Is a one-piece moulded case always better sealed?

A: In structure yes, in practice not always. The 0.15 mm groove depends on insert accuracy, and once that insert wears to 0.4 mm the moulded body leaks too.

Can a leaking assembled body be repaired on site?

A: Welded and glued seams cannot; a patch will not pass immersion. The screw-and-gasket version opens, takes a new strip and goes back to 1.2 N·m.

Why does a one-piece case still have a weld line?

A: Any cavity that splits the melt and rejoins it leaves one, here behind the latch ear at 65% of bulk strength, so the line sits where load is low.

How do I check a supplier's claim quickly?

A: Feel for a step on the inner rim, count the parting loops, weigh two cases of equal outline. If the three disagree, the body is not one-piece.

One-Piece vs Assembled Case Bodies: Where Seals Fail

Quick answer: A one-piece moulded case is shot as two single parts, lid and base, so the gasket groove, latch seats and hinge ears form in the same cavity as the walls and the rim carries no joint. An assembled body welds, glues or screws separate panels together, and each joint is a route for water. On a 520 mm body the moulded groove floor holds 0.15 mm flatness against 0.5 to 0.7 mm assembled, worth nine points of gasket compression in the same 4.5 mm groove.

Where the Joint Disappears
Three Joints, Three Leak Paths
Groove Flatness: 0.15 mm vs 0.6 mm
From -20 °C to 60 °C
Five Checks Without Cutting a Case
When an Assembled Body Wins
FAQ

1. Where the Joint Disappears

The melt fills one cavity and the lid comes out whole; the base follows. Gasket groove, latch bosses and hinge ears are formed in that single shot, so nothing is joined afterwards. A 520 × 380 × 210 mm body runs on a 1,600 t press at 68 s a shot, and the parting line sits at mid-height with a step under 0.08 mm.

Sealing rests on three dimensions: a groove 6.5 mm wide and 4.5 mm deep, its floor flat to 0.15 mm, and a 7 mm hollow silicone cord at 28% compression. Each groove wall gives 0.3 mm, so the cord is squeezed into a rectangle rather than sheared.

2. Three Joints, Three Leak Paths

An assembled body splits lid panel, skirt and base plate into separate shots, then joins them three ways, and each method fails somewhere else.

One-piece case: sky blue body closed on white, moulded pads at both front corners, shallow finger recess, two hinge lugs at the back

Ultrasonic welding runs a 0.8 mm bead with 1.2 mm melt depth round the skirt and holds 1 m of water for 30 minutes at 92% continuity; break that bead 3 mm at a corner and water enters.

Adhesive bonding cures a 0.3 mm polyurethane layer in 24 hours, and 0.05 mg of oil per square centimetre on the bond face takes 40% off peel strength.

Screws and a gasket rework easiest: M4 screws every 80 mm at 1.2 N·m on a 3 mm EPDM strip, where over-torque punches through and under-torque leaks.

Joint method	Added cost	Where it leaks	Rework
Ultrasonic weld	3.8	Interrupted bead at corners	Re-weld only
Polyurethane adhesive	5.2	Oil and voids on the bond face	Scrap on opening
Screw and gasket	7.6	Uneven torque	Strip replaceable

3. Groove Flatness: 0.15 mm vs 0.6 mm

Moulded in one shot, the groove floor is cut into a single insert and holds 0.15 mm. On an assembled rim it lands on the joint between skirt and lid panel, two operations stack tolerances, and measurement falls between 0.5 and 0.7 mm.

That 0.45 mm splits compression into two zones: the 7 mm cord needs 28% to keep reserve, and where the floor stands high it reaches only 19%. A 1 m immersion leak almost always appears along the highest 30 mm of groove.

Groove floor flatness	Compression	1 m immersion, 30 min	Verdict
0.15 mm	28%	Dry	Pass
0.35 mm	23%	Dry, droplets on strip-down	Concession
0.60 mm	19%	2 to 4 drops in the arc	Reject

Moulding is not automatically sound. With the gate at lid centre the melt meets again behind the latch ear, and PP welds at 65% of bulk strength, so a 0.15 mm groove behind a 65% ear still breaks at the ear.

4. From -20 °C to 60 °C

PP expands at 1.5×10⁻⁴ per kelvin and PC at 0.7×10⁻⁴, so splitting the materials between lid and base leaves a 480 mm length 2.3 mm apart over a 60 K swing. Screws constrain it into 0.2 mm of slip, two thirds of a 0.3 mm adhesive layer. Once slip passes the shear limit the bond face opens and water travels along the interface. Cold embrittles the polymer, with notched impact falling from 6 kJ/m² to 2.5 kJ/m² at -20 °C. Boss stress moves too, since a boss contracts and does not return fully on warming, so the gap grows cycle by cycle. Stack those effects and the body creeps forward every cycle rather than failing once, and by cycle 25 most samples show a white stress line along the joint.

One-piece case: light green body shot from the front, front pads merging into the finger recess, two hinge lugs at the rear edge, rounded corners

Preload has its own line: 25 cycles of four hours at -20 °C and four at 60 °C cost an M4 screw 18% of its preload in a PP boss, while the washer mark grows from 0.4 mm to 0.55 mm.

A one-piece body has no joint, so the swing only expands it as a whole and change stays inside 0.7 mm. The price is the tool, where a 520 mm body needs an 800 mm mould base.

5. Five Checks Without Cutting a Case

Inspection rarely allows a section cut, and five external features separate the structures:

Parting line: one loop at mid-height, no step across it
Inner rim: a 0.2 mm step outside the groove floor means assembly, a continuous curve means moulding
Latch seat: same colour and compound, no glue smear or flash at the root
Weld witness: a 0.8 mm band inside the lid in the matching position
Weight: 0.18 to 0.25 kg heavier at equal outline from the overlapping skirt

Tapping agrees: a short note on a moulded body, a dull one trailing a ring near the joint on an assembled body.

6. When an Assembled Body Wins

The barrier is tooling and tonnage. Two moulds for a 520 mm body cost 46,000 and tie up a 1,600 t press for 68 s a shot, while five small moulds plus a 6,000 welding fixture come to 34,000, dropping tooling per unit from 2,300 to 1,700 on a 200-piece run.

A 1,200 mm body needs a 4,000 t press and tooling from 120,000, which few plants hold stable.

Condition	One-piece	Assembled
Volume	Above 1,000 units	200 to 800 units
Outline	Under 900 mm	Over 900 mm
Tooling lead time	45 days	25 days
Design change	Cut the tool, costly	Change a panel, cheap

Mixed materials force assembly anyway: a clear PC lid over a PP base cannot share a cavity.

Conclusion

The argument ends at the rim. Moulded, the groove comes off one insert at 0.15 mm flatness; assembled, it sits across two operations at 0.5 mm and beyond, compression falls from 28% to 19%, and the leak shows along the highest 30 mm. Moulding is not universal either: over 900 mm, under 200 units, or a lid and base in different materials all favour assembly.

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