JP4916019B2 - Vibration welding structure of resin molded products - Google Patents

Description

  The present invention relates to an improvement in a vibration welded structure of a resin molded product obtained by welding and integrating a first resin panel and a second resin panel by vibrating them in a pressure contact state.

Patent Document 1 discloses a lid of a glove box that is attached to an instrument panel of an automobile as a vibration welded structure of a resin molded product. The lid is composed of a lid inner panel and a lid outer panel. A plurality of welding ribs are integrally projected on the mating surface side of the lid inner panel with the lid outer panel so as to extend in the vibration direction, and the front end surfaces of these welding ribs are provided. The lid inner panel and the lid outer panel are welded and integrated by vibrating in a state of being pressed against the back surface of the lid outer panel.
JP 2004-58468 A (paragraph 0028 column, FIGS. 1 and 4)

  By the way, in the resin molded product, the outer surface is hard with a solid layer, but there are cases where the inside is a foamed layer to reduce the weight.

  When two resin panels are integrated by vibration welding, when one resin panel has a foam layer inside as described above, the front end surface of the welding rib projecting from the other resin panel back surface, If vibration welding is attempted in a state where it is pressed against the back of one of the resin panels having the foam layer, the foam layer has a lower rigidity than the solid layer, and the solid layer is thin and does not have sufficient surface rigidity to bend. For this reason, the solid layer cannot resist the pressure contact force during vibration welding, and the weld rib tip may penetrate the solid layer and enter the foam layer.

  The state is shown in FIG. FIG. 12 corresponds to FIG. 1B of the first embodiment to be described later. In FIG. 12, a is a first resin panel constituting the front side of the resin molded product, and the resin density is formed on the outer surface of the first resin panel a. The solid layer b is high and hard, and the foam layer c having a resin density lower than that of the solid layer b is formed inside. d is a skin made of a non-woven fabric applied to the surface of the first resin panel a. e is a welding rib integrally protruding on the back surface of the second resin panel constituting the back side of the resin molded product, and the tip of the welding rib e is melted by vibration welding and integrally welded to the back surface of the first resin panel a. And penetrates the solid layer b and enters the foam layer c. f is a molten resin portion where the tip of the welding rib e and the solid layer b are melted and raised during vibration welding. g is a vibration welding jig that is set with the first resin panel a facing the back side up.

  As described above, when the tip of the weld rib e enters the foam layer c, the foam layer c having a lower rigidity than the solid layer b cannot sufficiently support the weld rib e, and the tip of the weld rib e and the first The welding strength with the resin panel a also seems to be lowered, and it becomes impossible to obtain a resin molded product in which two resin panels are firmly integrated by vibration welding.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a resin molding in which two resin panels are firmly integrated so that the tip of the welding rib does not enter the foam layer during vibration welding. Is to get the goods.

  In order to achieve the above object, the present invention is characterized in that the solid layer of the vibration welded portion is reinforced.

  Specifically, the present invention presupposes a vibration welded structure of a resin molded product obtained by welding and integrating the first resin panel and the second resin panel in a state where they are in pressure contact with each other. Measures were taken.

  That is, according to the first aspect of the present invention, the first resin panel has a solid layer formed on the outer surface and a foam layer formed on the inside, and the first resin panel has a solid layer on the back surface of the first resin panel before vibration welding. The plurality of projecting solid portions extend in parallel to each other in a direction intersecting the vibration direction so that the plurality of protruding solid portions extend in the vibration direction, and a welding rib is provided on the back surface of the second resin panel on the first resin. The protruding solid portion of the panel is integrally protruded so as to extend in the vibration direction corresponding to the protruding area, and at the time of vibration welding, at least one protruding solid portion of the plurality of protruding solid portions is welded. The welding rib tip is located opposite to the rib tip surface, and melts itself while being melted by the pressure contact vibration with the protruding solid part located at the opposite position, so that only the solid part is welded. It is characterized by .

  The invention according to claim 2 is the invention according to claim 1, wherein the projecting solid portion is composed of three or more, and the remaining portions on both outer sides of the projecting solid portion located opposite the front end surface of the welding rib A projecting solid portion is located, and both side surfaces of the weld rib tip are projecting shapes located on both outer sides by the molten resin portions of the projecting solid portion and the projecting solid portion facing the weld rib tip surface, respectively. It is characterized by being fused and integrated with the opposite side surface of the solid part.

