Metal Detector Cases: Search Head & Conveyor Protection

A metal detector is one of the few machines on a food, pharmaceutical or personal-care line that trades microvolt-level signals for inspection performance. Inside the search head, the transmitter coil, the receiver coil and the oscillator plus phase-demodulation circuitry form an extremely narrow balanced window. The sensitivity that was calibrated at the factory with Fe 1.0 to 1.5 mm and SUS 2.0 mm test pieces rarely survives a drop, a localised squeeze, or an afternoon spent leaning against a steel rack. JUNZHIJIA works from a simple premise: the job of a metal detector parts case is not to hold the components, it is to move the head's electromagnetic balance, the belt's geometry and tension, and the electronics' electrostatic and vibration state from point A to point B unchanged. The shell is only the envelope; the liner zoning, non-magnetic contact parts and static-dissipative design are what actually decide the outcome. This article covers four component groups, the search head assembly, the conveyor and idlers, the controls and signal unit, and the test pieces and accessories, and gives a packing scheme that can be written straight into a procurement technical agreement.

Metal detector relocations and overhauls tend to happen under pressure: the line schedule is tight, the shutdown window is one or two shifts, and the people stripping the machine are maintenance technicians rather than the original equipment engineers. The predictable result is a search head wrapped in stretch film, a belt folded in half and pushed into a carton, and a control cabinet set down on a forklift pallet. Days later the machine is back online with higher baseline noise, a shifted phase zero, a test piece that will not register, or nuisance rejects. None of this raises an alarm on the day of delivery. It surfaces over the following days or weeks as intermittent missed detection, and the cost is usually a product recall. The value of this article is that it turns the workshop rule of thumb, that heads hate magnets and hate being dropped, into an acceptance-testable cushioning G level, compartment dimension, liner material grade and hygiene requirement.

Contents

• Why a Metal Detector Parts Case Is a Sensitivity-Retention Device, Not Ordinary Packaging
• Teardown List and Case Split: Search Head, Conveyor, Controls, Accessories
• Search Coil and Head: The Physics Behind No-Magnetism, No-Drop, No-Deformation
• Aperture and Shielding Structure: Keeping the Liner Clear of Electromagnetic and Metal Interference
• Conveyor Belt and Idlers: No Reverse Folding, No Point Loading, and Winding Geometry
• Controls and Signal Processing Unit: Vibration, ESD and Connector Protection
• Food-Grade Cleanliness: GB 14881 and Washable Liners
• Liner Design and Compartments: Material Trade-offs, Machining and Fit Tolerances
• Case Structure and Sealing: IP Rating, Pressure Equalisation Valve and Latches
• Transport and Rigging: Designing Tests Around ISTA, ASTM D4169 and GB/T 4857
• Field Reassembly and Pre-Calibration Condition Checks
• Procurement Specification and Acceptance Points
• Common Misconceptions and Failure Case Reviews
• Frequently Asked Questions (FAQ)
• Conclusion and Related Reading

Why a Metal Detector Parts Case Is a Sensitivity-Retention Device, Not Ordinary Packaging

A metal detector works by filling the aperture of the search head with a controlled high-frequency alternating magnetic field, then reading how the receiver coil reacts when that field is disturbed. Any conductive or magnetically permeable contaminant inside the window produces a small dip in amplitude and a phase shift, and the electronics classify that dip against a threshold to trigger a reject. Because the raw signal being processed is so small, the effective sensitivity of the machine is essentially the cleanliness of the electromagnetic baseline multiplied by the stability of the mechanical geometry.

That definition exposes the three weak points.

The first is electromagnetic. The Q factor of the coils, the resonant frequency and the symmetry of the balanced bridge are all set by winding geometry and former dimensions. If a former develops a hairline crack during a drop, or turns slightly oval under lateral compression, the mutual inductance and phase relationship between the coils move. On the shop floor this shows up as zero drift, a raised baseline with the belt empty, or a test piece that registers only sometimes. What operators call losing a sensitivity step is usually this kind of geometric drift rather than a failed circuit board.

The second is mechanical. The aperture is both the product passage and the spatial criterion for sensitivity. Search heads are generally aluminium or stainless steel enclosures with relatively thin walls, and the form tolerance is carried by the flange faces and end covers. A lateral squeeze in transit, a heavy part stacked on top, or a lifting sling cinched around the middle can all introduce measurable ovality at the aperture ends.

The third is environmental. Washdown water, flour and sugar dust, brine and chlorinated sanitisers in a food plant attack both the electronics and the coil lead seals. Add the variable-frequency drives, servo amplifiers and welding sets that are ubiquitous on production floors, and any damage to the conductive continuity of the shielded housing, whether at the flange overlap, the screw holes or the earth stud, permanently reduces the interference margin once the machine is back in service.

