Hot melt thermoplastic adhesives have been widely used in industry for adhering many types of products, and are particularly useful in applications where quick setting time is advantageous. One application for hot melt adhesive which has met with considerable commercial success is the fabrication of cartons wherein the quick setting time of hot melt adhesive is useful in assembling the flaps of a carton in high speed cartoning lines.
A number of dispensers have been employed to deposit hot melt adhesive onto the flaps of cartons, or on other substrates where quick setting time is required. For example, on type of adhesive dispenser is a gun formed with an adhesive passageway connected to a nozzle having a discharge orifice. The adhesive is pumped through the gun and ejected from the discharge orifice of the nozzle in the form of a relatively thick bead of molten thermoplastic adhesive which is applied to the substrate. Another substrate is then placed into contact with the first substrate to "flatten" or spread out the adhesive bead over a larger surface area so that an acceptable bond is produced between the substrates.
One disadvantage of adhesive dispensers which discharge an adhesive bead is that a relatively large quantity of adhesive is required to obtain the desired bond. Molten thermoplastic adhesive is highly viscous and does not readily spread over the surface of one substrate even when a second substrate to be bonded thereto is pressed against the adhesive bead. As a result, a relatively large quantity of adhesive is required in forming the bead to ensure the surface area of the bond between the substrates is sufficient to adhere the substrates together.
Several attempts have been made in the prior art to lessen the quantity of thermoplastic adhesive required to bond two substrates together while obtaining acceptable bond strength between the substrates. In one prior art apparatus, the hot melt adhesive is transmitted under pressure to the discharge orifice of a nozzle. When the hot melt adhesive is ejected into the ambient air, it atomizes and forms a spray or mist of tiny droplets which are deposited onto the substrate. These small droplets cover a larger surface area than a single adhesive bead, and since bond strength is dependent in part on the surface area covered by the adhesive, a lesser quantity of adhesive in droplet form can be employed than is required with an adhesive bead.
One problem with spraying molten thermoplastic material in tiny droplets onto a substrate is that in order for the adhesive to completely atomize before it reaches the substrate, the nozzle must be positioned a relatively large distance from the substrate. As a result, the small droplets are exposed to ambient temperatures and tend to cool before they reach the substrate. It has been found that with some types of hot melt adhesives the droplets either harden before they contact the substrate or fail to retain sufficient specific heat after they reach the substrate to permit bonding to another substrate. Additionally, nozzles of the type designed to spray thermoplastic adhesive in highly atomized form can produce elongated strings or fibers of adhesive instead of droplets when the nozzle is first turned on and/or when the nozzle is shut off. These strings of adhesive tend to clog the nozzle and/or are deposited in that form onto the substrate.
Another attempt to reduce the quantity of adhesive utilized for cartoning applications and the like is found in U.S. Pat. No. 3,348,520 to Lockwood. The apparatus disclosed in the Lockwood patent produces relatively large drops of molten thermoplastic adhesive which are deposited onto one substrate for bonding with another substrate. The individual drops of adhesive are obtained by alternately opening and closing valves located in the adhesive supply lines upstream from nozzles connected to the supply lines. One problem with apparatus of the type disclosed in the Lockwood patent is that the valves which form the adhesive drops must open and close at extremely high rates to keep up with the speeds of modern cartoning lines, and they tend to wear or fail after relatively short periods of use.
Another approach in the prior art for spraying hot melt adhesives is found in U.S. Pat. No. 4,721,252 to Colton. This patent discloses an apparatus in which molten thermoplastic adhesive is ejected through the discharge orifice of a nozzle and a tube carrying pressurized air is positioned in the center of the adhesive stream ejected from the nozzle. As the pressurized air emerges from the tube, it expands radially outwardly and breaks up the hot melt adhesive in the stream to form droplets or blobs of adhesive which are then deposited on the substrate. Multiple air delivery tubes can be employed to control the width of the spray pattern of droplets formed.
The apparatus disclosed in the Colton Patent No. 4,721,252 produces thin, disk-shaped droplets and a relatively large amount of strings or strand-like fibers of adhesive between the droplets. The problem with thin, disk-shaped droplets is that they have a relatively short "open time", i.e., lower mass, thin droplets tend to cool and lose their ability to bond to another substrate in a relatively short period of time. Moreover, the strings or strand-like fibers formed in between the flat droplets cool so rapidly that they contribute little or nothing to the bond created between two substrates and constitute a waste of adhesive.