A parcel sorter is a machine that reads each parcel's label and pushes the parcel into the lane for its destination, at thousands of parcels an hour. It is used in the sorting centres of courier companies, and at the end of large warehouses where orders are packed.
Parcels enter the sorter on a conveyor, one behind another. A ring of cameras above the belt reads the barcode on each label, whichever way up the parcel lies. The barcode holds the parcel's tracking number. The sorter's software looks up that number and finds the parcel's destination, such as a pin code or a delivery centre. Before each shift, every destination is assigned to a chute, a slide that ends in a bag or a cage. When the parcel reaches its chute, the sorter pushes it off the belt.
There are several kinds of sorter. In a cross-belt sorter, each parcel rides on a small carriage that has its own short belt, running across the direction of travel. At the right chute, that small belt moves for a moment and drops the parcel off the side. Other sorters use trays that tilt, or rows of small wheels that push the parcel off at an angle. Many sorters also weigh each parcel and measure its size as it passes. The courier then compares the real weight with the weight the seller declared, and charges the seller more if the declared weight was too low.
Say a courier company's sorting hub in Indore receives trucks from 40 sellers' warehouses every evening. Its cross-belt sorter handles about 12,000 parcels an hour. A parcel for a buyer in Ujjain passes under the cameras. The software reads the tracking number and finds pin code 456001. That pin code belongs to the Ujjain delivery centre, which is assigned to chute 37 tonight. A few seconds later, the carriage drops the parcel into chute 37. When the Ujjain bag is full, an employee seals it and scans it, and it goes on the 2 a.m. truck. A parcel whose label cannot be read goes to a separate chute, where an employee enters the number by hand.
The sorter depends on good labels. The labels come from the seller's shipping software, which prints each label with the courier's barcode on a label printer. A smudged or crumpled label sends the parcel to the manual chute and slows the hub. Every scan at the sorter is also a tracking event. The express courier network passes it on, and the buyer sees "reached hub" in the order's tracking page.
Sorters are one part of a hub's automated material handling, together with the conveyors that bring parcels in and the machines that load them out. A hub without a sorter sorts by hand, with employees reading each label and carrying the parcel to a pile for its city. Hand sorting can cope with a few thousand parcels a day, but not with a festive sale, when the number of parcels can rise several times over.