The resultant material bond and seal strength of a heat seal packaging process depends on several factors, including: sealing dwell time, sealing temperature, sealing pressure, characteristics of the sealant and even the test method used to measure the seal strength. Seals must be strong enough to withstand the rigors of shipping and handling, yet at the same time facilitate easy access for the end-user to open the sterile package using aseptic presentation techniques. Optimizing the heat sealing process and consistently producing packages with the appropriate seal strength is of critical importance because it can have a direct impact on product efficacy and patient safety.
Sealing Dwell Time
Sealing dwell time refers to the time the heating elements of a packaging process (clamps, plates, bars, etc.) are in direct contact with the substrate(s). It can be either one-sided or two-sided heating. The two materials come together to form a bond or seal. It is important to understand how the equipment measures the dwell time to determine “true dwell time,” which is when the webs are actually pressed together. That’s because controls for heat sealing equipment vary and cycle time may or may not include travel time for the machine to engage into its final closed position. Any variation in the rates of materials reaching their seal initiation temperature, even by fractions of a second, can have a measurable effect on seal strength.
The heat sealing process, as previously described, marries two materials and creates a bond. To achieve this bond, one of the materials typically carries a surface sealant layer. The heating elements of heat seal packaging equipment are raised to a temperature high enough to either melt or activate the sealant.
The primary heat seal factors of time, temperature and pressure are interactive. A change or tweak in one typically requires a change or tweak in one or more of the other factors. A balance between time, temperature and pressure must be met to achieve the desired seal strength and visual seal transfer. Depending on the activation temperature of the sealant, raising the temperature and lowering the dwell time or lowering the temperature and raising the dwell time could produce more consistent seals without transparentizing the Tyvek®. Temperatures must be high enough to activate the sealant layer, while not overheating the surface of the material and inducing extreme transparentization.
It is recommended to optimize the heat sealing process using tools such as Design of Experiments. This not only results in a heat sealing process that consistently produces acceptable seals, but also can optimize energy output and operational throughput and efficiencies.
The third key factor of heat sealing is the pressure at which the equipment brings together and holds the two materials together to form the seal. It is important to know the actual pressure the substrates are exposed to because this value is often not equivalent to the input pressure or the setpoint on the machine controls. There are many techniques to measure sealing pressure—from sensor paper to more sophisticated electronic sensor technologies. For most heat seal materials, pressure is the least significant of the three factors required to make a heat seal.
The three main factors for heat sealing—dwell time, temperature and pressure—can produce significant variability in seal strength. However, there are other factors related to time, temperature and pressure that can also affect heat seals. Some common factors include: