Single-Cavity vs Multi-Cavity Molds: How Many Cavities Do You Need?

Early in any tooling discussion comes a deceptively simple question: how many parts should the mold make per shot? One cavity keeps the tool cheap and simple. Eight cavities make parts at one-eighth the machine time each—behind a much more expensive tool. There’s no universal right answer; there’s a right answer for a volume. And because cavity count silently shapes both quotes you’ll receive—the tooling quote and the piece price—mismatched cavitation assumptions are one of the most common reasons two suppliers’ numbers refuse to line up. This guide connects to mold cost, the part cost breakdown, and press size.

The Core Trade

Every added cavity moves cost from the piece price into the tool:

	Fewer cavities	More cavities
Tooling cost	Lower—less steel, less machining	Higher—every cavity is cut, fitted, and balanced
Piece price	Higher—each part carries a full cycle’s machine time	Lower—machine time per shot divides across cavities
Press size	Smaller	Larger (more projected area → more tonnage)
Capacity	Limited—one part per cycle	Multiplied—volume per press-hour scales with cavities
Risk & lead time	Simple, faster to build, easier to perfect	More complexity; one damaged cavity affects the set

The driver is lifetime volume. At low volumes, the tooling premium for extra cavities never pays back—a low-volume program usually belongs in a single-cavity (or two-cavity) tool. At high volumes, the per-part savings multiply across millions of shots and the multi-cavity tool pays for itself, sometimes quickly. Somewhere between is a crossover point that a supplier can calculate for your actual volumes—worth asking for explicitly, with the assumptions shown.

What “More Cavities” Demands

Multi-cavity molds aren’t just copies of a single-cavity design—three things have to be engineered:

• Filling balance. Every cavity must fill at the same time and pressure, or some parts pack well while others run short or flashy. Runner layout (and hot-runner systems on larger cavitation) exists largely to deliver this balance.
• Cavity-to-cavity consistency. Each cavity is its own little mold; dimensional consistency across cavities is a tooling-precision question, and serious programs measure parts by cavity—cavity ID marks on parts exist for this reason.
• A bigger press. More cavities multiply projected area and shot size, stepping the tool up a tonnage class—with the higher machine rate that brings. The cavitation decision and the press-size decision are one decision.

The Family-Mold Caution

A family mold puts different parts in one tool—say, a housing and its lid—sharing tooling cost across components. It can be sensible for matched sets at modest volumes, but it carries a known trap: different parts fill differently, making balance genuinely difficult, and the tool can only ever run the set together at one material and color. Family molds deserve skeptical, case-by-case evaluation rather than default enthusiasm—and a supplier who pushes back on a family-mold request may be protecting your quality, not padding the quote. The full decision framework, including the cavity-balancing mechanics and the questions that expose the risk, is in family mold vs dedicated molds.

What a Buyer Should Do

• Bring real volumes to the RFQ—annual and lifetime. Cavitation is a volume calculation; vague volumes produce arbitrary cavitation and incomparable quotes. (The RFQ template covers where this belongs.)
• Make cavitation explicit when comparing quotes. A 2-cavity quote and an 8-cavity quote differ everywhere—tooling, piece price, press, lead time—and aren’t comparable until normalized.
• Ask for the crossover. “At what volume does the extra cavitation pay back?” is a question good suppliers can answer with arithmetic.
• For precision parts, ask how cavity consistency is verified—per-cavity measurement at trial is the meaningful answer.

This is an independent buyer resource. The right cavitation depends on your volumes, part, resin, and supplier’s equipment—treat the trade-offs here as the questions to ask, and the arithmetic as your supplier’s to show.

Buyer FAQs

What does cavity count mean in injection molding?

It’s how many parts the mold produces per shot. A single-cavity tool makes one part per cycle; a 4-cavity tool makes four identical parts simultaneously; high-volume caps and closures may run dozens of cavities. More cavities mean a more expensive tool but less machine time—and therefore lower cost—per part.

How do I decide between a single-cavity and multi-cavity mold?

By lifetime volume. The extra cavities cost real tooling money up front and pay back through lower piece prices across the production run—so the question is whether your volumes reach the crossover where the savings overtake the premium. Low-volume programs usually justify one or two cavities; high-volume programs usually justify more. Ask suppliers to show the payback calculation at your actual volumes.

Why do multi-cavity molds cost so much more?

Each cavity is individually machined, fitted, and finished—much of a mold’s cost scales with cavitation. Multi-cavity tools also need engineered runner balance (often hot-runner systems) so all cavities fill identically, a larger mold base, and a bigger press class. The result is parts that are individually cheaper from a tool that’s collectively more expensive.

What is a family mold, and is it a good idea?

A family mold molds different parts—often a matched set, like a housing and lid—in one tool to share tooling cost. It can work for sets at modest volume, but balancing dissimilar parts in one tool is genuinely hard (one part fills easily while another struggles), and the parts are forever locked to the same material, color, and production schedule. Evaluate family molds case by case, and treat supplier hesitation as engineering judgment rather than upselling.