Precision Heat Sealing— Art or Science?
In recent years the methods and means of sealing plastic films and joining plastic components has advanced from an art to a science by the development and introduction of processes, designs, and controls that virtually guarantee perfect bonding every time. The development of this capability has been driven by the demand for polymeric materials and packaging in critical applications where failure is not acceptable. In addition, it has been shown that precision processing reduces the rate of rejection and thereby reduces cost and increases profit margins. Many new polymeric materials are being developed constantly with qualities that expand their usefulness but in spite of superb qualities they come with increased cost. It is therefore all the more important to eliminate rejects and reduce the waste of expensive materials.
The Evolution of Heatsealing Perfection
The art of heat sealing plastic materials began more or less concurrently with the development of the plastic materials themselves. The basic plastic materials would have much less commercial value if they could not be sealed or joined efficiently.
Initially the sealing was accomplished by manual means with simple hot wands or bars. When sealed manually the timing and the quality of the seal was dependent upon the skill of the operator because the seal was still hot when the force was released. Seals on plastic materials do not develop much strength until cooled; so the integrity of the seal was dependent upon the ability of the film layers to adhere to each other while still hot from the sealing process; so called “hot tack”. In the meantime the web of film must be handled with care to avoid damage to the strength or appearance of the warm seal.
Plastic sealing machines with hot bars use sealing bars that are constantly heated with cartridge heaters that are positioned within the bar as carefully as possible to deliver uniform heating to the sealing surface of the bars. Thermocouples or RTD’s are also positioned as skillfully as possible to monitor and control the temperature of the bar. In spite of the best efforts to exercise accurate temperature control, the temperature of the constantly heated bar fluctuates because of the delayed response of the thermocouple and the further delay in re-heating the bar to desired temperature. The inevitable result is overshooting of the bar temperature followed by a significant loss of heat when the bar contacts the work piece.
Some plastic film materials have a wide temperature window of opportunity and will seal successfully in spite of a wide variation in sealing temperature but many films do not offer a wide tolerance in the temperature sealing range. Commercial applications for pouches and plastic assemblies cannot risk the uncertainty of loose tolerances because of the nature of the application. This is particularly true in packaging for healthcare, food, and precious or noxious materials.
The art of successful packaging or product assembly with constantly heated devices required the constant vigilance of skilled operators and a willingness to accept a significant reject rate as the penalty for less than precise control.
In the mid 1940’s Impulse heat sealing was introduced into the market by the Vertrod Corporation. This was an important contribution to the art of heat sealing because it provided the opportunity to provide a momentary pulse of power through a relative thin heating element band that was pressed against two or more layers of plastic film. The energy supplied to the band produced enough heat to join the materials; and the jaws could remain closed long enough for the seal band to slightly cool. The impulse sealing technique gradually gained acceptance and earned a significant share of the market especially for sealing polyethylene.
Early impulse sealing machines are controlled by adjusting the voltage that is applied to the heatseal band and adjusting the duration of the sealing cycle. On simple machines the duration of each cycle is sometimes determined from cycle to cycle by the operator.
With the advent of automatic or semi-automatic machines, the attention of a skilled operator was still required because the temperature of jaw bars of the machine become warmer with every cycle. When an equal amount of energy is added with each cycle the sealing temperature soon becomes too warm and must be adjusted. In addition, the heat seal band will overheat if energy is applied to the band before the jaws are closed. This limitation creates two problems. The heatseal band is restrained from expanding freely when it is heated under pressure and the physical stress within the band when combined with the thermal stress causes premature band breakage. The inability to preheat the band before contact prolongs the sealing cycle time and reduces the production rate.
Several arrangements have been tried in an attempt to automatically adjust or compensate for these limitations but none of these are totally satisfactory. Nonetheless, the first steps had been taken to advance the heat-sealing technology to become less art and more science.
TOSS Technology
In the 1970’s, TOSS GmbH, in Germany, recognized the need for a high response temperature control system. The system would have to constantly monitor the temperature of the heatseal band, raise its temperature to a predetermined set-point in milliseconds, and hold it at a precise set-point for a pre- determined sealing time to allow the seal to bond and cool.
