Wide selection of flour bags, packing equipment fits a variety of needs
September 01, 1993
by Teresa Acklin
This is the first article in a two-part series on flour bag packing and warehousing. It was written by David Sugden, a grain industry consultant.
Flour bags come in all shapes and sizes. These range typically from 500 grams (1 lb), up to 50 kilograms or even heavier at 63 kg (about 140 lbs). The materials used range from cotton, jute, polypropylene or other man-made fibers to paper. Kraft paper generally is considered the most hygienic.
Cotton thread, even string, is widely used to close cotton or jute bags. Paper bags are self-sealed or closed with adhesive or cotton thread. Polypropylene can be closed in several different ways.
Intermediate bulk containers of about 1 tonne also are occasionally used. The container sits on a platform weigher and usually is fed from a spout. The weigher stops the feed when full to a preset weight.
Bag types for household flour fall into two major categories. The first involves pre-formed paper bags, which are placed in a magazine on a machine and then packed. The second involves reels of paper, which the machine forms into bags and then packs.
Packing speeds for these types of machines vary from about 80 bags per minute to as much as 120 on the most sophisticated machines, with the reel-feed type being the faster for up to 2-kg bags. Flour packing speeds increase as bag weight increases.
Industrial-scale paper packing machines for household flour perform many functions of the packing process, including accurate flour weighing; automatic check weighing and rejection of underweight bags; continuous weight recording and feedback for compensation; metal detection; bag closing, usually by adhesive compounds; and either shrink wrapping or placement in cardboard boxes.
Flour bags for bakeries and other industrial uses consist of two main types, valve and open mouth. Bag capacity commonly ranges from about 10 kg to 50 kg (22 lbs to 110 lbs), and either type typically is available in two, three or four ply. The open-mouth type perhaps is used most commonly worldwide because, in the simplest application, it does not require special machinery.
Valve bags and open-mouth bags differ in several ways, and each has different advantages.
Valve bags can be self-sealing, eliminating the need for an operator and closing equipment. They make a very effective “square” pack for palletizing and storage at the mill, on the truck or in the customers' premises.
Some advocates assert that valve bags are dust-free when sealed, but this is debatable because any bag packed with any flour will expel air and dust when settling. Still, valve bags tend to emit less dust than open-mouth bags.
Another advantage of valve bags is that, when opened, there is neither string nor thread to contaminate a bakery. Open-mouth bags offer this benefit only when they are adhesive sealed.
Price is an advantage of open-mouth bags; they are cheaper because they are simpler to manufacture. They also offer a choice of different closure methods, such as cotton thread, tape or various adhesive systems. This flexibility can benefit the miller, depending on customer preference.
Valve and open-mouth packing machines both may pack up to about 800 bags per hour, and they share other features. These features include automatic bag placing; weighing; check weighing recording and feedback; bag closing; automatic bag number sequencing and printing on the bag; flour type printing; metal detection; bag flattening, palletizing and overall shrink wrapping, then warehousing.
As with the bags themselves, open-mouth bagging machinery provides flexibility. An open-mouth bagging system can be as simple as a hopper with flour, a bag clamp and a weigher; or as sophisticated as the carousel packing machine lines.
Open-mouth bagging machinery can fill bags made of cotton, jute, polypropylene and other materials, as well as paper, while the valve machine can fill only paper bags.
The more advanced open-mouth carousel-type machine can have up to 10 different bins with different flours fed to it. This means changeover from one flour type to another is not only virtually instantaneous, but also is cross contamination-free. With valve packing machines, fewer bins, and therefore fewer flours, can be so easily handled.
Speeds of open-mouth carousel systems can be around 800 bags per hour for the so-called six spout version. This version is essentially manless except for empty bag magazine filling and general supervision until the pallet is ready to move into the warehouse.
The above are the main comparisons when choosing a machine. However, the principal element in bag type choice is market demand in other words, get this wrong, and the whole system is wrong!
Used bag disposal.
Disposal of empty bags after use varies. Paper bags can be baled and recycled by the waste paper industry, where the recycled paper is used for newsprint, kraft or other lower-grade uses. Paper bags therefore are environmentally friendly.
Cotton and jute bags find a myriad of uses, from garden waste removal to cleaning materials. Polypropylene and the like are notoriously difficult to recycle effectively, but find many uses.
The important issue in this first article is not only the different materials used for flour bags, but especially the bag type because type is almost always a customer-driven preference. However, most millers will want to steer or influence customers according to what suits best. Accordingly, a study should be made with the help of the various supplying engineers before commitment is made to new machinery.