Closures

The term closures refers to caps and lid, Because they provide the seal in many packaging systems, they are often critical components of packages. All forming processes result in some variation in the dimensions of the final product. When sealing is critical, packagers often depend on a closure to compensate for the manufacturing inaccuracies not only in the closure, but also in the container. To achieve a tight hermetic seal that is capable of protecting control of the closure itself, as well as provision of some resilient sealing surface which can deform to compensate for materials can be successfully used, thermoplastics are increasingly the material of choice for plastic closures, and injection is PP, HDPE, LDPE, and PVC, in that order, Thermoset phenolics and urea used to be commonly used for closures, but have increasingly been replaced by thermoplastics. Thermoplastics have also replaced a large share of the metal closure market. Three major categories for plastic closures can be identified: friction, snap-on, and threaded.

Friction Closure

Friction closures have the simplest design, The closure is designed o deform as it is pushed into the mouth of the container. The resilience of the material causes it to attempt to return to its original dimensions, providing a seal by the tight fit between the closure material and the container. Friction between the outside of the closure and the inside of the mouth of the container provides the force that resists removal of the closure. A cork oldest designs of plastic friction closures is used for inexpensive bottles of champagne, In Is hollow inside, and has a ridged outer surface.

      In the last several years, an increasing problem of cork taint, estimated to affect 2 to 10 percent of bottles of wine, is causing bottles to be more receptive to the use of alternatives to natural cork. Bottlers are increasingly turning to metal screw caps or to plastic corks for lower-end wines, and are even sometimes using them for higher-end wines, In contrast to the plastic cork pictured above, these corks are shaped like solid cylinders, essentially identical in shape to the natural cork they replace, Some are produced by molding, and others by extrusion, The plastic corks do not generally provide as tight a seal as screw caps, but, surprisingly, this may provide an advantage, An absolutely hermetic seal evidently may lead to development of some off-flavor in wine as it ages, while the plastic corks more closely mimic the tight, but not absolutely hermetic, seal of natural cork. Another advantage of plastic corks is no more worry about cork crumbs falling into the wine!

Snap-Fit Closures

A snap-fit, or snap-on, closure is made of a resilient materials and is designed to deform as it passes over a protruding feature on the container. Removal of the closure requires deformation again t “snap” the closure back over the protruding feature, which is typically a retaining ring. When the closure is in place, some resilient part of the closure system that is in contact with the container remains deformed, and provides a seal as it attempts to return to its original dimensions, This category of closure is widely used, and a variety of designs exist. Examples include plastic lids on coffee cans, as well as “line-up-the-arrows” child-resistant caps on medicine bottles.

      One of the major advantages of snap-fit closures is that they can be applied very quickly. A disadvantage is that they cannot be used for containers with internal pressures exceeding one atmosphere, since the pressure may act to snap the closures back off of the container.

Threaded Closures

Threaded closures are applied by screwing them onto a container, and removed by screwing them off. They contain a set of threads that is designed to match the threads on the container. These threads are usually continuous, leading to the designation CT closure. Threaded closured are very versatile, and can be used for packages that contain internal pressure, such as carbonated beverages, as well as for vacuum packages and those at atmospheric pressure. Chile-resistant designs of several types are available, as well.

      The amount of force required for application and removal of the closure is determined by how far the closure is rotated on the container. Charts are available that provide recommended torques for various sizes of containers. Removal torques are generally less than application torques, unless there has been some interaction between the liner and the contents, resulting in sealing the liner to the container. Removal torque typically declines with time for the first several days to a month or so after application, and then stabilizes. The change is caused by stress relaxation and creep in the liner, closure, and/or container. Recommended minimum removal torques are typically about half recommended minimum application torques. If an application torque is too low, the container may not be sealed adequately. On the other hand, if the application torque is too high, consumers may have difficulty removing the closure. It should also be noted that if application torques are too high, a variety of additional problems can result. The capping machinery may not be able to reliably the torque that is set. The forces involved may result in permanent deformation of the cap or the container, resulting In poor sealing, or even leakers. There may be excessive wear of the machinery, etc.

      The sealing action is provided by deformation of a resilient surface that is either built into the closure design, or provided by a liner used in conjunction with the closure. As is the case with the other closure designs, the attempt of the resilient material to return to its original dimensions exerts force against the container and provides the seal. The liner may contain plastic as the product contact layer, plastic foam or paperboard for resilience, and may also contain aluminum foil for barrier. Liners can be glued in place, but most often are held in place by being snapped behind a retaining ring built into the closure. Linerless closures are designed to provide a resilient sealing surface without requiring the use of a separate liner component. This resilient feature is molded into the closure itself. Usually the resilient feature is made of the same material as the rest of the closure and is produced during molding of the closure, providing resilience by a combination of its geometry and the nature of the plastic used fir the closure. In some cases, it is a distinct plastic material, with superior resilience characteristics, produced by a technique such as coinjection molding.

      The size of CT closures is specified by the nominal outside dimension of the container opening in millimeters, plus a number that represents the style of finish. Both container and closure finishes are standardized so that, at least in theory, a closure of a given size and style should fit any bottle of that same size and style, from any manufacturer. U.S. closure standards have been established by the closure Manufacturers Association, standards for glass bottles by the glass packaging institute, and for plastic bottles by the American society for testing and materials. Common closure diameters are 22-120 mm.

      Critical dimensions for closure performance include T, the diameter of the root of the thread inside the closure; E, the inside diameter of the closure; H, the distance from the inside top of the closure to the bottom of the closure skirt; and S, the distance from the inside top of the closure to starting point of the thread.

Specialty Closures

In addition to the basic design described above, a wide variety of special closure designs are available. Many of these are designed to provide a dispensing function, such as pumps, sprays, flips open caps, etc. They range in design from rather simple to extremely complex.

      An important set of specialty closures are those that are designed to make packages child-resistant, difficult for young children to open. Child-resistant packaging, known as “special packaging” in the relevant regulations, is required on most prescription drugs, aspirin, and other over-the counter drugs, and on household chemicals that pose a serious risk to children if they are accidentally ingested. The regulations prescribe standards that must be met for packages to quality, and include the ability to successfully prevent opening by young children, while at the same time permitting opening by adults, including the elderly.

      Some closures also are designed to provide temper-indicating features, typically, in closures, by incorporating some type of tear-off ring. Closure liners that are designed to release from the cap and seal to the bottle mouth can also be used to provide tamper-evidence. In the U.S, over the counter drug products are required by law to contain some tamper-evident feature, designed to alert consumers if a package has been opened and may have been tampered with. Such features are increasing found on packages for food and other products as well, although they are not required in these applications. Closures and liners, of course, are not the only way of providing tamper evidence. Other common mechanisms involving plastic packaging include shrink bands package necks and shrink warp around containers.

Fitments and Overcaps

Fitments are another set of devices associated with closures. These are components that are used in conjunction with a closure to provide some added utility by regulating the flow of product out of the package. A common example is the shaker-top on a spice jar. When dispensing device is built into the closure, instead of in a separate piece, the result is sometimes referred to as a fitment closure.

      Overcaps, as the name indicates, are designed to be applied to over the closure. They may serve purely a decorative function, but most often they are designed to offer some protection to a dispensing closure so that it is not activated prematurely.