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Hot Melt Adhesives: Application Methods and Variables to Consider

According to Markets and Markets, the global hot melt adhesives market is expected to be worth $9.64 billion by 2020. From automotive and packaging to assembly and the tape and label market, these adhesives are versatile and can serve a range of applications in diverse markets.

Let’s take a closer look at the types of hot melt application methods and variables to consider:

Application Methods

Hot melt adhesives can be applied in three main ways:

1. Extrusion:

  • Slot Extrusion: This type of extrusion is used for applications that require a wide bonding area. Typically, this method fits tape and label application needs.
  • Bead Extrusion: Bead extrusion pumps the adhesive through a nozzle and forms a bead-shaped adhesive amount onto a substrate. This type of method works well for rigid packaging applications, which don’t require a wide surface coverage area for case and carton sealing.

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2. Spray: 

The spray application method is similar to bead extrusion; a nozzle is used to push the adhesive out. However, it also uses a stream of air that creates a swirled pattern of the adhesive on a substrate.

This method is beneficial for temperature sensitive substrates, as the air helps cool the adhesive prior reaching the substrate’s surface. Additionally, it is used for applications that need a wide bonding area but don’t require much adhesive, such as in nonwovens or assembly.

3. Roll Coat:

The roll coat method is used when to bonding large panels together. The adhesive is metered by the gap between rolls onto the primary substrate. It moves down a conveyor where a second panel is placed atop the first panel, and then nipped together. Typically, this adhesive application method is used for truck and RV applications as well as garage door panels.

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Application Variables to Consider

To optimize maximum hot melt adhesive bond strength and minimum cost, consider the following variables:

Application Temperature: the temperature of the adhesive as it exits the nozzle

  • Hot melt adhesive application temperature should be as high as necessary to achieve the desired wet-out and penetration of the substrate.
  • Spray applications usually require higher temperatures due to the heat loss as the adhesive fiber travels from the nozzle to the substrate.
  • Other considerations to make include:
    • Adhesive heat stability
    • Substrate heat tolerance
    • Equipment limitations in terms of open time/compression time

Add-On (coat weight): the amount of adhesive applied to make a bond

  • An application that requires a higher coat weight means the adhesive will take more time to set up. Set up is the process of the hot melt losing its heat and solidifying. This is because more heat is present when more adhesive is applied to the substrate, and the heat must be dissipated for the adhesive to set up.
  • The amount of adhesive affects the primary and secondary substrate bond especially in bead extrusion applications. The more adhesive applied to the primary substrate, the more area it will cover when molten and compressed. Having more adhesive enables it to stay molten for longer and wet out better to the second substrate.
  • The coat weight also determines the surface area of the finished bond.

Compression: bringing the two substrates into physical contact by the use of pressure

  • Compression mainly affects the bond to the secondary substrate. It forces the hot melt adhesive to flow into the primary and secondary substrates. Compression time must be long enough to allow the adhesive enough time to develop the cohesive strength to overcome substrate memory or stress. Insufficient compression time results in an improper or inadequate bond.
  • To facilitate maximum wet-out and bond surface area, compression should take place as soon after adhesive application as possible.

Open Time: the time (seconds) measured from the point of adhesive application to the point of compression with the secondary substrate

  • Ideally, open time should be as short as possible to achieve maximum wet-out and penetration of the secondary substrate.
  • However, keep in mind these other considerations:
    • Set speed of the adhesive
    • Physical space on the line
    • Pressure sensitivity of the adhesive.
  • PSAs generally have a longer effective open time.

For more information on hot melt adhesive application methods and variables to consider, call 800-7-BOSTIK.

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