Shipping scrap carbide so the box arrives whole

Cemented carbide, tungsten carbide grains held in a cobalt binder, is close to twice as dense as iron. A 2023 study measured two commercial grades at 15.17 and 14.91 g/cm3, the higher-cobalt grade being the lighter one, against 7.874 g/cm3 that NIST lists for pure iron.

Put that into a box you know. The USPS Medium Flat Rate Box is 11 x 8 1/2 x 5 1/2 inches inside, about 514 cubic inches. Seventy pounds of the 14.91 grade, packed solid, takes up only about 130 cubic inches of that, roughly a quarter. Fill a Large Flat Rate Box (12 x 11 3/4 x 5 1/2 inside) solid with the same carbide and it would weigh about 418 pounds.

Loose inserts and broken end mills leave air between the pieces, so a real box weighs less than a solid block. Still, the rule holds: pack carbide by weight, never by how full the box looks. Weigh each box on a bathroom or shipping scale as you go.

Sources: OSTI, cemented carbide grades study (2023); NIST X-ray mass attenuation tables (element densities); USPS Priority Mail.

USPS says sturdy paperboard or corrugated boxes are best up to 10 pounds. Past that, its packaging rule for dense loads is the one to follow: "High-density items (such as tools, hardware, and machine and auto parts) weighing from 20 to 45 pounds must be packaged in fiberboard boxes constructed of a minimum 200-pound test board", or in an equivalent wood, metal or plastic container.

A few container choices cost extra with the carriers, so it pays to know them before you pack:

• Pails and buckets. UPS's 2025 guide puts barrels, buckets, cans, drums and pails under additional handling, and the same applies to an outer container that is not corrugated cardboard, such as metal, wood or hard plastic.
• Cans and wooden or metal boxes at USPS. USPS adds a nonstandard fee to a can or a wooden or metal box, while its own Flat Rate packaging is exempt from all nonstandard fees.
• Flat Rate boxes. The flaps have to close within the normal folds. You may tape the seams to reinforce the box, but you may not enlarge it or rebuild it.

For most sellers that points to several small, strong corrugated boxes, each well under the carrier's limit, rather than one big one.

Sources: USPS: preparing packages; USPS DMM 601: mailability; UPS Rate and Service Guide, 2025 Daily Rates; USPS Priority Mail; USPS Flat Rate quick reference.

Grinding sludge is carbide too, and we buy it, but it travels differently from solid tools. A 2023 paper by Pacini and colleagues notes that WC-Co (tungsten carbide and cobalt) grinding sludge often needs chemical recycling because it holds oil, refrigerants and other contaminants, and that wet grinding commonly runs a water-oil emulsion as coolant. That liquid is what makes a shipment of sludge a different question.

• Drain the free liquid and keep the sludge in a sealed, leak-proof container with a lid that locks or is taped down, bagged inside its box.
• Mind the dust. Breathed in, cobalt metal dust may scar the lungs and sensitize airways and skin, OSHA warns, and the dust can form explosive mixtures in air. Keep the lid on so dried fines cannot blow out.
• Tell us the coolant or oil it was ground with, on the form or by email, before anything ships.

The carriers put the burden on the sender. USPS says the mailer, not the Post Office, must identify a material's flashpoint, toxicity or other hazards before deciding it can be mailed, and a package holding liquid must be marked as liquid on the outside, with orientation arrows. FedEx Ground takes hazardous materials only from shippers it approved beforehand. Whether a particular sludge counts as hazardous depends on its oil or solvent, which we cannot judge from here, so ask us first. In California, Arizona, Utah and Nevada, the simplest answer for sludge is usually a pickup; see sludge, grindings and powder.

Sources: Pacini et al. 2023, Materials 16(4):1347; OSHA: cobalt dust health hazards; USPS Publication 52, section 323; USPS DMM 601: mailability; FedEx: how to ship hazardous materials.