In injection molding, part quality is decided long before the plastic reaches the cavity. The runner system — sprue, runners, and gates — controls how melt flows into the mold, while vents control how air escapes. Get these right and the mold fills evenly, packs out completely, and ejects cleanly. Get them wrong and you invite short shots, weld lines, burn marks, warpage, and high scrap rates.
This guide brings together the fundamentals of runners, gates, and vents: what each element does, the main design options, how to size and place them, and the classic troubles to avoid.
The Melt Path: Sprue, Runner, Gate, and Cavity
Follow the plastic through a typical molding cycle. Pellets fall from the hopper into the reciprocating screw, where shear heat turns them into a homogeneous melt. The screw rams forward, pushing melt through the nozzle into the sprue bushing — the tapered channel that carries plastic from the machine nozzle to the mold’s runner system. An undercut at the large end of the sprue pulls the sprue out of the bushing when the mold opens.
From the sprue, melt flows through runners — the channels that distribute plastic to each cavity — then through a gate, the constricted opening between runner and cavity, and finally into the cavity itself, the void shaped like the part. Cooling channels freeze the plastic, the mold opens, and part plus runner system are ejected.
Runner Design
The runner is the passageway carrying melt from the sprue to the gates, and its design is critical to filling. Runners that are too small starve cavities, causing short shots and high molded-in stress; runners that are too large extend cooling time, waste material, and lengthen the cycle. Well-designed runners minimize pressure drop, fill all cavities simultaneously, and reduce sink marks, weld lines, and internal stress.
Runner cross-sections. The two most common profiles are the full-round and the trapezoidal runner. The full-round runner — machined half into each mold half at the parting line — is the most efficient: a frozen skin forms against the cooled steel while the hot core flows through the center, and the round section gives the lowest surface-to-volume ratio, hence the least pressure drop and the easiest ejection. It demands precise alignment of the two halves. The trapezoidal runner is machined entirely into one plate, which makes it simpler to cut, but the shape imposes more flow restriction. It is the usual choice for three-plate molds or where a full-round layout is impractical.
Runner sizing. Diameter is chosen for the resin and the part; a common range is 3/16 in. to 3/8 in. Larger runners reduce pressure loss, warpage, and sink marks — but runners must stay as small as possible to limit material waste and cycle time. Resin suppliers publish recommended runner sizes for their materials; use them. Because the runner’s projected area counts toward the mold’s total footprint, runner size also affects how much tonnage your mold needs.
Balanced layouts. In multi-cavity molds, every cavity should fill at the same time and pressure. The classic solution is a geometrically balanced (H-pattern) layout, where melt travels the same distance from the sprue to each gate. Balanced filling gives uniform packing, consistent part weight, and minimal warpage. For more, see our multi-cavity runner balancing guide.
Runner surface. Cutter marks left from milling the runner should be polished out with rotary abrasives — a smooth runner flows better and releases cleanly.
Gate Design
The gate is the link between the runner and the part. It must pass enough melt to fill the cavity quickly, then allow extra material to pack out shrinkage as the part solidifies. Gate type, position, and size have a remarkable influence on the molding process — affecting part quality, appearance, and dimensional accuracy. Gate position also decides how melt crosses thin features: for living-hinge parts, gate so flow crosses the hinge — see our living hinge design rules for PP and PE.
Side (edge) gate. The most common gate. Melt flows from the runner through the gate directly into the edge of the cavity; typically half the round runner and the full gate are cut into the cavity half. It leaves only a tiny mark that can often be torn off by hand, or trimmed with nippers or a trim fixture.
Sprue gate. For large single-cavity molds: melt flows from the nozzle through the sprue directly into the center of the part, then radially outward. Center gating gives easy flow, the lowest pressure drop, and eliminates weld lines and trapped air. Almost any resin or filler works because the gate cross-section is large. Downsides: a large gate mark that must sit on a non-cosmetic surface, plus possible flow lines and sink opposite the gate. Degating requires trimming.
Ring gate. For long, hollow round parts such as pipes: melt feeds from a runner around the periphery, then flows radially inward through the ring so the whole circumference fills evenly. Ring gates avoid weld lines, trapped air, and stress concentration at the gate. If the bore is large enough, a disk gate (the inverse) keeps the witness mark off the outer cosmetic surface.
