Gate Size and Gate Type Selection: The Arithmetic Behind the Choice

Ask a supplier what gate they are proposing and you usually get a type: edge, sub, pin, hot tip. Ask what size and the conversation often stops. Gate dimensions get treated as shop knowledge rather than a specification.

That is backwards. Gate type mostly decides where the mark lands and how it is trimmed. Gate size decides how hard the material is sheared on the way in. That is where jetting, gate blush, brittle gate areas, and premature freeze-off come from. Size arithmetic first, then type. Sits alongside the broader gate design guide.

The Formula Suppliers Actually Use

Gate dimensions are not guessed. A published formula appears in mold-design references and technical tutorials, with two parts.

Gate depth scales with the wall it feeds:

h = n × t

where t is the part’s wall thickness at the gate and n is a coefficient for the resin.

Gate width scales with how much cavity has to be filled:

W = n × √A ⁄ 30

where A is the surface area of the cavity, and 30 is an empirical constant.

MISUMI’s worked tutorial runs the polyacetal case at n = 0.7. Writing in Plastics Technology, mold designer Jim Fattori works the same formula for a PVC box lid measuring 10 × 5 × 2 in with a 0.100 in wall: surface area 110 in², n = 0.9, giving a gate width of 0.314 in.

Two honest caveats before you use it.

Published coefficient tables disagree. Different references assign different n values to the same resin. One puts polyacetal at 0.6 where MISUMI’s example uses 0.7, and PC appears at both 0.6 and 0.7 depending on the source. The pattern still holds: stiffer, more shear-sensitive materials get bigger gates (PVC around 0.9, nylon 0.8) and free-flowing ones get smaller (PE and PS around 0.6).

Fattori’s own view is that the formula runs conservative (wider than he would use). He recommends it anyway, as a reality check on a number you already have in mind rather than as the answer.

Cross-check against an independent rule and the two roughly agree. Eastman’s mold design guidelines put gate thickness at 50–80% of the wall thickness at that location, with a floor for their polyester materials of 1.65 mm (0.065 in). That is the same territory as h = n × t for n between 0.6 and 0.8.

The Rule of Thumb That Burns Parts

An old rule says gate width should be twice gate depth. Fattori’s demonstration of why that is dangerous converts an abstract shear worry into a number anyone can feel.

Take a single-cavity mold on a machine with a 3 in diameter barrel injecting at 2 in/sec. That is a flow rate of 14.1 in³/sec through the sprue, runner, and gate. Hold gate depth at 0.050 in and vary the width:

• At 0.100 in wide (exactly the old two-to-one rule) the material goes through the gate at 161 mph.
• Every doubling of the gate width halves that speed.
• The flow rate never changes. Only the velocity does.

161 mph through a restriction is where shear-sensitive material degrades. It is the mechanism behind a burnt or brittle patch at the gate on an otherwise fine part.

The asymmetry matters for buyers. Making a gate wider than strictly necessary costs a bigger gate scar and a slightly harder trim, and it lowers the pressure needed to fill. Making it too narrow costs material damage you cannot see until something fails.

But there is a floor as well as a ceiling

Wide is not free either. The failure mode at that end is subtler. If a gate is so wide that material creeps through it, the gate can freeze off before the part is packed out. That reads as sink, voids, or dimensions that drift, and gets chased as a packing problem.

Fattori’s example: a 5 in diameter cylindrical part with an internal disk gate has a 15.7 in circumference. At the same 0.050 in depth, flow through that ring gate runs at 1 mph. That is why fan, film, curtain, ring, and disk gates often have to be cut deeper than expected. The gate styles chosen for gentle, even filling are the ones most exposed to premature freeze-off.

So the two failure modes sit at opposite ends of the same dimension. Neither is visible on a drawing:

Gate too narrow	Gate too wide
High velocity, high shear	Very low velocity
Jetting, gate blush, burning	Premature freeze-off
Degraded, brittle material at the gate	Under-packed part: sink, voids, dimensional drift
Higher fill pressure needed	Larger gate scar, harder trim

Matching the Gate Type to the Part

With size settled, type is mostly about three things: where the mark can land, whether it self-trims, and what flow pattern the part needs.

