Plastic bags

Plastic bags, available in virtually all shapes, sizes, colors, and configurations, have replaced paper in most lightduty packaging applications. Paper has been more difficult to replace in heavy-duty applications (see Bags, heavyduty plastic). Light-duty plastic bags are generally described in one of two ways: by the sealing method or by application. This article explains the methods used to produce plastic bags and defines the various types of plastic bags in terms of their intended use. A plastic bag is defined here as a bag manufactured from extensible film (see Films, plastic) by heat-sealing one or more edges and produced in quantity for use in some type of packaging application.

METHODS OF MANUFACTURE

By definition, all plastic bags are produced by sealing one or more edges of the extensible film together. The procedure by which this heat-sealing occurs (see Sealing, heat) is typically used to identify and categorize types of plastic bags. Three basic sealing methods are used today that include the following: sideweld, bottom seal, and twin seal.

Sideweld Seal.

A sideweld seal is made with a heated round-edged sealing knife or blade that cuts, severs, and seals two layers of film when the knife is depressed through the film material and into a soft rubber backup roller. The materials are fused by a combination of pressure and heat (see Figure 1).

The sideweld seal is the most common bag-sealing method. Typical high usage bags, e.g., bread bags and sandwich bags, are produced using the sideweld technique. The term ‘‘sideweld’’ is derived from the fact that many of the bags produced in this fashion pass through the bag machine with the length (or depth) of the finished bag perpendicular to the machine floor. The film fed into the machine is either prefolded, i.e., J stock, or folded during the in-feed process (see Figure 2).

Bottom Seal.

The bottom-seal technique seals the bag at the bottom only. Tube stock is fed into a bag machine, the single seal is produced at the bottom of the bag, and the bag is cut off with a knife action that is separate from the sealing action. A bottom seal is generally made by a flat, heated sealing bar, which presses the layers of film to be sealed against a Teflon (Du Pont)-covered rubber pad, i.e., seal pad (see Figure 3), or another hot-seal bar (see Figure 4). A separate cut-off knife is used to separate the bag from the feedstock while the seal is made or immediately thereafter. Both bottom-seal mechanism designs produce a bag with only one seal, unless the tube has been manufactured by slit-sealing (see below). The small amount of unusable, wasted film between the edge of the seal and the cutoff point, called the ‘‘skirt’’ of the bag, is an important factor in the total cost of the bag. Any disadvantages caused by the presence of the unwanted skirt are usually offset by greater control of the sealing process. The sideweld method actually melts through the plastic film, and overheating of the film resins can change the physical structure of the plastic molecules when the seal cools. In contrast, the bottom-seal method controls the amount of heat and the dwell time, i.e., the time that the heat is applied to the film, to produce a seal that does not destroy the film or change its physical properties. In addition, the total amount of film sealed together is usually larger, because the seal bar has a fixed width and none of the film material is melted or burned away.

The bottom-seal method is commonly used to produce (HDPE) merchandise bags and (LDPE) industrial liners, trash bags, vegetable and fruit bags, and many other types of bag supplied on a roll. In contrast to the sideweld method, designed primarily for high-speed production of bags made from relatively light-gauge films — i.e., 0.5–2.0 mil (13–51 µm)—bottom-seal methods are often used to produce bags from film from 0.5 to Z6 mil (13–≥152 µm) at slower production speeds. Bags manufactured by bottom-seal methods are delivered through and out of the bag machine with the length (or depth) of the bag parallel to the machine direction. Because all of the bags are produced from tube sock, multiple-lane production of bags is limited only by the widths of the machine and the bags and the film-handling capability of the bag machine (see Figure 5).

Twin Seal.

The twin-seal method employs a dual bottom- seal mechanism with a heated or unheated cutoff knife located between the two seal heads (see Figure 6). The unique feature of the twin-seal mechanism is that it supplies heat to both the top and bottom of the film material and makes two completely separate and independent seals each time the seal head cycles. Like the bottom-seal method, the twin-seal technique can supply a large amount of controlled heat for a given duration. This ability makes the twin seal useful in sealing heaviergauge films as well as coextrusions and laminates. Because two seals are made with each machine cycle, the twin-seal method can be used to make bags with the seals on the sides of the finished bag, i.e., like sideweld bags, or on the bottom of the bag in some special applications, such as retail bags with handles. Many special applications call for the use of a twin-seal-type sealing method, but it most often is used in production of the plastic ‘‘T-shirt’’ grocery sack.

Slit Seal.

Another type of sealing method, the slit seal, involves sealing two or more layers (usually only two) of film together in the machine direction through the use of a heated knife, hot air, laser beam, or a combination of methods. The slit-seal technique is usually used to convert a single large tube of film into smaller tubes. In the production of grocery sacks, for example, a single extruded 60-in. (152-cm) lay-flat tube of film (see Extrusion) can be run through two slit sealers in line with the bag machine. This process results in three tubes of 20-in. (51-cm) layflat material being fed to the bag-making system (see Figure 7).

APPLICATIONS

Most plastic bags are characterized in terms of their intended use, e.g., sandwich bags, primal-meat bags, grocery sacks, handle bags, or bread bags. To provide some order to this user-based classification and definition system, it is convenient to separate the bags into commercial bags and consumer bags.

Commercial Bags.

A commercial plastic bag is used as a packaging medium for another product, e.g., bread. Typical commercial bags and seal methods are listed in Table 1.

Commercial Plastic Bags Table 1.

^aWith dual bottom seal.

Consumer Bags.

Consumer bags are purchased and used by the consumer, e.g., sandwich bags, trash bags. The plastic bag is the product. Typical consumer bags and seal methods are listed in Table 2.

Consumer Plastic Bags Table 2.

In addition to the conventional bags, an enormous variety exists of special bags that have been created by bag producers to meet commercial and consumer needs. These bags are usually described as specialty bags, but they are all variations of the standard bags described above. Specialty bags include rigid-handle shopping bags, sine-wave handle bags, pull-string bags, patch-handle bags, double-rolled bags, square-bottom bags, roundbottom bags, deli bags, etc. Examples of both common and specialty bags are shown in Figure 8.