Bag making machinery

Heavy-duty bags, i.e., shipping sacks, of multiwall paper or single-wall mono- or coextruded plastic are used to package such dry and free-flowing products as cement, plastic resin, chemicals, fertilizer, garden and lawn-care products, and pet foods. These bags typically range in capacity from 25 to 100 lb (11.3–45.4 kg), although large plastic bulk shipping bags may hold as much as a metric ton (see Bags, paper; Bags, heavy-duty plastic; Intermediate bulk containers).

Although there are dozens of variations in heavy-duty bag constructions, there are only two basic styles: the open-mouth bag and the valve bag. The former is open at one end and requires a field-closing operation after filling. Valve bags are made with both ends closed, and filling is accomplished through an opening called a valve. After filling, the valve is held shut by the pressure of the bag’s contents.

MULTIWALL-BAG MACHINERY

Traditionally, multiwall bags are manufactured in two operations on separate equipment lines. Formation of tubes takes place on the tuber. Closing of one or both ends of the tubes to make the bags is done on the bottomer. Multiwall bags have two to six plies to paper. Typical constructions are three and four plies. Polyethylene (PE) film is often used as an inbetween or innermost ply to provide a moisture barrier.

Tube Forming.

The tuber (Figure 1) starts with multiple giant rolls of kraft paper of a width that will finish into the specific bag width. At the cross-pasting station, spots of adhesive are applied between the plies to hold them together. The material is then formed into a tube that is pasted together along the seam. The tube may be formed with or without gussets. During seam-pasting, the edges of the various plies form a shingle pattern. When they are brought together to form a seam, these edges interweave so that each ply glues to itself. This provides optimal seam strength.

Flush-cut tubes are cut to the appropriate sections by a rotating upper and lower knife assembly. With steppedend tubes, perforating knives are used to cut stepping patterns on both ends of the tube. The tube sections are then snapped apart along perforations that were made prior to crosspasting. This snapping action is accomplished by sending the tubes through two sets of rollers, with the second set moving slightly faster than the first. Once the tubes have been flushcut or separated, they proceed to the delivery section of the line.

A universal tubing machine. Figure insert shows components: 1, flexoprinter; 2, unwind stations with reel-change arrangements; 3, automatic web brake; 4, web-guider path rollers; 5, web guider; 6, vertical auxiliary draw; 7, perforation; 8, cross pasting; 9, longitudinal register rollers; 10, seam pasting and auxiliary draw; 11, tube forming; 12, cut/register regulator; 13, cutting and tearoff unit; 14, variable-length drive-unit; 15, packet delivery unit; 16, takeoff table. Figure 1.

About one third of the multiwall-bag market is accounted for by bags with an inner, or intermediate, ply of plastic film. Flat film, used as an inner or intermediate layer, is formed into a tube along with the paper plies and pasted or, if necessary, hot-melt laminated in place. Another possibility is the insertion on the tuber of openmouth film liners, the open end of which can project beyond the mouth of the paper sack. Stepped-end and flush-cut tubes are usually made on differently equipped tubers, but a universal model also can be adapted to produce either type.

Flush-cut vs. Stepped End.

Flush cutting is the most inexpensive tubing method in terms of both original equipment investment and tubing productivity, but these gains are lost in the subsequent bagmaking operations. The bottom of a flush-cut tube is normally sewn, and sewing is also the traditional method of field closure for many products such as seed and animal feeds. There was some use of flush-cut tubes as valve bags, particularly in Europe, but they are not widely used today because a pasted flush-cut bottom is structurally weak. The gluing of the bottom takes place only on one ply. To compensate for this weakness, a patch would normally be added to the bottom of the bag. Sewing is a widely used bottoming method in the United States because there is a great deal of flush-cut tubing and sewing equipment in place, and replacing it in many instances would result in only a marginal return on investment. Unfortunately, sewing has many drawbacks. It is labor-intensive, and because the equipment has a large number of delicate moving parts, maintenance and repair costs are high. Also, the needle holes created by sewing weaken the bag, allow sifting, and make the bag more accessible to rodents and other pests.

The stepped-end tube makes the strongest bag. The ends of the bag have shingle-like stepping patterns that intermesh at the gluing points. In the bottoming process, ply one is glued to ply one, ply two to ply two, and so on. Generally speaking, all bag manufacturers have their own stepping-pattern designs.

Bottoming Equipment.

