GLUE STICK AND APPLICATOR

Abstract: In a hot-melt adhesive system, a glue stick (2) is provided with a transverse cross section which is acircular. A glue gun is provided with an acircular opening in the gripper housing and an acircular opening in the heating element (4) to accommodate a glue stick (2). In a preferred embodiment, the major dimension of the glue stick (2) is approximately one-half inch while the smaller dimension is slightly more than one-quarter inch. The acircular glue stick (2) provides excellent heat transfer in the heating chamber (14) and permits a simpler manufacturing process. The gripper housing (34) includes a gripper element (38) which engages a major part of the width of the glue stick and can transmit a large advancing force without becoming embedded in the stick.

Description: Technical Field This invention relates to the art of adhesives. More especially, the invention relates to a stick of hot-melt adhesive and an applicator for use with that stick. Background Art In a known hot-melt adhesive system, a stick of hot-melt adhesive material is cylindrical and has a diameter of substantially 1,27 cm (one-half inch). While it is not clear why this particular dimension was choosen originally, there are several factors which have dictated the use of a cylindrical stick having a diameter of approximately this dimension. The cylindrical stick is manufactured through extrusion, and the circular cross section is maintained by twisting the stick as it comes from the extruder and passes to a cooling bath. The 1,27 cm (one-half inch) diameter allows a fairly efficient manufacturing process, and more sophisticated process controls are required to maintain the circular cross section as the diameter increases. In addition, if the diameter were larger than 1,27 cm (one-half inch), the tendency of the extruded stick to sag would increase, thus requiring additional steps in the manufacturing process. Moreover, a 1,27 cm (one-half inch) stick cools on the outside fairly quickly to prevent sagging of the stick, even while the inside is still soft. If the diameter were increased beyond 1,27 cm (one-half inch), the cooling of the outside surface would be inadequate to prevent sagging of the stick during the normal extrusion process. Further, if the diameter were increased, the glue stick would likely not cool (unless the cooling time were increased) before cutting or packaging, which would result in further manufacturing and shipping problems. The cylindrical glue stick shape is also critical to the design of a traditional glue gun. In the traditional glue gun, the glue stick is pushed into a heating chamber where it is melted. The unmelted portion of the glue stick thus acts as a seal for preventing the melted glue from flowing out of the heating chamber. Also, the hard glue stick is a piston which provides pressure to force the melted glue out of the nozzle. Thus, a good seal must be provided between the glue stick and the heating chamber. Glue which leaks out of the heating chamber cools in the mechanism of the gun and jams the mechanism of the glue gun. The diameter and shape of the glue stick also affect the heating efficiency. In the heating chamber, the glue stick is heated from the outside surface, and a round stick does not transfer heat efficiently because the surface area of the stick is small compared to its volume. Adhesive material is generally a thermal insulator, and one problem faced in the design of the heating chamber is that the center of the glue stick may still be un melted when the outer portion is melted. One traditional solution to this problem is to provide fins in the heating chamber for penetrating the chamber and carrying heat to the center of the glue stick. The shape and diameter of the glue stick also affect the choice of glue material. If the material is too soft, the glue stick will deform to such an extent that it prevents application of adequate pressure in the pressure chamber. If the glue stick is too hard, it will not deform enough to form the necessary seal with the heating chamber. If the diameter of a cylindrical glue stick is substantially less than 1,27 cm (one-half inch), the choice of material becomes critical because it must be formed of harder materials to have the necessary rigidity. The shape of the glue stick also has an impact on the design of the glue gun. A stick is typically advanced into the heating chamber by a mechanism which grips the stick. If the adhesive material surface hardness is high, the gripping mechanism may not be able to engage the stick properly, thus preventing proper advancement of the stick into the heating chamber. If the material is too soft, the gripper may become embedded in the glue and not released properly at the end of the stroke, also preventing proper advancement of the stick. One objective is to distribute the gripping force over the stick, but this has not been practical with cylindrical sticks. FR-A-2 513 905 has for object an applicator and a stick, the cross section thereof being cruciform. US-A-3 285 475 relates to a portable melting and dispensing tool using a rod having a V indentation. US-A-2 979 234 relates to an applicator adapted to receive a stick and provided with a barrel. The bore for receiving the stick of adhesive is specially shaped, the cross section thereof is lenticular. While the problems noted above with respect to cylindrical glue sticks have traditionally been addressed by modifications in the heating chamber, the advancing mechanism, or the composition of the glue stick, Applicant has determined that greater flexibility is available if the shape of the glue stick is acircular. Therefore it is an object of this invention to provide an improved hot-melt adhesive stick having an acircular transverse cross section and another object to provide an improved glue gun capable of receiving a glue stick having an acircular transverse cross section. In accordance with the invention there is provided an elongate glue stick of the type which comprises an adhesive material which melts at