Taken together, a competent metal detector parts case has to do four things at once: give every sensitive item an independent space that is never squeezed; keep every surface touching the head non-ferromagnetic and non-conductive; hold the transport shock peak inside the limits stated by the machine builder; and remain cleanable in the way a food plant cleans. None of the four can be solved by adding more foam. They have to be designed in from the liner outward.

Teardown List and Case Split: Search Head, Conveyor, Controls, Accessories

The first step is not choosing a case, it is writing down the disassembly sequence. For a metal detector the sensible order is: stop the line and clear product, isolate compressed air and power, record belt tension graduations and head mounting datums, remove the conveyor section and idlers, remove the search head assembly, remove the control cabinet and operator panel, then collect test pieces, spares and special tooling. Every step should be photographed and tagged, because those records are the geometric datum for reassembly.

Grouped by weight and sensitivity, the parts fall into four categories with quite different packing strategies.

Case A carries the centre of gravity of the whole packaging problem. Search heads typically weigh in the tens of kilograms with the mass biased toward the coil side, and the most common handling error is for two people to carry the assembly by its two ends. That grip applies a torsional moment directly to the flange faces. The packing design should predefine lifting points, or at minimum carry a clear instruction that the assembly may only be supported from underneath and never lifted by the end covers.

Case B is consistently underrated. A conveyor belt looks like a roll of flexible material, but it is a composite of cover, carcass and underside, and one reverse fold can delaminate the carcass. The belt then looks perfectly fine and still runs off-centre once installed. Idlers and pulleys belong to a different category entirely, precision bearing-seat components where a single hammer blow or a drop onto concrete can take the journal roundness out of tolerance.

Case C is about static and vibration, not strength. Variable-frequency drives, transmitter units and servo amplifiers are most at risk from sustained low-amplitude vibration loosening plug-in connections, and from human-body electrostatic discharge during handling in a dry environment.

Case D is about uniqueness. Standard test pieces are the reference for calibration and routine verification. Once they are mixed together, contaminated by magnetic material, or scratched, every subsequent sensitivity check loses its meaning. These small items belong in a compartmented soft liner, with a test-piece list and number cross-reference on the inside of the lid.

All four cases can share a single size family, common latches and common sealing components, which makes field management and spares replenishment much simpler. When JUNZHIJIA takes on this class of equipment case, the usual approach is to issue drawings as a model family plus zoning layouts, so that a future capacity expansion requires only a new liner, not a new case. The general logic of case selection is set out in the protective case selection guide.

Search Coil and Head: The Physics Behind No-Magnetism, No-Drop, No-Deformation

No magnetism is the rule most often overlooked. The housing of a search head is usually a non-magnetic metal such as aluminium or, on some models, stainless steel, but the fixing screws, end covers, hinge pins and mounting blocks around the coil former may still be ferrous. So can anything magnetic stored nearby: permanent magnets, loudspeakers, magnetised screwdrivers, magnetic level switches. Local residual magnetism does not break the machine immediately. It shows up as a zero that is always slightly off and a machine that needs frequent recalibration. Contact parts in Case A should therefore be standardised on 304 or 316 stainless steel, brass or engineering plastics. Liners must be free of iron powder, carbon-steel inserts and magnetic closures. Magnetic gasket strip has no place inside the case, and a magnetic label holder on the outside is a poor idea as well.

No dropping has to be expressed as a number rather than an instruction to be careful. A coil former is far less impact-tolerant than a solid metal part of the same size, because it is fundamentally a thin-walled structure carrying windings. As a rule of thumb, a search head dropped 200 mm onto a hard floor can already crack a former or strain the lead exit, and this type of damage often does not appear during an empty-belt self-check. It appears later, when product moisture changes and the product effect grows, as a missed detection. The design response is to hold the clearance in the single-part compartment to 5 to 10 mm, use a conforming EVA or XPE liner to restrain movement in all six directions, and put a 20 to 30 mm high-density cushioning layer underneath to cut the peak acceleration. The case itself needs enough lateral stiffness that stacking loads are not transmitted into the head.

No deformation means preserving the aperture and the flange faces. The correct technique is contoured support: machine the liner to the outside profile of the housing, grip it circumferentially near both flanges, and relieve the middle so the head is held by two rings plus a floor pad. Two techniques are always wrong. The first is blocking foam into the aperture from the open end, which compresses the aperture directly. The second is cinching a nylon strap around the middle of the housing, which applies a concentrated load exactly where the wall is thinnest. If a customer requires vertical packing, because the centre of gravity and envelope suit it, then use a full-contact base saddle and fit a temporary shape-retention ring at the open end, cut from liner offcuts of the same batch, to lock the roundness.

