Multiwall bags
INTRODUCTION
Multiwall bags are concentric tubes of 2–6 layers or paper with a choice in the type of end closure. Designs differ mainly with respect to whether the sack is to be filled through an open mouth or valve. This depends on the product or volume to be handled. Valve designs are closed automatically as a consequence of their design and there are various methods, including sewing and tying for the closing of the open mouth paper bag.
Prior to the United States Civil War, two industrious merchants in upstate New York, Arkell and Smiths, had been looking at cotton sacks to ship flour and grains to the larger southern cities; and from this vantage point, they were also investigating the use of paper sacks for marketing flour. Arkell received the first U.S. patent for paper sack making machines in the 1860s; this was just in time to permit paper sacks to replace the difficult-to-obtain cotton bag fabric during the war.
At the same time, salt was being packaged and marketed in cotton bags, until the 1890s, when Adelmer Bates developed a concept wherein a ‘‘valve’’ was incorporated into one corner of the cotton bags. Its advantages provided a faster method of filling cotton bags with salt, and it was patented in 1899. In 1901 John Cornell worked with Bates on the development of a valve sack filling machine, and in 1902 they were awarded a patent on a valve for paper sacks, but almost two decades passed before the paper shipping sack was widely used.
In 1919 R. M. Bates, Adelmer’s brother, was visiting in Norway, where he saw a paper sack of a truly different construction. Having five walls made from long strips of kraft paper with each edge of the strip pasted to its adjacent edge, it formed tubes of paper. These were cut to length and were open at both ends. At the place where they were to be filled, one end was gathered together and tied off; when filled, the top end was also tied with twine or wire. These were flat paper tubes.
While Bates was working on his ideas of filling these tubes through the valve corner, his partner Cornell conceived the idea of having valve corners inserted into a gusseted tube, or ‘‘in-fold’’ feature. These were first made in the mid-1920s, and their advantages were promptly recognized. During the first year of production, millions were manufactured and sold, and by 1927 about 130 million were made annually.
While the cotton bags with their paper-filling sleeves had solved the bag-filling problem, the other problem of their return and reuse to reduce bag costs still existed. Into this breach came the multiwall paper sacks (either with or without filling sleeves), and thus the paper shipping sack industry was born.
Portland cement and masonry cement plants, grain products and flour factories, and saltworks were the earlier multiwall sack users, because these new bags solved the dusty conditions that had been so dangerous, healthwise. Now the workers were freed from breathing undesirable dust and airborne mineral deposits, and working conditions were greatly improved.
The terms multiwall bags and paper shipping sacks are often used synonymously. Other industries followed suit as soon as more producers of multiwall bags were established, and the shipping sack industry prospered accordingly. WorldWar II caused severe shortages in burlap and manila fibers due to the extremely long distance from the primary sources in Southeast Asia; concurrently, cotton fiber for bags was in short supply, due to U.S. military and naval needs (as well as for civilian clothing). These shortages resulted in a large expansion of the use of paper shipping sacks, and the cotton bag never recovered its earlier dominance, just as cotton and burlap had replaced the rigid containers such as wood boxes, crates, and barrels.
During World War II the multiwall bag really came into common useage. With a wide range of special papers to control moisture and insect infestation plus the ability to tailor-make whatever package strength was required to fit the rough military handling, multiwalls came to be the approved specification packages for government purchasing.
From these early beginnings the sack industry has grown into a multi-billion-dollar business with shipments of over three billion sacks annually for over 2000 different products in the United States. Over four billion sacks are used annually in Europe (1). These end-uses fall into four major categories: agricultural products and supplies, food products, chemical products, and rock and mineral products.
The primary advanges of paper shipping sacks are low tare weight, flexibility, ease of filling and handling, low cost, minimum storage space, biodegradability, and good graphics. Other advantages include the basic material, which is a renewable resource; protection of contents from moisture absorption; control of contents from moisture loss; protection of contents from chemical action; control of seepage or penetration of hot-packed products; provision of loss of gas or vapor; prevention of product sifting or contamination; good stacking and utilization of warehouse space; FDA approval for human food products; excellent graphics; and ease of use in merchandising displays.
