Crab Claws Through the Film: Puncture Testing That Matches the Load

Seven packs out of 600 leaked on a single consignment of live crab from Cà Mau to Shanghai. The film met its spec, the seals were sound, and every failure was a crescent-shaped hole about 4 mm across on the face of the pack that had been facing down. The crab had done it in the first two hours, before the gel was even cold enough to matter.

Puncture is the failure mode nobody tests for, because the standard tests — seal strength, drop, compression — all assume a smooth or rounded contact surface. Shellfish are not smooth. Neither are frozen fish tails, bone-in lamb, or the corner of a stainless tray.

What actually penetrates a pack

A 70 µm film has a puncture resistance that is measured in newtons, and a shrimp rostrum applied to a 0.5 mm point concentrates force enough to exceed it at a load you can generate with a thumb. Frozen crab claw tips are harder still. The load does not have to be large; it has to be concentrated.

The two variables that matter are the radius of the contact point and whether the film is cold. A film at -18 °C is 40-60 percent less resistant to puncture than the same film at 20 °C, because the polymer chains lose mobility. So the worst case is a sharp contact plus a cold pack plus a box that gets vibrated for six hours on a truck.

The test you can build in a warehouse

Take ten filled packs from a production batch, condition them to the temperature they will ship at, and place them in a box with 2 kg of the actual product on top — shells, claws, tails, whatever the load is. Close the box and run it on a vibration table for two hours if you have one, or strap it to a pallet in a moving truck if you do not. Open it and count holes. Then submerge each pack in water for ten minutes and look for a wet line inside the film.

That test costs an afternoon and finds the failures that a lab puncture test misses, because it uses the real load at the real temperature with the real compression.

The counter-intuitive result on film thickness

Going from 70 µm to 100 µm of the same blend made things worse in one trial: 11 failures against 7. The thicker film was stiffer, so it did not conform around a claw tip; it bridged it, took the load on a smaller area, and cracked. A 70 µm coex film with a nylon core layer — same total thickness, different structure — dropped failures to two in 600.

Thickness is not toughness. What you want is a film that stretches before it yields, with a ductile core that absorbs the point load rather than transmitting it to a brittle outer skin.

Design fixes that beat a better film

• Put a 3 mm corrugated pad between the pack and the shell-on product; cheapest fix by a wide margin.
• Never place a pack face-down in direct contact with shell-on product in a box that will be stacked.
• Bag the product in a liner so claws cannot migrate into the gel zone during handling.
• Specify a rounded or edge-sealed pack; a fin seal on the face is a stress riser.

The economics are stark. A puncture costs the pack ($0.60), the carton ($1.40), the labour to re-pack, and often the product. Seven failures in 600 is a 1.2 percent rate that looks trivial until you multiply it by a weekly volume and add the customer conversation.

Run the vibration test with real shells before you approve any film change, and keep the failing packs. Photographs of a crescent hole with the shell that made it are the most persuasive argument you will ever have with a film supplier. When you spec the pack, ask for the puncture figures at -18 °C rather than at room temperature — the SAP powder range page lists the film options by service temperature.