In packaging, any of a wide variety of flexible containers, made from plastic, paper, foil, or other materials.
Most people are familiar with a variety of paper bags, such as kraft grocery bags, small paper lunch bags, and many types of small, often printed bags used to package goods at retail outlets. In packaging applications (as distinguished from consumer bags), literally billions of paper bags are manufactured each year. In the packaging industry, there are three primary classifications of paper bags: single-ply bags, duplex bags (or two-ply bags), and multiwall bags (three-ply or greater). (Duplex bags are also often called multiwall bags.) Multiwall bags, the most commonly used in industrial and consumer packaging, often include one or more layers of plastic, foil, or other non-paper material, depending upon the degree of vapor-barrier performance required. Such bags often require some level of moisture or air protection, especially those that are designed to contains foods or other materials which can be damaged by prevailing environmental conditions.
'Materials'. Paper bags, as their name indicates, are primarily manufactured from paper, specifically kraft paper, a strong, durable paper made from bleached or unbleached wood pulp, specifically sulfate pulp (see Sulfate Process). Kraft paper for paper bag manufacture is manufactured at basis weights of 25:60 lbs., at a basic size of 24 x 36 inches. Kraft paper used for industrial or commercial paper bags is often referred to as shipping-sack kraft, which is stronger than so-called "grocery kraft" paper. Paper used for bags can come in a variety of finishes, in particular machine finish, or a type of moderately glossy finish produced by a calender. Plastics are also used in multiwall paper bags—specifically polypropylene, polyvinyl chloride, and other polymers—either as separate plys or as paper coatings. Most multiwall paper bags consist of just kraft paper, but about one fourth of all the multiwall bags produced contain a plastic ply or plastic component.
'Bag Styles'. There are five basic paper bag configurations recognized by the packaging industry. Sewn open-mouth bags are sewn at the bottom, the top remaining open to facilitate filling. After filling, the top may be sewn, taped, stapled, or glued shut. SOM bags are often used for granular materials. Sewn valve are similar to SOM bags, except that both the top and bottom are sewn shut prior to filling, filling being accomplished by means of a small valve commonly located at a corner of the bag. Smaller granular materials are often put in SV bags. Pasted open-mouth bags are analogous to SOM bags, except that pasting rather than sewing is the primary sealing means. Pasted-valve stopped-end bags are analogous to SV bags, both ends being pasted shut prior to filling, with filling accomplished by means of a small valve. Filled PVSE bags are characterized by a distinctively square shape which facilitates further printing, shipping, stacking, and storing. Finally, pinch-bottom open-mouth bags are manufactured by pinching shut and gluing the bottom, filling the bag, and using a hot-melt adhesive to glue the top shut. The PBOM bags are increasing in popularity, as the tightness of the seal produced by the pinching and the use of hot-melt adhesive result in a more secure closure than is possible by traditional sewing, reducing leakage and contamination significantly.
'Filling and Closure'. The bags are filled either by impellers which force material though the valve, by a belt and pulley arrangement, by a horizontal, screw-like auger, by air flow means, or by simple force of gravity. Bags are closed either by sewing (the traditional method), which may then be covered over with tape, or by using heating elements to melt the plastic plys of a bag, effectively sealing it.
'Printing'. The printing of distinctive graphics on commercial, industrial, or consumer bags is more often than not performed using flexographic presses, which have increasingly been able to print at a high quality on a wide variety of substrates and with a wide variety of inks. In some cases, gravure presses—the original presses used for packaging—are also used.
Paper bags—of any of the above varieties or configurations of equipment—are used in five primary markets: agriculture and food (such as feed for livestock and pets, as well as flour, sugar, coffee, rice, etc.), construction material (such as cement mix, insulation, etc.), chemicals (such as fertilizer, salt, etc.), minerals (such as clay and lime), and a broad category of absorbent materials (such as kitty litter).
Plastic bags have replaced paper in many areas, but paper bags are still preferred in many heavy-duty applications.
'Manufacture'. Plastic bags are categorized in one of two ways: by application (see below) or by the method by which the sheets of plastic are sealed together to form a bag. A sideweld seal is created when a heated blade cuts and seals two layers of film. The knife passes through two sandwiched layers of plastic and, as it cuts through both sheets, the combination of heat and pressure fuses the sheets together. This type of bag is used in bread bags, ice bags, sandwich bags, vegetable bags, and some trash bags. In a bottom seal, a special seal bar is used to fuse the layers of plastic together at the seam, but a separate cutting action removes the bag from the rest of the bag stock. This latter method has advantages over the sideweld method in that the dwell time is easier to control. Additionally, the combination cutting/fusing action of the sideweld device can change the physical structure of the plastic at the point of the seal. The bottom weld is used on vegetable bags (especially vegetable bags that come perforated on a roll, as in a grocery store produce department), dry cleaning bags, freezer bags, and trash bags. A twin seal uses the same principle as the bottom seal, only it includes—as its name indicates—two seal bars, separated by the cutting knife. The interesting wrinkle with the twin seal method means that two separate seals on two separate bags can be made at the same time. This configuration also allows it to be used to seal bags on both sides (rather than the bottom) simultaneously. The twin seal is used on grocery bags primarily.
'Application'. Bags are primarily classified in terms of their ultimate end uses, such as "bread bag," "trash bag," etc. The two primary divisions, however, are commercial and consumer. Commercial bags are those used to package another product, such as bread. Consumer bags are those sold and purchased as bags, such as sandwich bags, trash bags, etc.
There is also an extremely large specialty bag market which advanced bag-making technology has made possible, such as "handle-tie" bags, drawstring bags, deli bags, etc.
'Heavy-Duty Plastic Bags'. Less than 10% of all the heavy-duty bags used in the United States are plastic; paper is still the material of choice for most heavy-duty usage. Plastic is used for materials—such as fertilizer, rock salt, ice melters, etc.—that require strong barriers to moisture. Some materials—such as industrial sulfur, pet foods, etc.—need to "breathe" in the bag, which reduces the occurrence of a variety of deleterious effects, such as the growing of mold or the spontaneous bursting into flame. Heavy-duty plastic bags are most often formed from a single ply of a thermoplastic film, such as polyethylene, either in high-density or low-density varieties. Two-ply bags are also manufactured. These bags can either be open-mouth or valve bags, which have the same basic configuration as open-mouth and valve paper bags. Valve plastic bags tend to square at the bottom when they are filled, and are thus also known as "square-bottom bags." Various form/fill/seal technologies have been applied to heavy-duty plastic bags. As with most other types of bags, flexography is the printing method of choice.
Although heavy-duty plastic bags are more expensive to produce than their paper counterparts, plastic can be more cost-effective in the long-run, due to its increases resistance to moisture and chemicals. In the garden products industry, for example, it can be more cost- and space-effective to store overstock outside, which plastic makes possible.