The liner material most often misapplied in this context is conductive foam. Carbon-black-filled foam does dissipate static, but wrapping it fully around a search head places a weak conductor in a ring just outside the magnetic window, which introduces additional coupling and loss in certain frequency bands. The correct division of labour is this: use non-conductive, non-magnetic PE, XPE or EVA cushioning around the head, and reserve static-dissipative materials for the circuit boards and connectors that are actually static-sensitive. This is routinely reversed in electronic equipment cases. The right way to implement a static requirement is covered in the ESD shielded case article.

Aperture and Shielding Structure: Keeping the Liner Clear of Electromagnetic and Metal Interference

Sensitivity inside a search head is not uniformly distributed. It is at its lowest near the centre of the aperture, where the signal is weakest and detection is hardest, and it rises toward the window edge closest to the windings. That is why the same machine can give different answers for a test piece passed through the centre of the aperture and one passed near the edge. What the packaging has to protect is precisely the geometry whose sensitivity distribution has already been characterised and calibrated.

Structurally, a search head can be read as three layers: coils and former at the core, magnetic shielding and balancing components in the middle, and the load-bearing, water-resistant metal housing outside. The packing design touches the outermost layer, but the objective is to preserve the relative positions of the innermost layer. That yields three hard constraints.

The first is that the liner must contain no foil, no wire mesh, no metallised film and no conductive coating. Aluminium-laminate pouches and metallised vacuum bags are common in electronics, but they cannot be wrapped directly around a search head. A foil bag may be used as an outer moisture barrier for the whole case, but only after the head has been placed in a non-conductive liner compartment with adequate dielectric separation.

The second is that the conductive continuity of the shielded housing must not be compromised. The metal housing is itself the best shield, and its effectiveness depends on clean flange overlap faces, intact screw holes and undamaged earth-stud threads. During packing, place a non-metallic protective shim between flange faces to prevent mutual scratching, bag the screws individually with hole positions labelled, and cap the earth stud.

The third is that nothing loose and metallic may be inside the case. Loose screwdrivers, hex keys, clamps and banding inside Case A will migrate with every vibration cycle, scoring the coating or, during handling, bearing down on the aperture end face. Cases D and the tooling must be physically separated from Case A.

There is one recurring argument worth settling: should the search head be wrapped up to keep moisture out? The answer is to handle it in layers. Use vapour-phase corrosion inhibitor film or desiccant on the inside to control humidity, and a sealed case with a pressure equalisation valve on the outside to control differential pressure, but do not lay foil tight against the housing. Tight foil abrades and punctures during handling, and every puncture becomes a local corrosion site while adding clean-up work at the next unpacking. The interaction between sealing and pressure equalisation is covered in the pressure equalisation valve article.

Conveyor Belt and Idlers: No Reverse Folding, No Point Loading, and Winding Geometry

A metal detector conveyor system is typically built from a belt, a drive pulley, a tail pulley, idlers, tensioning hardware and guide rails, with variants such as cleated, sidewall or elevating belts on food lines. These parts share one packing characteristic: they are not afraid of weight, they are afraid of the wrong attitude.

The first thing to control is the bend radius of the belt. The belt is a laminate of cover, carcass and underside. The cover is usually food-grade PU or PVC and the carcass is polyester fabric or steel cord. Reverse bending, that is, folding against the cover side, puts the carcass in tension. One fold may leave no visible trace, but the crease becomes a permanent stress concentration, and after installation it appears as belt tracking error, wrinkling or a split splice. The only correct attitude is therefore winding in the direction the belt bends in service, at a large diameter.

The relationship between winding diameter and belt thickness follows a useful rule of thumb: the winding bore should be no less than 40 to 60 times the belt thickness. For belts with sidewalls or cleats, the cleat height also matters. The usual approach is to isolate the cleat root with soft padding and then either wind the assembly, or lay the belt flat across a series of pallets rather than winding it. If the belt is longer than 3 m, intermediate support is needed so that self-weight sag does not create a sharp local fold.

For idlers and pulleys, the packing priority is the bearing seats. Journal ends, bearing housing mounting faces and taper-lock mating faces are precision fits, and a single impact or drop can push roundness or coaxiality out of tolerance. Three things are needed: coat the mating faces with rust-preventive grease and add protective sleeves in plastic or non-woven fabric; locate each idler in its own compartment so they cannot rest against each other; and lay pulleys on end supports with the journals carried in V-blocks or half-round saddles. Never let a pulley rest on its outer diameter, because that surface is usually rubberised or coated, and a dent there will bias belt tracking.

Springs, cylinders and gas struts in the tensioning assembly should be relieved or removed as the machine builder requires, and left in the free state during transport. Tensioning screws should be fully retracted and secured so that vibration cannot rotate them. Corrosion matters more than usual in a food plant, since carbon-steel parts develop red rust quickly in the presence of residual chlorinated sanitiser. A neutral rust-preventive treatment before packing plus adequate desiccant inside the case is the cheapest available countermeasure. Salt spray test methods are described in GB/T 10125.

