Three thousand anodized aluminum faceplates left Dongguan in a container, packed the way they had always been packed: foam between layers, cartons on a pallet, stretch wrap, done. Thirty four days later they were opened in Rotterdam with white blooms across about a fifth of the surfaces and dark water marks along every carton edge. The parts had not been scratched, dented or lost. They had simply been wet for a month inside a sealed box, and the customer rejected the lot. (Composite example; details changed.)
The fix was not a better carton. It was eighteen grams of desiccant per carton, a barrier bag, and a vapor corrosion inhibitor sheet, which together cost about eight cents per unit. Against a claim that ran to five figures, that is the cheapest insurance in the whole order.
We ship machined parts from Dongguan to Europe and North America every week. The floor runs more than 200 CNC machines, the shop has been at this for more than 23 years, IATF 16949 and ISO 9001 and ISO 13485 are in the quality system, and quotations go out inside 24 hours with free DFM. Packing is the part of the job customers think about last and remember longest, because it is the only part they see when the crate is opened. Here is how we specify it.
Why Metal Corrodes in a Sealed Container
The mechanism is not rain. It is condensation, and it happens inside the container.
A steel container crossing the Indian Ocean goes through a daily temperature cycle. Air inside warms during the day and holds more moisture, then cools at night, and the moisture condenses on the coldest surfaces, which are usually the metal parts and the container roof. The water that drips from the roof is called container rain, and it falls on cargo that has never seen the weather.
Two numbers make this concrete. At 30 degrees Celsius, a cubic meter of air at 80 percent relative humidity holds about 24 grams of water vapor. Cool that air to 20 degrees and it can hold about 17 grams, so roughly 7 grams per cubic meter condenses out. A 40-foot container holds about 67 cubic meters of air; the water that condenses out of it is nearly half a liter, looking for somewhere to land. Wooden pallets and cartons make it worse, because they release moisture absorbed in a humid factory.
This is why a machined part can rust in transit without ever being rained on, and why "we packed it dry" is not a sufficient answer. The water was in the air.
The Three Barriers, and What Each One Does
Export packing for machined parts is built from three barriers, and confusing them is the usual mistake.
Vapor corrosion inhibitor, VCI. VCI paper, film or foam releases a vapor that adsorbs onto metal surfaces and forms a molecular layer a few molecules thick, which raises the corrosion potential. It works in an enclosed volume, so it only protects what is inside the bag with it. It protects ferrous metals well, copper and brass reasonably, and aluminum less reliably, particularly if the surface is anodized. It does nothing for a part sitting in an open carton.
Desiccant. Silica gel or clay absorbs water vapor from the air in the enclosure, lowering the relative humidity below the threshold where condensation forms. It has to be sized for the volume and the transit time, and it only works if the enclosure is sealed. Eighteen grams in an unsealed carton is decoration.
Barrier packaging. An aluminum foil laminate bag or a metallized film bag with a heat seal creates the enclosure that both of the above require. Without it, VCI escapes and desiccant saturates in days. With it, a part can sit for a year.
The sequence matters: part, then VCI, then desiccant, then sealed barrier bag, then cushioning, then carton, then pallet or crate. Skip the seal and the other two are wasted money.
Sizing the Desiccant Properly
The unit is grams of desiccant per cubic meter of enclosed air, adjusted for transit time and for the moisture already in the packing materials.
A working rule for a sealed barrier bag: 16 units of desiccant per cubic meter for 30 days of transit, where one unit is 33 grams of silica gel, and add 50 percent if the packing includes wood or paper inside the bag. A 30 litre bag encloses 0.03 cubic meters of air, so the sum works out to about half a unit, which is roughly 16 grams of silica gel, and that is why small parts get a 10 gram sachet and large crates get 500 gram bags.
Two practical points. Desiccant has to be dry when it goes in, so it stays in its sealed foil until the moment of packing. And it has to be visible at receiving, or nobody knows whether it was there, which is why we tape a humidity indicator card inside the bag where it can be read through a window.
Matching the Pack to the Part
Four part types, four different answers.
Bare carbon steel and cast iron. The highest risk. VCI paper wrap, sealed barrier bag, desiccant, and a light oil film only if the customer's process allows it. Parts with machined bores should have the bore plugged or bagged separately, because a trapped pocket of humid air is where the rust starts.
Stainless steel. Corrosion resistant, not corrosion proof. Chlorides are the enemy, so never pack stainless against untreated wood, never use PVC film (it releases chlorides as it degrades), and never ship it in a crate that has held salt water. Passivation buys time rather than immunity, so a sealed bag with desiccant is still the right answer for a long sea transit, and the surface condition that makes passivation work is set during machining, which our stainless steel machining guide covers.
Anodized and coated aluminum. The risk is cosmetic and it is real. Fresh anodizing is porous until it stabilizes, and a humid enclosure produces white bloom that cannot be polished out without cutting through the coating. Separate every part with interleaving foam or paper, seal the bag, and use a neutral, pH-buffered paper rather than newsprint. Type III coatings are more resistant to handling damage but no more resistant to trapped moisture, as the thickness limits in our hard anodizing guide make clear.
