Foreign Body Control on Quick-Freezing Lines: Metal Detection Before or After Freezing

<p>Foreign body complaints rarely originate in the freezing stage, but investigations often have to include freezing as well. Whether the detection point is placed before or after freezing determines the downstream line layout, rejection method and record-keeping approach. This article works through six aspects: sources, location, interference, cycle matching, equipment and records.</p> <p><img src="/images/articles/foreign-body-control-on-freezing-lines-01.jpg" alt="Liquid nitrogen quick-freezing cabinet"></p> <h2 id="i-which-stages-foreign-bodies-enter-from">I. Which stages foreign bodies enter from</h2> <p>Beyond raw material acceptance, four locations on quick-freezing lines are easily overlooked:</p> <ul> <li><strong>Loading stage</strong>: plastic fragments from broken turnover baskets, cable ties, glove fragments, tools. Workshops that rely on manual tray loading are more prone to problems at this stage.</li> <li><strong>Mesh belt and conveying stage</strong>: stainless steel mesh belts run at low temperatures for long periods; broken wires, deformation and edge wear can all drop into products, and can only be found through inspection.</li> <li><strong>Equipment body</strong>: bolts, gaskets, welding slag, insulation debris. The risk concentrates at the first startup after equipment maintenance.</li> <li><strong>Packaging stage</strong>: poor sealing, bag material offcuts, nails and tape used for cartoning.</li> </ul> <p>Where seams, drainage and removable structures for cleaning are poorly executed, foreign bodies and accumulated material remain inside the equipment. For this part, see <a href="/en/tech/hygienic-design-and-materials-for-freezing-equipment/">Hygienic Design and Material Selection for Quick-Freezing Equipment</a>.</p> <h2 id="ii-detection-point-before-or-after-freezing">II. Detection point before or after freezing</h2> <p>Both placements are in use; which to choose depends on product form and packaging method.</p> <table> <thead> <tr> <th style="text-align: left">Location</th> <th style="text-align: left">Advantages</th> <th style="text-align: left">Costs</th> </tr> </thead> <tbody> <tr> <td style="text-align: left">Before freezing (after loading, before entering the machine)</td> <td style="text-align: left">Material is loose and thin, so the detection machine&rsquo;s pass-through height is small; problems found before freezing are simple to rework</td> <td style="text-align: left">Stages after loading can still introduce foreign bodies, so detection does not cover the whole process</td> </tr> <tr> <td style="text-align: left">After freezing, before packaging</td> <td style="text-align: left">Covers the freezing stage and mesh belt stage; most foreign bodies are intercepted before packaging</td> <td style="text-align: left">Products are frozen hard with frost and water, causing strong detection interference; once detected, the whole batch is already frozen product, and handling costs are high</td> </tr> <tr> <td style="text-align: left">After packaging (whole cartons or bags)</td> <td style="text-align: left">Covers the packaging stage; records correspond one-to-one with shipping batches</td> <td style="text-align: left">Aluminum foil and laminated bags reduce the detectable aperture; metal detection basically fails and must be replaced by X-ray</td> </tr> </tbody> </table> <p>Aquatic, poultry and prepared meat lines mostly use &ldquo;after freezing, before packaging&rdquo; as the main detection point, plus a coarse screening at the raw material stage. For aluminum foil packaged products, metal detection is not viable and must be replaced by X-ray, usually placed after packaging.</p> <h2 id="iii-what-interference-frozen-products-cause-to-detection">III. What interference frozen products cause to detection</h2> <p>Freezing does not change the electromagnetic properties of metal; the trouble comes from other things:</p> <ol> <li><strong>Product effect</strong>. The moisture content, salt content and additives of frozen products change the phase response of the detection machine. The signal of the same product differs before and after freezing, so sensitivity must be recalibrated for the frozen state.