Heat Seal Failure Modes

Once the seal is made and the package transported, the end-user needs to be able to open the package. Some heat seals are made to be lock-up seals, where the package must be destroyed or cut with scissors to open. Many consumer food packages are supposed to have easy-open, or peel seals. There are two common ways to 

One method is to add a second polymer to the heat seal layer that melts at a higher temperature than the sealant it should also melt at a higher temperature than is expected to be used on the packaging equipment for sealing. If this polymer is dispersed in the heat seal polymer in tiny domains, it acts to disrupt the continuity of the heat seal and weaken the seal strength to the point where the seals can be peeled apart easily. 

A second method of achieving a peel seal is to have the sealant break away from the next layer in the package and fail under tension. 

Drawings of examples of these types of packages are shown in Fig. 6.12. In this drawing, the EVA blend fails due to cohesive failure within the sealing layer, the first type of mechanism for achieving peelable seals. In the second sketch, the 

Seal failure mechanism examples in cereal bags 

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While preventive maintenance is concerned with regularly testing, and reconditioning equipment to prevent failures in service and premature deterioration, it follows that predictive maintenance procedures are concerned with the ability to predict when the equipment will fail and then developing schedules to implement timely repairs. Predictive maintenance does not imply that with the use of these techniques, failure modes in equipment can be prevented rather, it suggests that the occurrence of failure can be predicted and thus planned for. An appropriate example would be the inspection and change of a major compressor face-type oil seal where random heat checking (FM) has been observed over the years. 

Degradation of instrument air systems can lead to failures in safety related systems, including possible common-mode failures. Failures have occurred in several systems, including auxiliary feedwater, residual heat removal, main steam isolation, BWR scram, service water, emergency diesel generators, containment isolation, and fuel pool seals. 

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