  According to a third aspect of the present invention, in the above premise, the first resin panel has a solid layer formed on the outer surface and a foam layer formed on the inner surface. The projecting solid portion is integrally projected so as to intersect the vibration direction at intervals in the vibration direction at a plurality of locations, and the projecting solid portion of the first resin panel is provided on the back surface of the second resin panel. Corresponding to the region, the welding ribs are integrally protruded so as to extend in the vibration direction, and at the time of vibration welding, the plurality of protruding solid portions are positioned to face the welding rib tip surface, and the welding rib tip is It is characterized in that the pressure contact portion of each projecting solid part is melted by the pressure vibration with each projecting solid part and melts itself and is welded only to the solid part.

  According to the first aspect of the present invention, the solid layer on the back surface of the first resin panel is reinforced by the plurality of projecting solid portions extending in the vibration direction and has high surface rigidity. Even if the surface is brought into pressure contact with the projecting solid portion facing the front end surface of the welding rib, the solid layer in the projecting solid portion projecting region can sufficiently resist this pressure contact force and does not flex, and the solid layer Since the solid portion is thicker than the protruding solid portion, the weld rib tip does not penetrate the solid layer and enter the foam layer.

  Therefore, the weld rib is stably supported by the solid layer in the protruding solid portion projecting region having a hard and high surface rigidity, and the weld strength between the weld rib and the first resin panel is increased, A resin molded product in which the second resin panel is firmly integrated by vibration welding can be obtained.

  According to the second aspect of the present invention, both side surfaces of the welding rib tip are disposed through the melted resin portions of the welding rib tip and the projecting solid portion positioned opposite to the welding rib tip surface which are melted during vibration welding. Since it is integrated with the opposite side surface of the protruding solid portion on the outer side, the welding area between the tip of the welding rib and the solid layer is widened, and a sufficient welding strength can be obtained.

  According to the third aspect of the invention, the solid layer on the back surface of the first resin panel is reinforced by the projecting solid portions that are integrally projected so as to intersect the vibration direction at intervals in the vibration direction at a plurality of locations. Therefore, even if the weld rib tip surface is pressed against each projecting solid part facing the weld rib tip surface during vibration welding, the solid layer in the projecting solid part projecting region is The solid layer is thick enough for each projecting solid part to resist this pressure contact force, and the tip of the weld rib does not penetrate the solid layer and enter the foam layer. .

  Therefore, the weld rib is stably supported by the solid layer in the protruding solid portion projecting region having a hard and high surface rigidity, and the weld strength between the weld rib and the first resin panel is increased, A resin molded product in which the second resin panel is firmly integrated by vibration welding can be obtained.

  In addition, the welding rib tip melts each projecting solid part during vibration welding, so that each projecting solid part is inserted into the welded solid so that the welding area between the welding rib tip and the solid layer is widened. Strength can be obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
2 and 3 show a substantially rectangular package tray 1 as a resin molded product to which the vibration welding structure according to Embodiment 1 of the present invention is applied. The package tray 1 includes a first resin panel 3 and a second resin panel 5 and is configured to be welded and integrated by vibrating the two in a state in which both are pressed. In FIG. 3, broken lines indicated by reference sign L are welding lines, and in this example, five welding lines L are provided in parallel along the longitudinal direction of the package tray 1.

  A skin 7 made of non-woven fabric is integrally attached to the surface of the first resin panel 3 from the standpoint of improving appearance (see FIG. 2), and a holding recess is provided at the center of one long side. 9 is formed. A fitting recess 11 is formed on both short sides of the second resin panel 5, and the fitting recess 11 is detachably fitted to a support pin formed on a side trim of a cargo compartment at the rear of the hatchback vehicle body, for example. By combining them, the cargo compartment is divided into upper and lower parts by the package tray 1. In FIG. 3, the skin 7 is not shown for convenience.

  As shown in FIG. 1A and FIG. 4, the first resin panel 3 is formed with a solid layer 13 having a high resin density on the outer surface and a lower resin density than the solid layer 13 inside. The foam layer 15 is formed, and the solid layer 13 on the back surface of the first resin panel 3 before vibration welding has three elongated solid portions of the same quality as the solid layer 13 in the vibration direction (on the package tray 1). The projections are integrally projected in parallel so as to approach each other in a direction intersecting (orthogonal) with the vibration direction so as to extend in the longitudinal direction). Thereby, the solid layer 13 on the back surface of the first resin panel 3 is reinforced by these three projecting solid portions, and the surface rigidity is increased. The projecting solid portion projecting region in which these three projecting solid portions are projected corresponds to the welding line L after vibration welding. Hereinafter, the projecting solid portion at the center is referred to as a first projecting solid portion 17a, and the projecting solid portions located on both outer sides of the first projecting solid portion 17a are opposed to both side surfaces. This is referred to as the solid portion 17b. The first projecting solid portion 17a has a smaller projection than the second projecting solid portion 17b, and is melted away after vibration welding (see FIG. 1B).

  As described above, the solid layer 13 is formed on the outer surface, the foamed layer 15 is formed inside, and the three first and second projecting solid portions 17a and 17b are integrally projected on the solid layer 13. The 1st resin panel 3 is shape | molded as follows, for example.

  First, during molding, a foaming promoting substance such as a chemical foaming material that generates gas by a chemical reaction or an inert gas (physical foaming material) such as carbon dioxide gas and nitrogen gas, and fibers such as glass fiber are mixed. A thermoplastic resin (eg, polypropylene) is prepared.

  Then, this thermoplastic resin is injected and filled into the cavity of the mold in the mold closed state. On the molding surface of the mold, three elongated concave grooves are formed corresponding to the first and second protruding solid portions 17a and 17b on the back surface of the first resin panel 3. In the cavity, the thermoplastic resin is solidified and a skin layer is generated in the vicinity of the molding surface of the mold. This skin layer has not yet fully solidified.

  Next, in the process of solidifying the thermoplastic resin, the cavity volume is expanded by, for example, twice or more. At this stage, the portion of the thermoplastic resin that comes into contact with the molding surface of the mold is cooled early due to the influence of the mold temperature, so that the outer layer becomes a solid layer 13 having a high resin density and no voids. Constitute. On the other hand, the inner part of the thermoplastic resin is hardly affected by the mold temperature and is in a highly viscous gel state.

  Therefore, the expansion of the cavity volume causes the thermoplastic resin that has been compressed in the mold until now to be pulled to the molding surface of the mold, and expands and expands due to the gas generated by the chemical reaction in the thermoplastic resin, an inert gas, or the like. To do. At this time, the fibers in the thermoplastic resin are also elastically restored by reducing the compression, and the thermoplastic resin expands also by this elastic restoring force (spring back phenomenon).

  As a result, the solid layer 13 having a high resin density and no voids is formed on the outer surface, and the foamed layer 15 has a large number of voids (not shown) inside and has a lower resin density than the solid layer 13. The first resin panel 3 formed with is molded. Further, three first and second projecting solid portions 17a and 17b are integrally projected on the solid layer 13 on the back surface of the first resin panel 3. And this 1st resin panel 3 can achieve weight reduction compared with the case where it does not have the foaming layer 15 but consists only of the solid layer 13 with high resin density. It is not always necessary to mix fibers such as glass fibers in the thermoplastic resin. Moreover, you may perform foaming expansion of a thermoplastic resin only by the elastic restoring force by fibers, such as glass fiber.

  On the other hand, on the back surface of the second resin panel 5, five welding ribs 19 are integrally parallel to each other so as to extend in the vibration direction corresponding to the five protruding solid portion projecting regions of the first resin panel 3. Projected. The welding rib 19 is stably supported by a plurality of reinforcing ribs 21 from both sides so as not to laterally shake during vibration welding (see FIG. 2).

  The first resin panel 3 and the second resin panel 5 thus configured are vibration welded. At this time, as shown in FIG. 1 (a), the first resin panel 3 is set on the lower vibration welding jig 23 so that the first and second protruding solid portions 17a and 17b face upward, The second resin panel 5 is set on an upper vibration welding jig (not shown) so that the welding rib 19 faces downward, and the first projecting solid portion 17a is opposed to the front end surface of the welding rib 19, and the two The second projecting solid portion 17b is positioned on both outer sides of the tip of the welding rib 19. From this state, the upper vibration welding jig is pressed against the lower vibration welding jig 23 from above, and the front end surface of the welding rib 19 of the second resin panel 5 is brought into contact with the first protruding solid portion 17a of the first resin panel 3. The upper vibration welding jig is vibrated in the longitudinal direction of the package tray 1 while being pressed.

