The Root Causes of Adhesive Stringing & How To Address Them
Adhesive stringing is euphemistically known as ‘angel hair’, and less colourfully as ‘tailing’, due to its thin, almost gossamer spiders web appearance on the inside of hot melt adhesive machines. Unfortunately, the reality is less wistful than it sounds. The phenomenon occurs when thin filaments of dried adhesive remain attached to the nozzle and the substrate after application, creating undesirable trails. At best, this is a minor nuisance, but at worst, stringing can have significant consequences for production efficiency, product quality, and equipment maintenance. In this article, we look at the root causes of stringing and how you can take corrective actions to mitigate its impact.
What causes adhesive stringing?
Adhesive stringing is rarely caused by a single factor, but is typically the result of a combination of issues related to temperature, material properties, and application settings within your hot melt adhesive machine. Some of the usual suspects include:
Incorrect operating temperature
Temperature is the most important variable in any hot melt adhesive machine application. If the adhesive is not hot enough, its viscosity will be too high, preventing a clean break from the nozzle on shut-off. This results in the adhesive being pulled into a fine thread as the applicator moves away from the substrate. On the other hand, if the temperature is too high, the adhesive can degrade or char, altering its chemical structure and flow characteristics. This can also lead to stringing!
Poor adhesive quality or formulation
The quality of the hotmelt adhesive itself also plays a major role. Adhesives with a high molecular weight or a high melt viscosity are inherently more prone to forming angel hair. The cohesive strength of the material can cause it to resist separating cleanly, so it’s crucial to purchase a quality adhesive with a formulation that is appropriate to your application speed, substrate, and hotmelt machine. Using an incompatible or low-grade adhesive can introduce inconsistencies into your assembly line that lead to stringing.
How stringing affects production and quality
Adhesive string is messy, unattractive, and difficult to clean, but the real impact goes beyond what it looks like. These fine adhesive filaments can quickly accumulate on machine parts, disrupting your operation and potentially damaging your hot melt machinery. This is one of the reasons why we recommend hot melt tank cleaning at regular intervals. On the final products, meanwhile, stringing can interfere with downstream processes like printing and folding, increasing the risk of product rejection due to poor cosmetic experience, and can even cause stacks of products to stick together. Cumulatively, these minor irritations can slow your line speed and increase the need for manual cleaning, raising both your labour costs and material waste.
Can stringing be prevented?
Stringing is an inherent risk when working with hot adhesives, but yes, a proactive approach focused on equipment maintenance and process control can be effective at mitigating the risk and preventing adhesive stringing in most hot melt adhesive machines.
In particular, fine tuning your hotmelt adhesive machine settings is essential for achieving a clean application:
v Nozzle distance: the distance between the nozzle and the substrate should be minimised. A large gap increases the risk that the adhesive will be stretched into a string when cut off.
v Application pressure: air pressure that is too high can force the glue out too quickly, while pressure that is too low may not provide a sharp enough cut off. Experiment with your settings to find the optimal pressure for your adhesive and application.
v Nozzle condition: inspect your nozzles regularly for wear or partial clogs as part of your hot melt tank cleaning routine. A damaged or obstructed nozzle will not deliver a clean and consistent bead, making stringing almost inevitable.
v Distance of jetting: the ideal distance for jetting adhesive from the nozzle tip to substrate is 12mm. Most frequently we see cob webbing when the customer is jetting 30-60mm, then the angel hair is caused by the glue cooling in the air before it makes contact with the substrate.
v Draughts: simple things that can cool the area around the application gun can also cause stringing. A compressed air line leaking, guard doors open or even the factory door being left open can all cool the adhesive.
Find out more
Regular and thorough hot melt tank cleaning is an important part of stringing mitigation. We can remove the charred particles and contaminants that accumulate in the tank, which can otherwise enter the adhesive flow and disrupt application. If you would like to find out more or request a quote, please click here to contact one of our experienced team.
