General jaw design
HOW DO I MAKE A FUNCTIONING HEATSEAL JAW?
Heatseal jaw design is relatively simple. It is made even easier by the use of Jørgen Bork Electronic ApS components that are available to facilitate the construction. Good sealing depends upon equal and even contact between the jaws, for the full length of the jaws. This means that the jaw bars must be straight and evenly loaded to avoid bowing of the jaws during the sealing process. When building long jaws it may be necessary to utilize a supplementary bar as a stiffening back- up. Sealing is accomplished when the seal area cools, therefore, the ability of the jaw bar to absorb heat is important . Aluminum is usually used in heatseal bars because it has a high thermal conductivity, it is light, and it is easy to machine.
The force required to effect a good seal or cut is a function of the material composition, the band shape, the hardness of the back-up material, the hardness and thickness of the pad on the opposite jaw, and the desired result. We know of no means by which the required force can be calculated. Optimum results can only be attained by testing. As a starting point, one should set-up using a total force equal to 1kg/cm2.in. of heatseal band surface. More force may be needed to get a clean cut on hard materials or non-woven fabrics, as example, and less might be desirable on delicate materials.
HEATSEAL BAND TENSION
Heatseal bands expand when they are heated. Therefore, a means must be provided to take up the slack or the heatseal band will buckle and break prematurely. The heatseal band is also an electrical conductor so the band must be electrically insulated from the bar. The bar must, therefore, be fitted with end connector blocks that can provide tension on the band and remain electrically insulated. Jørgen Bork Electronic ApS can provide Jaw End Kits that can be conveniently mounted on the ends of the jaw bars to serve this function. Jørgen Bork Electronic ApS Jaw End Kits are designed to fit on to the ends of 10x60 mm., or larger bars. This is usually a convenient size and it provides a reasonable amount of heat sink capacity for most applications.
The face of the jaw bar must also be electrically insulated. This can be done with a variety of materials. The most convenient of these materials are adhesive backed to facilitate attachment. Teflon tape, Durit, and Siglaha are all available from Jørgen Bork Electronic ApS with self adhesive backing. The proper selection is a function of the sealing mode.
The opposite jaw bar of a set of heatseal jaw bars can be either heated or not, usually not. Either way the opposite jaw is usually faced with a silicone rubber strip that is covered with Teflon tape. It is important to have a resilient component on one of the two jaw bar faces in order to absorb the shock of fast moving jaws and to provide some forgiveness and compensate for minor misalignment or distortion of the bars.
To cool or not to cool, this is sometimes the question. Antiquated systems often use water-cooled jaws as a means of maintaining the jaw at a constant temperature, then supplying a generous amount of energy, each seal would be about the same, and the excess heat would be absorbed by the water cooled jaw. When using high response temperature control the controller supplies only enough heat to make the seal. Much of the heat is carried away by the material that has been sealed, the remainder is not too great and will be absorbed by the mass of the aluminum heatseal jaw bar. If the jaw bar gets warmer during prolonged operation the controller will continue to sense only the actual temperature of the heatseal band and will automatically reduce the amount of energy to the band; thereby assuring that the band temperature, during impulse, is always the same. When doing impulse sealing at high speeds, 40-60 seals per minute, cooling of the heatseal jaws can be helpful in reducing the temperature of the heatseal band at the end of the impulse cycle. This reduces the time required for the seal to set-up and permits operation at higher speeds. Some polymeric materials, like polypropylene may become embrittled when cooled too rapidly. It is, therefore, important to know the character of the film before developing a processing protocol for it. In any event, if one elects to use cooling water it is imperative that the heatseal jaw not be over-cooled. A cooled jaw will absorb heat and demand more power of the impulse cycle. In addition, if cooled below the dew point of the environment, condensation will collect on the jaw and cause electrical leakage and corrosion. Air cooling is occasionally employed to accelerate the cooling of the heat sealed material. This technique is more often utilized to cool seals that have been made by the constant heat mode.
MULTI - BAND DESIGNS
Some applications require two or more heatseal bands mounted in parallel on one jaw. In the case of thick materials, it may be desirable to heat the seal area from both sides. More than one band can often be powered by one controller/transformer system. This can be done as long as the total power requirement does not exceed the capacity of the controller (see 9.Temperature Controller Selection). If more than one band is mounted in parallel on a single jaw it is imperative that each band be mounted to individual jaw end blocks so that each heatseal band will be individually tensioned. In addition, if two or more heatseal bands are mounted physically in parallel they must also be wired electrically in parallel. If this is done correctly, the electrical potential (voltage) at any point along the length of one heatseal band will be the same as the potential at the corresponding point on the second band. If the bands come in contact with each other or are short circuited across the bands, no damage will be done. The situation is the same in the case of heatseal bands that are mounted on opposing jaws.
CUT AND SEAL WITH TWO SEPARATE BANDS
A typical application for this is where it is necessary to seal the top of a plastic pouch with a conventional seal and concurrently make a second seal/cut to remove excess material from the top of the pouch. In this instance a Jørgen Bork Electronic ApS Tapered Band can be mounted in parallel with a Jørgen Bork Electronic ApS Beaded Band of equal length and cross-section. The bands are then wired in parallel and energized by a single control system. The heat can be adjusted to provide a perfect seal with the Tapered Band and the Beaded Band, by its nature, will run slightly hotter along the bead to effect a cutting action. This may not work too well on multi-layer structures depending upon the character of the composite. Again, it is always well to be knowledgeable about the structure and seal-ability of plastic films before proceeding to process them.