  As a result, as shown in FIG. 1B, the tip of the welding rib 19 melts itself while losing the first projecting solid portion 17a due to the pressure vibration with the first projecting solid portion 17a. Thus, only the solid portion is welded, and the first resin panel 3 and the second resin panel 5 are welded and integrated.

  In FIG. 1B, reference numeral 25 denotes a molten resin portion in which the tip of the welding rib 19 and the first protruding solid portion 17a are melted and raised during vibration welding.

  As described above, in the first embodiment, the solid layer 13 on the back surface of the first resin panel 3 is reinforced with the three first and second projecting solid portions 17a and 17b extending in the vibration direction to increase the surface rigidity. Therefore, even if the front end surface of the welding rib 19 is pressed against the first projecting solid portion 17a during vibration welding, the solid layer 13 in the projecting solid portion projecting region cannot sufficiently bend against this pressing force. In addition, since the solid portion of the solid layer 13 is thickened by the amount of the first projecting solid portion 17a, the tip of the welding rib 19 can be prevented from penetrating the solid layer 13 and entering the foamed layer 15. .

  Therefore, the welding rib 19 is stably supported by the solid layer 13 in the projecting solid portion protruding region, which is hard and has high surface rigidity, and the welding strength between the welding rib 19 and the first resin panel 3 is increased. The package tray 1 in which the first resin panel 3 and the second resin panel 5 are firmly integrated by vibration welding can be obtained.

(Embodiment 2)
5 to 7 show a package tray 1 to which a vibration welding structure according to Embodiment 2 of the present invention is applied.

  In the second embodiment, the package tray 1 is configured by integrating the first resin panel 3 and the second resin panel 5 by vibration welding, and the skin 7 made of nonwoven fabric is integrally formed on the surface of the first resin panel 3. It is attached so that the solid layer 13 is formed on the outer surface of the first resin panel 3 and the foamed layer 15 is formed inside, and the welding rib 19 is extended on the back surface of the second resin panel 5 in the vibration direction. And the three first and second projecting solid portions 17a and 17b cross (perpendicular to) the vibration direction so that the solid layer 13 on the back surface of the first resin panel 3 extends in the vibration direction. Although it is the same as that of the first embodiment with respect to being integrally projected in parallel with an interval in the direction, the distance between the first protruding solid portion 17a and the second protruding solid portion 17b is the first embodiment. Slightly more than It is different from the first embodiment in that it is Ku formed. Then, due to the molten resin portions 25 of the welding rib 19 and the first projecting solid portion 17a that melt at the time of vibration welding, both side surfaces of the welding rib 19 are opposed to the second projecting solid portion 17b on both outer sides. It is welded and integrated with the side. In addition, the same code | symbol is attached | subjected to the same structure location as Embodiment 1, and the detailed description is abbreviate | omitted.

  Therefore, in the second embodiment, similarly to the first embodiment, the welding rib 19 is stably supported by the solid layer 13 in the projecting solid portion projecting region which is hard and has high surface rigidity, and the welding rib 19 and the first rib. By increasing the welding strength with the first resin panel 3, it is possible to obtain the package tray 1 in which the first resin panel 3 and the second resin panel 5 are firmly integrated by vibration welding.

  Further, in the second embodiment, the both ends of the welding rib 19 tip are made to be the second projections on both outer sides by the melted resin portions 25 of the welding rib 19 and the first projecting solid portion 17a that melt at the time of vibration welding. Since the welding is integrated with the opposite side surface of the solid part 17b, there is an advantage that the welding area of the welding rib 19 and the solid layer 13 is widened and a sufficient welding strength can be obtained.

(Embodiment 3)
8 to 10 show a package tray 1 to which the vibration welding structure according to Embodiment 3 of the present invention is applied.

