Injection Mold Cavitation and Tolerance: What 2-16 Cavities Mean for Your Order

Cavitation is the number of identical cavities machined into one mold, and it sets how many bottles a single machine shot produces, from 2 up to 16 or more for small parts. More cavities raise output per minute and lower the cost per bottle, but they also make uniform filling and cooling harder, so dimensional consistency depends on tighter process control. Choose cavitation from your volume and your tolerance need, not from a default the factory happens to own.

When a supplier quotes a bottle, the mold behind it has a fixed number of cavities, and that number quietly shapes your price, your lead time and how interchangeable the bottles are across a carton. Buyers rarely ask about it, then wonder why one run costs less but varies more. This guide explains what cavitation means, how 2 to 16 cavities change the economics and the quality, and what tolerance to write so every bottle fits the same cap and label.

What Is Mold Cavitation?

Cavitation is the number of identical cavities cut into a single mold. In cosmetic bottle production the mold may form preforms for stretch blow molding or, for some processes, the finished body directly. Each cavity produces one part per machine cycle, so a 16-cavity mold yields 16 parts per shot.

The count is a design choice, not a fixed property of the bottle. The same 100 ml bottle can be run on a 4-cavity mold or a 16-cavity mold, and the difference shows up in how fast the order fills and how the cost divides across the run. The mold is the asset; cavitation is how hard that asset works each cycle.

For buyers, cavitation matters because it is baked into the quote. A supplier quoting from a high-cavitation mold they already own can price a high-volume order keenly, while a low-cavitation mold suits a small first run without over-investing in tooling. Our custom mold guide covers when to open a new mold at all.

How Does Cavitation Change Unit Cost?

Machine time is the scarce resource. A 16-cavity mold makes eight times the parts per shot of a 2-cavity mold, so it uses far less machine time per thousand bottles. Since machine time is a large part of the conversion cost, higher cavitation usually means a lower unit price at volume.

The saving is not free. A high-cavitation mold costs more to build and maintain, so it only pays back across a large enough order. Below that volume, the tooling amortisation outweighs the machine-time saving, and a simpler mold is cheaper. The break-even is why cavitation should follow your forecast, not the supplier's inventory.

Lead time moves the other way. A high-cavitation mold fills a big order faster, which can shorten the wait when you are capacity-constrained. For a rush reorder, cavitation can matter as much as price. The development timeline guide shows how tooling choices flow into delivery dates.

Why Does High Cavitation Threaten Consistency?

Every cavity must fill, cool and shrink identically, and that gets harder as their number grows. With 16 cavities, a small imbalance in melt flow or cooling reaches the part as a weight or dimension difference between cavities, so bottles from the same shot are not quite the same.

The symptoms are practical. One cavity may run slightly heavier, another may show a faint flash, and a third may sit at the edge of the tolerance. None is a defect on its own, but across a carton the bottles vary more than the brand expects, and the cap or label fit drifts.

The defence is process control, not fewer cavities. Balanced runner design, precise temperature control and validated process windows keep high-cavitation molds consistent. A supplier who can show cavity-to-cavity weight data is controlling the process; one who cannot is hoping. The QC guide lists the checks that confirm consistency on arrival.

What Tolerance Should You Put on the Specification?

Tolerance is the allowed variation on a dimension, and for bottles the critical ones are neck finish, overall height and weight. Write them as a range, not a single number, because plastic always varies within a band.

1. Neck finish tolerance. State the allowable variation on diameter and thread so every cap still fits, since the neck is the capped interface.
2. Critical dimension tolerance. Height and any feature that meets a label or collarpiece, with a stated plus or minus in millimetres.
3. Weight tolerance. A gram range on the bottle, which also proxies wall consistency across cavities.
4. Verification method. How the supplier confirms the range, for example cavity-to-cavity weight checks on production samples.

These lines keep bottles interchangeable across a carton and across reorders, which is what lets a cap or label bought once keep fitting forever. They turn "same as last time" from a hope into a measured result.

How Do You Choose the Right Cavitation?

Start from volume and tolerance need, in that order. If your forecast is large and your product tolerates normal variation, a higher-cavitation mold lowers unit cost and can shorten lead time. If your first order is small or your product is tight on fit, a lower-cavitation mold avoids paying for tooling you will not use.

Then confirm the supplier can hold tolerance at the chosen cavitation. Ask for cavity-to-cavity weight and dimension data on the pre-production sample, and make it a shipment condition. The number of cavities is only as good as the control behind it.

At ShiJin we run a library of 1,000+ open molds across cavity counts, MOQ from 5,000 units, 15-25 day production, with tolerances confirmed on pre-production samples. Share your volume and fit requirements and we will propose cavitation that balances cost against the consistency your product needs.

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