A gate is the small orifice connecting the runner to the mold cavity, and it decides fill balance, weld line position, shear rate and cosmetic quality. Specify it late and you inherit defects no process setting can remove. These are the gate design questions buyers ask us most often.
Key takeaways
- A gate should feed the thickest section first, so the melt arrives last at the point that freezes first.
- Gate thickness is typically 50–70% of the nominal wall, with a land length of 0.5–1.5 mm.
- Gate location fixes weld line position and warp direction — both are design decisions, not process fixes.
- Edge and fan gates suit flat cosmetic parts; hot tip and valve gates suit high-volume automated production.
- Every gate leaves a vestige. Agree its size and finish before steel is cut, not after.
1. What is a gate in injection molding, and what does it control?
A gate is the deliberately reduced cross-section between the runner and the cavity. It is the narrowest point in the entire flow path, and that is exactly why it does so much work: it meters the flow rate into the cavity, it generates shear heat that keeps thin sections filling, it freezes off to seal the part from the runner, and it defines where the part is separated from the runner after ejection.
Because it is the narrowest point, the gate also sets the shear rate and the molecular or fibre orientation at the point of entry. That orientation is frozen into the part, which means the gate position quietly decides which way the part will shrink and warp. Two identical cavities with gates in different positions will not produce identical parts.
The gate also controls pack-out. Hold pressure can only reach the cavity while the gate is still open. If the gate freezes too early the part is under-packed — sink marks, voids and underfill appear even though the cavity filled. If it stays open too long, cycle time grows because the gate area has to cool before ejection. Gate sizing is therefore a cycle-time decision as much as a quality decision.
2. Which gate type should you choose?
Gate type follows part geometry, cosmetic requirements and volume. The common options:
- Sprue (direct) gate — the runner feeds straight into the part. Lowest pressure loss, used for large single-cavity parts and thick walls. Leaves a large vestige that needs machining.
- Edge / side gate — a rectangular gate on the parting line. The default for flat parts: easy to machine, easy to degate.
- Fan gate — the gate widens from the runner into the cavity, spreading the melt across a broad front. Reduces jetting and orientation, and suits flat cosmetic panels.
- Tab gate — a small tab absorbs the initial jet of melt and the shear spike before flow enters the cavity, protecting a visible surface from gate blush.
- Submarine (tunnel) gate — angled under the parting line so it shears off automatically during ejection. Ideal for two-plate tools running unattended.
- Cashew gate — a curved tunnel that allows gating onto a hidden or internal face when no visible gate mark is acceptable.
- Pin-point gate — a three-plate tool feeds the part away from the parting line, often near the centre of the part.
- Hot tip and valve gates — the nozzle touches the cavity directly. No runner scrap and a shorter cycle, but higher tool cost and a temperature control layer to maintain.
As a rule of thumb: cosmetic flat parts point to edge, fan or tab gates; hidden faces and automated degating point to submarine or cashew gates; high-volume parts with a visible surface point to hot tip or valve gates.
3. Where should the gate be located?
Location matters more than type. Five rules cover most of it.
Feed the thickest section first
Melt flows most easily through the thickest available path. Gate into the thickest wall and the flow front reaches the thin extremities while the gate is still open, so hold pressure can pack the whole part. Gate into a thin wall and the flow hesitates, the thin section freezes early, and you get short shots, sink marks and high fill pressure.
Keep flow length within the material’s limit
Every resin has a practical flow-length-to-thickness ratio, typically in the range of 100:1 to 150:1 for unfilled commodity resins and lower for glass-filled grades. A 1.5 mm wall therefore fills roughly 150–225 mm from a single gate before pressure becomes unrealistic. Longer parts need a second gate, a hot runner, or a thicker wall.
Put weld lines where they cannot be seen
Every additional gate creates a weld line where two flow fronts meet. Weld lines are not only cosmetic — they are the weakest line in the part. Place them behind a rib, inside a corner, or on a non-visible face, and keep them out of load-bearing sections.