a temperature substantially above room temperature and is adapted to be received in a heating chamber of an applicator, which is characterized in that the cross section of said glue stick is oval and comprises circular end portions, said cross section comprising linear portions between said end portions. The invention relates also to a glue applicator comprising a heating chamber for receiving and melting a stick of adhesive and gripper means for advancing said stick into said chamber, which is characterized in that said heating chamber comprises an inlet having an oval transverse cross section and said gripper means comprise a gripper housing having an oval transverse cross section, said oval transverse cross sections comprising circular end portions and linear portions between these end portions, whereby a stick of adhesive having an oval cross section comprising circular end portions and linear portions between these end portions may be fed into said chamber by said gripper means and said gripper means comprises a gripper element attached to said housing, said element extending across a larger transverse dimension of said housing for engaging a surface of said glue stick having a larger transverse dimension. In a preferred embodiment of said glue applicator, the transverse cross section of said heating chamber tapers from said oval inlet to a portion having a circular transverse cross section. The above shape of the glue stick results in a stick having a dimension from the surface of the stick to the center which is less than that of a cylindrical glue stick. This reduced dimension improves the heat transfer out of the glue stick during the cooling portion of the production process and provides greater heat transfer into the stick when in the heating chamber during use. The glue stick, thus, cools faster during production, allowing greater simplicity in the production process. Similarly, the transfer of heat in the heating chamber is faster and results in a wider range of choices for the adhesive material and the heating elements. The greater surface area provided by the above cross sections provides greater sealing at the inlet to the heating chamber, thus permitting the use of lower viscosity materials. The acircular cross section stick of the invention provides a rigidity which is similar to that of a cylindrical stick having a radius equal to the maximum dimension of the new oval stick. This increased rigidity permits the use of compositions having low Durometer values without requiring extensive redesign of the feed mechanism in the glue gun. The oval cross section specified above has a longer dimension on two sides, and the gripper of the improved glue gun of the invention is designed to engage this longer dimension. This allows a greater feeding force to be applied to the gripper, but the gripper is not caused to penetrate the stick because the greater force is spread out over a larger area. The glue gun according to the invention provides a heating chamber having an inlet with a transverse cross section similar to the acircular cross section of the glue stick of the invention. An advancing mechanism similarly has a transverse cross section which substantially matches that of the glue stick to receive the glue stick and feed it into the heating chamber. Other parts of the glue gun, such as the drag ring and the seal to the heating chamber have similar cross sectional shapes. One advantage of this design is a glue gun which has a smaller dimension in one direction. Such a gun is more compact and is useful for many purposes, such as fitting in tight corners. The glue stick having an acircular cross section has other advantages. For example, because the sticks are not cylindrical, they do not roll off of a work table. The flat side of the glue stick can be printed with the product identification without significant distortion of the printed identification in contrast to the distortion caused by printing on a cylindrical glue stick. Brief Description of the Drawings Figure 1A is an end view of a glue stick in accordance with the invention showing its cross sectional shape. Figure 1B is a side view of the glue stick shown in Figure 1A. Figure 2 is a side view of a heating element for use in a glue gun in accordance with the invention. Figure 3 is an end view of the heating element shown in Figure 2 showing the inlet end. Figure 4 is an end view of the heating element shown in Figure 2 showing the outlet end. Figure 5 is a top view of a silicone sleeve for use with the heating element of Figure 2. Figure 6 is an end view of the sleeve showing the inlet end. Figure 7 is a front view of a drag ring for use in a glue gun in accordance with the invention. Figure 8 is a side view of a housing for a gripper for use in a glue gun in accordance with the invention. Figure 9 is a front view of the housing shown in Figure 8. Figure 10 is a side view of a gripping element for use with the housing shown in Figure 8. With reference to Figures 1A and 1B, a glue stick 2 is generally elongate and has a transverse cross section, shown in the end view of Figure 1A, which is a modified oval. Figure 1B is a side view of a glue stick in accordance with the invention, the stick having a length of approximately 3,08 cm to 25,4 cm (two to ten inches). The acircular transverse cross section shown in Figure 1A is formed by two circles having a diameter of approximately 0,74 cm (0.290 inches) and linear portions connecting the two circles. The overall width of the glue stick is preferably about 1,30 cm (0.513 inches). Thus, the glue stick shown in Figures 1A and 1B has a larger transverse dimension of approximately 1,27 cm (one-half inch) and a shorter transverse dimension just larger than 0,63 cm (one-quarter inch). Figure 2 is a side view of a heating element 4 which is adapted to fit in a glue gun as is known in the art. Heating element 4 includes an inlet 6 for receiving the glue stick shown in Figure 1A. An outlet 8 is adapted to receive a nozzle having a spring-loaded ball valve for permitting application of