Why is shell resistance on the Medical anti-static precision instrument case set near 1×10⁵ Ω?

With 12% carbon-black masterbatch the shell lands between 1×10⁴ and 1×10⁶ Ω, so the middle of that band is taken as reference; below 8% loading insulating patches appear, above 16% thin walls short-shot.

Why is the insert two decades above the shell?

The insert bleeds charge slowly through a continuous 6 mm layer into the shell; too fast a discharge drives a surge in dry air, and too slow a one lets charge pool inside the pockets.

How much drift is acceptable after 200 ethanol wipes?

After 2 h of rest, drift must stay inside half a decade; chlorine disinfectant whitens the surface at 50 wipes and lifts resistance to 1×10⁸ Ω, which retires the case from clean areas.

How does the 0.4 mm closing gap relate to dust fallout?

An opening gap lets dust ride the airflow, so fallout is held under five particles per square decimetre, and a gap grown to 0.6 mm after 3000 cycles calls for a new gasket.

Why fit a 1 MΩ resistor at the earth terminal?

The resistor limits current on accidental contact while still allowing static to bleed off within 2 s; oxide on the ring can lift 0.1 Ω past 2 Ω at the six-month check.

Medical anti-static precision instrument case

Quick answer: A Medical anti-static precision instrument case holds surface resistance near 1×10⁵ Ω on the shell against 1×10⁷ Ω on the insert, keeps drift under half a decade after 200 wipes with 75% ethanol, and stays below 0.1 Ω from earth terminal to shell through a 1 MΩ resistor.

Shell resistance band of the Medical anti-static precision instrument case
Discharge path through the insert
Surface stability after disinfectant wiping
Sealing and gaps for clean transfer
Earth terminal and potential equalisation
FAQ

1. Shell resistance band of the Medical anti-static precision instrument case

The compound carries 12% carbon-black masterbatch in a PP base, which puts shell surface resistance between 1×10⁴ and 1×10⁶ Ω, with 1×10⁵ Ω taken as the factory reference. Below 8% loading the masterbatch breaks into strings and leaves insulating patches above 10⁹ Ω; above 16% the melt index drops, thin walls short-shot, and the colour darkens enough to hurt identification in a clean area. Resistance is read at five points by four-probe method — lid centre, both long sides, base and beside the handle mount — and the highest reading may not exceed the lowest by more than five times.

Static build-up is capped at 100 V: a nylon cloth rubs the shell 20 times, the meter must stay under 100 V, and decay from 100 V to 10 V must finish inside 2 s. Those two figures track handling better than resistance does, because a case dragged across a trolley charges immediately.

Item	Band	Verdict
Shell, five points	1×10⁴–1×10⁶ Ω	Max/min ≤5
Insert	1×10⁶–1×10⁹ Ω	Discharge not blocked
Earth terminal	≤0.1 Ω	Continuous to shell

2. Discharge path through the insert

The insert is anti-static EPE held between 1×10⁶ and 1×10⁹ Ω, two decades above the shell, so charge bleeds slowly into the shell rather than dumping at once. A foam that discharges too fast can drive a momentary surge in dry air, which is worse for sensitive parts, while one that discharges too slowly lets charge pool in the pockets. A continuous 6 mm layer runs between pocket wall and base as the route to the shell, and once that layer is cut the insert becomes an island.

Medical anti-static precision instrument case standing open, grey shell with blue divided tray under clear lids, black foam pockets on the other half

No conductive coating is applied, because a coating worn away by repeated handling jumps several decades locally and reads worse than bare foam. EPE density is set at 30 kg/m³; at 45 kg/m³ the anti-static agent migrates more slowly and surface resistance creeps toward 1×10¹⁰ Ω within six months.

3. Surface stability after disinfectant wiping

Two hundred wipes with 75% ethanol, one direction, 30 cm per stroke, 10 s between strokes for the solvent to flash off, then 2 h of rest before resistance is read again: drift must stay inside half a decade. Chlorine disinfectants attack the carbon-black system, whitening the surface after 50 wipes and pushing resistance from 1×10⁵ Ω to 1×10⁸ Ω, and a whitened patch powders under a fingernail — such a case should not return to a clean area. Hydrogen peroxide sits between the two, drifting about 0.8 decade over 100 wipes with no visible dulling.

Wiping trials run on flat panel only. Corners and the mouth land are left out because flow is most disturbed there and masterbatch distribution is least even, so a reading from those spots cannot stand for the whole case.

Disinfectant	Wipes	Resistance change
75% ethanol	200	≤0.5 decade
Hydrogen peroxide	100	About 0.8 decade
Chlorine based	50	Rises to 1×10⁸ Ω

4. Sealing and gaps for clean transfer

Spray testing follows an IP65 benchmark, with a double land at the mouth: the outer land stops water, the inner land stops dust, and a 1.5 mm relief groove sits between them. Closing gap between lid and shell is held under 0.4 mm, since dust rides the airflow once the gap opens, and fallout is kept under five particles per square decimetre. Every outside corner runs at R3 or more with no square grooves or blind holes, so wiping leaves no dead corner, and the hinges are concealed with no exposed pin to collect dust. The gap is re-read after 3000 open-close cycles, and a growth to 0.6 mm calls for a new gasket.

Medical anti-static precision instrument case reference shell, small dark green case open on a timber bench, orange handle, two metal catches, hinges exposed

Gap and land are judged separately: the gap describes how lid meets shell, while the land describes whether both steps are intact, and a chipped land lets dust through however tight the gap looks.

5. Earth terminal and potential equalisation

One M4 stainless earth terminal sits in the side of the case, its seat moulded in one shot with the shell and machined to a 0.8 mm ring, tightened to 1.2 N·m. Resistance from terminal to any point on the shell stays under 0.1 Ω, read with a milliohm meter between terminal and base centre. A 1 MΩ resistor sits inside the terminal to limit discharge current on accidental contact while still letting static bleed away within 2 s.

The terminal is re-checked every six months, and the screw comes out to inspect the ring for dark oxide, which lifts resistance from 0.1 Ω past 2 Ω. The resistor is not removed on its own; it is replaced together with the terminal.

Conclusion

The Medical anti-static precision instrument case ties 1×10⁵ Ω shell resistance, a 1×10⁷ Ω insert, a 6 mm continuous discharge layer, half a decade of drift after 200 ethanol wipes, a 0.4 mm closing gap and 0.1 Ω at the terminal into one static and cleanliness chain on the case side. The six readings sit on the shell, the insert, the wipe bench, the spray rig and the milliohm meter, and none replaces another. The shop keeps 12% masterbatch, 30 kg/m³ insert density, 1.2 N·m torque and the six-month interval on one parameter sheet. All of it covers shell and insert specification only.

This article discusses the case side of the specification only and does not cover handling, loading or use of anything carried inside.