Controls and Signal Processing Unit: Vibration, ESD and Connector Protection

The control section is the part of a metal detector that looks the most robust and behaves the most fragile. Power boards, signal processing boards, display panels, terminal blocks, encoder interfaces and communication modules mostly fail not because they were struck, but because sustained low-amplitude vibration and ambient humidity wore them out.

The first principle of vibration control is to eliminate degrees of freedom. Boards are retained inside the cabinet by card guides and retaining strips. If the whole backplane is simply laid into a case with no lateral restraint after teardown, transport vibration lets the boards work back and forth in the guides until the plug-in plating wears and the terminals loosen. The right approach is either to transport the cabinet whole with the original transit screws or temporary blocking in place, or to remove the boards from the guides, bag them in static-shielding material, clamp them in a dedicated compartment and cap every bus connector.

The second principle is to separate hard vibration from soft vibration. A drive or servo amplifier contains electrolytic capacitors, large inductors, heat sinks and fans, all dependent on screw torque and thermal interface material. Sustained vibration changes the contact pressure between heat sink and power device, which in the long run can cause thermal trips. At case level, the useful action is to keep the case off a rigid steel pallet. The case should sit on a cushioned dunnage layer in the vehicle, not directly on a steel pallet lashed down tight.

Static control has to be considered together with moisture control, because the two requirements frequently conflict. Loading a case with desiccant can drive relative humidity below 20 percent, at which point walking, peeling stretch film and rubbing bubble wrap all generate significant charge, and with nowhere to bleed, that charge finds the thinnest insulator available, usually a MOS gate. The balanced approach is to hold relative humidity inside the case between 40 and 60 percent, neither condensing nor excessively dry; to bag boards in metallised shielding bags before placing them in a cushioned compartment; and to fit an earth terminal to the case so that the field technician can put on a wrist strap before opening it.

The third area is connectors and cabling. Circular connectors, M12 connectors, network ports, fibre ferrules and service USB ports all need caps. Cables should be bundled by function and labelled, with a bend radius of at least 6 to 10 times the cable diameter, and considerably more for fibre. No cable should be routed against a coil channel, or the next reassembly may see it landed on a sensitive position.

Food-Grade Cleanliness: GB 14881 and Washable Liners

Equipment packaging in a food plant has to satisfy two sets of logic at once: transport protection and food hygiene. The central requirement in GB 14881, the national food safety standard for general hygiene in food production, is that equipment in contact with food, or present in food exposure areas, must be easy to clean and sanitise, corrosion resistant, must not become a contamination source, and must not present crevices, gaps or shedding risk that are difficult to clean. That requirement propagates directly into packaging.

First, anything that has touched product, including belt, idlers, guide rails, reject paddles and hoppers, must be cleaned before packing: remove powder and syrup residue, wipe off excess food-grade grease, and dry before packing. Sealing a wet component into a case is simply incubating mould and odour.

Second, no timber, cardboard, waste paper or starch-based adhesive may be used inside the case. Wood fragments and paper dust are genuine foreign-body hazards, and a single shed particle landing on the production line is a real food safety event. Acceptable fill and separation materials are PE, XPE, EVA, EPP, silicone and non-woven fabric supplied to a no-shed specification.

Third, liner surfaces must be cleanable. Closed-cell materials, XPE, EVA and EPP, do not absorb water and can be wiped with a neutral detergent. Open-cell materials and flocked surfaces feel better and resist scuffing, but in a food application they need a sealed coating, otherwise powder is retained over the long term. If the plant operates a wet washdown regime, the case should be IP65 or better with silicone seals, and removable liners should be fully dried before being refitted.

Fourth, packaging marking should follow GB/T 191, with additional special markings such as food contact components and do not ship with chemicals, because equipment cases are frequently transported in the same vehicle as detergents and lubricants. If the whole machine or its parts are delivered through a cold chain, for instance to a dairy or frozen food plant, low-temperature embrittlement of the case material also needs consideration; the material selection logic is discussed in the cold chain food case article.

Fifth, build a documented cleaning trail. Clean and record before packing, place desiccant and a humidity indicator card inside, and have the customer confirm condition at unpacking. With those three steps in place, a foreign-body complaint at least has a traceable chain of custody. Cleanable design is not an extra burden on a food line equipment case. Write it into the specification, and the liner process of any competent supplier will move in that direction on its own.

Liner Design and Compartments: Material Trade-offs, Machining and Fit Tolerances

The liner is where the real engineering content of this packaging lives. The same case with a different liner material and compartment layout can deliver a search head that has lost a full sensitivity step after transport, or one that arrives unchanged.

Compartment design follows three rules. First, one item per compartment, particularly for the search head, pulleys and transmitters, which must never share a pocket; two parts in one pocket strike each other during a drop, and this is the most common hidden damage source in the field. Second, the load path must be continuous, with a high-density cushioning layer running from the heavy part straight to the case floor, never left bridging or supported on one side only. Third, leave a retrieval allowance, using a finger scallop or pull hole in each pocket so that nobody reaches for a pry bar or screwdriver, since a pry bar is precisely the search head's worst enemy.

