Multi-Cavity Mold Balancing: Why One Cavity Always Runs Thin

The Problem Appears as a Lot Defect

A four-cavity mold that produces one thin cavity will show up as a random reject rate, because only a quarter of the output is affected.

The lot average looks acceptable and the inspection sample often misses the affected cavity, so the defect travels downstream.

Measuring by cavity is the only way to see it, and it is rarely done.

Why Cavities Differ

Vacuum distribution is never perfectly equal. A cavity further from the vacuum manifold sees lower suction and builds a thinner wall.

Slurry flow and temperature also vary across the head, and the first cavity to fill sees different conditions from the last.

Tool manufacturing tolerance is a smaller factor than most assume. Balancing usually fixes a machine condition, not a tool defect.

How to Measure

Mark each cavity with an identifier that survives forming, or track by position in the mold.

Measure wall thickness at a fixed point on ten parts per cavity, and record by cavity, not by lot.

A spread above 0.1 mm across cavities means balancing is needed, regardless of whether each cavity is in tolerance.

Balancing Steps

First, check the vacuum manifold for leaks and blocked ports. A partial blockage is the most common cause of one weak cavity.

Second, check the screen condition per cavity. A blinded screen on one cavity looks exactly like a balancing problem.

Third, adjust individual vacuum control if the head has it, or adjust the tool's flow restriction. Only then consider tool modification.

Tool-Level Correction

If a cavity is persistently thin after the machine checks, the fix is a flow restriction or a screen change on that cavity.

Machining the tool is the last resort, because it changes the cavity permanently and cannot be reversed.

Record the correction against the cavity number. Without that record, the next shift will re-adjust the wrong thing.

Ongoing Control

Measure per cavity weekly, and after every screen change. Cavity comparison is the cheapest early warning on a forming line.

Rotate the measurement point across the part so a local tool defect is not hidden by measuring the same spot.

Record the result with the cavity number, the screen age and the vacuum reading. Three numbers explain most variation.

What It Saves

A thin cavity produces weak corners that fail downstream, and the reject appears at the customer rather than at the forming station.

Balancing typically recovers 3 to 8 percent of yield, and more importantly removes an intermittent defect that is hard to trace.

The measurement cost is minutes per week. The alternative is a claim with no clear cause.

Questions Buyers Ask

Why does one cavity produce thinner walls?

Usually unequal vacuum distribution, a partially blocked port or a blinded screen on that cavity. Tool tolerance is a smaller factor.

How is cavity variation measured?

Wall thickness at a fixed point on ten parts per cavity, recorded by cavity. A spread above 0.1 mm needs balancing.

Can the tool be machined to balance a cavity?

Only as a last resort, because it is permanent. Check the vacuum manifold, ports and screens first.

How often should cavities be compared?

Weekly and after every screen change. Cavity comparison is the cheapest early-warning measure on a forming line.

How much yield does balancing recover?

Typically 3 to 8 percent, plus the removal of an intermittent defect that is otherwise very hard to trace.

Next Step

Send the cavity count and the current inspection method. You will receive a cavity measurement plan with balancing steps in order, in one reply.

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