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

How many parts should the mold make per shot? One cavity keeps tooling cost and complexity down. Eight cavities cut machine time per part to roughly one-eighth, behind a much more expensive tool.

There is no universal right answer. There is a right answer for a given volume. Cavity count drives tooling cost and piece price together. When suppliers assume different cavitation, their quotes will not line up.

The Tooling vs. Piece Price Trade-Off

Every added cavity moves cost from the piece price into the tooling budget.

Single or low-cavity tools use less steel, less machining, and simpler engineering. Each part absorbs the full machine cycle, so piece price stays higher. That fits low-volume programs where multi-cavity tooling never pays back.

Multi-cavity tools cost more because every cavity must be cut, fitted, and balanced. They need a larger press for projected area and tonnage. Machine time divides across cavities each shot. At high volume, piece-price savings stack across many shots and often repay the tool. Ask suppliers for the crossover volume on your annual numbers.

Engineering the Multi-Cavity Tool

Multi-cavity molds are not single-cavity designs pasted side by side. They need their own engineering.

Filling balance: Every cavity must fill at the same time and pressure. Otherwise some parts pack while others short or flash. Runner layout, and hot-runner systems on higher cavitation, exist mainly to hold that balance.

Cavity-to-cavity consistency: Each cavity is its own mold. Dimensional consistency across cavities is a hard test of tooling precision. Serious programs measure parts by cavity during qualification.

Press size: More cavities multiply projected area and shot volume, which pushes tonnage class up. Higher tonnage means a higher hourly machine rate. Cavitation and press size are the same decision in practice.

Physical Limits on Cavitation

Two machine limits cap cavitation before budget arguments do. Suppliers should run this math early in quoting.

Clamp force limit: Required tonnage equals total projected area of all cavities and runners times the cavity pressure the resin needs. General-purpose resins such as PP and PE need roughly 0.3–0.5 tons/cm². High-viscosity materials such as PC or PEEK need 0.5–0.7 tons/cm², plus a 10–15% safety factor. A plan built for polypropylene can fail if the resin switches to polycarbonate.

Shot size limit: Total volume of cavities plus runner should stay at or below 80% of rated barrel capacity. Above that, shot weights drift and material sits long enough to degrade. Machines generally run best between 20–80% of maximum shot capacity. A single-cavity tool on an oversized press uses too little of the barrel. Material sits and bakes, and degradation defects follow that no process tweak will clear.

Before arguing tooling cost, ask what tonnage and shot-size math actually allow.

The Family Mold Trap

A family mold puts different parts, such as a housing and its lid, in one tool to share cost. That can look efficient at modest volume. The engineering risk is real.

Dissimilar parts fill differently. Balancing both parts in one shot is hard. Parts from a family mold are locked to the same material, color, and schedule. Treat family molds with skepticism. Supplier pushback is often engineering caution, not upselling. Full framework: family mold vs dedicated molds.

Managing the RFQ Process

State annual and lifetime volumes clearly in the RFQ. Vague volume produces arbitrary cavitation and quotes you cannot compare.

Normalize cavitation when you score bids. A 2-cavity quote and an 8-cavity quote differ in tooling cost, piece price, press size, and lead time. They are not direct substitutes.

Ask for the payback volume on added cavities. For precision parts, require per-cavity measurement at the T1 trial to prove cavity-to-cavity consistency.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, provide quotes, or operate a supplier directory. Costs, terms, and timelines are supplier- and program-specific. Confirm them in writing with your suppliers.