Packaging Sealing Methods
The method for heat sealing a particular structure depends on the type and form of the structures being sealed, as well as the type of package and product. The following are the most important sealing methods used in packaging:
Bar or Thermal Sealing
Thermal sealing uses heated bars to press together the materials to be sealed, with heat from the bars conducted through the materials to the interface, melting the heat seal layers and fusing them together. When sufficient time has elapsed. The bars, release and the material is moved out of the seal area. At this point, the materials are still hot, and the seal does not have its full strength, but the materials must be able to adhere to each other well enough to insure the integrity of the seal. This ability of the materials to remain together while they are still hot is known as hot tack. The full strength of the; seal develops as it cools to ambient temperatures. Proper seal formation requires the correct combination of heat, dwell time (the time the material is held between the sealing bars), and pressure. Too little of any of these will prevent an adequate seal from forming. On the other hand, excessive heat, time, or pressure will result in too much flow in the heat seal layers, weakening the material.
The edges of the heat-seal bars are often rounded so that they do not puncture the packaging material. Often the contact surface of one of the bars contains a resilient material to aid in achieving uniform pressure in the seal area,. Bar sealing is the most commonly used method of heat-sealing packaging materials, and is often used in form-fill-seal operations.
A variation on bar sealing uses only one heated bar, with the other bar not heated, resulting in heat conduction occurring only in one direction. Another variation uses heated rollers instead of bars, with the materials sealed as they pass between the rollers. In this type of system, preheating, slow travel through the rollers, or both, are generally required due to the very short contact time between the rollers. A third variation uses shaped upper bars for sealing lids on cups and trays.
Impulse Sealing
Impulse sealing is another common heat-seal method. Impulse sealing uses two jaws, like bar sealing, but instead of remaining hot, the bars are heated intermittently by an impulse(less than one second) of electric passed through a nichrome wire ribbon contained in one or both jaws. The jaws apply pressure to the materials both before and after the current flow. The current causes the ribbon to heat, and this heat is conducted to the materials being sealed. After the pulse of current is passed through the wire ribbon, the materials remain between the jaws for a set length of time, and begin to cool. Thus, impulse sealing provides for cooling while the materials are held together under pressure. This method allows materials with a low degree of hot tack to be successfully sealed, as well as permitting sealing of materials that are too weak at the sealing temperature to be moved without support. The sealing jaws can be water-cooled for faster cooling of the materials being sealed. Shaped impulse seals are used for sealing lids on cups and trays.
Impulse sealing produces a narrower seal than bar sealers, resulting in a better looking but weaker seal. Maintenance requirements tend to be heavy, since the nichrome wires often burn out and require replacement. A fluoropolymer tape on the jaws, covering the nichrome wire, is often used to keep the plastic from sticking to the jaws, and may also require frequent replacement.
Band Sealing
Band sealing, illustrated, like impulse sealing, provides a cooling phase under pressure. This high speed sealing system uses two moving bands to provide pressure and convey the materials past first a heating station and then a cooling station. The primary disadvantage of this method is the tendency for wrinkles in the finished seals. Preformed pouches that are filled with product are often sealed using this method.
Hot Wire or Hot Knife Sealing
This method, as its name describes, uses a hot wire or knife to simultaneously seal and cut apart plastic films. The wire or knife causes the substrates to fuse as it is pushed through, cutting them off from the webstock. The seal produced is very narrow and often nearly invisible. It is also relatively weak, and does not provide a sufficient barrier to microorganisms to be used when a hermetic seal is required. However, it is very economical due to its high speed, and is an excellent choice for relatively undemanding packaging applications with materials that seal readily, such as LDPE bags used in supermarket produce sections.
Ultrasonic Sealing
In ultrasonic sealing, two surfaces are rubbed together rapidly. The resulting friction generates heat at the interface, melting the surfaces of the substrates and producing a seal. Since the heat is generated only in the seal area, ultrasonic sealing is particularly useful for thick materials where conduction is inefficient. It is also useful when exposure to heat for a sufficient time to conduct heat to the seal can damage the substrates, such as in sealing highly oriented materials, which can lose their orientation and shrink when heated.
Friction Sealing
Friction sealing, often called spin welding, like ultrasonic sealing uses friction to produce heat. It is most often used for assembling two halves of a rigid or semirigid plastic object, such as a deodorant roller or a container, or sometimes for sealing caps to bottles. The two halves are mostoften circular in cross section, and one is rotated rapidly while the other is held in place. The halves are designed to fit together only with some interference, so there is considerable friction, generating heat that welds them together. The sealing mechanism usually has a sensor that measures the amount of resistance to rotation, and the object is released when the resistance reaches the level determined. A variation of this method allows sealing of non-circular materials, using an oscillating motion rather than rotation.
Hot Gas Sealing and Contact Sealing
In hot gas sealing, the sealing surfaces are exposed to a gas flame or to hot air, also to avoid the need to conduct heat through the materials being sealed. The heat melts the sealing surfaces, and then the two materials are pressed together between cooled jaws. Contact sealing is similar to hot gas sealing, except that the sealing surfaces are touched to a heated plate.
Radiant Sealing
In radiant sealing, heat is transferred to the sealing surfaces primarily by radiation rather than by conduction or convection . It is most commonly used for materials which distort excessively under pressure. The most common application is for spun-bonded high density polyethylene (e.g. Tyvek TM from DuPont) used for medical device packaging. Other applications include sealing highly oriented materials, sealing uncoated PET, or for producing shaped seals. Radiant heating is often used in thermoforming, as well.
Dielectric Sealing
In dielectric sealing, an oscillating high frequency electrical field is used to seal polar materials. Polar molecules exposed to the field attempt to line up with the electrical charge, so they oscillate as they are attracted first in one direction and then in the other. This generates heat, melting the materials and producing a seal. Dielectric sealing is ideal for heavy PVC materials, especially for textured PVC, as it experiences considerable distortion with methods that rely on conduction. Non-polar materials, such as polyolefins, are not affected by the field, so cannot be sealed in this manner.
Magnetic Sealing
Magnetic sealing relies on a similar idea, with an oscillating magnetic field that causes magnetic iron compounds to attempt to line up with it, producing heat as the field, and consequently the iron particles, oscillate. Non-magnetic materials are not affected. Since magnetic iron compounds are not normally found in plastic packaging materials, this method relies on the use of special gaskets or coatings containing magnetic iron. Magnetic sealing is rarely used, but does have some applications with cap liners or lids.
Induction Sealing
Induction sealing is also a method to generate heat near the sealing surface. It is often used to apply tamper-indicating inner seals on plastic bottles and jars. It uses an alternating magnetic field to induce an electric current in any metal within the field, most often a layer of aluminum foil. The electrical current heats the foil and is conducted to a neighboring heat-seal layer, resulting in the sealing of the liner to the container. The original inner seal structure for induction sealing, which is still sometimes used today, consisted of a heat seal coated foil attached with wax to a paperboard backing glued into the closure. When the assembly is placed in an alternating magnetic field and current induced in the foil, in addition to sealing the liner to the bottle rim, the heat melts the wax layer, releasing the foil liner from the cap, and thus allowing the consumer to remove the cap to open the container. This structure has now generally been replaced by other designs. These inner seals provide an excellent barrier against gain or loss of moisture, oxygen, and other components, as well as providing a useful tamper-indicating feature.