What separates an engineering plastic box from a case?

Cavity and wall. A box stays under 20 L at a 2.2 mm wall with a carry handle; a case passes 30 L at 3.2 mm to stack and travel.

Is a 2.2 mm wall enough on an engineering plastic box?

With lid ribs, yes: flat 2.2 mm deflects 3.8 mm under 20 kg, ribbed it holds 1.5 mm, and without ribs the wall goes to 3 mm.

How long does a living hinge last?

About 10000 cycles, a year at 20 opens a day; below −10 °C, a steel pin hinge gives 30000.

Should compartments be moulded or cut from foam?

Mould above 2000 units a month with a fixed layout, cut foam for changing ones; shallow pockets under deep slots.

Can a 500-piece order change colour?

Yes, at 1.5% masterbatch; purging takes 8 kg, adding 0.3 CNY per unit at 500 pieces.

How to Choose an Engineering Plastic Box: 5 Criteria

Quick answer: An engineering plastic box is specified from the inside out: cavity first, then a 2.2 mm wall with lid ribs, then hinge and catch life, foam density and order volume. Get one wrong and the lid will not seat or the pockets will not line up.

How an Engineering Plastic Box Differs from a Case
Cavity and Compartment Layout: Content First, Outline Second
Hinges and Catches: Life Numbers for Small Parts
Lid Stiffness and Stacking: Controlling Thin-Wall Deflection
Foam Density, Colorant and Minimum Order
FAQ

1. How an Engineering Plastic Box Differs from a Case

The order reverses against a case: a case starts with resin, an engineering plastic box with the cavity, because the contents are fixed before the drawing opens.

Five criteria follow: cavity, wall and lid ribs, hinge and catch, foam, colorant and minimum order. The sharpest split is wall: 3.2 mm on a case against 2.2 mm on a box, the saving spent on portability and repaid in ribs.

Criterion	Box	Case	Failure when wrong
Cavity first	Layout drives outline	Outline drives layout	Pockets misaligned
Main wall	2.2 mm	3.2 mm	Lid sags, sides flex
Hinge	Living PP hinge	Steel pin hinge	Stiff or cracked hinge
Liner	Cut foam pockets	Solid foam pad	Contents stick
Minimum order	500 units	300 units	Tool cost unabsorbed

2. Cavity and Compartment Layout: Content First, Outline Second

An engineering plastic box cavity is derived from the contents: a 280 mm torque wrench, a 150 mm caliper and ten hex keys fill 290 × 220 mm with clearance, so the cavity becomes 300 × 230 mm and the outline 340 × 270 mm.

Engineering plastic box: open black box on white, foam cut-outs holding a torque wrench, hex keys and a digital caliper, two lid ribs

Height follows the tallest item: a 45 mm wrench head plus 8 mm of foam above and below gives a 78 mm cavity and a 95 mm outline. Moulded compartments suit fixed layouts above 2000 units a month at 3 mm walls and 1° draft; cut foam changes in days.

3. Hinges and Catches: Life Numbers for Small Parts

A living PP hinge folds on a thin web: 10000 cycles at 0.35 N·m, cheap and assembly-free, but it whitens and cracks below −10 °C.

A 3 mm stainless pin hinge reaches 30000 cycles, and 60 N catches hold an engineering plastic box lid through a 1 m drop.

Type	Life	Force or torque	Use
Living PP hinge	10000 cycles	0.35 N·m	Ambient, low cycle
Steel pin hinge	30000 cycles	0.4 N·m	High cycle, cold
Two side catches	8000 cycles	60 N	Carrying by hand

4. Lid Stiffness and Stacking: Controlling Thin-Wall Deflection

The lid is where an engineering plastic box fails first. A flat 2.2 mm panel deflects 3.8 mm at the centre under 20 kg; a 45 mm grid of 3 mm ribs pulls that to 1.5 mm on the inside face.

Engineering plastic box: open black box on a wooden bench, empty foam compartments, a pressure valve and two catches in front

Stacked eight high at 3 kg, the bottom unit carries 24 kg through four corner pads, and a 1.0 m six-face drop calls for 0.3 mm of extra corner wall.

5. Foam Density, Colorant and Minimum Order

EVA at 45 kg/m³ sheds no crumbs and suits metal tools, PU at 30 kg/m³ conforms to odd shapes, EPE at 25 kg/m³ is cheapest but furs up; keep 6 mm between pockets.

Colorant sits at 1.5%: light shades need that much to kill the yellow cast, and excess leaves flow marks on a thin wall. A 380 t press with two cavities cycles in 42 s; a 120000 CNY tool costs 240 CNY per unit at 500 against 24 CNY at 5000, so share a frame below that.

Liner	Density	Process	Suited engineering plastic box
EVA	45 kg/m³	Die cutting, lamination	Metal tools, gauges
PU	30 kg/m³	Direct foaming	Odd shapes, instruments
EPE	25 kg/m³	Hot wire, splicing	Light parts, one-way transit
XPE	33 kg/m³	Die cutting	Frequently handled fittings

Conclusion

Cavity first, then wall and ribs, then hinge, catch, foam and order volume. Get the cavity wrong and nothing fits; skip the ribs and the lid sags; pick the wrong hinge and it cracks in a cold store. Work the five criteria in order and an engineering plastic box holds both feel and cost.

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.