One Cavity or Four? The Wrong Choice Costs You Thousands

Before starting any mold design, two questions must be answered: What is this mold for? and How many parts do we actually need? The answers determine not just the cavity count, but the entire approach to the tool.

Here’s a practical framework based on four common production scenarios.

Scenario 1: Prototype or Trial Molds

The goal is simple: get a few samples to check form and fit. Production efficiency doesn’t matter — speed and low cost do.

For this purpose, many features can be simplified or eliminated. High precision isn’t required. Holes can be drilled after molding instead of molded in. Side cores can be avoided. Cooling systems and ejection mechanisms can be minimal or even omitted.

A single cavity is sufficient. The mold should be quick to build and inexpensive — even if the sample parts cost more per piece than fully molded ones.

Scenario 2: Material Testing or Shrinkage Studies

Here, the part shape must be accurate, but production volume remains low.

The mold still needs proper cooling and ejection systems because those affect material behavior. However, cost savings can come from material selection — low‑carbon steel, aluminum, or brass may work just fine. Heat treatment and polishing can be skipped where they don’t affect test results.

A single cavity is still sufficient. But remember: thermal conductivity of the mold material matters for accurate shrinkage data, so choose accordingly.

Scenario 3: Pilot Production (Engineering Samples)

This is for limited runs — typically a few hundred to a thousand parts — often used for market testing or customer trials before full‑scale production.

The mold must produce correct shapes, but simplification is still possible. A good rule of thumb: if skipping a feature saves more mold cost than the post‑molding labor to add it, skip it. Side holes can be drilled later. Simple two‑plate molds with direct or edge gates often work fine, though runner systems may affect part appearance.

For runs under 1,000 pieces, a single cavity is usually adequate. But if you have 2–4 cavities, production becomes more efficient and closer to real production conditions.

Scenario 4: High‑Volume Production

This is where the numbers really matter. The mold must run for hundreds of thousands or even a million cycles, delivering consistent quality throughout its life.

The key drivers here are: delivery time, per‑part cost, and mold investment. Cavity count directly impacts all three.

Let’s look at an example.

Assume annual demand is 4 million parts with a cycle time of 10 seconds.

• Single‑cavity output: 360 parts/hour → 11,111 hours to produce 4 million parts

• Two‑cavity output: 720 parts/hour → 5,555 hours

Assuming 5,400 working hours per year (24 hours/day, 5 days/week, 50 weeks/year, at 90% efficiency), a two‑cavity mold barely meets the target — with no allowance for downtime or maintenance.

For reliable production, three or four cavities are the more practical choice. The extra cavities provide a buffer for unscheduled stops and keep per‑part costs under control without compromising delivery.

Final Takeaway

Cavity count is not a one‑size‑fits‑all decision. It depends on your project stage, volume, and risk tolerance. Prototypes and material tests can often be single‑cavity. Pilot runs may benefit from 2–4 cavities. For high‑volume production, always calculate your capacity gap and add a safety margin — it’s the most cost‑effective way to ensure on‑time delivery without over‑investing in tooling.