Adhesives and Sealants

 

2.5.6 Hot Melt Adhesive Guns for Jetting

We will study in detail all the hot melt applications in another chapter in Volume 4 of this Handbook. However, we briefly mention here the main equipment used for hot melt application.

The difference with hot melts is that they are applied in a melted state, at temperatures ranging from 120 to 200°C according to the formulations and end uses. Usually hot melts are applied in beads or lines with special hot melt guns, such as the ones shown in Fig. 30.

But hot melts may also be sprayed, and the main technique is fiberisation or swirl application, where the bead of adhesive coming out of the gun head is swirled in the air to produce a loop pattern as shown in Fig. 31. Several equipment manufacturers: Nordson, Graco and others supply these orbital guns.

This technique gives very thin ‘hairs’ of adhesive, which may be sprayed usually at moderate temperatures (from 120 to 160°C). During the passage, in the air between the gun head and the substrate, the hot melt cools down quickly, and its temperature is only 50 to 70°C when it reaches the substrate, so that it may be sprayed on heat sensitive materials such as polyolefin films or polystyrene foam without damaging it.

The adhesive pattern on the substrate may be a narrow band, 8 to 20 mm wide, which can be delimited quite accurately, as shown on Fig. 31.

Several types of adhesives may be applied with this technique:

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pressure sensitive hot melts for disposable items (baby diapers, feminine napkins),

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packaging hot melts for sealing polyolefin film packagings, cartons,

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bonding heat sensitive flexible foams,

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also other types of adhesives (non hot melts). For instance Graco supplies Precision Swirl applicators to the automotive industry where it is used to apply Hem flange adhesives based on epoxies and other adhesives to bond sound deadeners or other panels (see Fig. 32).