Point (pin) gate. Used with three-plate molds: a small restricted gate feeds each cavity, and the mold automatically separates part, runner, and sprue on opening (sucker pins break the gates). The tiny gate cross-section is problematic for filled, low-viscosity, long-fiber, or heat-sensitive resins — shear heating at the restriction can degrade the material.
Sub (tunnel) gate. A tunnel machined below the parting line shears off automatically as the part ejects — automatic degating with a tiny witness mark (about 0.020-0.060 in. diameter), easily hidden on a non-cosmetic area. Gating from the bottom up also aids venting. Same resin restrictions as the point gate. For more on controlling that mark, see our guide to gate vestige control for cosmetic parts.
Tab gate. Melt passes through an edge gate into a tab attached to the part, then into the cavity. The tab absorbs jetting and keeps shear stress out of the part, giving a stronger part with better flow — at the cost of a larger mark to trim and the same resin limits as an edge gate. Keep the tab on a non-cosmetic, non-functional area.
Fan gate. A wide, fanning gate for large or thick sections, giving a smooth transition from runner to cavity; it also spreads flow in thin parts to reduce flow lines. The large cross-section suits almost any resin or filler. Trim with a fixture and keep the (large) witness mark off cosmetic surfaces.
Flash (film) gate. A very thin, long gate along a straight edge — ideal for thin, large flat parts such as dials — distributing melt uniformly across a wide front with a minimal witness mark that is easy to remove by hand. The thin land can be troublesome for low-viscosity or highly filled materials.
Resin and filler considerations. Small-restriction gates (point, sub) struggle with filled, low-viscosity, or long-fiber resins and can overheat heat-sensitive materials; large-area gates (sprue, ring, fan) accept nearly any resin. Always match the gate to the material data sheet.
Venting
As melt enters, it must displace the air already in the cavity. If air cannot escape fast enough, it is trapped and compressed — the temperature spike burns the plastic (the diesel effect). Poor venting also causes short shots, splay marks, weak weld lines, higher injection pressure, and excessive internal stress.
Some air escapes naturally along the parting line, ejector pins, slides, and inserts, but that is rarely enough for the volume that must leave in a fraction of a second. The designer must anticipate trapped-air areas and add dedicated vents.
Vent types. The most common is the parting-line vent: a shallow, narrow land milled at the parting line, backed by a deeper relief slot that keeps the land open. Part size, geometry, and resin viscosity dictate the land depth — deep enough to let air out, shallow enough that plastic cannot enter and flash. Ejector-pin vents use small flats ground on the pin; air travels along the flats into a relief and out of the mold. Vents can also be built into inserts, cores, and leader-pin bores.
Vent placement. Vent where the air ends up: the last place to fill, typically opposite the gate. Never place vents where flash would hurt part function or cosmetics. Thin-wall parts fill in a fraction of a second at high injection speed, so vents at the last-to-fill areas must handle extreme air displacement — see our thin-wall molding machine requirements for the vent depths and machine speeds that keep up.
Vent maintenance. Parting-line vents cut too wide are prone to coining — the steel compresses under clamp force and seals the vent shut. Vents cut too narrow plug with residue quickly, and inconsistent depths across multiple vents cause filling imbalance.
Common Design Troubles — and How to Avoid Them
- Unbalanced filling — use geometrically balanced runner layouts so all cavities fill together.
- Burn marks (diesel effect) — add or deepen vents at the last-to-fill areas; never starve a cavity of venting.
- Weld lines in visible areas — reposition gates (center or ring gating) so welds form off cosmetic surfaces.
- Gate blush and jetting — use a tab gate or re-aim the gate to break the melt stream.
- Excessive scrap and cycle time — keep runners as small as the resin allows.
- Flash at vents — size vent lands to the resin viscosity and keep vents out of sealing and cosmetic zones.
Design the melt-delivery and air-evacuation systems together, not as afterthoughts: a balanced runner, a gate matched to part and resin, and generous, well-placed vents are the cheapest insurance for stable, high-quality injection molding.