Gate type	Flow behavior	Mark and trimming	Typical fit
Sprue (direct)	Melt enters straight from the sprue	Mark left at the part center, trimmed manually	Single-cavity parts where a central mark is acceptable
Edge (side)	The most widely used form	Separates easily; balances readily across cavities	General purpose, multi-cavity
Overlap	Melt strikes the cavity wall before entering the part	Harder to trim cleanly	Parts prone to flow marks
Fan	Spreads flow over a wide, thin entry	Trimmed	Flat or large-area parts; helps prevent distortion
Film / curtain	Gate as wide as the part; very even flow front	Trimmed	Flat, large parts where warpage is the concern
Ring	Feeds a cylinder from all around	Trimmed	Tubular parts; keeps the core from deflecting
Disk / diaphragm	Feeds outward from a central opening	Harder to trim	Cylindrical parts or parts with a large central hole
Submarine (tunnel)	Enters below the parting line into the part side	Self-severing on ejection, inconspicuous mark	Where a manual trim step is undesirable
Banana / cashew	Like a sub gate but curved	Self-severing; the gate deforms as it releases	Needs a resin with enough elongation to survive it
Pin / point	Perpendicular onto a flat face	Auto-cuts as the mold opens; small vestige	Three-plate and hot-runner tools; multi-point feeding

Three patterns are worth pulling out of that table.

Self-trimming is a cost decision, not a cosmetic one. Sub, banana, and pin gates remove a manual operation from every part for the life of the program. That is usually a bigger number than the tooling difference.

The wide gate styles are chosen for flatness. Fan, film, and ring gates exist to spread the flow front and prevent warpage on large or tubular parts. Per the section above, they are exactly the ones that need extra depth to avoid freezing off.

Banana gates need the right material. The gate deforms as the part ejects, so a resin with poor elongation will crack rather than release. That is a material-and-tooling interaction, not a free choice.

What a Buyer Can Do With This

You are not going to size the gate. Three questions land differently once you know the arithmetic exists:

• “What gate size are you proposing, and how did you arrive at it?” A supplier who answers with a wall-thickness ratio or the width formula has calculated it. One who answers “standard for this part” has not.
• “If we change the wall thickness, does the gate change?” It should. h scales with t. A late wall-thickness revision quietly invalidates a correct gate.
• “If we change resin, does the gate change?” Also yes. Eastman notes directly that tooling designed for one material may need a different gate size for another’s viscosity. A resin substitution on an existing tool is a gate question, not just a purchasing one.

If a supplier proposes shrinking a gate to reduce a visible mark, ask what that does to velocity and pack at that location. You may be trading a small cosmetic defect for an invisible weak spot.

Buyer FAQs

How is injection molding gate size determined?

By calculation, not by eye. The common published approach sets gate depth as a coefficient times wall thickness at the gate, and gate width as that same coefficient times the square root of cavity surface area, divided by 30. The coefficient reflects the resin: higher for stiff, shear-sensitive materials like PVC and nylon, lower for free-flowing ones like PE and PS. Eastman’s independent guidance putting gate thickness at 50–80% of the wall lands in the same range. Ask the supplier to show their size math before you approve the gate.

Why does gate size cause defects like jetting and burning?

Because gate size sets material velocity, not flow rate. The volume entering the cavity is fixed by the machine; the gate decides how fast that volume squeezes through. One published example has material passing through a narrow gate at 161 mph, enough to shear-damage a sensitive resin and produce burning or a brittle area at the gate. Every doubling of gate width halves that velocity. Do not accept a shrink-the-gate cosmetic fix without checking velocity and pack.

Can a gate be too big?

Yes, and the failure mode is the opposite one. If the gate is very wide, material moves through slowly and the gate can freeze off before the part is packed out, producing sink, voids, or dimensions that drift. This is a known issue with fan, film, curtain, ring, and disk gates, which is why those styles often have to be cut deeper than expected. If packing problems appear with a wide gate style, ask about gate depth before chasing process alone.

Which gate type should I ask for?

Usually none. The type follows from where a mark is acceptable, whether you want the gate to self-trim, and what flow pattern the part needs. What is worth stating is the constraint: which faces are cosmetic, whether a manual trimming step is acceptable at your volume, and whether the part has a flatness requirement that argues for a wide, spreading gate. Put those constraints in the RFQ and make the supplier propose type and size against them.

How This Page Was Put Together

The gate depth and width formulas are quoted from published mold-design tutorials and trade-press guidance, with the worked examples reproduced as printed. Coefficient tables for the resin factor vary between published sources; where they disagree, that is stated rather than resolved. The gate-type table describes behaviors documented consistently across gating references, and is a summary of common practice rather than a standard. Gate dimensions for a real part depend on the resin grade, part geometry, cavity layout, and machine, and are the supplier’s engineering decision.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, design tooling, or certify suppliers. Gate sizing and gate type selection for your part should be confirmed with your supplier against that specific part, resin, and tool.