For machining, EVA and XPE liners are normally CNC routed, with mating faces held to roughly plus or minus 0.5 mm and precision interfaces tightened to plus or minus 0.3 mm. Volume production uses heat-pressed tooling, which gives excellent repeatability once the tooling cost is amortised. Allow 1 to 2 mm of assembly clearance between liner and case to prevent swelling under humidity cycling, and where a liner is built in two layers, insert a thin PE sheet at the interface to prevent layer misalignment.

Maintainability matters too. Head pockets are opened and closed repeatedly in the field, edges shed material easily, and the answer is a chamfer and coating at the pocket mouth. Label compartments by laser marking or oil-resistant label, never paper. On this class of inspection equipment case, JUNZHIJIA normally offers a workflow that scans the search head surface and rebuilds the liner in three dimensions, so the customer only needs to supply the physical assembly or a 3D model; the process detail is described in the EVA foam insert custom process article.

Case Structure and Sealing: IP Rating, Pressure Equalisation Valve and Latches

The structural requirements for a metal detector parts case differ noticeably from a general instrument case. It is heavier, with single cases commonly in the 30 to 80 kg range, it is handled by forklifts and slings more often, it is stored in damp areas, and many customers treat it as long-term storage rather than one-way packaging. Those facts make repeated opening and closing, heavy-load handling and cleanability the design premises.

The essence of sealing is not the rating number but the seal interface design. An O-profile or rectangular silicone or EPDM gasket must sit in a case groove and be compressed evenly by a lid rib, with compression typically held between 25 and 35 percent. A moulded endless ring is preferable at the joint; a cemented ring tends to separate after humidity and temperature cycling. Latches should be over-centre types so that every latch point delivers consistent compression. The longer the case, the more latch points are needed, and the mid span requires ribs or metal latch inserts, otherwise the lid crowns and the gasket never compresses. That is the real reason so many cases marked IP67 actually leak. A more detailed comparison of latch, hinge and gasket materials is given in the toolbox hinge latch and seal article.

Heavy sealed cases also need to handle air pressure. A well-sealed case flown by air, moved over high ground, or simply subjected to a large temperature swing will develop a differential. Sealed at low altitude and moved to high altitude, internal positive pressure lifts the gasket. Moved from warm to cold, internal negative pressure holds the lid shut, makes it hard to open and can draw moisture past the seal. The answer is a pressure equalisation valve with a microporous membrane, which passes gas while blocking liquid water and releases the differential while preserving the ingress rating. Selection criteria are covered in the pressure equalisation valve article.

Handling features must match the weight. Above 30 kg, provide side grips or metal handles. Above 60 kg, provide lifting eyes or forklift clearance under the base, and mark the centre of gravity and lifting points. If castors or a pallet base are added, verify the wheel load rating against the floor conditions. For the shell material, the choice between HDPE, PP copolymer and rotomoulded or injection-moulded construction depends on the priority: injection moulding gives tight dimensions and uniform wall thickness and suits precision liners, while rotational moulding gives better impact and distortion resistance and suits very large or very heavy items.

Transport and Rigging: Designing Tests Around ISTA, ASTM D4169 and GB/T 4857

The purpose of transport testing on a metal detector parts case is not to produce a presentable report. It is to answer three specific questions: has the peak acceleration on the search head been reduced into the permitted range, has anything inside the case moved relative to its compartment, and are the case and its seals still intact after vibration, drop and stacking?

Test items are normally combined according to the delivery route. For mainly domestic road transport, the vibration, drop and stacking methods of the GB/T 4857 series apply. For export and multimodal routes, ISTA 1A or 2A for individual packages, or ISTA 3A for parcel delivery, are common, and the distribution cycles of ASTM D4169, such as a less-than-truckload or full-truckload combination, can be used instead. Where a customer needs an environmental robustness argument, MIL-STD-810H is often cited by method number to describe the methodology for vibration, shock and temperature, with the clear caveat that citing methodology is not a military certification.

A few details derail test programmes. First, the sample must be packed exactly as it will ship, including liner, desiccant and every accessory; testing an empty case proves nothing. Second, drop attitudes must cover the worst case, which for a search head means the end face along the flange direction and the side edge, not the base. Third, mark the liner surfaces before a vibration test and check afterwards whether every part has returned to its starting position; this is how a liner that looks conforming but lets parts swim is found. Fourth, the stacking load must reflect the actual number of storage tiers with a safety factor, and account for the effect of temperature and humidity on case stiffness.