Multiwall bags exclude single-ply bags and bags of duplex construction, but do include those with three or more plies of paper or other barrier protectors. They are produced by combing several layers of paper (or other substrate) over a metal former that nests the walls into a long continuous tube (flat or gusseted). These walls are bonded together with adhesives so that each tube is independent of the other tubes, and therein lies the bag’s strength. The tubes may be cut straight (all at once) or in stepped-end formation where each wall is first perforated, then simultaneously snapped apart.
Next, in order for the tubes to be filled and used, the tubes are delivered to bottoming machines in the bag plant where they are either sewn closed at one or both ends. Or, the tubes may go to a pasting bottomer to form either pasted open-mouth, stepped-end pasted valve, or pinch-bottom open-mouth sacks (see Figure 1).
TYPES OF MULTIWALL SACKS
There are two basic multiwall types: the open-mouth type and the valve type bags. Multiwalls are custom-made to order according to the customer’s requirements and for adequate strength to ensure the safe arrival of their contents to the destination locations.
Whether the product is packaged with high-speed openmouthweighing and filling machines with automatic closing and sealing machinery, or with even faster valve packers and automatic bag placers, a proper and successfully designed and printed multiwall bag may be readily developed.
Most multiwall bag suppliers have very well trained sales personnel, as well as experienced factory technical representatives with a wide range of user experience who are well qualified to recommend the best bag for any product requirement.
OPEN-MOUTH BAG TYPE
Sewn Open Mouth (SOM)
This style, with any number of walls, has a factory sewn bottom and open top, and may be either flat or with side gussets. These bags are used primarily to package granular or large-particle products. The product is delivered into the open top, and the top may be closed by sewing or other means. If a polyethylene liner ply is specified, this may also be heat-sealed. These types of packaging lines may be fully automated.
Pasted Open Mouth (POM)
This style has a pasted bottom and open-mouth top, and it is sealed after filling by folding and pasting, or, in some cases, sewing the top. This latter style is not preferred for packaging and shipping free-flowing products because a finlike (nonflat) package is obtained. But it is ideal for containing a number of previously filled and sealed unit bags–for example, 12.5-lb sugar or 5- to 10-lb bags of potatoes. These are called master container or baler bags.
Pinch-Bottom Open-mouth (PBOM)
This style has the plies cut in a stepped fashion with the bottom sealed by gluing or heat sealing at the bag factory. After filling at the user’s plant, the open mouth top is folded over and sealed by reactivating preapplied hot-melt adhesive. This style of bag provides completely moistureproof and sift-proof packages. PBOM bags are the fastestgrowing segment of the total multiwall bag industry because of the secure and strong closure ends, and full control of sifting or infestation.
VALVE BAG TYPE
Sewn Valve (SV)
These are made with the plies at the top and bottom ends sewn at the bag plant usually with sewn-through crepe paper tape. The bags are filled with a valve packing machine by forcing the product through a built-in valve corner. For small particles and large powders, the contents may be retained without any leakage, by hand-folding in the paper valve extension.
This old style with sewn tape at both ends has fallen out of favor in recent years, and the modern pasted valve style has supplanted it.
Pasted Valve Stepped End (PVSE)
As in the pinch-bottom open-mouth bag, the plies are cut off in a stepped configuration; then the bag factory bottoming machine folds and, with adhesive, seals both top and bottom ends of the tubes. An added valve sleeve is built into a corner, permitting rapid bag placement on a valve packer; and when the filled package falls to a takeaway conveyor, the valve automatically is closed by the internal pressure of the product.
PVSE-type bags, introduced in 1956, are the present standard package for most automatic or high-speed operations such as cements, powders, feeds, fertilizers, or other chemical or building products. When stacked or palletized, they provide stable and attractive three-dimensional containers.
