When an operator is experiencing issues with seal integrity, their first instinct is usually to increase the heat on the sealer. Unfortunately, in most cases, this is the wrong adjustment. There are two much more common causes of improper sealing than temperature, and making corrections for those can solve most issues.
The first question you should always ask when something is off is “what has changed since it was running right?” Find this and you will usually find the problem. Generally, the change is not that the temperature on the seal bar has dropped. So, why start troubleshooting by raising the temperature?
1: Decreased Seal Pressure
Good seals come down to four factors: time, temperature, pressure, and absence of seal tension. The first three are interrelated, meaning the problems caused by a decrease in one can be solved by an increase in another. For this reason, an increase in temperature can improve seals when seal pressure is causing issues, but there is a tradeoff. Higher seal temperatures thin out the film, making the seals weaker. In other words, by solving one problem, you may create another.
Instead, operators can solve the problem at the source. Unlike temperature that stays constant on a machine, seal pressure can change over time from wearing on the seal pads, misaligned seal jaws, shims under the seal pad, or issues with air cylinders in pneumatic systems. All these issues can be tested for and solved without impacting the quality of the shrink-wrapped film.
Measuring seal pressure is difficult to impossible, but inspecting the seal pad for damage and/or replacing the seal pad is a quick and easy step to alleviate a seal pressure issue. While the pad is out, check for shims under the pad that may have been used to overcome a misalignment or weakened pad. Check the seal jaw alignment by making a manual seal (without heat) on a piece of pressure-sensitive or carbon paper. Examine the impression on the paper to ensure the marking is consistent across its length. If one side seems lighter than the other, that side may be a little high and may not be generating the needed pressure. On pneumatic systems, users should check incoming air pressure. If it’s outside of the 80-100psi range, air cylinders may be leaking or binding.
2: Presence of Seal Tension
Unlike issues with seal pressure that can be solved by temperature at the expense of film thickness, seal tension problems cannot be remedied by anything other than a seal tension solution.
As products travel through the wrapper, they pass through two layers of film that form a tube. The seal bar comes down between products to cut and seal film around the individual packages. Those two layers of film need to be fully relaxed between products or there will be seal tension resulting in open or weak seals.
There are four typical sources of seal tension:
- Products are too close together. This can be spotted by monitoring items before and after the seal bar. If goods on either side are pushed away from the bar during sealing, they are too close together.
- Product spacing is inconsistent. Does the seal bar periodically hit a product? Follow the packages upstream and watch the spacing between them. The point where the spacing changes from consistent to inconsistent is where the problem is located.
- Sealing is occurring above or below the seal plane. Ideally, the upper and lower seal jaws should meet at the center of product height. Sealing above or below this can pull too much film from the top or bottom and cause tension issues.
- Film is experiencing tension outside of the sealer. This can happen for a variety of reasons, such as upstream film dragging on a worn surface, a bad bearing in the unwind system, film routed improperly through the unwind, and static pinning the film to a surface.
Running through these potential problems and solutions will solve most seal problems. If you need assistance with any of these options or have an issue outside of seal pressure or tension, a Texwrap technician is always available and happy to help.