</li> <li><strong>Frost and dripping water</strong>. Frost layers and free water form a water film on the belt, causing signal drift and a higher false rejection rate. Adding an air knife or water-absorbing roller before the machine works better than repeatedly adjusting sensitivity.</li> <li><strong>Material layer thickness</strong>. Stacked frozen products increase the pass-through height, and the detectable aperture shrinks accordingly. Tunnel line output is a material stream, so spread it flat before entering the machine; cabinet machine output is whole trays, and whether to pass whole trays or split trays depends on tray material.</li> <li><strong>Packaging materials</strong>. Aluminum foil, aluminized film and metal clips overwhelm the signal; aluminum-containing packaging can only use X-ray.</li> </ol> <p>Sensitivity is implemented according to the iron, non-iron and stainless steel test piece apertures calibrated with the detection machine. Different aperture sizes and different products vary greatly; copying someone else&rsquo;s parameter table is meaningless.</p> <h2 id="iv-how-to-match-with-cycle">IV. How to match with cycle</h2> <p>The detection section is one of the few non-bufferable links on the line: if products cannot pass, the tunnel ahead must slow down or stop.</p> <p>Tunnel lines are continuous flow. Taking DKN-LFT-1206 as an example, the mesh belt is 6000 mm × 1200 mm, the cycle from feed end to discharge end is adjustable from 6 to 30 minutes, and the mesh belt speed is variable-frequency. Leave a short belt transition between the discharge end and the detection machine, with the two speeds separately adjustable; otherwise frozen products stack at the inlet, and once the pass-through height changes, false rejection and missed detection occur simultaneously. A buffer table is needed after the rejection device; rejected products must not stop on the main conveyor awaiting handling.</p> <p>Cabinet machines are batch flow. Taking the four-zone DKN-LFC-1200 as an example, the inner cavity is 2000 mm × 1500 mm × 1750 mm, single-zone load is 0～1200 kg, working temperature is -150℃～room temperature, and temperature control accuracy is ±2℃. After batch discharge, whole trays pass detection, or small trays are split off during tray loading; the detection point is often at the same workstation as weighing and labeling, with personnel movements following the material flow.</p> <table> <thead> <tr> <th style="text-align: left">Matching item</th> <th style="text-align: left">Tunnel line</th> <th style="text-align: left">Cabinet machine</th> </tr> </thead> <tbody> <tr> <td style="text-align: left">Material flow form</td> <td style="text-align: left">Continuous mesh belt flow</td> <td style="text-align: left">Whole-tray batches</td> </tr> <tr> <td style="text-align: left">Main control variable</td> <td style="text-align: left">Speed matching between mesh belt and detection machine belt</td> <td style="text-align: left">Single-tray weight and tray loading thickness</td> </tr> <tr> <td style="text-align: left">Common issues</td> <td style="text-align: left">Material stacking at inlet, frost and dripping at discharge end</td> <td style="text-align: left">Missed detection when splitting trays, tray material affecting signal</td> </tr> <tr> <td style="text-align: left">Record unit</td> <td style="text-align: left">Time period (shift cumulative)</td> <td style="text-align: left">Batch number</td> </tr> </tbody> </table> <h2 id="v-what-can-be-reduced-on-the-equipment-side">V. What can be reduced on the equipment side</h2> <p>Half of foreign body control rests on equipment structure and daily inspection.</p> <ul> <li><strong>Mesh belt</strong>: choose stainless steel mesh belt wires with adaptive tensioning structure for stable tension at low temperatures, reducing deviation and broken wires. Inspection focuses on edge fraying and loose joints.</li> <li><strong>Insulation and sealing</strong>: reinforced ribs with low-temperature foaming and intact sealing strips keep cold air from escaping, reduce frost, and thereby ease the water film problem in the detection section.</li> <li><strong>Cleaning structure</strong>: the shallow V-shaped bottom of the body with drainage outlet allows cleaning residue and standing water to drain cleanly, leaving no fragments from the previous batch.