/ KNOWLEDGE · GATES

Injection moulding gates — types, location, and the marks they leave

The gate is the smallest opening in the whole tool — a few square millimetres where the runner meets your part — and more decisions pass through it than through any other feature. Its position decides where your weld lines fall; its size, how hard the material is sheared and how long the part can be packed; its type, what mark is left and who removes it. All of it is cut into steel before you see a first article. This guide teaches the families and the location rules, so you can approve the gate plan on the drawing — where a change costs a conversation, not a re-cut cavity.

What the gate actually does

The gate is the last section of the melt’s journey — sprue to runner to gate to cavity — and the most violent. At that abrupt change of section the melt sees elongational stress on top of shear (Goodship, §2.6.4), which is why gate size and material sensitivity have to be matched. After filling, the gate changes jobs: it is the valve through which holding pressure keeps feeding the shrinking part, and the moment it freezes, packing ends — which is why gate decisions resurface later as dimensional ones; our tolerances guide tells that side.

The gate families — and what each one trades

Edge gates are the workhorse for larger and thin-walled parts: placed along a side, they give the widest moulding window of all because their shear rates are low; the recommended thickness is about 0.40 to 0.50 times the nominal wall where the gate sits (Harper, §1.4.3.1). Their price is a visible vestige on the part’s edge that must be trimmed. Fan gates spread the same idea wider — lower pressure and lower clamp demand, excellent for short flow lengths — but the broad connection is hard to trim cleanly (Harper, §1.4.3.2). Tab gates run into the part at up to the full wall thickness, but if you want them removed without special tooling, they should not exceed half the nominal wall (Harper, §1.4.3.8).

Pinpoint gates (in three-plate tools) and tunnel gates (cut below the parting line) belong to the self-degating family: the tiny gate shears as the tool opens — no operator trims anything — which is why tunnel gates dominate small parts and connectors. The trade is high shear through a small opening, which narrows the process window and puts the material supplier’s gate recommendations in charge (Harper, §1.4.3.3–1.4.3.4). Sprue gates feed big single-cavity parts dead-centre for even flow; expect a sink mark behind the gate and a degating fixture (Harper, §1.4.3.7). Ring and diaphragm gates exist for round parts over core pins: fed evenly all around, the pin is not pushed sideways by unbalanced flow — no core-pin shift, no thinned wall (Harper, §1.4.3.5–1.4.3.6). And a hot-runner drop replaces the cold gate entirely — a decision that belongs to the runner system, covered honestly in the tooling guide.

Where the gate goes — five rules that survive every part

• Gate into the thickest section, so flow runs thick to thin — the reverse invites voids and sinks (Harper).
• Let the melt meet a wall. A gate firing into open space makes the melt jet and “worm” across the surface instead of filling smoothly — Harper’s rule is that flow should arrive against a wall, core pin or feature (Harper).
• Put the vestige where nobody looks. Every gate leaves a scar; the drawing should say which faces may carry it and which may not (Harper).
• Keep the gate away from in-service stress. The gate region carries high residual stress — it is the wrong place for a load, a snap or an impact (Harper).
• Respect the air and the holes. Gas ahead of the melt must escape without burning, and flow around core pins and holes creates weld lines behind them (Harper).

Weld lines are countable — before the tool exists

Weld lines form wherever two melt fronts meet, and their number is not a mystery: an uninterrupted single-gated part has none, two gates give one, and in general the count is the number of gates minus one — plus one for every hole or interruption the flow must split around (Harper, §1.5.3.8). They affect strength and appearance; gate placement can move them, never abolish them. Ask at quotation time: where will my weld lines fall? Mould-flow analysis shows the map before steel is cut. What weld lines look like when they go wrong — and what else can — is the territory of the defects guide.

How big should a gate be? Two honest answers

Harper’s rule of thumb puts an edge gate at 0.40 to 0.50 times the nominal wall — the figure our design-for-mouldability rules publish. Rosato’s worked example sizes a gate at two-thirds of the wall, with the width twice the depth. Both are respectable handbooks; they disagree; and that is the real lesson: gate size is a per-part calculation, not a constant — it depends on material, wall, flow length and packing. Expect the calculation, shown. Undersize the gate and the shear rate through it can quietly exceed what the material tolerates — test any proposed gate in thirty seconds with the gate shear-rate calculator.

The four sign-offs before steel

Before a cavity is cut, four things deserve your initials: the gate type and what it implies for degating labour; the gate location, marked on the drawing with the faces that may and may not carry a vestige; the weld-line map from mould-flow, judged against where your part carries load; and the gate size arithmetic, with its shear check. On a drawing they cost nothing to change; in hardened steel they cost real money. Send us the part and you will get all four back with the quotation — an engineer replies within 48 hours on working days.