How Tool Cavitation (1-Cavity vs. 8-Cavity) Drives Piece-Price Reductions

Eight impressions per cycle cut press hours. They do not divide the invoice by eight.

Cavitation is a throughput lever. Material per part is almost unchanged. Flash risk is multiplied. Balance can throw away whole shots. The only number that scales near-linearly is how many impressions you get each time the press opens, *if* every cavity is good.

What 1-cavity is paying for, honestly

A 1-cavity LSR tool is a process lab that can also ship parts.

You pay less steel, less cold-runner complexity, less tryout. Cycle time is the full cure+open+close for each part. Operator attention can be high without wrecking the P&L. Scrap is one part at a time. Engineering changes hit one pocket.

Piece price at low volume is dominated by setup and tool amortization, not by the fact that the press makes one. Adding cavities before those two shrink is how you own a large, idle tool.

Use 1-cavity when: design is young, annual volume will not keep an 8-cavity fed, the part is large relative to the press, or the shutoff is already a fight.

What 8-cavity is actually buying

Eight impressions per cycle means, in a perfect world, roughly one-eighth the press hours for the same count. That is the conversion win.

What you also buy:

More steel and more plates. Eight cavities, eight gates, a manifold that must stay cold and balanced, more heaters, more thermocouples, more vacuum real estate. Class 103 for eight pockets is not “8 × 1-cavity.” It is a different machine.

Balance. LSR will take the easy cavity. Family tools (different parts in one tool) are worse. Even identical cavities go off if cooling/heating or runner lengths differ. One weak pocket can force you to block it, and then you own a 7-cavity with 8-cavity flash geometry.

Scrap geometry. A short in one cavity can scrap the shot or force a sort. Start-up shots waste eight times the compound. A 2% cavity-level defect rate is not a 2% shot-level rate if defects cluster.

Trim. Eight flash lines per cycle. If shutoff is imperfect, you hired eight trims. Cavitation without shutoff quality is a labor multiplier.

Maintenance. Eight lands to spot. Eight gates to wear. One damaged cavity may idle seven good ones unless you designed spare inserts.

MOQ and batch physics. You will not want to set up an 8-cavity cell for 80 parts. The factory will set a batch floor that protects their changeover. Piece price looks pretty at 50,000 and hostile at 500.

The shot math without a fake quote

Let *T* be cycle time (cure-dominated for LSR; extra cavities do not shorten cure of a thick section). Let *n* be cavities. Press time per good part ≈ *T* / (*n* × yield).

If yield is 0.98 on a 1-cavity and 0.90 on an 8-cavity because two pockets flash, the 8-cavity is not 8×. It is 8 × 0.90/0.98 ≈ 7.3× impressions, minus extra sort labor.

Runner waste: a cold-slug 1-cavity wastes one slug. A poorly designed 8-cavity cold runner can waste a tree. A valve-gated cold runner reduces that and adds tool cost. Put runner mass per part on the quote. It is a cavitation tax or a cavitation savings, depending on design.

Steel amortization: if the 8-cavity tool costs on the order of several times the 1-cavity (not eight times, not two times; it depends on whether you duplicated inserts or built a manifold), you need enough lifetime parts for that delta to fall below the press-time savings. Thick parts (long *T*) love cavitation. Tiny, fast parts still love it, but the tool delta is a larger share. Very low annual volume never pays it back.

Freight and packing barely care. Labor at the press can drop per part (one operator, eight parts) until quality sorting wakes up.

2- and 4-cavity: the unglamorous winners

For custom silicone, 2-cavity and 4-cavity are often the adult answers.

• 2-cavity: almost 1-cavity simplicity, a real cut in press hours, still sane for moderate batches.
• 4-cavity: where many frozen LSR gaskets and buttons live, if the press platen and cold runner can balance.
• 8-cavity: when the part is small, the volume is real, the grade is easy, and you have spare inserts.

Jumping 1 → 8 because a spreadsheet divided by eight is the classic overseas-tooling regret. The first article comes back with two cavities shiny and two cavities hungry.

Interactions with class, finish, and scrap

Higher SPI class on eight cavities is expensive, and it is where high cavity counts belong. An 8-cavity Class 105 is a contradiction: prototype construction with production layout.

Finish: eight polished medical cavities is eight polish quotes. Texture the same.

Scrap: never assume the 1-cavity scrap rate. Require a cavity-balance study in tryout (shot weight per pocket, fill movie, flash map). If the shop will not weigh eight parts from one shot, they cannot claim 8-cavity economics.

RFQ so cavitation is a decision, not a flex

Ask for 1-cavity and 4-cavity (and 8 only if volume is serious) on the same grade and flash spec, with:

• Tool construction (class, runner type, spare inserts)
• Cycle time estimate and whether cure or machine motion dominates
• Runner waste per part
• Batch / MOQ implication of that cavitation
• Assumed yield

The cheaper piece price is the one whose tool delta + press hours + runner + trim + scrap wins at *your* 12-month quantity and *your* release size.

Sentence for the PO: “Do not quote 8-cavity as a unit-price trick. Show 1-cavity and 4-cavity stacks at our real batch size. 8-cavity only if annual volume and spare-insert plan are stated.”

FAQ

Does an 8-cavity LSR tool cut piece price by eight?

No. It cuts press hours per part if every cavity is good. Material per part barely changes. You also buy more steel, a balanced cold runner, multiplied flash risk, start-up waste times eight, and a higher honest min batch.

Why can a small cavity-level scrap rate wreck 8-cavity math?

Press time per good part is cycle divided by cavities times yield. If two pockets flash, you do not have 8x impressions. A short in one cavity can scrap the shot or force a sort. Never assume the 1-cavity yield on eight pockets.

When are 2-cavity and 4-cavity the better buy than 8?

Custom silicone often lives there: almost 1-cavity simplicity with a real cut in press hours, still sane for moderate batches. Jump 1 to 8 because a spreadsheet divided by eight is how first article returns hungry cavities. 8-cavity belongs when the part is small, volume is real, the grade is easy, and spare inserts exist.

Why is an 8-cavity Class 105 a contradiction?

Prototype construction with a production layout. High cavity counts belong on a class that can hold shutoffs. Eight polished medical cavities are also eight polish quotes.

What must tryout weigh before anyone claims 8-cavity economics?

Shot weight per pocket, a fill movie or short-shot, and a flash map. If the shop will not weigh eight parts from one shot, they cannot claim the conversion win. Quote 1-cavity and 4-cavity at your real batch size; 8 only with annual volume and a spare-insert plan stated.