Flour bag packing and warehousing
October 01, 1993
by Teresa Acklin
Total concept should be considered when choosing equipment.
This is the second article in a two-part series on flour bag packing and warehousing. It was written by David Sugden, a grain industry consultant.
Flour bag packing and warehousing is a total concept, and various issues require resolution. Mill capacity, desired loadout, palletizers, warehouse type, and manpower ratios are some of the factors to be addressed.
Diagram A above is only one example of a multitude of possibilities, yet it serves as a demonstration.
The diagrams represent a system for a flour mill that runs six 24-hour days per week, or 144 hours, but packs only 40 hours per week (five eight-hour days). The mill uses bags only, with no bulk loadings. The mill feeding such a layout would be about 250 tonnes of wheat per day, or 4,400 cwt of flour per day.
The packing system comprises three open-mouth carousel bag packers, each with four spouts. The minimum bag weight is 25 kg (50 lbs), and the packers deliver direct to truck or to warehouse.
Although the nominal capacity of each carousel is 600 bags per hour, the average rate is calculated at only 400 bags per hour. The lower rate is used to account for truck delays, a very common problem. Thus, the effective capacity of the entire system is 48,000 bags per week, or 9,600 per day.
Each packing line is equipped with an automatic bag placer, stitchers, printers, bar coders and facilities for spillage and exhaust. Each line also includes check weighers and metal detectors.
Low-level-fed palletizers also are indicated. The palletizers include bag flattening and turning, as well as full pallet shrink wrapping. Pallet storage also is seen. The entire system is suitably exhausted to control dust.
Note in Diagram B the facility for returned or broken bags. The system uses a compressor or bailer to empty, or split, the bags. The recovered flour then is sieved and conveyed to a bin. Bulk flour intake also is possible, as the system has an in-line magnet.
Various options for truck loading are shown. Full pallets may be loaded directly or indirectly ex warehouse. Single bags not yet palletized also may loaded for road transport.
Palletizers may be one of two main types, the so-called high level (top fed) or low level (bottom fed). The merits of either depend on the exact circumstances, as both are reliable.
The high level palletizer is ideal when bag packing machines are one floor above because the feed of bags to the palletizer already is at or above that level. The low level palletizer typically is used when the bag packing machine is at the same floor level.
Both types are capable of palletizing at the necessary speed of 600 bags per hour. The load per pallet typically is 1 tonne, or about 40 25-kg bags, on a pallet size of 1,200 x 1,000 mm (48 x 40 inches). Variations are legion across the world, with pallets of 2 tonnes used sometimes.
Common pallet materials are wood, steel or plastic, the latter being some six times more expensive than wood. Wood is most commonly used, but wood pallets that are not in near-perfect condition will create problems on automatic equipment. That is one reason for plastic pallets, which also are more hygienic because they lack splinters, nails and the like. Steel pallets are not often used in flour milling due to cost, rust, weight and because they are prone to become mis-shapen.
Another palletizer type is the so-called skid sheet. Made of plastic, the sheet is a very tough, thick material onto which flour bags are deposited as with the pallet.
In the warehouse, a special fork lift truck places the loaded skid sheets on top of each other in slightly less stable stacks of up to two high. When loading a truck, the fork lift, with a special attachment, grips the plastic sheet tongue and pulls the whole onto the forks. The skid sheet, which can hold 1 tonne of bagged flour, is then loaded onto the truck base or empty pallet. The fork lift hydraulic attachment pulls the skid sheet away, leaving the bagged flour in place.
The cost of the special fork lift is about 50% more than standard. Moreover, it cannot function at heights of five meters (15 feet) or more, as can a reach fork lift. Thus, the advantages of the skid sheet are diluted because of cost and the inability to make maximum use of warehouse space.
Robots are sometimes used instead of dedicated automatic palletizers. Their advantage is that they are highly flexible. Their disadvantages include a much lower speed or capacity, along with the fact that a flour bag, being soft, is difficult to place accurately and neatly on a pallet. The standard palletizer scores all around.
The simplest warehouse is a weather- and rodent-proof shed without shelving. Depending on roof height, it is possible to stack three pallets safely. Above that level, stability begins to disappear.
In planning a warehouse area, 1 tonne per square meter (almost 10 square feet), a slightly generous size, should be allowed. There are two types of stacking, block and by rows. Block stacking maximizes fully the floor space available, but does not allow proper stock rotation. Inventory checking therefore is that much more difficult.
Clear row stacking with fork lift access to each allows almost perfect stock rotation. But the best method for inventory check and stock rotation is one shelf for one pallet. The cost, of course, is greater.
Even more expensive are the almost fully automatic warehouses. The concept comes from the engineering industry, where full automation of inventory deposit and retrieval is relatively commonplace.
Because of exceptionally high cost, only a very few flour mills are almost fully automatic. This system relies on shelving for each pallet, with fully automatic fork lift cranes and automatic guided vehicles, both on rails. The system is computer-operated and not yet completely manless.
Apart from cost, the other drawback is that no one yet has been able to resolve the problem of automatically depositing and retrieving individual flour bags according to customer demand.
Packing and warehousing including cleaning and truck loading are notoriously the highest manpower users in milling production. For the example illustrated in the diagram, the productivity manpower ratio should be about 1,500 bags (25 kg or larger) per man per day or better.
One pallet fork lift truck driver in a warehouse of the straightforward type described can shift 37 pallets per hour, the equivalent of about 1,500 bags. One man on a truck bed with a fully adjustable telescopic conveyor at shoulder height can stack at the rate of 500 bags per hour. If the conveyor is at hip height, the rate increases to 1,000 per hour, providing the bags are dropped, rather than lifted, into stacks.
Apart from safety and hygiene laws, bag weight laws of course must be observed. Weight laws consist of two types, minimum and average.
Average weight laws are more advantageous because they allow some short weight. But the term is defined differently among jurisdictions, so allowances should be researched locally. Minimum weight generally means no bags may be shipped out short weight.
COMPUTER PROCESS CONTROL.
The modern packing and warehouse system is fully process-controlled and therefore measured. Management information is readily available, including formulation, inventory, quality, weight, maintenance and diagnostic control and monitoring.
What about the manless warehouse? This seems doubtful, but more automation will come as the physical problems of flour bags and dust are resolved further. The question, as always, is one of reliability, not of ideas.