Hot melt guns

Another type of progressive-feed system is a self-contained applicator gun (portable hot-melt gun). This system requires no insulted hose, because the adhesive is melted at the gun, from adhesive cartridges, pellets (slugs) or... 

Figure 19 Hand operated portable hot melt gun using hot melt adhesive rods, and different sizes of nozzles.

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. 

Figure 30 Hot melt gun for automatic spraying/extrusion (source Steinmeyer, Germany).

The melting of certain thermoplastic polymers, which often corresponds to a softening (going beyond the flow threshold) due to the application of heat, is a reversible phenomenon. Thus a solid adhesive mass, deposited using a hot-melt gun, can set while cooling down. Three types of compounds are representative of Hot-Melt adhesives the ethylene vinyl acetate (EVA) copolymers O Fig. 14.9), the polyamides (referred to as food adhesives ), and the polyesters (tend to be reserved for high temperature applications). 

Manual units all operations reqnire handling by an operator. Common examples are the well-known mannal extrusion guns, manually operated peristaltic pumps, hand-held pinch valves and the hot-melt guns. 

Miscellaneous uses include textile bobbins, guns for hot melt adhesives and bilge pump housings. These materials are normally found in reinforced form. In addition to glass fibres, other fillers such as glass beads, talc and mica are used in conjunction with coupling agents. 

Hot-melt adhesives Hot-melt adhesives are thermoplastics that form good adhesives simply by melting, followed by subsequent cooling after the plastic has filled surface voids. Nylons are frequently employed in this manner. Electric glue guns typically operate on this principle. 

Most systems function with paste but Young et al. (10) described the use of a process whereby the extrusion was with a hot-melt system, the extrudate cut in a pelletizer (Randcastle RCP-2,0, Randcastle Inc. Cedar Grove. New Jersey, U.S.A.) into cylindrical pellets, which were then spheronized on a conventional spheronizer (Caleva Model 120, AC Compacting LLc, North Brunswick, New Jersey, U.S.A.) heated with a heat gun to 65-70 C while dusting with microcrystalline cellulose (MCC) to prevent agglomeration. The picture of the pellets shown to illustrate the product appears round but the process times of 45 and 80 minutes are considerably longer than those involved with the paste systems. 

Due to the considerably higher heat conductivity of metals in contrast to wood or plastics, it is advisable to preheat the adherends to be bonded to the temperature of the melt in order to achieve good adhesive strengths. (Hot-air gun, where appropriate, hair-dryer at highest heat level because of its electric conductivity never use a microwave oven )... 

High-pressure extrusion i.e. jetting—widely used for hot melt systems where a jet is fired by a pressurised gun. 

The use of hot melt or wax coating is limited by the ability to deliver the wax in a molten state. Although this is physically possible, significant limitations are realized with application of molten spray and associated liquid line tracing and gun to bed spray distance. Application of a hot melt coat is feasible in a suitably temperature controlled conventional pan. 

A new technological development for hot melts is their application by robotics (87). Robotics have been used to apply hot melts to structural components, e. g. , fiberglas-reinforced components. In one case, structural adhesives are applied at 480 F (249 C) with a specially designed hot-melt dispenser and gun marketed by Monsanto Co. Robotics have also been used to apply hot-melts to make PSA s. Robotics are useful in applying hot-melts to gaskets and seals. In general, robotics provide higher productivity, improved quality, and reduced cost. 

These systems heat only a small amount of adhesive at a time, thereby eliminating some of the problems inherent in a reservoir system. Several versions of the first-in first-out equipment are in use. In one type, granules or pellets of adhesive are loaded into a hopper. The adhesive melts on a heated grid, then flows to a gear pump and is immediately transferred under pressure, through an electrically heated hose, to a heated gun. This equipment can feed hot melts at the rate of 9.1 kg or more per hour, even though less than 500 grams of adhesive is maintained in the molten state. 

Hot-melt adhesives are also available in film form. In this form, the joint is assembled with the film in place. On healing, the hot-melt adhesive melts and flows. Some clothing items are bonded in this way with hot-melt fiber formed as a light mat. Hot-melt sealants are also available in rope form. These are often used as construction sealants with a special heat gun for on-site application. 

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. 

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