Hot runner valve gates and well-tuned submarine gates minimize both; direct gates maximize both.
  • Post-processing plan. Whether the vestige gets trimmed flush, sanded, or hidden by assembly. The plan decides which gate types are genuinely in scope.
  • Samples as the reference. Cosmetic limits argue best against physical samples: an approved T1 part becomes the standard the production parts are compared to, instead of two parties describing “acceptable” from memory.

    • Vestige requirements interact with the finish the surface carries — a textured face hides a modest mark that a gloss face would broadcast — so the pairing with the finish grade belongs in the same design note.

    What gate sizing changes downstream

    Gate dimensions are specified alongside type and position, and they ripple through the entire process window. An undersized gate shears the melt — heat generation, surface defects, degraded resin at the entry — and freezes early, cutting off packing and inviting sink and voids on thick features, the failure pattern mapped in the sink mark guide. An oversized gate freezes late, extends the cycle, and leaves a vestige that needs finishing work. The correct size follows resin viscosity, wall thickness at the gate and the packing the part needs — three numbers the mold maker should state, because a gate size quoted without its reasoning is just a guess with a drawing number.

    This is also where late resin switches bite: gate sizing follows the resin’s flow behavior, so changing material after the gate is cut can leave the delivery system sized for the wrong melt. The whole chain — resin, gate, packing — is one conversation, and the RFQ checklist is where it starts on paper.

    Special situations that change the gate decision

    Four scenarios bend the standard decision sequence enough to deserve their own rules, and recognizing them early keeps the DFM honest:

  • Multi-cavity molds. Every cavity must fill identically, so gates are chosen and sized as a balanced set, not one at a time. An unbalanced layout shows up as good parts and bad parts from the same shot — one of the more confusing T1 symptoms, and avoidable at design time.
  • Overmolding and insert molding. The gate must avoid flowing against an insert or substrate in a way that displaces it, and the vestige must not land where the overmold bonds later. Gate placement here is dictated by the assembly sequence, not just the part alone.
  • Very thin walls and long flow paths. Fill pressure limits dominate: gates may need to be larger or more numerous than the vestige preference would like, and the cosmetic compromise is negotiated openly rather than discovered as a short shot at trial.
  • Shear-sensitive resins. Some materials degrade when forced through tight gates at speed; for them, generous gate sections and gentler entry styles are process requirements, not preferences, and the resin choice conversation and the gate conversation are the same conversation.

    • In each case the pattern is identical to the base rules: the special condition gets named in the RFQ, the factory’s gate proposal answers it explicitly, and the reasoning lands in writing while changes are still free. The cost of gate engineering is a few design hours; the cost of gate archaeology after the steel is cut is measured in weeks.

    I can usually tell how a mold design conversation went just by looking at the gate. A gate placed with reasons — vestige zoned on the drawing, size derived from wall and resin, degating matched to volume — means the engineering happened before the steel. A gate sitting in the middle of the only visible face with no explanation means the buyer asked for a price and got one, and the real design decisions happened afterwards, alone. The gate is small enough to ignore in a quotation and expensive enough to regret; that combination is exactly why it gets its own line in our DFM documents.

    — Arthur, Tooling Engineer at aCatMould

    Frequently asked questions

    Which gate type leaves the smallest vestige?

    Hot runner valve gates leave the smallest and cleanest marks, followed by well-tuned submarine and pin-point gates. Direct sprue gates leave the largest. The right answer still depends on the face the mark lands on — a small vestige on a visible face can be worse than a larger one hidden in assembly.

    Can a gate be moved after the mold is built?

    Usually by welding shut the old gate and cutting a new one — a repair that works but leaves history in the steel and costs downtime. It is precisely the class of change the DFM stage exists to prevent, which is why gate position is argued on geometry, not discovered at T1.

    Why do some parts have multiple gates?

    Large or long parts need more than one flow front to fill within pressure and orientation limits. Multiple gates introduce weld lines where the fronts meet — so their count and positions are design decisions with named consequences, not defaults.

    Does the gate affect weld line location?

    Directly. Weld lines form where flow fronts meet, and the fronts radiate from the gates — moving a gate moves the weld lines with it. If a weld line cannot live on a loaded or visible zone, gate placement is the lever that relocates it.

    Is a hot runner worth it for small volumes?

    Rarely. The manifold investment amortizes over volume and runner-scrap savings; small programs usually return that math negative. At high volumes with appearance requirements, the same math flips decisively. The quotation should show the crossover rather than assert it.

    Should we specify the gate type ourselves?

    Specify the constraints — appearance zones, volume, degating preference — and require the factory to propose and justify the gate. Constraints you own, gate engineering they defend; the combination produces a decision both sides can sign.

    Key takeaways

    1. The gate decides flow origin: weld lines, packing path, sink risk and the cosmetic vestige all radiate from it.
    2. Seven common types — edge, tab, submarine, pin-point, direct, fan, valve gate — each trading vestige, fill control, degating and cost differently.
    3. The decision sequence: appearance surfaces first, then flow needs, degating method, and volume justification.
    4. Vestige is specifiable: location zone, height and halo, post-processing plan, approved samples as reference.
    5. Gate type, position and size belong in the DFM before the deposit — moving them afterwards is steel work, not a tweak.

    If your part has appearance faces or high-volume ambitions, send us the geometry and the cosmetic zones. You will get a DFM that proposes the gate with position, sizing rationale and the vestige zone marked on your drawing — from a named engineer, before any money moves.

    Sources
    HASCO — standard mold units including gate and runner components
    DME — global supplier of mold bases, components and hot runner systems
    Autodesk Moldflow — injection molding simulation for gate location studies

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