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Heat Sealing

Plastic on paperboard (i.e. plastic-coated board) can be used as a gluing medium, heat sealing, instead of adding an extra string of glue. What is needed is a source of heat, a pressure unit and time for the plastic to cool down without being stressed.

The heat can be applied directly to the polymer prior to sealing by using hot air or a gas flame in line with the erecting of the package. The heat can also be applied by pressure bars if they are heated by induction or ultrasonic means.
In most applications it is sufficient to have a plastic coating on one side of the board in order to achieve a good heat seal. Plastic coating on both sides will lead to a better seal. The properties of the plastic or technical/physical limitations in the plastic coating line can make two-sided coating unsuitable.

Heat sealing is well suited for high speed operations. In addition, there is no need to buy an adhesive or to clean the sealing line. These factors alone may be sufficient reason to use a plastic-coated paperboard.

Heat sealing characteristics

The sealability of a plastic coating lies in its ability to act as its own bond-forming agent without any extra hot melt adhesive. This property adds value at no extra cost. A bond can be rapidly formed by melting the plastic coating and pressing the two bonding surfaces together while the plastic resolidifies. A good seal will give full fibre tear.

Surface characteristics

Heat sealing works on most plastic coatings and plastic coating surfaces. The chemical nature of the different plastics will require different setting conditions in the heat sealing line. Some plastics are easier to heat seal than others because they require less energy (in the form of temperature, time or pressure). Using an electrical corona discharge can improve the heat sealing result but is often not an absolute necessity.

Essential requirements for successful heat sealing:

• The plastic coating must be molten when brought in
contact with the second surface.
• There must be enough plastic to form a bond.
• The pressure must be maintained until the plastic has cooled and solidified.

Description of heat sealing methods

Because plastic-coated paperboard is heat sealed no extra glue is added. The thin plastic coating is melted to a tacky state in the seal area and the other paperboard surface is applied under pressure, which is held until the seal has cooled enough to solidify. The most common methods used to obtain a seal are described below. 

Hot bar sealers are electrically heated jaws that are pressed over the plastic-coated areas to be sealed. The main use is in erecting tools for top-fed cartons made from plastic-coated paperboard. Bar sealers are convenient to use and easy to maintain. The drawback may be that they can cause visible markings on the board. 

Hot wheel sealers have a gas flame-heated wheel that is brought in contact with the seal as it moves past the wheel. They offer the same advantages and disadvantages as bar sealers.


Click to enlarge.

Hot air sealers melt the plastic with a jet of gas flame-heated or electrically heated air. The molten, sticky areas are then clamped by Teflon-coated (or similar) jaws or by a roller nip. Hot air sealers are convenient but the equipment tends to be noisy and to heat the surroundings. 

Radiation sealers work like hot air sealers except that the heating is done by precision-mounted heating elements. They are less noisy and do not emit so much excess heat. A limitation is that the heating elements must usually be made for one specific package size. 

Gas flame sealers are very similar to hot air sealers, the difference being that gas flames are used instead of hot air jets. They are less noisy and emit less excess heat but require the installation of a gas system. 

High frequency sealers are similar to hot bar sealers, the difference being that the energy is transferred as dielectric losses in the packaging material from a high frequency voltage applied across the jaws, which turns them into a capacitor. The heat is created inside the material much like the heating in a microwave oven. The technique offers a high-speed precision function. 

Ultrasonic sealers are similar to high frequency sealers

but here the heating is caused by mechanical friction losses inside the packaging material, which is clamped between a vibrating jaw and an anvil.

Testing and troubleshooting

As for all other conversion steps, regular testing is essential to ensure the desired heat sealing result. The procedure should be well documented with regard to test frequency and test method. Note that different heat sealing methods and different heat sealing applications may require different kinds of test methods.

It is very important that all pre-testing in the sealing and the packaging line is done with the packaging material intended to be used for the full scale run. This includes using the same printing and varnishing techniques as intended and the correct grade of plastic coating. Extra treatments and plastic additives can affect the heat sealing characteristics and must therefore be evaluated in advance to avoid unexpected problems.

All these heat sealing methods are based on using a certain amount of energy (in the form of heat, time and pressure) to bond two surfaces together. Even a small deviation in the energy applied can result in insufficient heat sealing and severe problems in subsequent conversion stages. The temperature readings in particular can deviate from the actual temperature of the heated bar or hot air jet. Good maintenance is crucial in this respect.

Side-seam heat sealing requires precise machine settings. The glue seam is fully developed after the appli­cation of pressure and the cartons must be cooled so that residual heat does not make them stick together.
Heat sealing is usually used in the erecting units for plastic-coated paperboard.

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