Is a thicker wall better for shielding?

A: No. In an EMI shielding case, wall thickness adds very little absorption, and going from 4 to 8 mm often moves the reading by a decibel or two. Surface conductivity, seam bonding and aperture size set the result, so the same material spent on a rebate returns more.

How can a plastic case shield at all?

A: By secondary treatment: conductive paint inside, thermal spray or a fabric liner, turning an insulating surface into a conducting one. The work is finished only when lid, body and interface plate are bonded to each other, otherwise the finish is decorative and measures close to nothing.

The seam is under a millimetre, so why treat it?

A: Because leakage scales with seam length, not width. A long thin slit behaves as a slot antenna and can pass more than a round hole. Fold the joint into a rebate so the gap turns a corner in depth, then clamp a gasket along its length.

Where should the ground strap attach?

A: To the conductive layer, not to paint or a latch. A stainless stud with a toothed washer bites through oxide, bonding resistance stays under 10 milliohms between terminal and shell, the braid stays short, and every bond runs to one equipotential body rather than in series. ---

What an EMI Shielding Case Is: Shielding, Grounding and Sealing

Quick answer: An EMI shielding case is a rotomolded box whose surfaces conduct, whose seams close and whose shell is bonded to ground. Performance comes from a continuous conductive surface, not from wall thickness, since extra polyethylene adds almost no absorption while one unbonded seam erases tens of decibels. Seams, apertures and cable entries are the three paths that leak, and grounding turns the shell into a low-impedance return. Miss any of the three and an EMI shielding case is only a plastic box with a metallic finish.

Definition: a continuous conductive surface
Shell: applying the layer and keeping it continuous
Three breaks: seams, apertures, cable entries
Grounding: a low-impedance return
Deciding when the premium pays
FAQ

1. Definition: a continuous conductive surface

An EMI shielding case separates inner fields from outer ones with a conductive shell. Its figure of merit is shielding effectiveness, the ratio of field strength without the EMI shielding case to that with it, in decibels. Surface conductivity, seam bonding and aperture size set that figure, not wall thickness.

Open case showing rim rebate and gasket groove

Polyethylene is an insulator, so untreated it blocks almost nothing. An EMI shielding case therefore means a coated, metallised or fabric-lined shell, not a plastic box with a new name. From one mould, coated and uncoated leave the shop as two products.

2. Shell: applying the layer and keeping it continuous

Three routes are common on an EMI shielding case: conductive paint inside, thermal metal spray and conductive fabric liners. Filled paint reaches 50 to 100 microns and under 0.1 ohm per square. Sprayed metal wears better but runs hot.

Silver-grey conductive coating over cavity ribs

Continuity matters more than thickness. A rotomolded part has shallow draft and deep cavities, so the gun thins out in recesses, latch seats and handle roots, where current turns a corner. Mask threads before coating, or the shell leaks.

3. Three breaks: seams, apertures, cable entries

The seam has the longest perimeter, and leakage follows length rather than width: a 300 mm slit 0.2 mm wide can pass more at gigahertz than a 10 mm hole, acting as a slot antenna. A stepped rebate folds the joint once and adds about ten decibels.

Draw latch clamping the rim beside a ground stud

Apertures cover vents, windows and ports. A hole stops passing energy near a twentieth of a wavelength, so ventilation needs honeycomb mesh. Cable entries are forgotten most: one untreated power lead can pull an EMI shielding case down to 20 dB.

Break	Why it leaks	First loss at	Case-side fix
Lid seam	longest perimeter, acts as a slot	above 300 MHz	stepped rebate plus gasket
Vents and windows	hole near wavelength, no cutoff	above 1 GHz	honeycomb or finer mesh
Cable entry	conductor uses the shell as an antenna	one lead drags the case	shielded gland and filtering
Ground bond	paint and oxide block the path	low-frequency drain	toothed washer onto coating

4. Grounding: a low-impedance return

In an EMI shielding case, grounding is not a lightning accessory but one leg of the shielding circuit. Current picked up by the shell needs a low-impedance path away, or a potential difference builds and radiates inward. Bonding resistance stays under 10 milliohms.

Case bonded to an equipotential bar in a store room

Bond onto the conductive layer, never onto decorative paint. A stainless stud with a toothed washer bites through oxide. Copper against aluminium corrodes when damp, so a plated transition goes between, and above a few megahertz every bond runs in parallel.

5. Deciding when the premium pays

Three situations justify an EMI shielding case: sensitive electronics inside, an emission or immunity limit, or a stated leakage requirement. Each must resolve into a band and a decibel figure, or a standard protective case will do.

Metal cases shield well but are heavy and dent. Moulded conductive plastics hold tolerance and stop at the mould size. Rotomolding with a coating wins on size, impact and corrosion, and loses on labour.

Conclusion

Judge an EMI shielding case in order: band and decibels first, the three leak paths second, the ground reference last. The first decides whether the work is needed, the second how well it is done, the third whether it survives on site. Grounding is usually left to the end and wastes everything before it, because a shell with no path under 10 milliohms keeps its charge.

This article covers container-side specifications and design implications only, not the handling, loading or use of the contents.