How is the usable volume of a Plastic Case calculated?

Answer: Deduct two wall sections from length and width, then 12 mm of lip and 3 mm of base from the height, so 420 x 300 x 180 mm becomes 414 x 294 x 168 mm.

How much does a moulded-in handle carry?

Answer: A 6 mm root at 40 mm wide holds 30 kg static. A bolt-on handle on two M4 screws slips at 18 kg, so anything above 20 kg wants a shoulder strap.

How many latches does a case need?

Answer: Two on a 1400 mm perimeter, no more than 500 mm apart, three past 1600 mm. Hold at 220 N and open at 30 N; an over-tight latch breaks sooner.

Does IP67 need a relief valve?

Answer: On routes with temperature swings, yes. At 0.35 bar an unvented case is hard to open, while an 8 mm vent passes air one way and stays watertight the other.

What texture and colour should be specified?

Answer: A 0.15 mm pebbled texture hides 0.05 mm scratches, with 1.2 ΔE within batch and 1.5 across, on a 650 tonne press and a 500,000 shot tool.

Plastic Case Selection: Five Criteria for the Closing Face

Quick answer: Judge a Plastic Case from its closing face, not its outline: usable volume, wall with corner radii, the load path through handle and hinge, latch count against holding force, and the sealing grade with pressure relief. The first four settle on one sample; the last decides whether it can travel in rain.

Five Criteria for Plastic Case Selection
Closing Face and Usable Volume
Handle and Hinge Load Paths
Latch Count and Holding Force
Sealing Grade and Pressure Relief
FAQ

1. Five Criteria for Plastic Case Selection

A Plastic Case differs from an open tote at the closing face, where lid, lip and gasket decide both water resistance and how the case opens. Compare outlines first and the rework lands on the latches.

Order matters: usable volume first, because the lip steals depth; then wall and radii, which decide where a drop splits; then the handle and hinge path, then latch clamping, and sealing last.

Criterion	Measure	Line to hold	Usual mistake
Closing face	Cavity and lip depth	Lip under 12 mm	Outline read as cavity
Wall and corners	Wall and radii	2.8 mm, R12	Thickest point only
Handle and hinge	Root and pin size	30 kg static	Judging by feel
Latches	Count, holding force	220 N	One latch on a long case
Seal and relief	Gasket and valve	IP67 with valve	Valve missing

2. Closing Face and Usable Volume

A cavity quoted at 420 x 300 x 180 mm loses 12 mm to the lip, leaving 168 mm of height, and a 2.8 mm wall shaves the plan to 414 x 294 mm.

Plastic Case: red shell with black overmoulded corners and a black lid, handle up, two black latches closed at the front

A tongue-and-groove Plastic Case lip uses a 3 mm tongue, holds a 0.5 mm fit and resists twisting. R12 outside with R6 inside is the tightest radius that fills at 2.8 mm; below R5 the corners whiten and split in a drop.

3. Handle and Hinge Load Paths

The handle carries the whole Plastic Case. A moulded-in handle 6 mm thick and 40 mm wide holds 30 kg static; a bolt-on handle on two M4 screws slips its threads at 18 kg.

A 4 mm stainless hinge pin rated at 50,000 cycles needs 0.15 mm of clearance per side; more and the lid rocks. A 3 mm triangular rib from handle root into the wall spreads the pull, or the crack appears in the lid beside it.

4. Latch Count and Holding Force

Latch count on a Plastic Case follows the perimeter: two on 1400 mm spaced no more than 500 mm apart, and three past 1600 mm, or the lip gapes mid-span and the gasket gives way first.

Plastic Case: a pale grey case tilted above a dark reflective floor, side handle and two latches facing the camera

Hold 220 N and open at 30 N for one-handed use. The seat fails before the latch body: 3.5 mm with an R2 root survives 50,000 cycles, while 2.5 mm cracks on the third cold drop.

5. Sealing Grade and Pressure Relief

IP67 puts the case in a metre of water for 30 minutes, which needs a 50 Shore A O-ring at 25 percent compression; past 40 percent the rubber takes a set and leaks on the second closing.

A relief valve is the part most often deleted: at 0.35 bar an unvented Plastic Case is hard to open after a high-altitude leg. Pins and springs take 72 hours of salt spray, and one rusted pin wears a 0.15 mm bore out to 0.4 mm.

Grade	Build	Suitable transfer
IP54	Flat lip, no gasket	Indoor, inside a vehicle
IP65	Tongue-and-groove, one gasket	Short open-yard runs
IP67	Two gaskets, relief valve	Rain, sea freight, air
IP67 locked	Two gaskets, padlock eyes	Long routes, handovers

Conclusion

The five criteria form a chain: closing face sets volume, radii set the drop result, handle and hinge set life, latches set closure, sealing sets the transport boundary. Loosen one link and the sample still passes on a bench, then fails in rain or at altitude. Write the five figures into the drawing and the inspection sheet. Choosing a Plastic Case means fixing those numbers on the closing face first.

This article discusses only the container (case body) side of specifications and design intent, and does not cover any handling, loading or use of the contents.