Avoid jetting
A gate that fires melt straight into an open cavity produces jetting: a snake-like track frozen onto the surface. Gate into a wall, use a tab gate, or enlarge the gate until the melt enters as a smooth front rather than a jet.
Respect fibre orientation
In glass-filled resins the gate position sets fibre direction, and fibre direction sets shrinkage. That is why warp control on filled parts usually starts with a gating change rather than a process change.
4. How do you size a gate?
Gate size is a balance between filling and freezing. The dimensions that matter are depth, width and land length.
- Depth — commonly 50–70% of the nominal wall thickness. Thin walls, high-viscosity resins and long flow paths push toward the upper end or beyond.
- Width — for an edge gate, roughly two to four times the depth. A wider gate lowers shear without delaying freeze-off.
- Land length — the straight section after the gate, usually 0.5–1.5 mm. A longer land increases shear and freeze-off risk for no benefit.
- Shear rate — keep it below the resin supplier’s limit. Exceeding it shows up as jetting, gate blush and burn marks at the gate.
Worked example: a 2 mm nominal wall points to a gate depth of about 1.0–1.4 mm and an edge-gate width of 2–4 mm. A gate that is too small raises fill pressure, causes early freeze-off and leaves the part under-packed. Too large and the gate area drives cycle time and leaves an oversized vestige to trim.
5. Hot runner or cold runner gate — when is the extra cost worth it?
A cold runner gate is cheaper to build, but the runner solidifies with every shot and is thrown away or reground, the cycle is longer, and the gate leaves a vestige that has to be trimmed. A hot runner gate keeps the melt molten right up to the cavity, so there is no runner scrap, the cycle is shorter, and the part can be degated automatically.
The trade-off is capital and maintenance: a hot half, temperature controllers, tip wear and a more complex colour change. Break-even is driven by runner weight as a share of part weight and by annual volume. For small parts with long runners, or expensive engineering resins, hot runners pay back quickly; for a few thousand parts a year in a cheap commodity resin they rarely do.
Gate vestige and degating
Every gate leaves a mark. Decide the acceptable vestige height and the degating method — hand trim, robot shear, or automatic tunnel break-off — before the tool is cut. Changing the vestige requirement afterwards means modifying the gate insert, not adjusting the press.
Our answer: how we gate a tool
Gate design is where a mold maker’s process discipline shows. Our five mold designers — averaging 8 years — model every tool in UG and run it through Moldflow before a single steel plate is cut, and return a written DFM report within 3 working days at no cost. That report states the gate type, gate position, gate depth and the expected weld line location, with the fill analysis behind each choice.
We machine gates on 13 CNC machining centres with travels up to 1,600 mm and finish them on 6 EDM machines, including Sodick mirror-finish and twin-head units. Our machining tolerance is ±0.005 mm on critical mold dimensions, and the molded part tolerance that tool can then hold is ±0.01 mm on critical dimensions, subject to the resin and the part geometry. Tools are built up to 48 cavities, maximum mold size 1,600 × 1,000 × 500 mm and up to 15 t.
We make molds in-house and run trials and production through vetted partner factories — a deliberate split that keeps us focused on tooling rather than competing with our customers for their own production. Trial delivery is samples, trial photos, a trial video and the full process parameter sheet, together with a CMM dimensional report, and every document is issued against your mold number so the record is traceable.
Gate design is settled at DFM stage, not on the press. If you want a second opinion on a gate before you commit to steel, send us your part and we will review it at no cost.
Related guides in this series
This article is part of our Mold Design & DFM Guide — a full walkthrough of the topic with the numbers and checklists behind each decision.
- Injection molding design guide — 15 essential rules for better plastic parts
- Seven critical DFM rules — the review we run on every design
- Mold flow simulation guide — reading a fill analysis
Next step
Send your 3D files and we will return a marked-up DFM report covering gate type, gate position, wall thickness, draft and tolerance targets — with the fill analysis behind each recommendation. No cost, no obligation. Request a free DFM review.