The finished tube sections are converted into bags by closing one or both of the tube ends in any of the following three ways:

1. One end of the tube is shut, forming a sewn openmouth (SOM) bag. After filling, the top of the bag is closed by means of a portable field sewing unit. 
2. A satchel bottom is formed on each end of the tube, with one of the bottoms provided with an opening or valve through which the bag is filled by insertion of the spout of an automatic filling machine or packer. The valve is closed by the pressure of the bags contents. Additional means are available to make the bag more siftproof. In a valve bottomer (Figure 2), tubes advance from a feeder to a tube aligner and a diverting unit for removing incorrectly fed tubes. The tubes pass through a series of creasing stations, and needle holes may be added under the valve for proper venting of the bag during filling. At the opening section, the tube is opened and triangular pockets are formed. Valves are inserted at a valving station. Valves are automatically formed by a special machine unit and then inserted into the bottom. They may be inserted and folded simultaneously along with the bottom or performed and automatically inserted. Preformed valves permit the use of a smaller valve size in proportion to the bottom of the bag. In Europe, reinforcing patches are customarily applied to both ends of the bag for added strength. The bags are discharged to a press section where they are conveyed in a continuous shingled stream. Powerful contact pressure of belts (top and bottom) ensures efficient adhesion. In most instances, the final station is an automatic counting and packeting unit. 
3. Stepped-end tubes with gussets and a special step pattern can be converted into pinch-bottom bags on which beads of hot-melt or cold adhesives (see Adhesives) are applied to the steps in the bottom (see Figure 3(a)). These, in turn, are folded over and pressed closed to make an absolutely siftproof bottom (Figure 3(b)). Beads of hot melt applied to the steps at the top of the bag are allowed to cool and solidify. After the bag is filled, a field-closure unit reactivates the hot-melt adhesive, folds over the top of the bag, and presses it closed. 

A valve bottomer. Figure insert indicates components: 1, rotary or double feeder; 2, tube aligner; 3, diverter for removing incorrectly fed tubes; 4, diagonal creasing and needle vent hole arrangements; 5, bottom center creasing stations with slitting arrangement for bottom flaps; 6, bottom opening station; 7, bottom creasing station; 8, unwind for valve patch; 9, bottom turning station; 10, valve unit; 11, bottom pasting; 12, bottom closing station; 13, bottom capping unit or intermediate pressing station; 14, flexo printers for bottom caps; 15, unwinds for bottom caps; 16, delivery, optionally with incorporated counting and packeting station. Figure 2.

In most instances, bags are collected in packets or bundles palletized for shipment to the end user. However, it is also possible to collect the bags on reels for efficient loading of automatic bag-feeding equipment in the field. The reeled bags form a shingled pattern held in place by the pressure of two plastic bands that are wound continuously around the reel along with the bags.

Other Development.

With conventional equipment, it usually takes two bottomers to keep pace with one tuber. This fact has generally discouraged the development of inline multiwall bagmaking systems in the United States. For example, tubers for cement bags typically operate at speeds from 270 to 320 tubes/min, whereas old-style bottomers run at 120–150 bags/min. Newer bottoming equipment can achieve speeds up to 250 bags/min, enabling one-to-one operation of tuber and bottomer on an in-line system. The tuber operates at less than maximum output, but the in-line system still produces more finished bags because of the increased efficiency resulting from the bottomer being continually fed with fresh tubes. Tubes where the paste has dried become stiff and difficult to handle. As paper is unwound from a roll, it quickly loses its moisture content and becomes less workable. These types of problems are alleviated with in-line bottoming.

In-line tube forming and tube bottoming also lend themselves to significant improvements in manpower utilization. The U-shaped in-line pinch-bottoming system shown in Figure 4 is capable of reducing the personnel requirements from nine to four. The key to the system is a unique turning station that rotates the axis of the tube by 901 for proper alignment with the bottomer. The ‘‘factory end’’ of the bottomer may be heat-sealed in-line (see Sealing, heat). On the ‘‘customer’’ end of the bag, hot melt or cold glue can be applied, or this end of the bag can be flush-cut for sewing in the field. A sewn top with a pinch bottom offers strength and siftproofness in this bottom style while allowing the customer to retain existing closing equipment. For a consumer product such as pet food, the pinch bottom allows the bag to be stacked horizontally on the shelf, still presenting a large graphics display area for the shopper.

