Seal area represents the most problematic part in food packaging for controlling the moisture and gas ingress and preserving product quality. Understanding the mechanism of heat sealing, which is a widely used method in flexible packaging, is critical for maintaining product quality throughout the storage and preventing food waste.Likewise, understanding the factors causing the leak formation in the seal interface helps to avoid failures and increase integrity for all seal types produced by heat sealing methods. This review looks at heat sealing and its mechanisms in flexible food packaging materials and particularly focuses on the reasons behind the encountered seal integrity problems that have a detrimental effect on food quality and shelf life.Heat sealing mechanisms, form fill seal systems and seal types were analyzed. Then, factors affecting the leak formation have been grouped as process parameters, material properties, contaminants, and further processes, which is uncharted territory in open literature. Finally, the details of these groups and their interrelationships were examined and discussed. Revealed key factors as shown in this study are expected to guide future research for understanding leak formation mechanisms in flexible food packaging.

Seal strength is a key indicator of heat seal quality in flexible packaging. In this study, the effect of seal bar geometry, material composition and food particle contamination on the seal strength of widely used low‐density polyethylene (LDPE)‐based compound films was examined. Additionally, the maximum level of allowable solid food particle contamination was determined for ground coffee particles and powdered sugar. The results showed that adding metallocene LLDPE compound decreases seal initiation temperature (SIT) and increases overall seal strength. Also, changing seal bar geometry from flat to grooved bar with 0.56‐mm pitch height enhanced the seal strength significantly. Moreover, pressure mapping and T‐peel tests at SIT pointed out that grooved bars alter the pressure distribution and first contact points through the seal surface. Contamination of ground coffee particles at the seal interface as occurs during the packaging process when a powdery product is dropped in a package did not affect the seal strength up to 10 g/m2 at 0.5‐s dwell time. Above that amount, seal strength dropped dramatically. In the case of powdered sugar, threshold contaminant level was 2 g/m2 at 0.5‐s dwell time. Consequently, it has been revealed that knowing the type and the amount of contaminant during the food packaging process is important to maintain the seal quality, to find optimum process values between dosing and filling and to choose the right seal bar design because they can have a critical influence, especially at seal initiation phase.

Seal integrity and seal strength are important requirements in the heat sealing of flexible packaging. In this article, the influence of talc compounds and different process parameters, such as film tension and contamination on the sealant films consisting of polypropylene and low‐density polyethylene blend were investigated. According to the results, increasing the talc ratio from 0% wt to 30% wt positively influenced both hot and cold seal strength. Among different polymer blends having same talc ratio, the sample having the lowest melt flow index (2.84 dg/min at 230°C and 2.16 kg) showed the highest cold seal strength with 9.07 N/25 mm. On the other hand, in the samples with higher melt flow indexes, less seal integrity issues were observed in the presence of contaminants. Elevated film tensions from 0 to 0.4 N/mm2 enhanced the seal strengths significantly at the seal initiation temperatures of each film. However, this situation changed at higher operating temperatures due to the increased orientation and brittleness. Besides, in the presence of contaminant coffee particles at the seal interphase, high film tensions adversely affected the seal integrity since the average leakage increased 2.7% for samples A and B, and 7.4% for sample C. In summary, it has been shown that talc incorporation can improve seal strength, high MFI can fill the gaps at the seal interphase and the high levels of film tension (above 0.16 N/mm2) during sealing as well as the contamination need to be avoided to ensure integrity sealing.

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