</li> <li><strong>Maintenance access</strong>: the whole-machine lifting structure facilitates opening the cavity for inspection; whether fasteners are loose or parts have fallen off can be seen in one opening.</li> </ul> <p>Both the feed end and discharge end of the tunnel line are equipped with oxygen concentration detectors, with the set range of 19.5%～21.5%, which must be reset and checked together after maintenance. For equipment structure, see the two pages <a href="/en/products/ln2-tunnel/">Liquid Nitrogen Quick-Freezing Tunnel Line</a> and <a href="/en/products/ln2-cabinet/">Liquid Nitrogen Quick-Freezing Cabinet</a>.</p> <p><img src="/images/articles/foreign-body-control-on-freezing-lines-02.jpg" alt="Discharge section of liquid nitrogen quick-freezing tunnel"></p> <h2 id="vi-records-and-failure-handling">VI. Records and failure handling</h2> <p>Daily verification of the detection machine is not complicated; the difficulty is persistence:</p> <ol> <li>At the start of each shift, pass iron, non-iron and stainless steel test pieces three times each; only full rejection counts as passing.</li> <li>Place test pieces at the actual product pass-through position, not on an empty belt.</li> <li>Confirm on the spot that the rejection action lands in the waste bin; do not rely only on the screen prompt.</li> <li>When verification fails, isolate the products of that period separately and trace back from the last passing moment; do not let them go on the assumption that &ldquo;there should be no problem&rdquo;.</li> <li>Record the verification time, test piece specifications, pass status and handler clearly, on the same line as the batch number.</li> </ol> <p>The approach to merging records is consistent with the two books described in <a href="/en/tech/freezing-temperature-records-and-traceability/">Freezing Temperature Records and Traceability</a>: equipment-side data is continuous and automatic, manual-side data is discrete, and only matching by batch number holds up.</p> <h2 id="frequently-asked-questions">Frequently Asked Questions</h2> <h3 id="the-product-is-already-frozen-to--18-will-metal-detection-sensitivity-be-much-worse-than-before-freezing">The product is already frozen to -18℃; will metal detection sensitivity be much worse than before freezing?</h3> <p>Freezing does not change the metal response; what changes is the product effect. The moisture content, salt content and frost layer of frozen products change the background signal, so sensitivity must be recalibrated for the frozen state. When the same product undergoes both pre-freezing and post-freezing detection, the two parameter sets are verified separately.</p> <h3 id="can-prepared-foods-in-aluminum-foil-trays-pass-through-a-metal-detection-machine">Can prepared foods in aluminum foil trays pass through a metal detection machine?</h3> <p>Basically no. Aluminum foil is metal packaging and will mask the signal. Such products switch to X-ray detection, placed after sealing, which can also reveal non-metal foreign bodies such as bone fragments, plastic and glass.</p> <h3 id="for-batch-production-on-a-cabinet-machine-must-every-tray-pass-the-detection-machine">For batch production on a cabinet machine, must every tray pass the detection machine?</h3> <p>It depends on customer quality requirements. Most practices are 100% inspection, either whole trays or split small trays; for sampling inspection, the sampling ratio and judgment rules must be stated clearly and cannot be decided on the spot by experience. When splitting trays for inspection, metal trays must be replaced with non-metal ones.</p> <h3 id="the-detection-machine-occasionally-gives-false-rejections-can-sensitivity-simply-be-lowered">The detection machine occasionally gives false rejections; can sensitivity simply be lowered?</h3> <p>Check the cause first: uncalibrated product effect, frost and water, excessive material layer thickness, belt residue — all of these can explain false rejections. Lowering sensitivity amounts to letting missed detections through, a greater risk than false rejection. Adjust parameters only after physical causes are ruled out.</p> <hr> <p>Send us the product form, packaging method, hourly output and existing line layout, and we will provide a layout suggestion based on the interface between the freezing section and the detection section. For details, <a href="/en/contact/">contact us</a>.</p>