Writing transport testing into the technical agreement has a second benefit: it makes the liner supplier responsible for the cushioning curve rather than for a feel. On inspection equipment cases, JUNZHIJIA normally issues a cushioning calculation based on the customer's weight distribution and allowable acceleration, keeps margin available at the prototype stage, and runs one full-case verification before series production, which controls risk and cost at the same time.

Field Reassembly and Pre-Calibration Condition Checks

The window between unpacking and completing reassembly is where all the earlier investment is either realised or written off. Metal detector site reassembly follows a clear sequence, and skipping any step tends to be paid for later.

1. Visual inspection and count. Check against the packing list, look for evidence of shifting, abnormal liner compression marks, rust spots or condensation.
2. Search head geometry check. Use a straightedge and feeler gauge on the flange faces, confirm aperture roundness with an internal gauge or a dedicated ring, and inspect the housing and end covers for impact marks.
3. Lead and shield check. Inspect coil lead exits for strain, verify pin alignment in connectors, and check continuity between shield and earth stud.
4. Belt and conveyor reassembly. Restore tension to the graduations recorded at teardown, turn the belt by hand to confirm it tracks, then run at low speed. If the belt wrinkles, stop immediately and check whether it was reverse-folded in transit.
5. Controls reassembly. Seat boards and re-tighten retaining strips, mate connectors by their tags, re-torque drive terminals to the maker's figures, and clean heat sinks and fans.
6. Cleanliness treatment. On a food line, wipe down and visually confirm cleanliness, removing any rust-preventive residue from the transport phase.
7. Empty and loaded verification. Run empty first to observe the noise baseline, then pass test pieces, Fe, SUS and non-ferrous, several times at front, back, left and right positions, and finally verify with real product.
8. Record and file. Compare post-reassembly sensitivity and phase values against the factory record. A difference beyond the permitted range means investigate the cause, not simply adjust it away.

One diagnostic rule of thumb is worth emphasising. If detection performance for a test piece in the centre of the aperture has dropped, the cause is most likely geometric or coil-related: deformation or a hairline crack. If the empty-belt noise floor has risen and the zero keeps drifting, the cause is more likely residual magnetism or shield continuity. If sensitivity is normal but nuisance rejects have increased, the problem usually lies at the electrical end, in earthing and interference, rather than in mechanical damage. Working in that order substantially reduces rework on site.

For customers who will use the case repeatedly, such as maintenance crews or third-party inspection service providers, it is worth making the case traceable: keep a status card in a fixed position inside recording the date packed, the person who packed it, internal and external condition, and the test reference. The administrative cost is close to zero, and it is extremely useful for telling whether a given defect is new or left over from last time. Reuse and service life assessment for inspection equipment cases is discussed in the protective case service life article.

Procurement Specification and Acceptance Points

Putting requirements into a specification is the most effective way to avoid disputes later. A metal detector parts case specification should include at least the following entries.

Acceptance should be staged in three parts: first-article acceptance, confirming liner fit, compartment correctness and conformity of non-magnetic parts; in-process sampling, using a sampling standard such as GB/T 2828.1 with an agreed AQL, focused on liner dimensions and appearance; and final case verification, running the agreed vibration and drop sequence and inspecting for displacement and damage after unpacking. Details of sampling and decision rules are in the custom case acceptance and AQL article.

On documents, ask at minimum for a liner material report covering density, hardness and flame rating such as UL94; case material certification; gasket material report; packing list and packing drawing; and the full-case test report. For export, consider the language of packaging markings, fumigation requirements for any timber packaging, and destination-country food contact material regulations.

Where OEM or ODM is required, the specification can state branding and appearance directly: case colour, screen-printed content, nameplate position and serial number rules. This falls within normal customisation capability; evaluation criteria for choosing a supplier are set out in the article on how to choose a case OEM factory.

Common Misconceptions and Failure Case Reviews

Misconception one: an ordinary plastic tote plus bubble wrap. This is the most widespread approach and the most failure-prone. A tote has no continuous seal and no compartmented liner, and bubble wrap gradually migrates under vibration until the search head is resting against the wall. The real problem is not that it is insufficiently strong, it is that it provides no restraint mechanism at all.

Misconception two: wrapping the head fully in carbon-black conductive foam. The intent is static protection, but the result is a conductor wrapped around the magnetic window. Handle the two requirements separately: cushioning in non-conductive material, static protection only for the electronics.

Misconception three: folding the belt in half to save space. This happens routinely on site. The carcass damage at the crease is invisible at installation and shows up days later as tracking error, at which point the belt has to be replaced. The only correct attitude is winding in the service bending direction at large diameter, or flat packing on segmented supports.

Misconception four: using the end face of the head as a load-bearing surface. To make the packing feel tight, foam blocks are pushed in from the open end, or several heads are stacked vertically so the end faces carry the stack. This compresses the aperture directly and is the fastest way to cause permanent sensitivity loss.