An out-of-line double feeder-equipped pinch bottomer produces pinch-bottom bags for the manufacturers not anticipating having the volume to fully utilize the more productive in-line system. The trend for bag users to reduce inventories and place more small orders is expected to continue indefinitely. For the converter, this has meant decreased productivity because of a disproportionate amount of time being spent in changeovers. New computer numerical control (CNC) bottoming equipment promises to reduce changeover time from an average of about 3 h to about 30 min. All gross adjustments of machinery for a particular setup are stored in the microprocessor and made on the machine by way of stepping motors. Although minor fine-tuning is still required, the starting adjustment point of each operator is the same, and settings are optimized according to a logical sequence designed into the control (see Instrumentation). In addition to faster setups, standardization of tuning procedures should result in more consistent and improved product quality.

In-line tube forming and pinch bottoming. Figure 4.

PLASTIC BAG MACHINERY

The procedure for making all-plastic, heavy-duty bags is similar to the procedure for multiwall bags; i.e., various bottoming techniques are used to transform a tube into a finished bag, generally either an open-mouth or valve bag, with or without gussets. The three basic differences are described below:

1. Plastic bagmaking almost always uses a single ply of material, either mono-extruded film, coextruded film, or woven fiber instead of the multiple plies used in paper shipping sacks. 
2. All bagmaking operations are performed on a single converting line. If the bag is made from a flat sheet, the tubing and bottoming operations are integrated into a single bagmaking line. Bags are often made from tubes of blown film or circular woven fibers, and no tubing step is necessary. 
3. The plastic-bagmaking line may incorporate in-line printing, although the outer ply of kraft paper used in a multiwall bag is typically preprinted off-line. 

Woven-bag Machinery.

Economy of raw materials and toughness are two features that make the woven plastic bag an attractive packaging medium for goods mainly intended for export.

A typical line for converting woven high-density polyethylene (HDPE) or polypropylene (PP) material into heavy-duty shipping sacks includes the following: unwind units for sheet or tubular webs, jumbo or normal size; a flexographic printing machine (see Printing) designed for in-line operation; a wax-application unit (see Waxes) to apply a hot-melt strip across uncoated material at the region of subsequent cutting to prevent fraying; and a flat and gusseted tube-forming unit. The flat sheet of coated or uncoated material is longitudinally folded into tubular form. Some machines have the ability to do this without traditional tube-forming parts. A longitudinal seam is sealed by an extruded bead of plastic. Output of the extruder is matched to the web speed by a tachogenerator.

A PE liner unit can be arranged above the tube-forming section to apply a PE liner to the flat web automatically. The principal element of this unit is a welding drum with rotating welding segments that provide the reel-fed PE with a bottom weld at the correct intervals. A Z-folding device enables a fold to be made in the crosswise direction for the provision of a liner that is longer than the sack. In addition, a crosscutting unit cuts the outer web and the PE insert, usually by means of heated rotating knives. In the bottoming unit, cut lengths are transferred to the bottoming equipment by conveyor. Bottoming is accomplished either by sewing or the application of a tape strip. Instead of folding the tape over the open end of the sack, the sack end can be folded once or twice and the tape can be applied in flat form over the folds. The delivery unit collects finished sacks into piles for manual or automatic unloading.

Plastic Valve Sack Machinery.

Plastic valve bags operate by the same principle as multiwall valve bags. On filling, the pressure of the product closes a valve that has been inserted in either the bottom or the side of the bag. If the material is granular (not pelletized), channels along the bottom of the valve sack would allow some of the product to sift out. These channels can be made siftproof by closing them off with two beads of hot wax during the bottoming operation. Only 5–10% of the plastic valve sacks made in the United States require this feature. Therefore, most plastic valve sacks are produced on high-speed lines that produce sacks at about twice the speed of the siftproof machinery.

