~ All images courtesy of Emerson
When it comes to fastening, joining, and assembling plastic materials, heat sealing has been a commonly accepted joining technique since the 1950s. It’s a fast, efficient process that’s still used extensively today.
However, as companies are under increasing pressure to become more sustainable 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 ’60s) and has also proven fast and reliable. What’s more: it offers several environmental advantages.
For example, it:
- Consumes less electricity
- Reduces the amount of plastic material required for a reliable seal
- Seals bio-derived and recycled materials more effectively
- Yields less waste
- Uses zero adhesives, solvents, or other consumables.
Together, these features mean that ultrasonic welding can cut the carbon footprint of an assembly line by up to 75%.
Heat versus ultrasonics
Heat seals are a relatively simple process. 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. The conduction sealing controls — including the temperature, pressure, and dwell time — are based on the melt characteristics of the plastics to be 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. It does so while controlling parameters such as force, pressure, and speed to optimize the weld results. The frictional heat melts the interior of these surfaces and, under pressure, creates a permanent bond.
Comparing energy use
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 into the weld zone and the surrounding environment — meaning, energy is wasted.
Ultrasonic seals 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. The Energy Comparison table shows the potential to reduce electricity use by 25%.
Comparing the material
To provide a high-quality closure, heat sealing generally produces a 0.5-inch seal. Let’s assume a package gets a 0.5-inch 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 only 0.25 of an inch of total package length (0.125 inches each on the top and bottom), resulting in a savings of 0.75 of an inch per package. The Material Comparison table shows the potential material savings using a six-inch bag as an example.
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 also lead to material damage.
This problem is eliminated with ultrasonics since heating is minimal and momentary. It can also be precisely set by the welder control.
One more consideration: 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 is applied. With heat sealing, the seal integrity can be compromised and, if so, the entire package and its contents must be discarded.
Ultrasonic welding offers important environmental and sustainability advantages compared to heat sealing and other technologies, including chemical sealants. However, the choice of which technology represents the ideal solution for a given manufacturing application is not always clear.
Source an expert in the field that offers quality technology and support for your specific application.
About the author…
Patrick O’Sullivan is a product specialist for ultrasonics at Branson Welding and Assembly at Emerson. He holds a BS in mechanical engineering from the University of Rochester and will earn an MBA from the University of Rochester. The Emerson team, with its Branson brand of welding and assembly systems, offers a process-neutral approach to recommending the ideal technical solution for each application.