  In the third embodiment, the package tray 1 is configured by integrating the first resin panel 3 and the second resin panel 5 by vibration welding, and the skin 7 made of nonwoven fabric is integrally formed on the surface of the first resin panel 3. The solid layer 13 is formed on the outer surface of the first resin panel 3 and the foamed layer 15 is formed inside the first resin panel 3, and the welding rib 19 is extended on the back surface of the second resin panel 5 in the vibration direction. Although it is the same as that in the first and second embodiments, it is different from the first and second embodiments in the shape and orientation of the protruding solid portion.

  That is, in the first and second embodiments, the three elongated first and second projecting solid portions 17a and 17b made of the solid layer 13 extend in the vibration direction so as to extend in the vibration direction (longitudinal direction of the package tray 1). Close to each other in the intersecting (orthogonal) direction, the first resin panel 3 has a solid layer 13 on the back surface of the solid layer 13 integrally projecting in parallel, and the central first projecting solid portion 17a is melted away after vibration welding. On the other hand, in the third embodiment, a plurality of short protruding solid portions 27 are crossed (orthogonal) with the vibration direction at intervals in the vibration direction (longitudinal direction of the package tray 1), and The solid resin 13 on the back surface of the first resin panel 3 is arranged so as to be aligned in a straight line, and the plurality of protruding solid portions 27 are formed on the second resin panel 5 at the time of vibration welding. The welding rib 19 is opposed to the front end surface of the welding rib 19, and the tip of the welding rib 19 melts the press contact portion of each protruding solid portion 27 by the pressure contact vibration with each protruding solid portion 27. This is different from the first and second embodiments in that it is welded. In FIG. 8B and FIG. 9, for the sake of convenience, the tip of the welding rib 19 that melts at the time of vibration welding and the molten resin portion of the protruding solid portion 27 are not shown. Further, the same components as those in the first and second embodiments are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the third embodiment, the solid layer 13 on the back surface of the first resin panel 3 has a plurality of protruding solid portions that are integrally projected so as to intersect the vibration direction at intervals in the vibration direction at a plurality of locations. 27, since the surface rigidity is enhanced by vibration welding, even if the tip end surface of the welding rib 19 is pressed against each of the protruding solid portions 27 at the time of vibration welding, the solid layer 13 in the protruding solid portion protruding region is in this pressure contact. Since the solid layer 13 is thick enough to have the protruding solid portions 27, the tip of the welding rib 19 penetrates the solid layer 13 and the foam layer 15. You can keep out.

  Therefore, in the third embodiment, as in the first and second embodiments, the welding rib 19 is stably supported by the solid layer 13 in the protruding solid portion projecting region which is hard and has high surface rigidity, and the welding rib 19. The package tray 1 in which the first resin panel 3 and the second resin panel 5 are firmly integrated by vibration welding can be obtained.

  Further, in the third embodiment, the tip of the welding rib 19 melts each protruding solid portion 27 and is integrally welded and integrated with each other during the vibration welding, so that the welding rib 19 and the solid layer 13 are integrated. And a sufficient welding strength can be obtained.

(Embodiment 4)
FIG. 11 is a perspective view showing a protruding solid portion protruding region on the back surface of the first resin panel 3 constituting the package tray 1 to which the vibration welding structure according to Embodiment 4 of the present invention is applied. In the fourth embodiment, the first and second projecting solid portions 17a and 17b of the first and second embodiments and the projecting solid portion 27 of the third embodiment are combined to form a projecting solid portion projecting region. Yes.

  Therefore, the surface rigidity of the solid layer 13 on the back surface of the first resin panel 3 is further increased as compared with the first to third embodiments, and the integration of the first resin panel 3 and the second resin panel 5 can be made robust. At the same time, the welding area between the welding rib 19 and the solid layer 13 is significantly widened, and the welding strength can be obtained.

  In the first and second embodiments, the three first and second projecting solid portions 17a and 17b are used. However, depending on the hardness and thickness of the solid layer 13, the number may be two or four or more. May be. Further, the number of protruding solid portions that are melted away by vibration welding with the welding rib 19 is not limited to one.

  Further, in the third embodiment, the plurality of short protruding solid portions 27 are crossed (orthogonal) with the vibration direction at intervals in the vibration direction (longitudinal direction of the package tray 1) at a plurality of locations. In addition, the solid resin 13 on the back surface of the first resin panel 3 is integrally projected in parallel so as to be aligned in a straight line. For example, the adjacent solid solid portions 27 are slightly displaced in a direction crossing the vibration direction so as to be staggered. Alternatively, one continuous long protruding solid part 27 may be arranged in a zigzag shape so as to extend in the vibration direction.

