What the runner and gate actually do
An injection mould is not one cavity with plastic poured in; it is a delivery system. The sprue carries melt from the machine nozzle into the tool, the runner carries it from the sprue to each cavity, and the gate is the final restriction where the melt enters the cavity itself. Everything the part becomes -- its weld lines, its sink marks, its dimensional stability, even how much plastic is thrown away each shot -- is set at those three junctions before the cavity is even full.
This is why gate and runner design belongs in the design review, not in the molder's discretion. A buyer who specifies the part but leaves the delivery system to chance is specifying the result and ignoring the cause. A manufacturer that runs the full chain in house, such as DAYIN's plastic injection molding operation, treats sprue, runner and gate as part of the same drawing, because the part cannot be right if the path to it is not.
Cold runner vs hot runner -- the first fork
The first decision is whether the runner is allowed to cool. A cold-runner tool lets the sprue and runner solidify with the part; they are ejected as regrind or scrap and the next shot pushes fresh melt past them. A hot-runner tool keeps the runner molten inside heated manifolds and drops, so only the part and the tiny gate freeze -- there is no solid runner to throw away. The trade-off is structural, not a matter of which is 'better'.
Cold runners are simpler and cheaper to build, easier to maintain, and kinder to heat-sensitive or easily-degraded resins, because nothing sits in a heated manifold between shots. Hot runners eliminate runner scrap, shorten cycle time (no runner to cool), and make multi-cavity and multi-material tools far cleaner to run -- but they add heaters, thermocouples, a temperature controller and a leak path, and they punish a resin that degrades if held hot. The table below sets the two against each other on the points a buyer actually feels.
| Criterion | Cold runner | Hot runner |
|---|---|---|
| Runner scrap per shot | Significant -- the whole runner solidifies and is ejected | Near zero -- only the part and gate freeze |
| Tool cost and complexity | Lower; no heaters, manifolds or temperature loop | Higher; heated manifold, drops, controller, more maintenance |
| Cycle time | Longer -- the runner must cool before ejection | Shorter -- no runner cooling step |
| Resin sensitivity | Safer for heat-sensitive or degrading materials | Risky for materials that degrade if held molten in the manifold |
| Colour change / purge | Simple -- purge the cold runner, start fresh | Slower -- manifold must be cleared of old colour |
| Best fit | Low-to-mid volume, frequent colour changes, sensitive resins | High volume, stable colour, many cavities, tight cycle |
The honest way to choose is to cost the runner scrap over the program life and compare it with the extra tool and maintenance cost of the hot runner. For a high-volume, stable-colour program the hot runner usually wins on total cost; for a program that changes colour often or runs a sensitive resin, the cold runner is the cheaper and safer route.
Gate type and where it lands on the part
Within a cold or hot runner, the gate itself comes in several forms, and the form decides the mark it leaves and the flow it allows. An edge gate is a simple rectangular opening in the parting line -- easy to cut, leaves a visible vestige, good for general parts. A submarine (or tunnel) gate is cut under the parting line so the ejector shears it automatically, leaving a cleaner edge on a cosmetic face -- common on enclosed housings. A pin-point gate is a tiny gate fed through a secondary runner, used where several cavities must be gated small. A fan gate spreads the melt across a wide entry to reduce stress on a long flat wall. A valve gate uses a pin driven by air or hydraulics to open and close precisely, giving a clean mark and a controllable fill -- the standard where cosmetics and repeatability matter.
The gate vestige is the one unavoidable trace of moulding on the part. A visible gate on a show face is a defect whether or not the dimension is right, so gate type is partly a cosmetic decision: submarine and valve gates exist largely to move or hide that mark. A molder who knows the part will be seen picks the gate for the face, not just for the flow.
Gate location decides weld lines, flow and warpage
Where the gate sits is the single most consequential choice in the delivery system, because it sets the flow pattern inside the cavity. Melt enters at the gate and pushes outward; wherever two flow fronts meet, they join without fully mixing and leave a weld line (or knit line). A gate placed for convenience -- nearest the sprue, or on the easiest face -- can leave a weld line straight across a load-bearing rib or a visible face. Moving the gate to a thick section, or adding a second gate, re-routes the fronts so they meet somewhere that does not matter.
Gate location also drives warpage. Fill that arrives unevenly cools unevenly, and uneven cooling is warpage: the part measures correctly out of the tool and drifts out of tolerance a day later. A gate that feeds the thick sections first lets the thin sections fill last and cool evenly; the reverse invites sink on the thick side and bow on the whole part. And fibre-filled resins -- glass-filled nylon, for example -- align with the flow, so a gate position that was convenient can leave a part strong in one direction and weak across it. If the part carries a load, gate position belongs in the design review, not the molder's set-and-forget.
Balancing a multi-cavity tool with mold-flow analysis
A multi-cavity tool fails quietly when its cavities fill unevenly: one cavity shorts, another flashes, and the whole tool is throttled to the worst cavity. Natural balancing -- making every runner the same length -- only works for symmetric layouts. Real tools are not symmetric, so the runner must be sized deliberately so each cavity gets the same pressure and fill at the same time. That is a flow problem, not a measuring problem, and it is solved before steel is cut.