Clean and medical. No VCI, because the inhibitor is a surface contaminant and the customer will have to validate it away. Double bag in cleanroom-compatible film, sealed and labeled with the cleaning date, and pack inside a controlled environment if the part is specified clean to a particulate class.
Crates, Pallets and the Wood Problem
Wood is regulated internationally, and the rule catches people out. ISPM 15 requires that solid wood packaging be heat treated or fumigated and stamped with the IPPC mark. Plywood and engineered board are exempt because they are processed, but a crate built from sawn timber without a stamp can be refused at the border, and the customer pays for the return.
For heavy parts we use a plywood crate with a plywood or engineered base, foam or wooden blocking to prevent movement, and bolts rather than nails on anything that will be opened more than once. A 300 kg machined housing that shifts 20 mm in transit arrives with a damaged mounting face, and no amount of foam inside the carton prevents that if the crate itself is not blocked to the pallet.
Weight and volume decide the mode as well. Air freight is charged on volumetric weight at a divisor of 5,000 or 6,000 depending on the carrier, so a light, bulky enclosure can cost more to fly than a heavy, compact shaft. Where the schedule allows, sea freight plus proper preservation is usually a fifth of the air cost, and the packing specification is what makes that trade safe. It is also why packing time belongs in the lead time you were quoted, not outside it, a point we make in our CNC machining lead time guide.
Incoterms, Insurance and Who Owns the Risk
Packing specifications are often written by the party that carries the risk, so the Incoterm matters more than most buyers expect.
Under FOB, the seller's responsibility ends when the goods are on board, and the buyer arranges the ocean leg and insurance. Under CIF, the seller pays freight and minimum insurance, and the buyer still carries the risk after loading. Under DDP, the seller carries everything to the door, which means the seller also eats the cost of a claim for water damage.
Our default for new customers is FOB Shenzhen or FOB Shekou, with the packing specification agreed in writing before the first shipment. When a customer asks for DDP, we quote the packing upgrade as a line item rather than absorbing it, because the difference between a carton and a barrier bag is the difference between a claim and no claim, and it should be visible on the invoice.
Documentation travels with the packing: packing list with carton count, net and gross weight per carton, HS code, certificate of origin where applicable, and a material certificate for each heat lot. For coated parts, the coating supplier's data sheet goes in the document pouch, because customs in some markets ask for it.
What We Check Before the Crate Closes
Packing is an operation with its own first article, and we treat it that way.
- Final inspection signed off, with the inspection report photographed and filed against the shipment.
- Surfaces clean and dry, no fingerprints, no rinse residue, no loose chips in a bore.
- Threads and precision bores protected, either plugged or capped, never taped with a residue-leaving tape.
- Correct barrier and desiccant for the route, with the indicator card visible.
- Carton weight and count against the packing list, photographed open and closed.
- Crate blocked to the pallet, banded, and photographed with the seal number recorded.
- Container loaded and photographed before the doors close, because that photograph is the only evidence of condition at origin.
The last one has settled more disputes than any document. A photo of a dry, well-blocked load with a readable seal number is the difference between a claim being paid and a claim being argued for three months.
FAQ: packing and shipping machined parts
Does VCI paper work without a sealed bag?
No. VCI relies on a saturated vapor concentration in an enclosed volume, and an open carton exchanges air continuously. In an unsealed box the inhibitor disperses within hours and the desiccant saturates within days, which is why the barrier bag is not optional.
How much desiccant do I need for a 30 day sea shipment?
As a starting point, 16 units per cubic meter of enclosed air for 30 days, where one unit is 33 grams of silica gel, and add half again if wood or paper is inside the enclosure. The number is a rule of thumb, not a substitute for a humidity indicator card that shows whether the specification worked.
Can stainless steel rust in transit?
Yes, though not in the way carbon steel does. Stainless is protected by a chromium oxide layer, and chlorides break that layer down. Untreated wood, PVC film and salt-contaminated crates all supply chlorides, so stainless should be bagged with desiccant and kept away from those materials.
Can you pack to a customer's own specification?
Yes, and we prefer to. We will pack to your drawing, your bagging standard or your supplier manual, and we will tell you if something in it will not survive the route. What we will not do is invent a specification and hope. The packing method is written into the order acknowledgment so there is no argument later.
Who pays if the parts arrive damaged?
It depends on the Incoterm and on the evidence. Under FOB and CIF the risk passes at loading, so a claim depends on proving the damage happened after loading, which is why the loading photographs and the seal number matter. Under DDP we carry the risk, which is exactly why we specify the packing rather than leaving it to the forwarder.
Tell Us Where the Parts Are Going
Send the part list and the destination, and we will tell you what packing the route actually needs: bag or no bag, how much desiccant, crate or carton, and whether air freight is worth the difference. If your current packing is fine, we will say so and ship it. If it is not, we would rather discuss it now than open a claim in six weeks.
Free DFM on the drawings, a quotation within 24 hours, ±0.01 mm on the features we commit to, and a shop in Dongguan with more than 200 CNC machines and a packing bench that has seen every routing there is. Send us the part list, or reach us on WhatsApp at +86 17620300785 and by email at gongtianbao@xhlmarketing.com.