Valve bags may also be purchased with paper tuck-in sleeves that, when folded in, will control all possible leakage.
SIZING MULTIWALL BAGS
First, the weight to be packaged and the product density must be determined. This latter may be done by using a box of known dimensions (1 ft × 1 ft × 1 ft) or 1 ft3 to determine the weight per cubic foot. With the known weight and volume requirements and with the bag type selected (by product characteristics), an educated estimated set of dimensions can be offered. A supplier can fine-tune these numbers by offering several handmade factory samples. When test-filled, these may be adjusted to supply a larger number of machine-made test packages to finalize all specifications such as dimensions, filling, handling and shipping, and palletizing conditions.
CONSTRUCTIONS
The paper most commonly used to fabricate multiwall bags is natural-colored virgin pulp brown kraft. Made primarily in the Southern U.S. states using a modified sulfate pulping process, it affords longer fibers that enhance the paper’s cross- and machine-direction strength. Typically brown, it may be lightened through bleaching, or semibleaching sulfate pulping.
Kraft is usually designated in basis weights ranging from 40 to 80 lb per ream of 500 sheets of 24 in. 36 in. in area. These grades are known as multiwall kraft and have higher strength specifications than grocery-bag kraft (NK) used for light-duty brown paper bags.
Other popular grades used in multiwall bags are extensible (XTK) and free-dry (FDK) process papers. These developments during the 1960s and 1980s resulted in improved cross- and machine-direction stretch specifications and greater overall bag strength. Other grades, such as high-finish, rough, calendered, machine-finish, and wet-strength, are available for specific end-use purposes (see Paper).
Subsequent to the mid-1940s with the development of tailor-made plastics, polyethylene (PE) films have been integrated into many bag constructions. Prior to this, asphalt laminated kraft (ALK) provided moisture barrier properties to multiwall bags, but it was difficult to fabricate on the tuber and bottoming equipment, and it is no longer in use.
Multiwall paper bags may be lightly coated with highshear, low-stress adhesives to overcome bag slippage when stacked on pallets or in storage. This inexpensive treatment is invisible yet very effective.
Heavy-duty single-wall plastic film bags are also available, however, when compared in cost with multiple-ply kraft or a kraft paper ply plus a lightweight poly ply or polyethylene-coated kraft, the single-wall plastic bags are not cost-effective. They also are slippery, and palletized filled bags cause shipping difficulties (see Bags, plastic).
Depending on the physical characteristics of the product being packaged in multiwall bags, they can be custom-designed to provide all the protection that is necessary. Low-or high-density polyethylene, poly(vinyl chloride), poly(vinyl dichloride), nylon, saran, foil, and polypropylene are some of the available substrates.
Most multiwall bag suppliers have well-trained sales personnel and specially trained factory technical representatives who are well qualified to recommend the best construction for any particular requirement.
SPECIFICATIONS
With the complete knowledge of a product’s physical and flow characteristics, a bag style may be selected. This is only a minor portion of the information required to design a new multiwall bag that will deliver the product in acceptable condition. The product density and the desired weight to be bagged must be known; from this, the required capacity (in cubic inches) is calculated.
General rules are usually followed to select the total kraft paper basis weight and the number of kraft plies. For instance, 50 lb of product might require three walls of 50-lb natural kraft to total 150 lb, and 80 lb would need three or four walls of combined 40- and 50-lb basis weight multiwall kraft, totaling ~180 lb.
The requirements for product protection must be carefully evaluated. This might include moisture protection and control, gas or odor control, grease or oil control, siftproofness, chemical resistance, mold protection, and toxicity or hazardous chemical protection.
