Last updated: 2026-09-01 · 5 min read
What Is Cavity Balancing in a Multi-Cavity Mold — and Why Does It Matter?
Cavity balancing means every cavity in a multi-cavity mold fills at the same rate, packs at the same pressure, and cools at the same speed — so that parts from cavity 1 and cavity 8 are dimensionally identical. When the balance is off, some cavities produce parts that are within tolerance while others produce parts that are not, and the process window shrinks to the overlap between the best cavity and the worst cavity.
Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.
Key Facts
Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.
Where the Imbalance Comes From
The most common source of imbalance is the runner system itself. In an H-pattern or X-pattern runner, the cavities closest to the sprue fill first because the flow path is shorter and the pressure drop is lower. The cavities farthest from the sprue fill last, at a lower pressure and with more cooled melt at the flow front.
Even when the runner diameters are adjusted to equalize the flow path length, the melt that travels through more turns and branches has a different temperature and shear profile than the melt that takes a direct path.
| Imbalance Source | Effect | Detection Method |
|---|---|---|
| Runner geometry | unequal fill time between cavities | short-shot study (fill at 95 %, weigh each cavity) |
| Shear-induced heating | melt near runner wall is hotter, lower viscosity | cavity pressure sensors at each gate |
| Cooling imbalance | different shrinkage per cavity | CMM measurement of parts from each cavity |
| Gate-size variation | different freeze-off time per cavity | gate vestige weight measurement |
| Venting imbalance | air traps in some cavities, not others | visual inspection of burn marks at flow end |
How to Verify Balance at T1
If the weight spread exceeds 2–3 % between the heaviest and lightest cavity, the runner system is not balanced.
Plot the values by cavity number. A systematic pattern (cavity 1 always high, cavity 8 always low) points to runner imbalance; a random scatter points to cooling or ejection variation.
If some cavities produce larger gate vestige than others, the gate freeze-off time is different — which means the packing pressure is applied for a different duration in each cavity.
For high-cavity molds (32+ cavities), full balancing at the runner-design stage is rarely achievable with standard H-pattern layouts.
The industry practice is to use a hot-runner system with individually adjustable thermal gates, which allows the process engineer to tune each drop independently during the T1 trial. The additional cost of the hot-runner controller with individual zone control is justified by the reduction in cavity-to-cavity variation.
What to Do When Balance Cannot Be Achieved
When the mold is already built and the imbalance is discovered at T1, the options are limited but not zero. The first step is to identify the worst-performing cavities and determine whether the root cause is runner geometry, cooling, or gate size.
If the root cause is gate size, the gate land on the affected cavities can be adjusted (opened or closed) by wire EDM or by replacing the gate insert. If the root cause is cooling, the affected cooling circuit can be re-routed or the flow rate increased. If the root cause is runner geometry, the options are more limited — runner re-cutting is possible but expensive, and a hot-runner conversion is the most effective (and most expensive) fix.
Frequently Asked Questions
Does a hot runner guarantee cavity balance?
No. A hot runner with individual thermal gates and zone control provides the adjustability to tune each drop — but if the drops are not balanced in length and diameter, the imbalance persists. The hot runner is a tool for correction, not an automatic solution.
How many cavities can be balanced with a cold runner?
For amorphous resins with moderate viscosity, 4–8 cavities can be balanced with a geometrically designed cold runner. Beyond 8 cavities, the runner length and the number of turns produce too much thermal variation, and a hot runner becomes the practical choice.
Is cavity-to-cavity variation always a runner problem?
No. Cooling imbalance (uneven water flow, blocked channels, or different channel distances from cavity to cavity) produces dimensional variation that is independent of the fill balance. A mold can fill perfectly balanced and still produce parts with different shrinkage because the cooling is not uniform.
Key Takeaways
- Cavity balance is verified by measurement, not by runner design — a short-shot study and a per-cavity CMM report are the evidence.
- The process window for a multi-cavity mold is the overlap between the best cavity and the worst cavity — the wider the cavity-to-cavity spread, the narrower the window.
- For molds with more than 8 cavities, a hot runner with individually adjustable drops is the most effective balancing tool — and the upfront cost pays back in reduced scrap and process-window risk.
The cavity balance question is not whether it can be achieved — it is whether it was verified. A mold quotation that does not include a balance verification plan at T1 is a quotation that defers the risk to production, where the cost of fixing an imbalance is measured in scrapped parts and missed shipments.
Sources
Beaumont Technologies — runner balancing
Priamus — cavity pressure technology
Plastics Technology — molding process data
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet