What is hot melt glue?
Hot glue (also called “hot-melt adhesive”) is a 100% solid thermoplastic that is applied in a molten state and forms a bond on cooling to a solid state. Most hot melts lose tack as they solidify; those that retain their tack are known as pressure-sensitive adhesives.
Typically, hot melts are formulated with polymers, to provide strength and flexibility; resins, that provide the initial tack, and plasticisers, to alter the viscosity and ease of application. Hot glues also include waxes to thin the adhesive, and antioxidants to preserve the adhesive at high temperatures during storage.
How does hot melt glue work?
As already mentioned, hot glue is applied in a liquid (molten) state after being heated to the adhesive’s application temperature. Ensuring the glue is hot enough guarantees a good “wet out” of the first substrate, with glue able to fill small crevices on its surface. The second substrate must be brought into contact with the first during the adhesive “open time” when it is still hot enough to provide sufficient wet out. Compression is important at this stage, helping to bond both surfaces together.
How is hot melt glue applied?
The first step in the application process is to load adhesive in a hot melt tank; with the adhesive conveyed, via a hose, to the applicator; melted adhesive can then be sprayed, extruded, rolled, or coated onto the substrate, depending on the application requirements.
- Spraying: a useful method for larger coverage areas, fast line speeds, and heat-sensitive substrates.
- Extrusion: nozzles that eject adhesive in bead or line patterns are ideal for closure systems where adhesive is required in a specific spot.
- Roll coating: a quick, efficient, and even way to apply adhesive to the flat surface of a substrate such as paper, wood, metal, or plastic.
- Slot die coating: offers high levels of coating uniformity, with the adhesive extruded from a slot in a die head onto the substrate.
Hot melt glue housekeeping
Applying hot melt glue at the right temperature, and pressure, in the right quantities, and with the right level of compression will help to deliver an effective adhesive bond. The guidelines below, offer an outline to help troubleshoot some of the most common issues found while using hot melt glue.
- Hot melt tank: applying adhesive at the right temperature prevents the hot melt from oxidising and burning, if the heat is too high, or alternatively, stringing and running poorly if the temperature is too low.
- Applicator: getting the application temperature right prevents excess adhesive from being applied, if the glue is too hot, or from experiencing poor jetting and an inconsistent adhesive application if it's too low.
- Pressure: if pump pressure is too low, the flow of the adhesive will be severely restricted and, ultimately, could starve the glue nozzles. If pressure is too high, it may splash, overfire and use an excess of adhesive, not only affecting the efficiency of the process but also presenting a hazard.
- Substrate materials: substrate temperatures above 15°C (60°F) and below 35°C (95°F) generally give the best results. When a hot melt is in contact with a cold surface, its temperature rapidly falls, creating a sharper setting speed and consequent wetting difficulties that can lead to defective gluing.
Hot melt glue application tips
- Compression: the right amount of compression will prevent spontaneous bond failure or poor long-term bond integrity, if compression is too low; or deformation of the substrates so that adhesive/substrate contact is inadequate with the bond broken during consolidation, if compression is too high.
- Wetting out: sufficient compression at a high enough temperature should allow difficult surfaces to be properly wet out, such as coated packaging materials that have low surface energy.
- Open time: the hot melt glues open time must be equal to that of the application. If the open time is too short, the substrate will not wet out and bonding will not take place or bonds will fail shortly afterward. If too long, then insufficient time will elapse for the hot melt to lose heat and acquire sufficient green strength to prevent the flaps from springing open when leaving the compression section.
- Adhesive quantity: The optimum amount of hot melt must be applied to affect an adequate bond. Too much will lead to open time problems, increased consumption, stringing, and squeeze out during compression – leading to machinery contamination and unnecessary cleaning. With too little adhesive applied, the open time and set speed will be reduced, as will the gap-filling properties, wet out of the adhesive, and ultimately, it's bond formation.
If you would like further information about Beardow Adams’ range of hot melt glues, or would like to talk to our technical team about the set-up of your hot melt application, drop us a message via the contact us page.