When it comes to fastening, joining and assembling plastic materials, heat sealing has been a commonly accepted joining technique since the 1950s. It is a fast, efficient process that is still used extensively today. However, as companies come under increasing pressure to become more environmentally friendly and reduce their overall carbon footprint, is heat sealing still the best option?
Ultrasonic sealing or welding has been in existence for nearly as long (since the 1960s) and has proved to be reliable and fast, and, when it comes to sustainability, it has many advantages, chiefly:
- It consumes far less electricity
- It reduces the amount of plastic material needed for a reliable seal
- It is more effective at sealing bio-derived and recycled materials
- It yields less waste
- It uses no adhesives, solvents or other consumables.
These factors, taken together, mean that ultrasonic welding can cut the carbon footprint of an assembly line by up to 25%.
Heat seals are simple: Component surfaces are held together, and heated sealing bars are applied on one or both sides. Heat flows (conduction) from the bars through the material to the seal interface, where the mating surfaces melt to form a bond. Conduction sealing controls are straightforward, with temperature, pressure and dwell time selected based on the melt characteristics of the plastics being joined.
Ultrasonic welding generates and applies heat and pressure differently. It generates heat by applying a vibration with a precise frequency and amplitude between the surfaces of the thermoplastic layers, while also controlling parameters such as force, pressure and speed to optimise weld results. The frictional heat melts the interior of these surfaces and, under pressure, creates a permanent bond.
Since the heat required for heat sealing is generated by a resistance-heated bar, the sealing elements are held at operating temperatures for 100% of the time. This generates a lot of heat that is transferred not only into the weld zone but also into the environment around the sealing equipment and, essentially, wasted. Ultrasonic seals, on the other hand, generate heat by high-frequency friction, with seals completed in as little as 100 milliseconds. The tooling is cool except during that brief time. Much more energy is required for resistance heating than for generating the ultrasonic sealing vibrations.
To provide a high-quality closure, heat sealing generally produces a 12mm seal. Assume a package, for instance, gets a 12mm seal at the top and bottom. That means a full inch of material goes into the sealing area alone. On the other hand, two comparable ultrasonic welds require just 6.35mm of total package length (3.175mm each on the top and bottom), resulting in a savings of 19mm per package.
For heat-sensitive materials, which include non-fossil-fuel-derived biopolymers and some recycled materials, the high temperatures and longer cycle times involved with heat sealing can lead to material damage. This problem is eliminated with ultrasonics since heating is minimal and momentary and can be controlled very precisely by the welder control.
Unlike heat sealers, ultrasonic welders are not affected by contamination in the weld area. Any product or residual oils in the sealing area during the filling process are squeezed out when the package is compressed and ultrasonic energy applied. With heat sealing, the seal integrity can be compromised, and if so, the entire package and its contents will need to be discarded.
This article has focused on the differences between ultrasonic welding and heat sealing. However, when considering the sustainability of the ultrasonic sealing process and its overall carbon footprint, it is only reasonable to look at another commonly used sealing approach … this one involving adhesives and solvents. Ultrasonics requires no additional consumables of any kind. In addition, dealing with solvents -- sometimes used to bond plastic materials or to clean adhesive application equipment -- necessarily represents a significant environmental concern, which should be avoided whenever possible.
While ultrasonic welding offers important environmental and sustainability advantages over heat sealing and other technologies, the choice of which technology represents the ideal solution for a given manufacturing application is not always clear. Emerson, and its Branson brand of welding and assembly systems, can offer invaluable support and assistance.
