Plastics packaging can be grouped into the general categories of rigid and semirigid containers, flexible packaging, and other forms. In the container category, bottles are the most common package type, but plastics are also widely used in tubs, tubes, drums, bins, trays, and other shapes. The flexible packaging category includes wraps, pouches, bags, sacks, envelopes, and similar packages. The "other" category includes, among others, cushioning (loose-fill and molded), blisters, caps, and lids.
As mentioned, the use of plastics in packaging has been growing steadily for a number of years. Rigid plastic containers (bottles, jars, drums, etc.) have replaced materials such as glass and metal. There is also a long-standing trend of replacement of rigid packages with flexible packages. For example, many institutional food service items are now distributed in multilayer pouches rather than in steel cans.
Production of flexible packaging begins with extrusion of plastic resin to yield sheet or film. For film, two main processes are used. In the cast-film process, the resin is extruded through a slit-shaped die onto a water-cooled chrome roller, where it is solidified. After the edges are trimmed off, the film is wound onto a roll. The film may be oriented by stretching it in the machine direction (uniaxi-ally oriented) or in both the machine and cross directions (biaxially oriented) to improve mechanical and barrier properties. This process is also used to produce plastic sheet for thermoforming or other applications.
In the blown-film process, the resin is extruded through an annular die and internal air pressure used to expand the bubble. After the resin solidifies, the bubble is collapsed and may be wound as a tube or slit and wound as rolls of flat film. Since the film is stretched by the take-off rollers as well as by the bubble expansion, it has biaxial orientation.
If a multilayer material is desired, it can be produced by coextrusion or by a subsequent laminating operation. Coextrusion has the advantage of producing a multilayer material in a single step and permits individual layers to be much thinner than is possible in a laminated structure. On the other hand, lamination permits use of nonplastic layers such as paper or foil. Coating, with plastics or nonplastics, can also be used to produce multilayer structures. Metallized film uses an extremely thin layer of aluminum, added in a vacuum deposition process, to greatly enhance barrier or, sometimes, to enhance the appearance of the material. Silicon oxide coatings (SiOx) are also used to enhance barrier. This technology is not yet widely used in the United States, though it is quite common in Japan. An advantage over metallization is that transparency is preserved, including transparency to microwave radiation so that these materials can be employed for packaging microwave-ready food products. Transparent aluminum oxide and other metal oxide coatings have also been developed.
Films may be used without further conversion as wraps, such as in pallet stretch wrap, or in shrink wrap. Most often, films are converted into bags, sacks, pouches, or envelopes. Usually, the package seams are formed using heat sealing. In many cases, packaging is done in a form-fill-seal operation in which the package is formed and the product filled in a single machine. If the package is printed, as is often the case, the printing is usually done while the plastic material is in the form of roll stock, prior to forming the package. Commonly, some type of surface treatment of the plastic is necessary prior to printing, to enhance ink adhesion. Corona treatment, in which the plastic is exposed to an electrostatic discharge that produces some oxidation of the surface, is used most often.
Plastic bottles are generally produced by blow molding. The most common process is extrusion blow molding, in which the plastic resin is extruded through an annular die, forming a hollow tube (parison). The parison is captured in the blow mold, and air pressure is used to expand it into the shape of the mold. This is a very versatile process that can be used to produce a wide variety of shapes and sizes, from tiny containers to 55-gal drums and larger. The containers may have hollow handles, offset necks, and a variety of other features. Some are even produced with two or more compartments in a single container. Extrusion blow-molding results in generation of scrap (flash) from the extra material that must be trimmed off from above the rim of the container and below the bottom (pinch). Handles result in generation of additional scrap from the area between the handle and the body. If a container has an offset neck, a considerable amount of scrap is usually generated from the neck area. In most cases, this flash is automatically collected, ground up, and metered back into the process. The amount of material required in each container can be minimized by controlling the distribution of plastic in the parison, and hence in the finished container, using such techniques as die shaping and parison programming.
