What Actually Moves the Unit Price of a Moulded Case

Two factories quote the same protective case from the same 3D file and the numbers come back thirty percent apart. Before you decide one of them is padding, it helps to know that the cost drivers behind a moulded silicone case unit price are not one number with a margin on top. They are six separate lines, each set by a different decision, and only about half of them are yours to move. A buyer who asks for "your best price" gets a haircut on the supplier's margin, which is the smallest line on the sheet. A buyer who asks the right question about part weight or trim labour changes the cost of every unit for the life of the tool.

This is the anatomy of that number, in the order the money actually accumulates on the floor.

The six lines behind one number

A moulded case leaves the factory carrying six kinds of cost. Quotes rarely show them separately — that is the first thing to ask for — but they exist whether or not anyone itemises them.

Two of those lines — compound and machine minute — are physics. Two — trim and decoration — are people. One is cardboard. And the last is the only one most buyers negotiate hard, usually while ignoring the five above it. The broader pattern of why two Chinese quotes for one specification diverge is covered in why China quotes for the same spec differ; what follows is the case-specific version.

Compound: you are buying grams, and the grams are a drawing decision

Silicone is sold by weight. Not by part, not by area — by the kilo, in a compound grade with a price that moves with raw feedstock. So the first question on any case quote is not "what does it cost" but "what does it weigh".

Part weight is set by the model, not the moulder

A protective case is a shell. Its mass is wall thickness multiplied by surface area, plus whatever solid features hang off it — corner bumpers, plugs, lanyard lugs, a thick bezel lip. Take a case designed at 2.5 mm nominal wall and thin the flats to 2.0 mm while keeping the corners heavy, and you have removed something close to a fifth of the material on a part whose protective performance is concentrated in the corners anyway. Nothing else on the quote has to change for that saving to land.

This is why the design conversation and the price conversation are the same conversation. A factory that runs mould design in house — WJM lists 12 R&D engineers handling 3D modelling, structural analysis, material selection and thermal simulation — can answer a "what does this cost in grams" question during design review rather than after tooling is cut. A factory that only quotes finished drawings cannot.

Grade and cure system

Materials on a case programme commonly include food-grade silicone, industrial silicone rubber, TPU, PC and ABS, and combinations of them in overmoulded constructions — the list WJM publishes. These are not interchangeable at one price. Food-grade compounds cost more than general industrial grades and may carry a post-cure requirement that industrial grades do not. High-temperature and UV-stable grades cost more again. If your case does not go near a mouth or a dashboard, paying for a grade specified for those environments is money spent on a certificate you will never use — and if it does, the reverse mistake is worse.

Colour, and the cost of changing it

Pigment itself is trivial. Colour changes are not. Every switch means purging and cleaning, and on a compression line it also means separated compound batches and a scrap allowance at the head of the run. This is the mechanism behind the MOQ-per-colour rule that surprises first-time buyers: five colourways at 1,000 pieces each is a materially different job from 5,000 pieces in black, even though the part count is identical.

The machine minute: cure time divided by cavities

The second physical line is press occupancy. Cost per part is the press rate per minute multiplied by the cycle time, divided by the number of cavities that cycle produces. Both halves of that fraction are engineering decisions.

Cure time is governed by the thickest section on the part, not the average. Compression moulding of silicone is a heat-conduction problem: the tool heats the compound from the outside in, and the last part to reach cure temperature sets the clock. As a working rule of thumb, cure time scales with roughly the square of the thickest section, which is why one heavy corner bumper or a solid lanyard lug can add more to the cycle than its volume suggests. Thinning a single feature on the model sometimes buys more than thinning the whole part. Compression moulding under temperature and pressure control — the process WJM lists for its silicone parts — is what delivers consistent wall thickness and surface finish, and it is also what makes the thickest section so expensive.

Cavitation is capped by the press, not by ambition. A 16-cavity tool needs the platen area and clamp tonnage to close it. Doubling cavities also roughly doubles tool cost, so it only pays above a volume where the extra steel is amortised — and it rarely halves the unit price, because flash pad area grows with the tool and trim labour grows with it. Ask what cavitation the quote assumes. A price quoted on a 8-cavity tool and a price quoted on a 2-cavity tool are not comparable numbers, and nothing on the quotation will tell you which you are holding.

One detail worth writing into the tooling specification: cavity identification numbers moulded into every cavity. They cost nothing at tool-cutting stage and they are the only way to trace a dimensional problem back to one cavity instead of scrapping a whole shipment. Without them, a single bad cavity in a 16-cavity tool produces a defect rate of about six percent spread evenly across every carton, which is exactly the kind of failure that survives sampling inspection and reaches your customer.

Injection-moulded plastic parts — the PC, ABS and TPU side, run on the factory's injection line — follow the same arithmetic with cooling substituted for cure. Wall thickness dominates there too, for the same conduction reason.

