Conveying Freeze-Dried Dog Food: Where the Pieces Break Before the Scale

A tray comes out of the freeze-dryer holding maybe 4 kg of chicken cubes, each one about 15 mm across and light enough that a handful weighs less than a biscuit. The operator tips the tray into the infeed hopper from a height of roughly 400 mm. Nothing dramatic happens. A few pieces crack, a little dust rises, and the line runs. By the end of the shift the fines tray under the weigher holds 6 kg, and the yield report says 97.8% — a number nobody questions until the quarterly review, when someone asks what 2.2% of a high-value product is worth.

Freeze-dried product is the most fragile thing on any pet food line, and almost all of the damage happens in the first metre after the dryer and the last metre before the bag.

1. The Moment the Tray Tips

2. What Freeze-Dried Product Is, Physically

3. Where Conveying Sits on a Freeze-Dried Line

4. Three Places It Goes Wrong

5. Why a Smooth Belt Rather Than a Mesh or a Vibratory Feeder

6. The Numbers That Decide the Design

7. Changeover and Cleaning Without Water

8. One Sentence for the Line

Freeze-drying removes water by sublimation: the product is frozen, the chamber is evacuated, and the ice turns directly to vapour without passing through a liquid phase. What remains is a skeleton of the original tissue — porous, light, and structurally closer to a dry sponge than to dried meat.

The variables that matter for conveying are unusually unfavourable, and they arrive together:

There are four conveying steps on a typical line, and they are not equally dangerous. The table is the useful part of this section.

Total belt length on such a line is often under 12 metres. The breakage does not scale with those metres. It scales with the number of vertical drops and the number of times product is accelerated sideways, which is why a line with 8 metres of belt and 5 transfers loses more product than a line with 14 metres and 2.

Segment 1 is where most of the loss occurs, and it is usually outside the conveyor scope — a manual tip, a dumper, or a chute that was drawn as an arrow on the layout. Segment 2 is where the design decisions are still open when the conveyor is specified, and it is the segment worth arguing about.

The tip and the hopper throat. Product falls into a hopper whose walls are stainless. At the impact point, pieces break, and the fragments land on other pieces and break those. The first 20 kg into an empty hopper takes the full drop; after that the pile absorbs some of it, but the pile also presses on the product at the bottom, and the hopper discharge is where that pressure releases into shear. The result is fines at the weigher and a product that photographs badly.

Cleats and the incline. A cleated belt is the obvious answer to "get it up 800 mm". Product arrives at the cleat, is pushed by it, and gets pinched between cleat and belt at the base — where a 1.5 g piece has no strength to resist. Low cleats or a steeper smooth belt with a guide rail are gentler, but a smooth polymer surface starts losing freeze-dried pieces backward above roughly 12–18°, depending on surface and how much dust is present.

Static and the dust loop. Dust generated at segment 1 does not stay there. It becomes airborne, settles on the belt, on rails, on the weigher's radial feeders and inside hoppers. Fill weight then drifts, because the weigher weighs product plus dust and dust does not pack the same way twice. The dust can also bridge in a hopper throat, and a bridge in a 1.5 kg batch is a 40% underfill. On lines running multiple proteins it is the allergen carryover route as well, and it travels further than expected because it is light and charged.

Three candidates get considered for segment 2. Each is defensible; the differences are in what they cost downstream.

Vibratory feeder handles fragile product well in principle — product slides rather than rides — but freeze-dried pieces are light enough that vertical vibration makes them hop. Hopping produces dust, dust adheres to the tray and the next pieces, and the tray needs cleaning more often than a belt does. It is a good answer for dense, hard product and a mediocre one for a product with the density of breakfast cereal and the strength of a dry leaf.

Smooth belt, low tension, minimal transfers. The product rides without being pushed by anything, the surface can be cleaned by vacuum and dry brush, and the number of breakage events equals the number of transfers, which can be counted and reduced. The trade is that inclines need more length, and the belt needs to be flat enough that product does not migrate to one side — light, dust-lubricated product tracks to the low side of a frame that is 5 mm out of level.

Six numbers, all of them known to the plant, and none of them replaceable by a throughput figure.

The number most often missing from an enquiry is bulk density. A line described as "300 kg/h" can be 1.5 or 3.7 m³/h depending on the product, and those two require different belt widths, hopper volumes and weigher configurations.

Freeze-dried product is cleaned dry, and the design has to make that possible.

Water plus powder equals paste, and paste in a belt seam or a frame crevice is a microbial habitat that later washing does not remove without disassembly. So the belt is lifted or released at one end, the frame is open on both sides, and the surfaces are reachable with a vacuum tool and a brush. Belts with a mechanical splice open more easily than endless belts, and the splice is also where product accumulates.

The breakage on a freeze-dried line is decided in the 400 mm between the tray and the hopper and in the 300 mm in front of the weigher — not along the six metres of belt that everyone photographs.

About us. Shanghai Xisheng Taolan Intelligent Equipment Co., Ltd. builds custom conveying equipment for food and pet food plants. For fragile, low-density products the design work is mostly about counting transfers and removing drops, which is a smaller job than it sounds and a much cheaper one than it looks when it is done after installation.

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