Conveying Baked Kibble After the Oven: Porous Pieces and a Cooling Curve That Cannot Be Shortcut

The oven band delivers baked kibble at around 90–110 °C, and the pieces look finished. They are not: each piece has spent an hour at oven temperature, and the middle is still giving up moisture while the surface is already dry enough to crumble. Everything between the oven and the bag happens to a product that is brittle in one direction and wet in the middle, and each fact takes its own toll.

1. A Moment at the Oven Discharge

2. The Sentence That Decides It

The band tips over the head drum at two in the afternoon, mid-shift, and the pieces fall about a hand's width into a chute. From the mezzanine it looks like any kibble stream — same brown, same shape, same rumble. Up close the difference is in the sound: an extruded pellet at the same drop lands with a knock, a baked piece lands with a knock and leaves behind a scattering of corners and slivers fine enough to sift through a screen. Nobody stops the line. The fines accumulate in the chute corner, in the dust extraction, and eventually in the bottom of every bag packed that shift.

Baked (slow-baked or oven-baked) kibble is made from a denser, softer dough than extruded product, sheeted or cut and then held at 90–120 °C for a long bake. What comes out the other end is physically the opposite of a pellet:

· Porous and light. The crumb structure is open, so bulk density sits below extruded kibble of the same piece size, and each piece carries more surface per gram.

· Brittle. With little fat on the surface and a dry, open crumb, the piece breaks at the corners first. Impact tolerance is a fraction of an extruded pellet's.

· Moisture gradient. Exit temperature of 90–110 °C, surface moisture already low, interior still equilibrating. The piece keeps drying for the next several minutes.

· Low surface fat. Coating is rare on baked product, which changes the cleaning problem completely: dust instead of grease.

None of that is a defect — it is the reason people buy the product — but every property has a corresponding cost in the conveying between oven and bag.

The segment runs from the oven band to the packaging infeed — four stages, each with its own belt decision:

The three middle stages are where residence time is bought: the temperature has to come down through the whole piece, not just off the surface, and the belt is the only place that happens at line speed.

Fines at the discharge, and what they become downstream. The drop off the oven band breaks corners. The mechanism is simple: an open crumb has thin webs between large voids, and a corner hitting steel concentrates the load exactly on those webs. The result is a fine fraction that grows through the day. Fines ride on the bed, sift through the stream, and arrive at the weigher, where they upset statistics and end up in the bottom of the bag — where the consumer finds them. Every extra centimetre of drop in the first two metres adds to that fraction.

Deep beds that cool on the outside and not in the middle. On a wide bed fed by a long oven, the instinct is to pile the product deep and let time do the rest. A porous bed resists airflow, which takes the path of least resistance — over the top and down the sides. The core of a 100 mm bed can still be 15–20 °C warmer than the exposed surface ten minutes later. The pieces go into the bag at spec, and the warm centre keeps driving moisture outward into the headspace. Sealed warm, the bag fogs, the shelf-life clock starts early, and the failure shows up weeks later as a returns claim — with nothing at the packaging machine to blame.

Dust and static at the packaging infeed. A dry, porous, low-fat surface generates fines through ordinary abrasion, and dry packaging-room air lets them carry a static charge. They migrate to the film, the seal area and the weigher optics — a cleaning-frequency problem first and a rejection-rate problem second, both traceable to how gently the product was handled upstream.

Two candidates cover this segment, and they cost different things.

Open mesh belt, metal wire. The case for it is airflow: an open area of 40–70% lets cooling air pass through the bed rather than over it, and it drains fines to a tray below instead of letting them ride. The costs are real: an open surface needs support underneath so pieces do not nest in the apertures, and a wire belt in a dry duty needs less washing but more inspection at the drum ends where fines collect.

Solid PU belt with low side walls. The case for it is gentleness and product containment: no apertures, a flat continuous surface, easy to wipe, and it carries the product round curves without spillage. The costs are the mirror image: no airflow through the bed, so cooling needs either a much longer belt or a shallower bed — often a third again the length for the same temperature drop — and the dry crumb dust accumulates on the surface instead of falling through, so the belt needs brushing on a schedule the mesh does not see.

On a high-moisture, fat-coated line the PU belt usually wins. On baked kibble the maths inverts: the cooling requirement, not abrasiveness, is the deciding load, and airflow through the bed is worth more than a flat wipe-down surface. This is the same selection logic described in our note on mesh selection for drying and cooling — open area against bed depth is the trade that sets the belt length.

The layout is decided by six numbers a line owner can answer before anyone draws anything:

· Oven exit temperature — typically 90–110 °C, and the exact figure sets how much cooling the belt has to buy.

· Piece size and single-piece weight — a 15 mm porous piece and a 25 mm one need different bed depths and different drop allowances.

· Cooling endpoint — usually stated as "within a few degrees of ambient, and below the condensation threshold at the bag". The second half is the one that gets missed.

· Bed depth the line can afford — 40–60 mm keeps the bed porous to airflow; deeper beds buy residence time and quietly give back evenness.

· Residence time on the cooling run — for baked product this is commonly several minutes, not seconds, and it sets the belt area more than throughput does.

· Peak versus average throughput — the oven surges; the cooling belt absorbs the surge or the bed depth pays for it.

The 1×/10×/100× rule applies with unusual force here: cooling is the segment most often shortened when the building turns out smaller than the drawing assumed, and shortening it after installation is the expensive version of the same decision.

Baked product is a dry, low-fat duty, and the cleaning regime reflects it. Brush and vacuum on the bed between recipe changes, a full wash on the weekly pattern, and attention on two places only: the fines tray under the mesh and the drum ends, where dry crumb packs into the gaps and turns to paste the first time it meets water. Water drains at the infeed end; the discharge end stays dry. The parts that come off are the fines trays and the side guides — minutes, not shifts.

The oven made the product porous and the cooling run has to respect that: the real constraint on this line is not oven capacity but how long the pieces spend on open mesh before the bag, and every centimetre of drop between the band and the bed is a small tax paid in fines at the weigher. A line can be re-tuned around a shallower bed and a longer mesh run; it cannot be re-tuned around a checkweigher drowning in corners. When the cooling curve is given its room, the bag ships without the fog, the fines or the claim.

Orange Zhou, Technical Sales Manager · orange@xstlconveyor.com