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Toward improved heat-seal performance

If you've signed on to a solid approach, your packages are being sealed and delivered with good consistency.
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FILED IN:  Machinery  > Filling/sealing  > Dry filling
     
When a heat-seal fails, costs accrue. And the later in the value chain that the failure occurs (or is discovered), the greater the costs.  When failures occur on the filling line, for example, the purchase price and other costs associated with getting the package to the line are sacrificed and can be increased by line stoppage, equipment readjustments, clean-up, etc.  Worst, when failures occur with the consumer, not only are accrued costs (including those related to the product) sacrificed, but future sales can be jeopardized.
 
Seal integrity is critical to a diversity of products, a short list being food, beverage, pharmaceuticals, health & beauty, detergents, and car-care.  Product forms can be liquid (thin to viscous) or dry (pellets, granules, grains, or powders), but what they share in common is the ability to flow, resulting in leakers when seals fail.  Package types can be flexible, such as pouches, bags, and sacks, or semi-rigid (or rigid) containers with flexible lids.  The degree of automation can range from intermittent to form/fill/seal (f/f/s).  But all the aforementioned variables notwithstanding, any company seeking higher seal integrity should have an approach built around the following:

Know the variables that affect line operations

Sealing is achieved by pressuring together two surfaces while heat is applied for a specific time.  By the same formula, one can make a grilled cheese sandwich; however, the package-sealing version is a bit more intricate.  The variables of heat, pressure, and dwell time are interrelated and should be manipulated for optimal results.  A fact that complicates that task is that the variables differ in how subject they are to manipulation.  Since equipment is built to a rated capacity (i.e. X-number of packages per minute) and since, ideally, the user wishes to run the equipment at a rate as close to capacity as practical, the variable of dwell time becomes a constant.  By similar constraints, pressure is a designed feature of the equipment, adjustable (at most) within a tight range.  It turns out, then, that the variable most receptive to manipulation is temperature.
      
Know the challenges posed by the product

The number one consideration is the product's potential to contaminate the sealing area.  No matter how neat and tidy the filling operation, there always is a possibly of a cast-off──a fine droplet or a tiny particulate that can make for a discontinuous seal.  Such discontinuities can be so small that they're difficult to detect visually, or even mechanically, but nonetheless can be large enough to serve as pathways for leakage.   Another consideration is the composition of the product.  Oleaginous substances (oils, fats, and greases), for example, can degrade a seal by penetrating and softening it.
      
Know the challenges posed by the packaging

Those challenges can be the result of materials or structure.  As an example of the former, aluminum foil, for all its superior barrier properties, is not a sealant; therefore, the sealing layer must provide adequate adhesion to aluminum.  An example of the latter is the highly popular stand-up pouch, with its gussets and folds that demand certain flow and strength characteristics on the part of the sealant.
 
Choose the proper sealant

Indeed, a sealant should be compatible with equipment, product, and package; and, because temperature is the operational variable most under the control of the packager, the sealant's temperature profile is fundamentally important to seal integrity.  Components of that profile include range and hot-tack.

The sealing temperature range is the span of temperatures under which the sealant melts enough to cover the seal area.  Every make of sealing equipment undergoes some temperature fluctuations; therefore, a sealant that has a wide sealing temperature range reduces the need for equipment recalibrations.  Be that as it may, a wide sealing temperature range has practical limitations. Whereas low-end temperatures are beneficial in terms of lower energy usage and lower maintenance and parts replacement, high-end temperatures might not be usable, if, for example, they cause other parts of the package to melt or otherwise distort.
 
Hot-tack refers to the sealant's ability, while yet molten (or at least before it's fully set), to hold the surfaces together.  It's important because forces have the potential to separate the seal before it is completely set.  One example is vertical f/f/s, wherein the bottom seal is under load from the weight of the filled product.  A second example is horizontal f/f/s, in which the “spring” from the gussets on a pouch exerts against the seals.  As a third, the vapor pressure given off by a hot-filled product stresses the sealed lid on a tray.

In addition to temperature-related considerations, as well as aforementioned others (such as the ability to seal through product contamination), any criteria for choosing a sealant should include whatever is pertinent to the individual product manufacturer.  To make that determination, the product manufacturer has to be committed to making the required investigation and analysis.
       
Have effective quality assurance

The next best thing to avoiding seal failures is to catch them at the earliest stage, before they've had a chance to pile on the costs.  Both depend on quality assurance, which, in turn, should be based on reliable testing.  One measure──the need for which almost goes without saying──is that of seal strength.  There are standardized methodologies, such as those promulgated by ASTM; however, since they aren't obligatory, as are regulations, a product manufacturer always should be willing to modify them as warranted or even to devise proprietary methodologies.
 
But no matter how proprietary, quality assurance always should travel with the packaging operation, wherever it takes place.  That is to say, if there is outsourcing to a contract packager, that third-party provider's quality assurance should be an extension of the product manufacturer's.  Then there are those situations in which the product's traditional package doesn't require heat-sealing and the contract packager is used so that the product can be offered in a package that does──for example, a trial sample, pack-on, or the like.  Even so, that doesn't absolve the product manufacturer of the obligation of being able to evaluate the heat-seal quality assurance of a contract packager.
 
In summary, there are a variety of ways to produce a heat-seal, whether with the use of wax, polyolefin, ionomer, EVA, copolymer, or something else.  Not only are the considerations multiple, but the stakes are high; for, in this ultra-competitive marketplace, leakers can cost big, making sealing one's packages and sealing one's fate intertwined.

Sterling Anthony is a consultant, specializing in the strategic use of marketing, logistics, and packaging.  His contact information is: 100 Renaissance Center-176, Detroit, MI 48243; 313-531-1875 office; 313-531-1972 fax; [email protected]; www.pkgconsultant.com.

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