The new system also required a unique alloy that would replace the old nichrome heatseal bands. TOSS Alloy-20,® was then introduced. This new alloy was able to predictably and measurably change its electrical resistance as a function of its temperature. An ultra-high response controller could then be used to monitor the actual temperature of the heatseal band by monitoring the change of its resistance. The controller could then automatically adjust the power supplied to the heatseal-band in conformance to a preset program. Previous attempts to accomplish this degree of control by using thermocouples or RTD’s failed because the sensors could only monitor the temperature at a single location and were far too slow to respond.
In addition, electrical components like transformers and current transformers that could predictably respond to signals generated by the controller were also needed. The coordination of this family of related components, together with special heatseal bands, heatseal bars and bar facing materials, form the complete system known as TOSS Technology.
This total system called TOSS Technology was globally introduced by TOSS GmbH in Germany and TOSS Machine Components in the USA for the impulse heat-sealing of polymeric materials. TOSS Technology is the systematic application of the various elements required to predictably produce “Perfect Seals... Every Time”.
TOSS Alloy-20® Heat-SealBands
It is absolutely essential that an alloy material used for the heat sealing element be durable and formable into the different shapes required for sealing and cutting. Whereas nichrome heat sealing bands were originally used because of their high electrical resistance, they are soft, less durable, and will not hold their shape when formed into the various configurations that are employed in the current “state-of-the-art”. The TOSS companies introduced TOSS Alloy-20 heatseal bands that are stronger and last longer in service, as well.
To achieve optimized precision sealing of plastic materials it is important to select a heatseal band configuration that is most suitable for the task at hand. Scientifically designed heatseal bands are available from the TOSS companies in over 300 different sizes and shapes. When selecting a band, the width is usually determined by the nature of the application.
The thickness, however, is more often related to the nature of the film or object to be sealed. Thicker bands, greater than 0.2mm, will heat-up and cool-down slower but they are more durable and, when pre-heated, they will store more heat to be delivered to the work piece and thereby accelerate the sealing cycle. Thicker films, greater than 6mil or thick fitment flanges will seal better by using a thicker band. In the case of thin materials, a thinner band, less than 0.2mm, will heat and cool more rapidly thus allowing a higher production rate.
Cutting wires or cut-seal bands are also available to cut and seal between adjacent packages or seal the edges of films while concurrently cutting away an adjacent part or scrap material. When cutting plastic materials, special bands are employed that are constructed with a raised bead that is heated at a slightly higher temperature than the adjacent portion of the heatseal band that creates a seal of the desired width. Standard bands are available in a variety of sizes. In addition, custom made bands can be made to produce ideal performance on specific plastic film compositions and thicknesses. Contoured bands are also available to cut curves, rounds, ovals, or irregular shapes for products like gloves or protective garments.
TOSS Heat Seal Jaw Bars
Heat-seals can be made in various lengths and configurations. This presents the next challenge because the perfect seal must be uniform throughout its entire length whether it is inches or several feet. There is a need for absolutely uniformity of force applied to the full length of the seal.
In the course of working through hundreds of applications, it became clear that the best heatseal jaw bars are aluminum bars with a generous cross-section. Because a perfect seal is not accomplished until the seal is cool, it is helpful to use massive jaw bars with a high specific heat so that the bars can absorb the residual heat during the cooling cycle. In the case of short bars, this is not a problem because the bars can be machined to be flat and true. Long bars present a different situation because, despite best efforts, the commercial tolerances of stock materials are not sufficiently accurate to assure the precision required for uniform force. Precision machining of long bars is also more difficult. The TOSS companies have resolved this issue by having their aluminum jaw bars custom extruded to the closer tolerances required. In addition, a relieved area on the contact face of the jaw bar is provided to receive a self-adhesive silicone rubber facing. The TOSS jaw bar is also designed to receive specially designed, insulated, and spring loaded, Jaw End Blocks that automatically ensure that the heatseal band is properly positioned and tensioned when an original or replacement heatseal band is installed. Now, using the available scientific technology, one can achieve a precise, predictable seal or cut every time using a self-regulating system and leaving very little to chance. What was “art” is now “science.”