The human hours, and the packing that outgrows the part

Deflashing is the line buyers never see

Compression-moulded silicone comes out of the tool with flash along the parting line. Somebody removes it. That somebody is either a person with a blade, a tumbling operation, or — best case — the tool itself.

Trim cost scales with the length and accessibility of the flash line, not with part size. A case with eight port cutouts, a camera window, a speaker grille and a scalloped bezel has an enormous perimeter to trim relative to its weight, and every one of those edges is a place where a hand-trim slip becomes a cosmetic reject. This is where a large share of a thirty-percent quote gap usually lives, and it is invisible on a drawing.

The fix is a tear-trim edge — a designed thin web at the parting line that lets flash be pulled off in one piece cleanly, instead of cut off feature by feature. It is a tooling decision made once that removes labour from every unit produced for the rest of the tool's life. Cryogenic deflashing, where parts are tumbled with media at low temperature so flash embrittles and breaks away, is the other route and suits small parts with awkward geometry. Ask which method the quote assumes.

Decoration is priced per colour, per pass

Silicone is a low-surface-energy material, which means ink does not want to stay on it. Printing a logo generally requires a primer, a silicone-compatible ink system, or a surface treatment step — a real per-unit cost, plus plate origination as a one-off. Each additional colour is another pass, another set-up and another chance to reject a good part for a smudged mark.

Moulding the mark into the steel instead costs essentially nothing per unit. The catch is that it is tone-on-tone and it is now in the tool: changing artwork means an insert change, not a plate change. Laser marking sits in between — fast, no consumables, but it works by ablating pigment, so again no second colour. The full comparison sits in logo decoration methods on silicone cases.

Packing can cost more than the part

A light silicone case in a printed colour box with a hang tab, an insert card, a barcode label and a polybag can carry more cost in its packaging than in its moulding. Retail-ready packing also adds handling minutes per unit that do not appear on any moulding line. If your channel is B2B or fleet replacement rather than shelf retail, a bulk polybag-and-carton spec is often the single fastest saving available, and it needs no tooling change at all. Packaging is one of the classic entries in the hidden costs of sourcing from China.

Ranked by how much they move a unit price against how much they cost you to implement, the realistic levers look like this:

That last row is the one with the biggest single effect, and it is binary. WJM publishes 80+ ready-made public moulds for POS terminal cases and states near-zero upfront mould cost with samples in as little as 72 hours on those models, against 10,000+ custom mould projects completed for everything else. If your device is in the library, an entire cost line disappears; if it is not, you are amortising steel and should model it as such. The mechanics of that trade-off are in what an 80-mould POS case library actually gets you and the volume maths in MOQ on a public mould versus custom tooling. General tooling amortisation practice is covered in tooling and mould costs in China manufacturing.

Common questions

Why did my supplier's price rise between quotation and order?

Silicone compound and engineering resins are commodity-linked, and a quote issued without a stated validity window is an opinion rather than an offer. Ask for the validity period in writing and for the compound to be named on the quotation, so a mid-order increase can be checked against a published feedstock movement instead of accepted on trust. Broader input-cost context is in China manufacturing costs.

Is a heavier case a better case?

Not reliably. Mass in the corners does useful work; mass in the flat panels mostly adds cost and bulk. A quote comparison that ignores part weight will systematically favour the thinner design, which may or may not be the one you want — so compare grams alongside price, not price alone.

The two quotes assume different cavity counts. How do I normalise them?

You cannot, from the outside. Ask both suppliers for the assumed cavitation, the tool cost at that cavitation, and the price at the volume you actually intend to order rather than the volume that makes their number look best. Then rebuild both as tooling plus per-unit, over your real horizon.

What to ask the supplier next

Send these with the 3D file, before anyone quotes. The answers are what make two quotations comparable.

1. What is the finished part weight in grams, and what compound grade and cure system is that priced on?
2. Break the quotation into lines — compound, moulding, trim, decoration, packing, tooling — even if the total stays the same.
3. What cavity count does the price assume, what is the tool cost at that cavitation, and what would the next step up cost in steel and save per unit?
4. How is flash removed — hand trim, tear-trim feature in the tool, or cryogenic — and is a tear-trim edge feasible on this geometry?
5. Are cavity identification numbers included in the tool as standard?
6. What is the quotation validity window, and what index or evidence would justify a revision?
7. Is a post-cure step required for this grade and application, and does that change lead time?
8. What is the packing specification priced on, and what does the same part cost in bulk polybag and carton?
9. What price applies at the volume I will actually reorder, not the volume that produces the headline number?

WJM Silicone is a reasonable case to run this exercise against, because its two production routes sit under one roof and price differently: silicone compression moulding under temperature and pressure control alongside plastic injection, with in-house mould design and fabrication, surface treatment, printing, assembly and inspection in a 12,000 m² Longgang plant of 200+ staff, roughly 80% of output going to the Americas and Europe. Payment is stated as T/T with terms confirmed per order, sampling at 72 hours from a public mould, and lead time confirmed per order — which means the commercial detail above is exactly the material you settle before the proforma, not after it.