Problematic Sealing on Automatic packing machine
It is more challenging to seal an oriented film because it tends to revert to its previous state upon being heated. This causes puckering in the area of seal. Thankfully, it is possible to overcome this problem through the use of multi-point sealers with elevated hot spots on the sealing jaws as opposed to a flat surface. This technique confines the puckering to isolated spots on the seal, although it cannot be employed for a hermetic seal.
Narrow Sealing Range Films
It is difficult to seal films that have a narrow heat range of sealing owing to the requirement of very close temperature control. While copolymerization can be employed to widen the range, dielectric heating is also another plausible strategy.
The sealing bars pass electric current of high frequency through/to the film. In case the film comprises of polar molecules, which is indeed the case with nylon and PVC as a case in point, the impact of the current causes these molecules to oscillate. This molecular agitation generates a strong enough heat to melt the film in the area of seal.
Things to consider
Packagers as well as machinery makers are constantly endeavoring to enhance the productivity of packaging machinery. From the standpoint of film, this implies that they desire a film that not only operates quickly and easily via the machine, but can also be heat sealed readily to make seals with sufficient mechanical reliability and sufficient gas impermeability to be able to address the unique requirements of the product being packaged. In this context, “sufficient” underscores the fact that all seals do not need to be hermetic and the ones on packages which contain an ounce or two of product are not required to be as robust as those designed for packages containing much heavier materials.
Manufacturers of film should convert these requirements into measurable characteristics, which makes it possible to determine whether or not a film can be operated or whether their standard film production are able to meet the requirements of the packaging machine.
Film-specific properties that are relevant to such requirements of operability include COF, stiffness, roll formation, sheet flatness, uniformity of thickness and a plethora of properties governing heat sealability:
- temperature range of deal
- jaw release
- degree of hot tack
- percent shrinkage in the temperature range of heat seal
- strength of deal
- melt viscosity
- melting point
A low COF essentially implies that the film would move easily and swiftly around the machine’s stationary components with which it establishes contact. Other attributes like roll formation, good flatness, stiffness, and thickness uniformity mean that the film would be able to proceed through the machine in a uniform manner rather than oscillating and impelling the operator to slow the pace of production to ensure alignment adjustments on a frequent basis.
On the other hand, a wide range of temperature seal, for example at least 50°F assumes significance because it is not easy to regulate temperatures of sealing bars with great precision. In case this range goes below 10°F, it will mean that a few packages would continue to be sealed incompletely whereas others would be burned through in the seal area even as the temperatures of sealing bar move back and forth close to the optimal temperature setting.
Films whose melting points and/or viscosity are high suffer a drawback because it takes longer to raise the film temperature to the range of a sealing temperature. Moreover, a longer dwelling time is necessitated at that temperature for producing sufficient resin flow to ensure the quality of the seal.
The implication of an essentially high strength of seal is as follows:
- fewer packages are rejected on account of inadequate seals
- It is not mandatory to precisely control sealing bar temperatures, which, in turn, prevents frequent shutdowns to make adjustments in bar temperature
- It is possible to lower the seal coating’s thickness (if any) on the film, thus reducing material costs
- It becomes possible to operate thicker packages on the machine as the seal area requires less heat to ensure the minimum required seal integrity.
In turn, less heat translates into fewer distortions and shorter dwell times, which eventually accelerates operation.
Film producers can implement a range of formulating techniques in order to rectify machinability defects in a film that is otherwise desirable. For example, additives like finely divided silica or acrylic coatings can bring down COF to acceptable levels. Additionally, coating a film with inadequate seal characteristics using a thin layer of plastic resin with sound heat seal attributes can help improve seal properties.
In fact, OPP as well as other oriented films which are otherwise not easy to seal without involving distortion or puckering are commonly coated in this manner only. Alternatively, PVDC can be used to coat OPP in order to form a seal layer which also makes improvements in the base film’s barrier attributes.