Each of these factors can be controlled by the use of one or more of the following available sheets: low-density PE (LDPE), high-density PE (HDPE), polyethylene-coated kraft paper (PEK), saran, Tyvek, saran-coated polypropylene (SCPP), polyfoil–polykraft laminate (PE-AL-PE-K) biaxially oriented polypropylene (BOPP), or even polycoated crepe kraft tape, or heat-sealed waxed kraft tape. About 25% of the multiwalls now being produced specify some kind of special liner ply.
Special barrier plies are commonly used in multiwall bags to protect the product from gaining or losing its intended moisture content. This is stated in terms of its water vapor transmission rate (WVTR). A wall of material may be laboratory-tested, to determine its WVTR rating in terms of g/m2 per 1001F at 95% RH (relative humidity) in 24 h. The more effective barriers usually are the heavy 2.5–4.0-mil HDPE and LDPE, or 0.75-mil BOPP film, while those most frequently needed are the less expensive 0.5-mil HDPE and 0.8–1.0-mil. LDPE films.
Some products have specific needs, such as controlled porosity to permit proper air or moisture transfer, grease or oil penetration, wet-strength protection with urea formaldehyde, or melamine formaldehyde, which enable the paper to retain its original strength when saturated by water. All these and more—toxicity, hazardous nature, odor control, acidity, or alkalinity—can be controlled.
Or, for a really attractive consumer package, the manufacturer might select a supercalendered high-finish fullbleached white sheet with gravure-quality printing.
Multiwall prices are developed by applying cost factors for bag style, bag size (sheet width and tube length), basisweight factors for kraft, and the plies of other substrates. To these the supplier adds the cost for printing by area and number of colors required. Bags are usually ordered by the carload or trailerload to obtain the lowest unit price.
In cases where a product is highly aerated and moisture protection is only nominal, one or more kraft plies may require allover pinhole perforations (AOPHP) of different diameter and spacing. These allow the internal pressure to be expended during bag filling and they prevent bag breakage.
Discussing the particular needs that are required for your product with a multiwall bag supplier will direct you to the correct and complete bag specifications from the weighing and filling stages to the customer’s ultimate place of use.
PACKAGING EQUIPMENT
For each style and size of bag used, there is a wide choice of equipment available for weighing, filling, and closing the packages. There are two main styles of bagging equipment: open-mouth and valve.
Open-Mouth Packers
The older, original method of open-mouth bagging required a scale, a vertical delivery spout, and a closing function. With today’s modern high-speed open-mouth equipment, there are automatic and accurate weighing scales, automatic bag placers, and finally, automatic bag closures.
These systems usually run up to 20 or more preweighed charges of up to 100 lb of material per minute with only token supervisory requirements. There are two preferred methods of closing open-mouth multiwall bags: sewn closure or heat sealing.
Most common of the sewn types is the stitching of the bag mouth with cotton or polypropylene sewing thread with an industrial sewing head. With more secure or special product requirements, either sewn-through tape, adhesive, or heat-sealed tape over sewing may be provided.
Valve Bag Packer
With the invention of the valve packer in the mid-1920s, the valve-style bag may be filled by forcing the product through a spout (usually horizontal) into the valve corner and then down into the bag. The product is either preweighed and then forced into the bag, or it is weighed in the package (gross-weighed).
Choices are available when weighing and filling valve bags, and they depend on the product’s physical characteristics, price, and production speed requirements.
Most valve packers use either the impeller-wheel method or the belt-feed method. With the former, rotating high-speed paddles force the material into the package. This causes a great amount of aeration to occur, and the PV or PVSE bags must be capable of withstanding great pressure. One operator with a four-spout impeller valve packer may produce up to thirty 94-lb (206-kg) filled bags per minute.
With the belt valve packer, the product is thrust between a moving belt and a rotating grooved pulley and then into the valve opening. This is gentler on the material, but not as fast as the impeller method. Fertilizers and small-pelleted materials are usually packaged on belt-style valve packers.