Injection blow molding is used primarily for plastic resins that lack sufficient melt strength for extrusion blow molding. Injection blow molding is also used for applications such as pharmaceutical products where very accurate finish (threaded area of the neck) dimensions are required to ensure good sealing of the container. In this two-step molding process, a preform or parison is produced by injection molding, and then it is transferred to a blow-mold for expansion into its finished form. In the injection-molding step, a mold is filled through one or more gates with melted plastic, which is then solidified, and the molded product ejected. The injection-molded parison may be immediately transferred to the blow mold and blown without reheating or may be totally cooled and then reheated and blown at a later time. Injection blow molding produces much less scrap than extrusion blow molding. It also often permits better control over the wall thickness of the finished container.
Stretch blow molding is a process designed to impart biaxial orientation to bottles or jars. In this process, a preform is produced (usually by injection molding-injection stretch blow molding) that is shorter than the final container. During the blowing step, a stretch rod stretches the preform in the vertical direction as it is expanded in the radial direction by blowing. Biaxial orientation can significantly improve the strength and barrier properties of the container.
Coextrusion blow molding is often used to produce multilayer plastic containers. A multilayer parison is produced, and subsequent operations are identical to ordinary blow molding. Reuse of flash is more difficult since it will be multilayer. In many cases, the flash is used in an inner layer in the container, either alone or combined with other recycled plastic. High-density polyethylene (HDPE) laundry product bottles, for example, routinely are made with a three-layer structure in which the inner layer contains flash blended with postconsumer recycled HDPE.
Multilayer injection blow-molded containers have been available for a much shorter time than multilayer extrusion blow-molded containers, as development of the process was more difficult. However, these too are now available. A prominent example is multilayer polyethylene terephthalate (PET)-based ketchup bottles, which contain three layers of PET, and two layers of ethylene vinyl alcohol (EVOH) as an oxygen barrier.
When containers will be exposed to elevated temperatures, such as in packaging of hot-filled products, container distortion may be a problem, especially for biaxially oriented bottles. In this case, a subsequent heat-setting operation may be used to reduce thermal stress and minimize deformation on subsequent exposure to elevated temperatures.
For many products, the labels are placed into the empty mold and sealed to the container during the molding process (in-mold labeling). This eliminates the need for a subsequent labeling step and produces somewhat recessed labels with a better appearance, including improved scuff resistance. In-mold labeling is used most often with extrusion blow molding but can also be used with injection molding.
Injection molding alone (see description above) is also used to produce some packaging containers and components. Most threaded closures (caps) for bottles are produced in this way since there is no other feasible way to produce accurate threads in plastic caps. Plastic tubs and other shapes are also often produced by injection molding. Injection molding excels at producing very tight tolerances on the finished parts.
In both injection molding and injection blow molding, the plastic flows to the injection mold through a system of runners. In most packaging applications, a variant of injection molding, hot runner molding, is used. In this process, the mold is designed to provide heating of the runners to keep the plastic in them from solidifying. When the mold is emptied and the next cycle begins, the first plastic to enter the mold is the still-molten plastic that was in the runners during the previous molding cycle. This significantly reduces scrap generation and also decreases cycle time.
Thermoforming is another common method of producing packages and package components. Most trays and blisters (dome of plastic attached to a backing material to enclose a product) are produced by thermoforming. In thermoforming, a plastic sheet is heated to soften it, and then it is shaped into or over a mold, using vacuum or a combination of vacuum and pressure.
There are a number of thermoforming variations, including the use of male or female (positive or negative) molds, prestretching with air (billow forming), and use of mechanical devices to assist in moving the plastic into the mold (plugassist). Matched-mold forming uses a combination of male and female molds to produce very good dimensional control. Twin-sheet thermoforming uses two molds with two plastic sheets to produce hollow objects such as pallets. In-line thermoforming builds thermoforming into the extrusion line, so that the plastic need not be totally cooled and rolled up before it is thermoformed, permitting some energy savings. Melt-to-mold thermoforming is a relatively new process that can be regarded as a cross between extrusion and thermoforming. In this process, the melted plastic is delivered to a roller that contains engraved molds, rather than cooled on a smooth roller for subsequent reheating and thermoforming.
Most thermoforming operations produce a relatively large amount of scrap in each cycle. Scrap rates of 30% are not unusual. If the sheet is produced in one location and thermoformed in another, or if the material is multilayer, reuse of scrap is more difficult than in extrusion blow molding.
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