Misconception five: storing a removed head next to a steel rack. Residual magnetism often originates not in transport but in temporary storage inside the plant. Once removed, the head should go straight into a non-magnetic liner compartment rather than being set aside until next week.

Misconception six: more desiccant is always better. Excess desiccant drives internal humidity very low, which increases static risk from material friction and accelerates ageing of some elastomers and rubber parts. Holding 40 to 60 percent relative humidity is the sensible band. Cleaning and dehumidifying after long-term storage is described in the article on how to clean a protective case.

Misconception seven: ignoring residual pressure in pneumatic and hydraulic parts. If tensioning cylinders, gas struts or pneumatic reject actuators are not relieved, rising temperature inside the case can push components out of position, or even force a liner compartment open. Relieve and tag them during teardown.

Misconception eight: accepting the case, not the packed assembly. A case that passes visual inspection and a liner that passes dimensional inspection do not together prove that the packed combination is acceptable. The acceptance object is the loaded case, not two separate items.

Checking against these eight points covers the great majority of failure modes seen in real metal detector parts case projects. Compressed into one sentence: this class of case exists to move an already-calibrated electromagnetic and geometric datum safely, so its design centre is always the liner, the compartment layout and material compatibility, never the thickness of the shell.

Frequently Asked Questions (FAQ)

Q: Why does a metal detector parts case have to be non-magnetic, and can ordinary steel fasteners really affect sensitivity?

A: Yes, and the way it happens is subtle. Inside the search head, a set of coils operates in a balanced state, and ferromagnetic material nearby changes the reluctance of the local magnetic circuit, introducing a small asymmetry into an otherwise balanced bridge. In the short term this shows up as a larger zero offset and more frequent recalibration. In the long term it raises the empty-belt noise floor, which is equivalent to pushing the detection threshold upward until a test piece registers only part of the time. Typical sources include ordinary steel fasteners, ferrous hinge pins, magnetic label holders and rubber pads containing iron powder. Contact parts in the head compartment should therefore be standardised on 304 or 316 stainless steel, brass or engineering plastics, the liner should be a grade of EVA or XPE free of carbon black and metallic fillers, and no magnetic closures or magnets should be stored inside the case. When JUNZHIJIA builds this type of case, all non-magnetic contact parts are written into the liner bill of materials as a hard requirement and confirmed item by item at first-article acceptance using a magnet test, which costs far less than correcting sensitivity later.

Q: Can the search head be packed upright, or will that damage the aperture?

A: Upright packing is acceptable, but only with shape-retaining support rather than simply standing the head in a pocket. The mechanical issue with vertical storage is that the head's own weight is transmitted through the end flange into the base of the liner. If that base is a plain foam surface, the contact area between flange and liner sees concentrated compressive stress, and after prolonged stacking the flatness of the end face can drift out of tolerance. Once aperture roundness changes, the sensitivity distribution moves with it. The correct approach is a full-contact saddle at the base, machined from a higher-density EVA, typically 80 to 120 kg per cubic metre, with a contoured recess so the load spreads across the whole flange face; a circumferential grip above the head to limit lateral movement; and, where necessary, a temporary retention ring at the open end to hold roundness within tolerance. Horizontal packing is generally safer, because the side of the housing is the primary load-bearing structure and the centre of gravity sits lower. Whichever attitude is chosen, mark the permitted orientation and tilt limit on the case and have the site follow the marking during handling.

Q: How should the conveyor belt be packed so that the carcass is not damaged?

A: The single governing principle is large-diameter winding in the direction the belt bends in service. The belt is a laminate of cover, carcass and underside, and the carcass is normally polyester fabric or steel cord. Reverse folding puts the carcass into tension and creates a permanent stress concentration. The belt looks intact but after installation it tracks off-centre, wrinkles or splits at the splice. As a rule of thumb, the winding bore should be at least 40 to 60 times the belt thickness, so a 2 mm belt needs 80 to 120 mm or more, and the mandrel should carry end discs to stop the coil collapsing. Belts with cleats, sidewalls or elevator flights are generally not wound at all; they are laid flat across a series of pallets with the cleat roots isolated by soft padding. For belts longer than 3 m, the mid length needs intermediate support, and single-point slinging or letting self-weight sag into a sharp angle is not acceptable. The belt should also be cleaned and dried before packing, because sealing food residue and grease inside the case creates both a corrosion source and a microbial growth site.

Q: Why do control cabinets and circuit boards need dedicated vibration and static protection in the packaging?