  In each of the above embodiments, the case where the resin molded product is the automobile package tray 1 has been described. However, other automotive resin molded products such as a glove box lid and a trunk board, and further resin molded products other than automobiles. It can also be applied to products.

  The present invention is useful for a vibration welded structure of a resin molded product obtained by welding and integrating by vibrating in a state where the first resin panel and the second resin panel are in pressure contact with each other.

FIG. 1A is an enlarged cross-sectional view showing a state before vibration welding in which the tip of the welding rib of the second resin panel is made to correspond to the projecting solid portion projecting region on the back surface of the first resin panel, and FIG. It is an expanded sectional view which shows the state which welded the welding rib front-end | tip to the shape solid part protrusion area | region. It is sectional drawing in the II-II line | wire of FIG. 3 is a perspective view of a package tray according to Embodiment 1. FIG. It is a perspective view which shows the protruding solid part protrusion area | region of the 1st resin panel back surface in Embodiment 1. FIG. FIG. 3 is a diagram corresponding to FIG. FIG. 3 is a diagram corresponding to FIG. 2 of the second embodiment. FIG. 5 is a diagram corresponding to FIG. 4 of the second embodiment. FIG. 6 is a view corresponding to FIG. 1 of Embodiment 3. FIG. 6 is a view corresponding to FIG. 2 of the third embodiment. FIG. 5 is a diagram corresponding to FIG. 4 of the third embodiment. FIG. 5 is a diagram corresponding to FIG. 4 of the fourth embodiment. It is a figure equivalent to Drawing 1 (b) of a conventional example.

Explanation of symbols

1 Package tray (resin molded product)
3 First resin panel 5 Second resin panel 13 Solid layer 15 Foam layer 17a First projecting solid part 17b Second projecting solid part 19 Welding rib 25 Molten resin part 27 Projecting solid part

Claims (3)

A vibration welded structure of a resin molded product obtained by welding and integrating by vibrating in a state where the first resin panel and the second resin panel are in pressure contact with each other,
The first resin panel has a solid layer formed on the outer surface and a foam layer formed on the inside.
In the solid layer on the back surface of the first resin panel before vibration welding, a plurality of protruding solid portions are provided in parallel and projecting in close proximity to each other in a direction crossing the vibration direction so as to extend in the vibration direction,
On the back surface of the second resin panel, a welding rib is integrally projected so as to extend in the vibration direction corresponding to the projecting solid portion projecting region of the first resin panel,
At the time of vibration welding, at least one of the plurality of projecting solid portions is located opposite to the tip end surface of the welding rib, and the tip of the welding rib is pressed against the projecting solid portion located at the opposite position. A vibration welding structure for a resin molded product, wherein the protruding solid portion is melted by vibration and melts itself to be welded only to the solid portion. In the vibration welding structure of the resin panel according to claim 1,
Protruding solid part consists of 3 or more,
The remaining protruding solid parts are located on both outer sides of the protruding solid part facing the welding rib tip surface,
Both side surfaces of the weld rib tip are welded integrally with opposite side surfaces of the projecting solid portion located on both outer sides by the molten resin portions of the weld rib tip and the projecting solid portion located opposite to the weld rib tip surface. Vibration welded structure of resin molded products, characterized in that A vibration welded structure of a resin molded product obtained by welding and integrating by vibrating in a state where the first resin panel and the second resin panel are in pressure contact with each other,
The first resin panel has a solid layer formed on the outer surface and a foam layer formed on the inside.
In the solid layer on the back surface of the first resin panel, protruding solid portions are integrally projected so as to intersect the vibration direction at intervals in the vibration direction at a plurality of locations.
On the back surface of the second resin panel, a welding rib is integrally projected so as to extend in the vibration direction corresponding to the projecting solid portion projecting region of the first resin panel,
At the time of vibration welding, the plurality of projecting solid portions are positioned opposite to the front end surface of the welding ribs, and the welding rib front ends are pressed against the projecting solid portions by pressing vibrations with the projecting solid portions. A vibration welded structure of a resin molded product characterized in that it melts itself while being melted and is welded only to a solid part.

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