How does an engineering plastic case differ from a crate?

A crate uses recycled PP at about 2 mm and cracks in a −10 °C drop; an engineering plastic case starts at 3.2 mm with ribs and a rated gasket.

Is a thicker wall always better in a drop?

No. Going from 3.2 to 4.5 mm adds sink marks, not strength. An R8 corner with an internal gusset beats brute thickness.

How many latches does an engineering plastic case need for IP67?

Two under 450 mm, three at 565 mm, and three plus a transverse rib past 700 mm, each at 250 N.

Which grade suits an engineering plastic case below zero?

PC/ABS above −20 °C, block PP at 4 mm below −30 °C, HDPE for −40 °C transport.

Can 300 units justify a tool for an engineering plastic case?

Yes, through a shared frame with inserts, near 60% of a dedicated tool, in about 35 days.

How to Choose an Engineering Plastic Case: 5 Criteria

Quick answer: Run five criteria in order: resin for the low-temperature limit, wall and ribs for impact, gasket and latches for the rating, hardware for service life, foam and volume for cost. The step buyers skip is the wall-to-rib ratio, where cracked corners begin.

The Five Selection Criteria for an Engineering Plastic Case
Resin Grade and Wall Thickness: Cold and Impact Trade-offs
Gasket and Latches: Turning a Rating into Numbers
Hardware and Stacking: Hinges, Handles and Rib Layout
Foam, Colorant and Batch Economics
FAQ

1. The Five Selection Criteria for an Engineering Plastic Case

An engineering plastic case is not specified by whoever moulds the thickest wall; five criteria lock in sequence, ordered by rework cost.

Criterion	Check first	Failure when wrong	Rework cost
Resin grade	Low-end temperature	Corner cracking in a cold drop	Scrap lot, new tool
Wall and ribs	Drop height and mass	Sagging lid, bulging side	Tool rework
Gasket and latches	Rating, cavity length	Water beads inside	New gasket, insert
Hardware	Cycles and load	Rusted pin, pulled handle	Supplier swap
Foam and volume	Layout, order size	Misaligned slots, overrun	Secondary rework

2. Resin Grade and Wall Thickness: Cold and Impact Trade-offs

The workhorse is a PC/ABS alloy: 45 kJ/m² notched Charpy, 105 °C heat deflection, so a 3.2 mm wall survives a 9 kg weight dropped 1.2 m into a corner.

Engineering plastic case: black foam strip along the lid rim, brushed aluminium corner extrusion on the box corner

Block PP stays ductile at −30 °C but needs a 4.0 mm wall; rotomoulded HDPE runs 6–8 mm, yet a 60 L body is 11 kg empty.

Grade	Impact	Temperature	Main wall	Suited case
PC/ABS	45 kJ/m²	−20 to 90 °C	3.2 mm	Instruments, equipment
Block PP	8 kJ/m²	−30 to 80 °C	4.0 mm	Cold chain, outdoor
HDPE	No break	−40 to 60 °C	6–8 mm	Large volume, heavy
PA66 + 30% GF	12 kJ/m²	−30 to 150 °C	2.8 mm	Hot workstations

Ribs matter more than wall: a 3.2 mm face stepped to a 2.4 mm root on a 110 mm grid lifts section modulus about 40% for 6% more weight.

3. Gasket and Latches: Turning a Rating into Numbers

Sealing rests on compound, compression and groove depth: a 55 Shore A EPDM cord in a 2.6 mm groove lands at 25–30% compression.

IP67 is 30 minutes at 0.5 m without ingress; IP65 is a 12.5 L/min jet for 3 minutes. Past 450 mm an engineering plastic case needs a third latch, each pulling 250 N.

4. Hardware and Stacking: Hinges, Handles and Rib Layout

A 304 hinge pin shows no red rust after 96 hours of salt spray; a zinc-plated carbon pin spots at 48 hours and seizes within three months. Size the handle for 1.5 times the load.

Engineering plastic case: two black cases on a workbench, a closed one with a wide latch in front and an open one showing foam cut-outs behind

Stacked five high, the bottom unit carries 72 kg, so lid flatness must hold within 0.8 mm or the second tier rocks.

5. Foam, Colorant and Batch Economics

PU at 30 kg/m³ suits light parts, EVA at 45 kg/m³ sharp metal, XPE at 25 kg/m³ is cheapest but sheds crumbs; inserts use a 20 mm base pad.

Each extra point of colorant costs about 2% of notched impact. A 0.31 m² projected area needs 1085 t of clamp at 35 MPa, so a 1300 t press cycles in 78 s.

Size	Outside	Inside	Wall	Empty
Small	420 × 330 × 180 mm	380 × 290 × 155 mm	2.8 mm	2.6 kg
Medium	565 × 390 × 215 mm	520 × 345 × 190 mm	3.2 mm	4.8 kg
Large	700 × 480 × 320 mm	650 × 430 × 290 mm	3.6 mm	7.9 kg
Extra large	900 × 600 × 420 mm	845 × 545 × 385 mm	4.2 mm	13 kg

Conclusion

Resin first, then wall and ribs, then gasket, hardware and foam. Get the resin wrong and every later step is wasted; get the rib ratio wrong and corners crack first. Choosing an engineering plastic case is arithmetic done before the tool is cut.

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.