If you’re struggling with stringing on your packaging machine, you’ve probably tried everything in your power to try to stop this problem and may even find the problem getting worse over time - especially after a stoppage!

If the usual fixes of increasing nozzle temperatures by 5C or moving the nozzles closer to the packaging havent fixed the problem - dont worry - lets get an understanding of why this is happening and how we can fix the problem.

In this article, we’ll delve deep into the mechanics behind stringing. We’ll explore the science that drives these annoying filaments and discuss innovative solutions that can help you overcome them. With advancements in materials like EnBA hotmelt adhesives making their way into the spotlight, there’s never been a better time to conquer this challenge head-on. Get ready to elevate your packaging game!

What is stringing?

Stringing is an undesirable occurrence that often plagues packaging machinery. It manifests as fine strands of material that form on packaging and on compression bars, creating a web-like effect often looking like candy floss

This problem typically occurs after the adhesive has been jetted from the nozzle and the nozzle fails to cut off properly leaving a ‘tail’ or ‘string’. This ‘string’ is often seen on both the packaging material and left on the machinery parts

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In many cases, stringing can affect not just aesthetics but also functionality. If you're using hotmelt adhesives for bonding purposes, these filaments can cause jams and downtime. In a worst case scenario - long opentime or particularly soft adhesives can transfer to belts, bars etc which can then transfer to finished products.

What causes stringing?

Hotmelt adhesives are thermoplastics which are designed to melt down to a low viscosity to allow the adhesive to jet cleanly and to ‘snap’ cleanly on exit without leaving a tail or string. There are 4 main factors which cause stringing but each factor involves the adhesive not cutting off cleanly at the nozzle.

1. Viscosity
2. Degradation
3. Polymer chain length
4. Miss-matched set speed/nozzle/distance configuration

Viscosity

Viscosity is often misunderstood, many people think that an ultra low viscosity will resolve all their bonding issues but while it may fix the issue of stringing at the nozzle - you may and up with spitting, poor cut off especially with electrically operated valves, and may cause problems with pop opens due to the reduced hot tack. In fact, the viscosity needs to be balanced to provide enough back pressure to open and close the valves effectively without dribbling. Thats said, when an adhesive is running colder than it is designed to be, There is a significantly higher risk of stringing as the adhesive is closer to its set point and too high viscosity. Will a lower viscosity adhesive resolve your stringing issues? Likely to make big difference to prevent stringing but be warned of the trade-offs with a low-melt.

Degradation

Not all hotmelts are made equal. Some hotmelts have been developed using a selection of low cost raw materials and there is a market for these types of products. The problem with using a selection of cheap waxes and resins is that they dont have the thermal stability of premium hotmelts which means that very quickly, the adhesive changes in its chemical nature which usually causes them to go thicker over a period of hours or days. This problem can be seen with adhesives which run at 120C or adhesives which run at 160C. If you’re experiencing problems especially after shut downs or stoppages, it is likely that the adhesive is degrading prematurely causing it to thicken while the machine has a stoppage causing issues on start up. These types of adhesives are designed for high throughput packaging machines where the tanks are replenished more than once per hour. IE. If you have a 16 Litre melt tank, you should be consuming more than 16 litres of hotmelt per hour. If you have a 4 litre melt tank and you’re only topping it up a few times daily - avoid these types of adhesives.

Polymer chain length

For most customers, you wont need to know what is a long chain polymer and what is a short chain polymer. For customers with high speed packaging lines battling stringing and tailing, then knowing if you are using a long chain polymer or a short chain polymer could be the difference between a good day and a bad day. A shorter polymer chain will ‘snap’ more readily and therefore is likely to be significantly cleaner running especially at high speeds. 

As a rule of thumb - see this guide below:

Miss-matched set speed/nozzle/distance configuration

The first thing we all try to do when we start seeing issues with stringing is engineer the problem away. There are certain things we can do to help by adjusting the settings on your packaging machine. The adhesives needs to be balanced to the line requirements, by balanced we mean:

• Fast setting enough to prevent cartons/cases popping open at the end of the line
• Not too fast setting so that it can be jetted from the nozzle and reach the packaging material before the adhesive has cooled down to its ‘stringing’ temperature
• High enough viscosity to give the adhesive enough hot tack to hold the case/carton closed in the compression area, and also to give the valves enough back pressure
• Low enough viscosity to jet cleanly with compromising hot tack
• Long enough open time to wet out on both substrates at the lowest machine speeds
• Adhesion profile to provide adequate adhesion to your finished products with failing in the field

Stringing can happen if the adhesive is too low in temperature, it can also happen when the nozzle is too far from the substrate. We have seen problems where the nozzles have too long an ‘engagement’ this is the length of the nozzle beyond the cutoff valve.

So what can we do to eliminate stringing?

1. The first thing to do is check that the temperatures are correct, If the stated adhesive temperature range on the datasheet says 150-170C, have the tanks at 150C, hoses at 160C and nozzles at 170C, if the adhesive is still stringing then increase the hose temperature to 170C.
2. Still stringing? Try moving the nozzles closer to the substrates without physically contacting the substrates.
3. Check for excessive air movement which could causing a sudden drop in nozzle temperatures.
4. Are the nozzles right for the adhesive?
5. If all of the above are still not fixing the problem, time to take a look at the adhesive polymer type, Request a trial sample

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Introducing the new EnBA hotmelt adhesives

The new EnBA hotmelt adhesives are game changer for many, developed because of a global shortage of EVA raw materials, the Ethylene n-butyl Acrylate offers improved adhesion to polar substrates while offering lower stringing properties.

What makes EnBA unique is its tailored molecular weight and polymer chain length. This results in a lower viscosity that enhances application versatility while minimizing issues like stringing.

EnBA offers an ideal balance of adhesion strength and flexibility. It bonds various substrates with ease, making it suitable for diverse industries—from packaging to automotive manufacturing.

With innovations like EnBA, manufacturers now have access to solutions that not only improve productivity but also enhance product quality across applications. Embracing this technology may very well be the future of efficient bonding solutions.

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