Auditing a Silicone and Injection Workshop in One Visit

You have four hours on the floor and a flight out of Bao'an in the evening. The shop makes protective cases, which means it runs two completely different processes under one roof: silicone compression moulding, where an operator loads a weighed blank into a heated tool and the part cures under pressure, and plastic injection, where a screw fills a cold tool in seconds. Knowing how to audit a silicone molding and plastic injection workshop is largely a matter of not spending your four hours in the meeting room, and of knowing which twenty minutes in each area actually tell you something.

Case factories cluster tightly in eastern Shenzhen — WJM Silicone runs a 12,000 m² plant on Xingnong Road in Baolong Street, Longgang District, covering silicone compression, plastic injection, a mould workshop and assembly on one site. A plant laid out that way is auditable in an afternoon because the whole chain sits in one building. A supplier whose moulding happens somewhere you are not being taken is answering a different question, and that is worth establishing before you book the ticket.

Fix these three things before you arrive

Confirm you are visiting the manufacturer. Match the entity name on the business licence against the name on the quotation and, later, against the bank account on the invoice. Where those three names diverge, you are dealing with a layer. CMH lists WJM's licensed entity as 深圳市维嘉美橡塑电子有限公司 with direct buyer contact and no trading-company intermediary; whatever supplier you are visiting, ask for the same three names and check them yourself. The tells that separate a factory from a trading company are mostly visible in an hour on site.

Ask for production to be running. Request a visit on a day when moulding is under way, and say in advance that you want to see a case product in production — yours if it exists, a comparable one if it does not. Sending the geometry and RFQ packet ahead of the trip means the engineer you meet has already looked at your part. A silent workshop is a showroom.

Bring your own instruments. Calipers, a Shore A durometer, a phone with a good macro camera, and the device the case is for. Half the value of the visit comes from measuring a part yourself, off the machine, before anyone has picked through the box — the same sample measurements you would run before cutting steel.

Settle the language question in advance too. The useful conversation on a mould floor is with a tooling engineer rather than a salesperson, and a shop with an in-house R&D team should be able to put one in front of you.

The walk: nine stations and what each one tells you

Walk it in production order, but keep one auditor habit: look at finished goods and shipping first if you can. It is the hardest area to stage and it shows the real output mix.

Station 1 — Raw material store and rubber mixing

Silicone gum arrives in sealed cartons with batch labels and a shelf life. Look for date-ordered stock rotation, a segregated area separating food-grade silicone from industrial grades, and pigment being weighed to a batch card rather than scooped. Ask to see the label on the compound named in your quotation. If the person walking you round cannot find it, the material specification on your quote is aspirational.

The tell here: ask what happens to a part-used carton of compound at the end of a shift. A shop with a real system reseals, relabels and re-dates it. A shop without one leaves it on the bench.

Station 2 — Silicone compression presses

This is the heart of a case factory and the area buyers under-read. Count presses actually cycling against presses idle under dust sheets — idle is normal, dusty is a capacity story. Then watch one operator for two full cycles.

What you are watching for is the blank. Compression moulding starts with a pre-form cut or extruded to weight, and the difference between a shop that pre-weighs blanks on a scale and one that lets operators eyeball a lump of gum is the difference between consistent flash and variable flash for the life of your tool. Under-fill shows as a short at a corner; over-fill shows as heavy flash and, on a thin-walled case, as a distorted rim after trimming. Ask to see the weight standard for the part running in front of you.

Check the temperature controllers on the platens, whether cure time is set on a timer or called by the operator, and whether there is a log for each press.

Station 3 — Deflashing and post-cure

Ask how flash comes off. Hand trimming with scissors or a blade produces a rim that varies with whoever is holding the blade; tumble or cryogenic deflashing produces a consistent edge. On a protective case the rim is a visible, hand-felt surface, so this single answer sets a large part of your perceived quality.

Post-cure ovens are the second half. Moulded silicone normally goes through a post-bake to drive off cure by-products and stabilise colour and odour. Ask for the oven schedule and whether cycles are recorded. An oven with a data logger and a rack system is a different quality proposition from one with a dial and a shift supervisor's memory.

Station 4 — The mould workshop and mould storage

If the factory cuts its own tooling, this is where a visit earns its cost. Look at the CNC machines and the electrical-discharge machining bench, then turn round and look at the racks.

The insider question is not "how many moulds do you have" — it is "show me the maintenance card for a mould that has run half a million shots." A mould record card carries the tool number, the customer, cumulative shot count, and every clean, polish and repair. A shop that keeps them can tell you when your tool will need refurbishment and who pays. A shop that cannot produce one is guessing, and you will discover that when flash suddenly appears at 200,000 parts.

Also check that moulds are tagged and racked rather than stacked on a floor, and ask where a customer-owned tool physically sits. Where a factory advertises a public-mould library — WJM lists 80-plus ready-made POS terminal moulds and more than 10,000 completed custom mould projects — ask to see the racks and pick two at random. Whether those public moulds can ever be used exclusively for one buyer is a separate commercial question worth asking in the same room.

Station 5 — The injection hall

Different physics, different checks. Note the machine brand and the clamp tonnage stamped on each press — WJM's floor runs Haitian injection machines alongside the silicone lines — and ask for the tonnage range and press count in writing, since the biggest tool you will ever run has to fit the biggest platen they own.

