Hot-melt adhesives (HMAs or hot glue) are formulated with several chemistries and a range of properties. Some formulas can be soft and rubbery, while others are rigid. Ethyl vinyl acetate (EVA) is the most common HMA consumers are likely to encounter. EVAs can be formulated with various properties and are good for general-purpose binding.
Polyolefin HMAs and other chemistries are also fairly common. Reactive polyurethane hot-melt adhesive undergoes secondary curing to form a thermoset plastic.
Hot-melt adhesives are well suited for rapid assemblies, typically using a glue gun. The adhesive is melted, applied, and allowed to cool. The hot formula wets the surface when applied and, upon cooling, hardens to make the glue joint.
HMAs are thermoplastic and do not change when heated. In fact, they can be remelted and used again. They also have no solvent. Hot melts are 100% solid plastic with low toxicity and a long shelf life.
With a high safety profile, the biggest risk of using hot-melt adhesives is direct contact with the hot glue as it is applied. Before use and after application, these adhesives are benign materials.
As with all tools, ensure the glue gun is safe and maintained. Industrial glue guns typically resemble the home variety used by crafters or hobbyists. Generally, the heat is supplied using an electric heating element. However, gas-powered glue guns are available for wireless use.
Within manufacturing, hot-melt adhesive can also be applied using other processes — such as dipping, rolling, and spraying.
Generally, working time is quick and fairly easy when using a hot glue gun. Proper solidification requires rapid clamping at the correct pressure to squeeze the molten adhesive into a thin layer. Sufficient pressure must be maintained until solidification, or the bond might fail or be weakened.
The substrates or parts to be bonded must withstand heat. Most HMAs are applied at temperatures between 100° to 230° C (212° to 450° F). There can be contraction upon cooling capable of distorting thin substrates and repositioning is typically not possible. Bonded substrates must also be kept cool to prevent the bond from failing.
Hot-melt adhesives bond many substrates, including paper, plastics, glass, ceramics, metals, wood, and others. The high-melt viscosity of these adhesives means they’re ideal for porous and permeable substrates.
The packaging industry extensively uses these adhesives, but they’re also used for bookbinding, furniture assembly, and bond fabrics or crafts. HMAs are also used to fix, insulate, and protect electronic components.
Overall, the benefits of hot-melt adhesives include rapid bonding, zero use of solvents, and the ability to join many different materials. Additionally, these adhesives produce little if any toxic fumes, which can be a significant advantage for industrial processes.
Hot-melt adhesives are often supplied as a cylindrical rod known as a glue stick. The standard diameters of the glue sticks are 12, 15, and 43mm.
The sticks are compatible with glue guns that:
- Combine a heat source to melt the adhesive
- Use a nozzle through which the molten adhesive is extruded
- Have a mechanism — whether a hand trigger or an automated drive — that pushes the glue stick through the heater and nozzle