Gate selection is one of the least visible decisions in mold design and one of the most consequential. The gate is the small channel that feeds molten plastic into the cavity, and its type, size and position decide how the part fills, how it packs, where it will show a witness mark and whether it will warp out of tolerance after ejection. This guide walks through the common mold design gate types, what each one is good at, and the checklist buyers can use to sanity-check a toolmaker's proposal before steel is cut. If you are new to mould sourcing generally, the plastic mould tools catalogue is a useful starting point.
What a Gate Actually Does
Three jobs happen through one small opening. First, the gate meters flow: a narrow gate freezes quickly, which prevents backtracking but demands higher injection pressure. Second, it controls packing: the gate must stay open long enough for the cavity to top up as the part cools and shrinks. Third, it leaves a mark - every gate leaves a witness point somewhere on the part, and on cosmetic pieces that mark decides where the gate can live. Most mould design arguments are really arguments about balancing these three jobs.
Common Gate Types Compared
| Gate type | Typical use | Mark left on part | Watch out for |
|---|---|---|---|
| Edge (side) gate | General-purpose parts, flat mouldings | Small witness mark at the split line | Manual degating; not ideal for automated lines |
| Submarine (tunnel) gate | Small to medium parts, auto-degate | Slight drag mark on ejector side | Shear-sensitive resins can degrade at the shear point |
| Pinpoint gate | Three-plate tools, small multi-cavity parts | Tiny point mark | High pressure loss; needs ample pack time |
| Fan gate | Thin flat parts, optical and lens parts | Wide feather edge | Wider gate zone to trim; flow balance across the fan |
| Diaphragm gate | Tubular parts, hubs, circular bosses | Ring witness mark | Extra trimming operation after moulding |
| Tab gate | Thick sections, engineering parts | Tab remnant | Manual degating; gate land design is critical |
| Valve gate (hot runner) | Large, complex or cosmetic parts | No gate mark | Higher tooling cost; hot runner maintenance |
The table narrows the field, but the real answer depends on resin, part geometry and how the part will be assembled downstream. Shear-sensitive materials such as POM behave badly behind pinpoint gates, while large flat panels often need fan gates to keep the flow front even.
Choosing a Gate: A Practical Checklist
- Will the part be degated by hand or automatically? Automation rules out several gate types immediately.
- Where are the cosmetic faces? The gate mark must land on a non-visible surface, which often fixes the gate's position before its type is chosen.
- How thick is the section at the gate? Thin sections need gates that freeze early; thick sections need gates that stay open for packing.
- Is the resin shear-sensitive or filled? Filled and shear-sensitive resins push designs toward larger, gentler gates.
- How many cavities? Multi-cavity tools demand balanced runner layouts, and gate type has to cooperate with that balance.
Gate Marks and Cosmetic Requirements
When a part is plated, painted or used in a visible assembly, the gate mark is a cosmetic defect in its own right. Valve gates and submarine gates are the usual answers, but each carries cost or process consequences. Agree in writing where the witness mark may appear, and request a first-shot sample with the gate area photographed before the tool is signed off. Defects such as flow lines and jetting often trace back to gate sizing rather than machine settings, which is why this topic pairs naturally with a review of plastic injection molding defects and their root causes.
Talking to Your Mould Maker About Gates
Ask the toolmaker to show the gate decision on the tool drawing, not just in an email. A short written rationale - gate type, land size, depth and expected mark location - is a fair request and tells you how the supplier thinks. If the proposal uses a process comparison to justify the choice, such as the trade-offs laid out in CNC machining vs injection molding, that is a good sign the toolmaker is designing for the part's real constraints.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