Three other less popular valve packer styles are available auger, gravity, and airflow. The latter is more modern than the other two and has the advantages of handling non-free-flowing finely powdered materials, without degradation with much better accuracy. The auger packer utilizes a horizontal screw within a hollow tube. These work well for non-free-flowing products, and since the screw’s rate of rotation can be adjusted, better weight accuracies are obtained. The last and least popular valve packing method is the vertical-gravity style. But, since it is used for very free-flowing or inexpensive materials, accurate bag weighments are sometimes difficult to obtain.
GRAPHICS
Previously, the fabrics of multiwall paper bags were printed with simple yet effective designs to identify the manufacturer, the product, the net weight, and some methods for using the contents.
Cotton and burlap bags had a second life, since they must either be returned, cleaned, and filled for reshipment or, in the case of the cotton bags, be washed and then sewn into towels or items of apparel at home.
With the conversion from fabric to paper bags came the important advantage of superior printing capabilities. With smooth surfaces and all-around six-sided printing of up to four colors, a company’s marketing message could be extolled, thereby differentiating itself from all its competitors.
If brown kraft is not attractive enough, a supplier can readily furnish semibleached (SBW) or full-bleached (FBW) kraft. Smoother or brighter papers and coatings in conjunction with modern printing capabilities provide appealing and forceful advertising that make the bags effective billboards for marketing their contents. Whether they were small-unit consumer bags or large industrialsize bags, they can utilize flexographic printing to carry the intended messages.
If more definition is required, prepress gravure printing using up to six colors will result in photographic quality printing, with the following advantages: excellent product and protection; unlimited supplier capabilities; biodegradable materials; and proven shipping and storage environments.
Pet-food and charcoal briquette bags are excellent examples of effectively printed packages found in all supermarket shelves today.
Multiwall bags are readily accepted and desirable as shipping containers in all domestic and export markets, including hundreds of uses for agricultural products, building materials, chemicals, food products, and minerals.
TRANSPORTATION
In the past 40 years the shipment of filled multiwall bags to the market where they are used has swung almost completely from railroad boxcars to over-the-road trailers or straight trucks. Also, pallets have replaced the individual handling of filling bags. Coincidentally, plastic film has in many cases been incorporated into the bag’s construction. These changes have resulted in lighter basis-weight requirements and lower costs. Multiwall sacks are covered by the Uniform Freight Classification for Rail Rule 40; National Motor Freight by Truck Item 200; Hazardous Materials Regulations title 49 CFR; and Federal Specifications UUS-48, sacks for government shipment (2).
ENVIRONMENTAL CONSIDERATIONS
At present, the essential element in all paper sacks is virgin fiber. Secondary or recycled fiber is not an option because such material can result in an inconsistent pack that would be weaker and unsuitable for automated filling and in handling systems. The bags would be more costly to produce and inmany cases would be unacceptable for direct contact with food. As a result of continuous research, there has been substantial reductions in the average construction weight of a multiwall paper sack. Recovered bags can be recycled. It has been suggested that specifications should be revised to maxmize the content of secondary fber. A levy on virgin fiber has been proposed in Europe.
USES
Some examples of products that are packaged in weights of under 20 lb are additives, animal food, baking products, cat litter, charcoal, chemicals, coffee, cookies, dried foods, grits, insecticides, microwave popcorn, plant food, theater popcorn, salt, and sugar. Industrial products weighing more than 20 lb and packed in multiwall bags are absorbents, abrasives, cement, lawn and garden supplies, and animal feeds (3).
Helpful Hints.
The list below is meant to be of use to a packager in selecting an appropriate multiwall paper sack.
- In the initial planning stage of a new use for multiwall shipping sacks, obtain the guidance of bag suppliers.
- Develop accurate product physical and chemical properties.
- Finalize decision on desired package weights.
- Determine annual production requirements.
- Select proper bag style and features.
- Consider styles of packaging equipment available.
- Decide on the transportation and distribution modes.
- Determine probable bag sizes and constructions.
- Plan bag transporting methods to provide minimum individual handlings.