Hot melt adhesives for packaging closing are a success story: the long-term efficiency, flexibility and process reliability are unrivaled. During the process, nozzles apply dots or beads of hot adhesive onto the surfaces to be bonded. This often has an undesired side effect, where the applied adhesive is pulling behind it a fine string of material from the nozzle. A widely used euphemism for this problem is angel hair. Experts in the field call it stringing.
Identifying the problem
The length of these fine strings determines how heavily the machine will be soiled over time in an industrial environment. A string of just a few millimeters on the single bead can add up to a considerable amount in a high-throughput process. Apart from wasting adhesive, this also increases the cleaning effort, leading to extended maintenance and production standstills.
The success of PO hot melt adhesives has made the problem even messier. In addition to high thermal stability in the melter, these adhesives are also characterized by a wide range of adhesion. They have a formidable capability to adhere to the most different materials—including machine parts. Cleaning and maintenance can therefore become more difficult. It is therefore even more important to ensure that the adhesive is applied precisely only where it serves a purpose. In the worst case, it may otherwise lead to soiled products which can no longer be sold and have to be tediously cleaned or even disposed of.
The stringing of hot melt adhesives is influenced by many factors. In unfavorable conditions, several factors can occur at the same time and lead to considerable cobwebs of adhesive strings. Depending on the amount of stringing, the production line can then look like the home of Aragog, the giant spider from Harry Potter. However, magic is not needed to get rid of this nuisance. All that has to be done is to identify and optimize the possible causes one-by-one, from the process to the applicator equipment and to the adhesive itself.
Optimizing the process
Start by verifying the process parameters, with a focus on the processing temperature. It determines the viscosity of the adhesive and largely influences the cut-off of the adhesive at the nozzle. The temperature at the applicator head is particularly important. An increase of only five degrees Celsius in that zone can already significantly reduce the stringing. The temperature in the last few millimeters prior to the application is generally decisive.
Sometimes, the actual temperature may not match the temperature setting in the machine. This could be caused e.g. old application equipment. Temperature fluctuations can also be a result of the length, location, mounts and angle of the hot melt hoses, depending on the speed and consistency of the adhesive flow—possibly a cause of irregular stringing.
Ambient conditions can also have an effect on the temperature of the adhesive. Open doors and windows, unfavorably located fans or high process speeds can lead to draft. This can cool the applicator head as well as the adhesive and increase the stringing. In addition, thin adhesive strings may be blown into the machine or across the room.
If the nozzle is too far from the substrate, the length of the strings will also increase because of the higher temperature difference in the adhesive: While the material is still extremely hot at the nozzle tip, it has already cooled down considerably on the substrate due to the distance. This promotes long strings. The spacing should therefore be reduced as much as possible in the process.
Easy to fix, yet often overlooked: the position. Beads and dots squeezed out past the edge of the substrate do not serve any purpose. They only increase adhesive consumption and soil machine parts. A short string beneath the bonded flap is not as bad as one which is visible. Make sure the adhesive is applied in the correct location and at a sufficient distance from the edge of the substrate.
The potential to optimize the cut-off of the adhesive string within the processes is, however, limited. These limitations are determined by the machines used. Ensuring that the packaging is safely closed is generally more important than keeping the equipment spotlessly clean. If the steps described above fail to provide a satisfactory result, check the following influencing factors.
The influence of the equipment
Another factor influencing the stringing behavior apart from the process conditions is the application equipment. The mechanism inside applicator heads can be either electro-pneumatic (air open/spring close, or air open/air close) or fully electrical without pressurized air. Both technologies facilitate accurate adhesive application. However, applicator heads with a high closing force can cut off the adhesive string more cleanly in unfavorable conditions.
Application nozzles are available in a wide variety of designs. The feature most important when trying to reduce stringing is the capillary volume, i.e. the hollow space inside the nozzle. A smaller volume at the nozzle tip before the valve seat is more conducive to a clean adhesive cut-off. Needle-seat valves therefore routinely provide better results in the field compared with ball-seat valves. Experience also shows that multichannel, long-slot or angular nozzles are more likely to cause problems compared to short nozzles with only one channel. The nozzle is the proverbial bottleneck for clean adhesive cut-off.
Adhesive change can be an opportunity
If the previous approaches have been ineffective or cannot be implemented, the option remaining is to change the adhesive. A clean material cut-off at the nozzle is generally easier with a low-viscosity adhesive. However, it is also possible to formulate adhesives which have virtually no stringing tendency with the right mix of raw materials.
In most cases, the clean alternatives are easy to find. The new adhesive then has to be tested to ensure that it meets the requirements, for example short cycle times, high restoring forces, demanding surfaces or special temperature resistance. Jowat draws on many decades of experience in the field and carries out the necessary testing with modern testing equipment and free of charge for its customers. The implementation in the production environment is then accompanied by the experienced application specialists of the adhesive manufacturer. Ideally, the solution chosen will not only reduce the stringing but also provides additional benefits.
The objective: Bonding processes with increased sustainability
A clean material cut-off at the nozzle tip also saves resources by reducing the consumption of adhesive and downtimes. Minimizing the stringing is therefore a key objective for Jowat in the development of sustainable adhesives. The importance of this matter to the adhesive manufacturer from Detmold is evidences by not only numerous cooperations with the scientific sector—most recently with the Hochschule München University of Applied Sciences—and leading engineering companies but also by its in-house research efforts in this field. Last year, Jowat invested into a state-of-the-art unit for the meticulous analysis of adhesive string cut-off. The unit can simulate a wide range of different process conditions. Be it temperature, pressure, spacing, speed or different applicator heads and nozzles: Everything can be freely adjusted and configured for accurate reproducibility. This is important because each adhesive reacts differently to the varied combination of these factors.
At the end, it may be summarized that there are several rules of thumb and general tips to improve the cut-off but there is no silver bullet which solves the problem in all individual cases. Nevertheless, the adhesive manufacturer is constantly developing new and optimized high-performance adhesives based on its extensive application know-how. The objective: Always be a cut above the market in terms of sustainability also.
The article is published in the magazine "packaging journal 05-06/2023".