A: Because their failure modes are gradual and do not announce themselves on arrival. On the vibration side, a board without lateral restraint in its card guide will work back and forth during transport, gradually wearing the plug-in plating and loosening terminals, which typically surfaces later as intermittent communication loss or a false alarm; drives and servo amplifiers contain heat sinks, large inductors and electrolytic capacitors, and sustained vibration changes the contact pressure between heat sink and power device. On the static side, the gate of a MOS device has a very low breakdown voltage, and in a dry environment peeling and friction can accumulate substantial charge. A single discharge may leave a device that has shifted parameters without failing completely, and such parts often only reveal themselves after high-temperature ageing. Practical measures include retaining boards with original transit screws or temporary blocking when the cabinet ships whole; bagging boards in metallised shielding bags inside static-dissipative compartments when they ship separately; capping every connector; holding internal relative humidity between 40 and 60 percent; and fitting an earth terminal so the technician can put on a wrist strap before opening the case.

Q: What are the hard cleanliness requirements for an equipment case used in a food plant?

A: The basis is the general hygiene requirement of GB 14881 for equipment and premises, which translates into four hard packaging constraints. First, every component that contacts product must be cleaned and dried before packing, removing residue and excess grease, because sealing a wet item into a case actively incubates mould and odour. Second, no wood, paper or starch-adhesive materials may be used inside the case for fill or separation, since wood fragments and paper dust are genuine foreign-body sources and a single particle landing on the line is a food safety event; acceptable materials include PE, XPE, EVA, EPP, silicone and non-woven fabric. Third, liner surfaces must be washable, and closed-cell materials such as XPE, EVA and EPP can simply be wiped with a neutral detergent, whereas open-cell and flocked surfaces used on a food line need confirmation of cleanability or they will retain powder over the long term. Fourth, packaging marking should follow GB/T 191 with additional dedicated markings such as food contact components and do not ship with chemicals, because equipment cases often travel in the same vehicle as detergents and lubricants. Where the plant operates wet washdown, specify IP65 or better.

Q: After removing the search head, can it be left in a corner of the workshop until it is packed next week?

A: This is the most typical and most expensive practice found on site. Workshop corners and temporary racks are usually steel or galvanised, and the surrounding area often contains welding sets, lifting electromagnets, permanent-magnet fixtures, loudspeakers and power tools. A search head left in that environment can pick up residual magnetism even without being dropped or struck, and the effect accumulates gradually, so by the time packing happens nobody can tell whether a problem is new or pre-existing. The correct sequence is for the head to go straight into a cleaned, lined, non-magnetic compartment with the lid closed and latched. If packing genuinely cannot happen immediately, place it on a timber or plastic bench at a suitable distance from any ferromagnetic equipment, and record the storage location and duration. For customers who cycle the case repeatedly, keeping a status card inside recording time, location and appearance at every packing gives a quick answer about which link in the chain caused any sensitivity anomaly.

Q: What testing should be done before packing, and what happens if a test fails?

A: Combine tests according to the delivery route rather than applying one generic report. For mainly domestic road transport, use the vibration, drop and stacking methods of the GB/T 4857 series. For multimodal and export routes, ISTA 1A or 2A and ISTA 3A are common, and ASTM D4169 distribution cycles can be used instead. Where an environmental robustness argument is required, MIL-STD-810H can be cited by method number to describe vibration, shock and temperature methodology, with the caveat that citing methodology is not a military certification. For this equipment, three points deserve attention: whether anything shifted inside the liner after vibration, whether the search head peak acceleration during drop stayed inside the permitted window, and whether the lid mid span took a permanent set that compromised sealing after stacking. The sample must be a fully loaded case in its actual shipping configuration including desiccant and accessories, because testing an empty case has no value. If a test fails, work in this order: first examine the liner compartment and contact faces, since local bridging or single-point loading is the most common cause; then check whether cushioning layer density and thickness match the mass; and only then consider stiffening the shell, because adding wall thickness rarely fixes a peak acceleration problem and always adds cost.

Q: Can one case be used repeatedly, and how should its service life be assessed?

A: Yes. Metal detector parts cases are used mainly for equipment overhaul round trips, so they are inherently returnable packaging and repeated use is normal. Service life assessment focuses on three areas. The first is the gasket, since silicone and EPDM harden and shrink under humidity cycling and contact with sanitisers, and once cracking or compression set appears the ingress rating can no longer be guaranteed; this is usually the first consumable to need replacement. The second is the liner, because closed-cell materials lose resilience after repeated compression, which shows up as a compartment that gets looser with every trip until the part can move inside it; once visible crushing or shedding appears, replace rather than repair. The third is the latch and hinge, since over-centre latch compression falls as the pin wears and hinge bore wear makes the lid sit off-centre, both of which degrade sealing indirectly. A simple inspection routine works well: record appearance, gasket condition and latch feel on every round trip, run an occasional spray or immersion spot check after a set number of trips, and replace liners on a planned cycle.

Conclusion and Related Reading

A metal detector earns its keep through detection rate, and detection rate is a state that has to be preserved rather than a property the machine simply owns.

Related Reading

• Industrial X-Ray Inspection Cases: Source and Detector Protection
• Checkweigher Cases: Load Cell and Reject Mechanism Protection
• Toolbox Hinge, Latch and Seal Selection