A typical system for the production of pasted PE bags from either flat film or blow tubes (Figure 5) consists of the following equipment:

• 1. An unwind unit for flat film or tubing incorporating automatic tension and edge-guide controls. 
• 2. A tube former in which folding plates form flat film into a tube. A longitudinal seam is bonded by an extruded PE bead. 
• 3. A rotary cross cutter in which the formed tube is separated into individual lengths by the perforated knife of the rotary cross cutter. Fraying of woven materials can be eliminated with a heated knife that bonds the tapes together. 
• 4. A turning unit in which, after the cross cutter, the tubes are turned 901 to bring the cut ends into position for the following processes. 
• 5. A tube aligner and ejector gate in which exact alignment of tubes in longitudinal and cross direction is achieved by means of stops affixed to circulating chains and obliquely arranged accelerating conveyor bands. Photocells monitor the position of the tube lengths. In the event of misalignment, leading to malformed bottoms and, therefore, unusable sacks, the photocell triggers an electropneumatic gate, which, in turn, ejects the tube length from the line. 
• 6. Pasting stations in which each tube end is simultaneously pasted by a pair of paste units using a special adhesive. 
• 7. An enclosed drying system evaporates and draws off solvent from the adhesive. 
• 8-10. Creasing, bottom-opening, and fixing of opened bottom in which a rotating pair of bars hold the tube length ends by suction and the rotary movement pulls the tube ends open sufficiently to enable rotating spreaders to enter and complete the bottom-opening process. The diagonal folds of the pockets are fixed by press rolls to avoid subsequent opening of the pockets. 
• 11. A valve-patch unit forms the valve from rolls of flat film and places it in the leading or trailing pocket, as required.
• 12. The bottom-closing station, where, after the valve is positioned, the pasted bottom flaps are folded over, one to the other, and the sack bottom is firmly closed.
• 13–17. The bottom-patch unit, bottom-turning station, flexo-printing units for bottom patches, pasting stations with drying, and unwind for bottompatch film, in which patches are formed from two separate rolls of film, flexo-printed (if required), and pasted to both sack bottoms. The bottom geometry is checked by photocells and faulty sacks are ejected through a gate. Just before they reach the delivery section, the bottoms are turned from a vertical to a horizontal processing plane.
• 18. Delivery with counter and packeting station, in which good adhesion of the cover patch to the sack bottom is assured by applying pressure to the shingled sacks with staggered springloaded disks. Having reached a predetermined count, the conveyor accelerates the shingled sacks to the packing station, where the counted sacks are collected into packets and discharged. To accommodate a user’s automated filling line, equipment is also available to wind the plastic valve sacks onto reels.

Systems for making all-plastic heavy-duty bags from (a) tubular film or (b) tubular or flat film. Components are described by number in the text. Figure 5.

Continuous Bagforming and Bagfilling.

Plastic valve bags have been used extensively, especially in Europe, for products such as plastic resin. However, continuous systems for forming, filling, and closing flat and gusseted plastic bags are becoming increasingly popular in the resin market. Such systems typically use prefabricated tubing for high strength. The tubular material is usually preprinted with random printing. Since resin weight varies from day to day, depending on ambient conditions and other factors, random printing allows the bag length to be adjusted according to the prevailing resin volume— weight relationship. In this manner, a tight and graphically appealing package is formed.

An integrated system for forming, filling, and closing of shipping bags would contain the following stations: unwind unit; compensator roller; hot-emboss marking unit; sealing station for bottom seam; bag shingling; separation of bags; introduction of bag-holding tongs; bag filling; supply of the filling product; sealing station for closing seam; bag outfeed conveyor; and control panel. Such a system can produce up to 1350 filled sacks per hour.

One-way Flexible Containers.

One-way bulk shipping containers are becoming very popular in Europe. These oversized bags are designed for handling by forklift trucks equipped with one of several specially designed transport devices. Called intermediate bulk containers, they range in capacity from 1100 lb to about a metric ton (0.5–1 t), and are constructed of woven PP or HDPE. Tubes are generally woven on a circular loom because elimination of the longitudinal seam gives the bag exceptional strength. The advantage of this bag is that it represents an exceptionally economical and efficient method of handling bulk quantities. Acceptance of the concept has been relatively slow in the United States because it requires bag producers, product manufacturers, and product customers all to invest in special equipment for bagmaking, product filling, or handling.

ELECTRONIC CONTROLS

Today’s bagmaking equipment is following the overall industrial trend toward the use of programmable microprocessor control systems of increasing complexity. Ancillary equipment such as printing presses and extruders already have a high level of control, and other units on the bagmaking line are quickly being adapted to the computer. The first objective in conversion to programmable control is replacement of cumbersome mechanical logic. The next is storage of setup and processing parameters for subsequent reuse. Microprocessors are being used for controlling temperatures, web tension, surface-tension treatment, adhesive application, ink, and registration. The most recent stage of automation has been provision of multiple outputs so that lines may be monitored or controlled by hierarchal computers.