Gate Types in Injection Molding: A Selection Guide for Design Engineers

Choose the gate by answering four questions in order: where can the gate mark be tolerated, how will the gate be separated from the part, how long must the gate stay open for packing, and what does the material do as it passes through a small opening. The answers point to an edge, submarine, hot-runner, or specialized gate; the gate’s size and location then decide fill pattern, weld lines, packing, and cosmetics for the life of the mold.

What the Gate Controls

The gate is the narrow opening between runner and cavity. It is small, but it sets four things that the process cannot fully correct later:

• Fill pattern. Melt spreads from the gate, so its position decides where flow fronts meet (weld lines), where air is trapped, and whether the cavities of a multi-cavity mold fill evenly.
• Packing time. A gate thinner than the part freezes before the thicker sections do. Once it freezes, hold pressure can no longer push material into the part, so a gate that is too small or too far from thick sections leaves them under-packed, which shows up as sink and voids.
• Shear and orientation. Melt accelerates through the gate. High shear can cause blush or degrade sensitive resins, and in fiber-filled grades the flow direction from the gate sets fiber orientation, which affects strength and warpage.
• The gate mark. Every gate leaves a vestige, a witness mark, or a trimmed area. Its location decides whether that mark is acceptable.

A gate is one of several causes of molding defects, alongside wall design, cooling, venting, material condition, and process settings. When a defect sits next to the gate or appears where flows meet, the gate is the first place to look; when it does not, the gate may be innocent.

Edge Gates

An edge gate is a channel cut at the parting line from the runner into the edge of the part.

• Strengths. Simple to machine, easy to enlarge during sampling, easy to inspect, and compatible with a wide range of resins.
• Limits. The part and runner come out joined, so the gate is removed by hand, by a fixture, or by a trimming step, and the vestige sits on the part edge at the parting line.
• Fit. Technical parts where the edge mark is acceptable, development tools where gate size may change, and parts where a trimming step already exists.

Starting with a gate that can be opened up during tool trials, then refining it, is a practical way to protect a project from an undersized gate.

Submarine (Tunnel) Gates

A submarine gate enters the part below the parting line through an angled tunnel. When the mold opens or the part is ejected, the gate is sheared from the part automatically.

• Strengths. Automatic degating removes the trimming step, and the vestige sits off the main cosmetic face.
• Limits. The gate opening is kept small so that it shears cleanly, which limits packing on thick parts. Clean separation depends on the resin’s toughness, the gate geometry, mold temperature, and ejection timing; tough or ductile grades can stretch or string instead of breaking. A shearing trial with the actual grade is the reliable test.
• Fit. Multi-cavity production molds where degating labor matters and the part has a non-cosmetic side wall for the tunnel to enter.

A curved variant (cashew or banana gate) reaches locations a straight tunnel cannot; it is harder to machine and to polish, and the curved runner must flex out of the steel during ejection.

Hot-Runner Gates: Thermal and Valve

A hot runner keeps the plastic molten from the machine nozzle to the gate, so there is no cold runner to scrap or regrind.

• Thermal gates rely on a small plug of plastic that freezes at the tip between shots and is remelted by the heated nozzle at the next injection. They have no moving shut-off and leave a small mark whose appearance depends on tip design and process conditions.
• Valve gates use a pin that opens for fill and closes mechanically. They give a clean, controlled shut-off and allow sequential gating, where several gates open in order to move or remove weld lines on large parts.

What a hot runner changes, and what it costs, depends on the job:

The decision is a cost comparison over the program, worked with quoted mold prices, the runner weight per shot, resin price, regrind policy, and expected maintenance. The broader tooling-cost side is set out in injection mold tooling costs.

Specialized Gates

• Fan gate. Widens into a thin, broad entry to fill flat, wide parts with a straighter flow front and lower velocity; it leaves a long trim line.
• Diaphragm (disc) gate. Feeds around the full inner circumference of a round part for even radial flow and no weld line from the gate; the whole disc is removed afterward.
• Tab gate. Gates into a sacrificial tab so that high shear and the vestige land on material that is cut off, not on the part.
• Pin gate (three-plate mold). A small gate fed from a runner on a separate plate, so parts and runners separate as the mold opens. It allows center or multi-point gating on each cavity at the cost of a more complex mold and longer opening stroke.

Selection Sequence

1. Cosmetic map. Mark the surfaces where no gate mark is acceptable, where a small witness mark is acceptable, and where a trimmed vestige is acceptable.
2. Packing need. Identify the thickest sections; the gate has to be close enough, and large enough, to keep packing them until they are sized correctly.
3. Flow pattern. Run a filling simulation or a flow-length estimate to place weld lines and air traps away from load-bearing or visible areas.
4. Degating method. Decide whether separation is manual, automatic by shear, or eliminated with a hot runner.
5. Material behavior. Check the resin supplier’s guidance for gate sizing, shear sensitivity, and, for filled grades, fiber orientation.
6. Volume and budget. Compare cold- and hot-runner options on total program cost, not mold price.

Gate placement interacts with wall thickness, so the two are designed together; the wall rules are in the guide to wall thickness for molded parts.

Gate-Related Defects and First Checks

The gate is cut into the steel early and is expensive to move, so its position, size, and type deserve a decision on paper before the mold is built, with room left to enlarge it after the first trial.