melted glue. Heating element 4 includes a cavity in lower part 10 for receiving a PTC heating element (element 16 of Figure 3) as is known in the art. Fins 12 extend into a heating chamber 14 to assist in transmitting energy from the PTC element into the heating chamber. As noted above, the fins may be eliminated for many compositions because of the increased heating efficiency resulting from the acircular shape. Figure 3 is an end view of the heating element 4 and shows PTC element 16 received in a cavity in portion 10. The inlet opening 18 is provided with a transverse shape which substantially matches that of the adhesive stick shown in Figures 1A and 1B. As shown in Figure 3, fins 12 extend to the heating chamber 14. With reference to Figure 4, which is an end view of the heating element 4 from the outlet end, an outer surface 20 of the heating chamber 14 is circular. The heating chamber preferably tapers from the modified oval shape shown in Figure 3 to a circular profile in the heating chamber 14. Thus, as the adhesive is melted, it fills a substantially cylindrical chamber prior to being expelled from end 8 through the ball valve (not shown). Figure 5 shows a sleeve 22 which is preferably made of silicone. One end 24 of sleeve 22 fits over inlet end 6 of heating element 4, and end 26 receives the glue stick 2. Figure 6 is an end view of the sleeve 22 showing an opening 28 in end 26, the profile of opening 28 substantially matching the profile of glue stick 2. This sleeve provides a tight seal with the outer surface of glue stick 2 to prevent the escape of molten glue from the heating chamber. As discussed above, the provision of an acircular cross section for the glue stick results in a greater contact between the opening 28 in the sleeve 22 and the outer surface of the glue stick. This provides a better seal and prevents leakage of the melted glue into the mechanism of the gun even at higher pressures. Figure 7 shows a drag ring 30 having a lip 32 forming a modified oval which also matches the cross sectional shape of the glue stick 2. The drag ring is located on the rear of a glue gun (not shown) and the glue stick is inserted into the drag ring when being inserted into the glue gun. Among other purposes, the drag ring prevents the glue stick from falling out of the glue gun accidentally. Figure 8 is a side view of a gripper housing 34, and Figure 9 is an end view of the gripper housing. Gripper housing 34 is mounted within a glue gun and is activated by the trigger mechanism of the glue gun. The gripper housing carries the glue stick forward as it is moved forward by activation of the trigger mechanism. This feeds the glue stick into the heating chamber 14 as is known in the art. Gripper housing 34 includes a hollow center 36 which again has a cross section which is a modified oval to accommodate the acircular profile of the glue stick 2 shown in Figures 1A and 1B. Figure 10 shows a gripping element 38 which is pivotally mounted by a pin (not shown) which passes through opening 40 in gripper element 38 and opening 42 in the gripper housing 34 (see Figures 8 and 9). Opening 44 in gripper element 38 receives a pivotal mounting pin of the trigger mechanism, and extension 46 receives a return spring. The disclosure of U.S. patent 4,523,705 is incorporated by reference for an explanation of how activation of the trigger mechanism causes rotation of the gripper element 38 to engage the glue stick and to advance it into the heating chamber. One advantage of the acircular shape of hollow center 36 is that the gripping surface 48 of gripper element 38 may be the length of the linear portion of the cross section of the glue stick shown in Figure 1A. Thus, the gripping surface 48 may be longer than that normally provided with a circular glue stick, thus distributing the force on the gripping surface over a larger area. This allows the application of a larger gripping force while maintaining a low pressure on the gripping surface to prevent penetration of the gripping surface 48 into the surface of the glue stick. By this construction, it is less likely that the gripping element will become embedded and stuck in the glue stick. It will be appreciated that a unique design for a glue stick and glue gun have been described. Modifications of the invention within the scope of the appended claims will be apparent to those of skill in the art.

Claims: An elongate glue stick (2) of the type which comprises an adhesive material which melts at a temperature substantially above room temperature and is adapted to be received in a heating chamber (14) of an applicator, characterized in that the cross section of said glue stick (2) is oval and comprises circular end portions, said cross section comprising linear portions between said end portions. A glue applicator comprising a heating chamber (14) for receiving and melting a stick of adhesive (2) and gripper means (34) for advancing said stick (2) into said chamber (14), characterized in that said heating chamber (14) comprises an inlet (18) having an oval transverse cross section and said gripper means comprises a gripper housing (34) having an oval transverse cross section, said oval transverse cross sections comprising circular end portions and linear portions between these end portions, whereby a stick of adhesive having an oval cross section comprising circular end portions and linear portions between these end portions may be fed into said chamber (14) by said gripper means (38) and said gripper means comprises a gripper element (38) attached to said housing (34), said element extending across a larger transverse dimension of said housing for engaging a surface of said glue stick having a larger transverse dimension. A glue applicator according to claim 2, characterized in that the transverse cross section of said heating chamber (14) tapers from said oval inlet (18) to a portion having a circular transverse cross section.

Inventors: MELENDY, PETER S.
Applicants: MELENDY, PETER S.
Application year: 1990
Main classification: B05C17
IPC codes: B05C17