The fastest tell in any injection hall involves the material dryers. Polycarbonate is hygroscopic: moulded wet, it produces splay marks and a part that is quietly brittle. So look for desiccant dryers with a dew-point readout on the hoppers feeding any PC or PC-blend job, and ask what dew point and residence time they run. A shop moulding PC out of an undried gaylord will tell you a lot in five seconds.

Then look beside the machines for regrind bins. Regrind is normal and often fine; undeclared regrind is not. Ask for the regrind policy as a percentage and get it written into your specification.

Station 6 — Overmoulding and two-shot

Case work frequently combines materials — silicone or TPU over a PC or ABS frame. Adhesion between the two is the whole game, and it is process-dependent: surface preparation or primer, insert handling and dwell before the second shot, and a bond test at the line. Ask what the operator's adhesion check is and how often it runs. "It has never come apart" is not a check.

Station 7 — Surface finishing, printing and decoration

Look for a colour-matching light box with standard illuminants rather than colour being judged under workshop lighting. Pigmented silicone is opaque and diffuse, it will not behave like an ink on paper, and colour arguments after shipment are expensive and unwinnable without a controlled viewing condition and a signed reference chip.

For logos, ask which methods the shop actually runs in-house — pad printing, laser, debossing — and see one running.

Station 8 — Assembly, inspection and the QC lab

Ask to see the inspection station for the job in production, the first-article board, and the retained golden samples. Then ask what is in the lab: a durometer, a tensile tester, an optical comparator or coordinate measuring machine, a colour spectrophotometer. The equipment list matters less than two things — whether each instrument carries a calibration sticker with a due date that has not passed, and whether the inspection records are in a format you will be allowed to receive with each shipment.

Station 9 — Finished goods and shipping

Cartons, labelling, drop protection, and how mixed SKUs are segregated. Also, quietly: whose brands are on the boxes. That tells you the real customer profile — and that your own cartons will be equally visible to the next visitor.

The one-page audit table

Documents to ask for while you are standing there

Photograph these rather than accept a promise to send them later.

1. Business licence, original — check the entity name, registered capital, scope of business and address against where you are standing.
2. The bank account details, in the same entity name. A request to pay a different company or an individual is a stop, not a negotiation.
3. Any quality or product-safety certificate, in original, with a certificate number, an issuing body and a validity period. Then verify the number on the issuing body's own database before you rely on it. A logo on a slide deck, a photocopy or a wall plaque proves nothing — this is exactly why CMH publishes WJM's quality management system as certification on file, details pending verification, rather than naming a standard it has not physically checked.
4. Test reports for a specific material grade, if your market needs them, issued in the name of a named compound rather than in the factory's name generally.
5. The inspection record for whatever is running today, in the format you would receive.

Ask for all five at the start of the visit rather than the end, and give the office an hour to assemble them while you walk. Where a supplier has already been through a third-party review — WJM carries CMH factory ID CMH-F-L224U9 with a 2026 review covering business licence and product scope — that shortens the paperwork conversation but does not replace it. A platform check tells you the entity exists and makes what it says it makes; only you can check whether the presses can hold your tolerance. The broader factory audit checklist covers the systems layer that sits above the shop floor.

Common questions

Four hours or a full day?

Four focused hours on the floor beats a full day that includes lunch, a slide deck and a showroom. If you have a day, spend the extra time watching one job run rather than seeing more rooms. The discipline for a compressed single visit is the one set out in what to inspect in one day at a Chinese factory.

Should I hire a third-party auditor instead of going myself?

For a systems audit, a third party is more thorough and cheaper than your airfare. For a moulding-capability judgement, going yourself is worth it once, because the questions that matter are specific to your part — blank weight, split line, dryer, the tonnage that fits your tool. Use the third-party audit for compliance and the personal visit for capability.

What if the factory will not let me photograph the floor?

That is common and not automatically a red flag; other customers' tooling is sitting in those racks, and the same protection applies to you. Negotiate specifics instead: no wide shots, but photographs of your own job, the machine plates, the calibration stickers and the documents. A blanket refusal covering even the licence is a different matter.

What to ask the supplier next

Send these before you book the flight, so the visit is spent verifying rather than discovering:

• How many compression presses and how many injection presses, and what is the tonnage range of each?
• What is the biggest mould base your presses will accept?
• Which post-cure ovens do you run and are cycles recorded?
• What is in your inspection lab, and when was each instrument last calibrated?
• Will you release first-article and in-process inspection records with each shipment?
• What notice do you need for a visit, and will an engineer who speaks English be available on the floor?
• Which of your certificates can be shown in original, with number, issuing body and expiry, so I can verify them independently?

A shop that runs 3D modelling, CNC mould fabrication, compression moulding, injection moulding, finishing, assembly and inspection under one roof — the layout at WJM Silicone in Longgang is one example, with roughly 200 staff including a 12-person R&D team — is the kind of site where one visit can answer the whole question, because nothing is happening at an address you were not shown. Before you go, decide which process your part actually needs; the compression moulding against liquid silicone injection comparison determines which half of the workshop you should be spending your four hours in.