This is where mold-flow analysis earns its place. A supplier that runs flow simulation -- DAYIN lists mold flow analysis (模流分析) as part of its design service -- can predict fill pattern, gate balance, weld-line position and sink before the tool is machined, and move the gate or resize the runner on the screen instead of after the first scrap lot. It is the difference between a tool that balances on the first try and a tool that is re-cut because the far cavity kept shorting. For any program above a handful of cavities, asking whether the molder runs flow analysis is a fair and decisive qualification question.
How gate and runner choices reach into scrap, cycle time and cost
The delivery system is a recurring cost, not a one-time tool cost. A cold runner throws away its runner every shot, so the scrap rate is baked into the part price for the life of the program; a hot runner removes it but adds heater energy and maintenance. Gate size sets pack time: a small gate freezes early and limits how long holding pressure can pack the cavity, which shows up as sink on thick sections, while an oversized gate is hard to de-gate cleanly. Runner diameter sets pressure drop -- too thin and the cavity never fills at the machine's rated tonnage, too thick and cycle time stretches while the runner cools.
None of these are independent. Picking a hot runner to cut scrap raises tool cost and adds a temperature loop to maintain; picking a submarine gate for a clean face shifts the weld line; moving a gate to fix warpage can unbalance a cavity. The discipline is to decide them together, on the drawing, with the flow predicted -- because every one of them is cheap to change before the steel is cut and expensive afterwards.
A short checklist before the tool is cut
A handful of questions settle most gate and runner decisions early. What is the annual volume and program life -- hot runner only pays back over volume. Does the part change colour often -- if so, a cold runner purges more simply. Where is the show face, and can the gate vestige live there -- if not, submarine or valve gate. Where does the part carry load, and will the gate leave a weld line across it. Is the resin fibre-filled, where flow direction sets strength. And, for more than a few cavities, will the molder run mold-flow analysis to balance the tool before cutting steel.
Answering those before the quote -- rather than after the first articles -- is what keeps the tool from being re-cut. The molder can advise, but the buyer who supplies the function, the cosmetic face and the volume gets a tool designed for the part instead of a generic one adapted to it.
Conclusion
Gate and runner design is the part of injection molding that decides part quality before the cavity is full: cold versus hot runner sets scrap and cycle, gate type sets the mark on the face, gate location sets weld lines and warpage, and mold-flow analysis is what balances a multi-cavity tool without cutting steel twice. None of it is a detail to leave to the molder; it is the delivery system that the part is the output of, and specifying it on the drawing is far cheaper than discovering it in the first scrap lot.
The practical move for a buyer is to treat the runner and gate as design inputs: state the volume, the colour-change frequency, the cosmetic face and the load paths, and ask whether the supplier runs flow analysis and monitors the cavity. A manufacturer that handles design, mold flow analysis, molding and assembly in one chain -- as DAYIN's injection molding process notes describe -- can fold those decisions into the tool rather than bolt them on after, which is the difference between a tool that balances first time and one that is re-cut to get there. For programs where the part has to measure and look right every time, that integration, not the machine tonnage alone, is what protects the result.
Frequently asked
Cold runner or hot runner -- which should I choose?
Cost the runner scrap over the whole program and compare it with the extra tool and maintenance cost of the hot runner. Hot runners win on total cost for high-volume, stable-colour, many-cavity programs because they remove runner scrap and shorten cycle time. Cold runners stay cheaper and safer for low-to-mid volume, frequent colour changes, or heat-sensitive resins, because there is no heated manifold to leak, maintain or purge.
Does gate location really affect part strength?
Yes, and not only through weld lines. Gate position sets the flow pattern, so fibre-filled resins align with the flow and a badly placed gate leaves the part strong in one direction and weak across it. Gate location also sets cooling uniformity, and uneven cooling is warpage -- the part can measure correctly out of the tool and drift out of tolerance later. If the part carries load, gate position belongs in the design review.
How many cavities can one runner feed well?
There is no fixed number; it depends on balancing. Natural balancing only works for symmetric layouts with equal runner lengths. Real tools need deliberately sized runners so every cavity fills at the same pressure and time, usually solved with mold-flow analysis before the steel is cut. Without balancing, the tool is throttled to its worst cavity -- one shorts while another flashes.
Can the gate type be changed after the tool is cut?
Sometimes, and expensively. Moving a gate or resizing a runner can mean re-cutting the cavity or the runner block, and a change that alters flow can shift weld lines and warpage in ways that need a new first-article check. The safe assumption is that a late gate change means tool work, which is why gate type and location are decided on the drawing, not after the first articles.
Why does my molder run a mold-flow analysis?
Because it predicts fill pattern, gate balance, weld-line position and sink before the tool is machined, so the gate and runner are corrected on screen instead of after the first scrap lot. For any tool above a few cavities it is the difference between balancing on the first try and re-cutting the tool to get there -- which is why asking whether the molder runs flow analysis is a fair qualification question.