Can I read wall thickness off a load table?

Answer: Only as a start. The 3.2 mm row assumes 42 kg over 380 mm; at 440 mm the same load needs 3.6 mm.

What press does a 560 mm cavity need?

Answer: 0.233 m2 at 35 MPa comes to 815 tonnes, so the 1000 tonne band; one band lower leaves 0.3 mm flash and 25 s of trimming.

Do more ribs mean a stiffer PP Plastic Case?

Answer: No. Ribs over 0.7 of wall leave a 0.2 mm sink outside, and pitches under 60 mm chill the melt before the rib fills.

How many latches should a case carry?

Answer: Load and length together. 42 kg over 560 mm takes 2; 60 kg over 640 mm takes 3, the centre pulling 40 percent.

What wall tolerance belongs on the drawing?

Answer: Plus or minus 0.15 mm, matching the 0.2 percent swing in PP shrinkage. Tighter adds 13 s of cycle.

Why do two quotes differ by 18 percent?

Answer: Wall and rib strategy, not resin price. 3.2 mm with 4 ribs weighs 2.4 kg; 2.8 mm with none weighs 2.1 kg and dents after 3 drops.

One Drop Test Line, Then the PP Plastic Case Wall Thickness

Quick answer: Specify a PP Plastic Case backwards from acceptance. A 1.2 m drop at 42 kg into a 560 mm cavity puts the 380 mm span at 3.2 mm of wall and a 90 mm rib pitch.

Working Backwards from One Line in the Test Report
Cavity Size Sets Projected Area and Clamp Force
Rib Pitch and Height Fill the Stiffness Gap
How Latches and Hinges Load the Wall
Turning the Chain into Inspectable Callouts
FAQ

1. Working Backwards from One Line in the Test Report

A test report carries one line: full load, 1.2 m, one drop on six faces, three edges and one corner, no rupture, no latch release. That line must become deflection before a wall is named, since on a PP Plastic Case the wall is an output.

A PP Plastic Case cavity of 560 by 380 by 210 mm sets the rest. At 42 kg the base spans 380 mm, homopolymer bends near 1500 MPa, a drop triples the load to 126 kg on 0.213 m2, and 2 mm of deflection returns 3.2 mm.

Load	Cavity length	Base span	Wall	Rib pitch
30 kg	460 mm	320 mm	2.8 mm	105 mm
42 kg	560 mm	380 mm	3.2 mm	90 mm
60 kg	640 mm	440 mm	3.6 mm	80 mm

2. Cavity Size Sets Projected Area and Clamp Force

Fix the cavity and projected area follows: 560 by 380 mm with 3.2 mm of wall and 1.5 percent shrinkage projects 0.233 m2. At 35 MPa the clamp needs 815 tonnes, so this PP Plastic Case is moulded in the 1000 tonne band.

PP Plastic Case in olive green lying on a grey steel shelf, two latches and a side handle visible, scuffed shell, white factory walls behind

Melt flow enters only here: flow ratio is longest path over wall, so a short-edge gate puts 470 mm at the far end, giving 147 and calling for 10 to 14 g/10 min. Tie bars 820 mm apart cap the PP Plastic Case mould at 780 mm, and a starved clamp leaves 0.3 mm of flash.

3. Rib Pitch and Height Fill the Stiffness Gap

Ribs bridge the gap between calculated wall and surviving wall: across 380 mm at 3.2 mm a bare panel sags 5 mm under 42 kg, and 4 ribs cut that to 1.6 mm.

Three numbers set geometry: rib thickness at 0.6 of wall, 1.9 mm; height under 3 times thickness, 5.7 mm; draft from 0.5 to 1 degree. A root fillet of R0.5 mm pulls stress concentration from 2.4 to 1.6; without it a PP Plastic Case cracks by the second drop.

Location	Wall	Rib thickness	Rib height	Draft
Base, longitudinal	3.2 mm	1.9 mm	5.7 mm	0.5 deg
Side wall, transverse	3.2 mm	1.9 mm	5.0 mm	0.8 deg
Lid stiffeners	2.8 mm	1.7 mm	4.6 mm	0.5 deg

A 90 mm pitch rests on that plate model: 1.4 mm of sag at 42 kg against 2.8 mm at 120 mm, past the 2 mm limit.

4. How Latches and Hinges Load the Wall

A latch is the only route by which lid load returns to the body. With 42 kg aboard, 2 latches and one pin, each pulls about 210 N in a drop, so wall under 2.6 mm at the seat tears, and on a PP Plastic Case the seat is thickened to 3.6 mm.

PP Plastic Case in navy blue open on a steel bench, egg-crate foam in the lid, cut foam trays holding modules, orange racking behind

The pin is 5 mm of 304 stainless in a 4 mm bore with 0.15 mm clearance; at 0.3 mm the lid sags 1.2 mm after 5000 cycles. A PP Plastic Case seals on the same chain: a 6 by 4 mm groove holds a 5.5 mm cord at 27 percent compression, and at 18 percent IP65 leaks in 3 minutes.

Part	Key dimension	Load	Typical failure
Latch seat	3.6 mm local wall	210 N	Tear at the root
Hinge pin	5 mm diameter	180 N	Bore wear
Gasket groove	6 mm wide, 4 mm deep	27 percent squeeze	Leak at the seal

5. Turning the Chain into Inspectable Callouts

Four callouts go on the drawing of a PP Plastic Case: wall 3.2 mm plus or minus 0.15 mm, flatness 0.4 mm, weight 2.4 kg plus or minus 3 percent, latch force 60 N plus or minus 10 N.

At plus or minus 0.08 mm the cycle stretches from 55 s to 68 s and costs 1.9 CNY more; at plus or minus 0.25 mm the seat misses by 0.4 mm. Sampling runs 5 parts per batch at 9 points in 210 s on patrol.

Conclusion

One 1.2 m drop line yields a wall, a rib pitch, a latch count and a tolerance set: load, cavity, wall, hardware. With four callouts on the drawing the shop can reproduce the calculation, and a PP Plastic Case closes cleanly after 3 drops.

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.