EP0756946A1 - Pencil of which the rubber collar part is attached by ultrasonic welding and specially adapted collar part - Google Patents

Abstract

A lead pencil (8) with a rubber (6) mounted on its end in a ferrule (18) fixed to the body of the pencil has the outer body (7) and the ferrule (18) made of compatible thermoplastic materials. The base of the ferrule (21) is fixed to the end of the pencil by localised inter-penetration of the thermoplastic materials on at least one zone of ultrasonic melding.

Description

The present invention relates to a pencil, with a central lead, the rear end of which is provided with an eraser housed in a ferrule. It also relates to a ferrule made of a thermoplastic material and specially adapted to serve as a housing for an eraser to be fixed to the end of a pencil.

Pencils with a graphite center lead are sometimes fitted, at their uncut end, with a small eraser, fixed in line with the pencil. This fixing is obtained by means of a part, called a ferrule, which allows the pencil and eraser to be assembled. Conventionally, it is a generally aluminum part, which has a cylindrical shape. The fixing itself is obtained by crimping, that is to say by locally deforming the cylindrical walls of the ferrule so that the deformations serve as anchoring zones both in the pencil and in the eraser.

According to the applicant, the traditional aluminum ferrule, if it is perfectly effective, has the disadvantage of being limited on the decorative level, on the one hand by the deformation of the crimping and on the other hand by the limited number of colors and shapes.

The aim set by the applicant is to propose a pencil with a central lead which is provided with a ferrule for a plastic eraser tip, the range of colors of which is not limited and the external appearance of which can take all possible forms, in particular if such a shell is produced by molding.

It is understood that the fixing of a plastic ferrule in the external body of the rear end of the pencil cannot be carried out by crimping, since the plastic material will not keep its permanent deformation as is the case for aluminum .

Plastic ferrules have already been proposed, with a central partition delimiting two housings, one for the rear end of the pencil and the other for one end of the eraser, the connection between the ferrule and the pencil being obtained by gluing. However, the bonding technique poses great difficulties of implementation in industrial production, in particular in the case of machine stops. In addition, the ferrule must be sufficiently fixed to the end of the pencil so as not to be able to be detached from the latter in the case where a certain traction is exerted between the pencil and the ferrule, in the case of erasing, which is not always respected by bonding.

It is the object of the invention to provide a pencil with a central lead which is provided at its rear end with an eraser housed in a ferrule, and which overcomes the aforementioned drawbacks.

Typically, the pencil body and the ferrule are made of compatible thermoplastic materials and the ferrule is fixed to the rear end of the pencil by localized interpenetration of said thermoplastic materials in at least one ultrasonic welding zone.

It is certainly known, in particular from document EP.0.505.262, writing articles of the pencil type with graphite lead which are produced by coextrusion of thermoplastic material and of which the wooden material, which constitutes the body surrounding the lead material, is in a thermoplastic material.

Admittedly, the possibility of using the ultrasonic welding technique is also known to fix parts of all kinds between them, including in the field of writing articles, in particular in document GB.2.264.676.

However, nothing allowed the person skilled in the art to think that it would be possible to fix by ultrasonic welding, at the rear end of a pencil, a ferrule made of thermoplastic material having sufficient attachment. This is obtained on the one hand thanks to the selection of thermoplastic materials, compatible, between the shell and the body of the pencil surrounding the mine and on the other hand thanks to the interpenetration of the materials in the welding zone. Indeed for the attachment to be sufficient it is necessary that there is a certain interpenetration of the materials constituting the shell on the one hand and the peripheral body, surrounding the mine, on the other hand.

Preferably, the thermoplastic materials of the shell and of the body of the pencil are both based on ABS (Acrylonitrile Butadiene Styrene).

Thanks to the invention, it was possible to produce ferrules of all colors and all shapes, giving decorative effects, in particular contrasting between the pencil and the ferrule.

It should however be emphasized that for the bonding between the ferrule and the pencil to be sufficient, it is desirable that the proportion of dyes in the thermoplastic material constituting the ferrule is substantially the same as that existing in the material constituting the peripheral body. pencil. Too large a difference in the proportion of dyes between these two elements significantly reduces this bonding.

Furthermore, it was noted that the attachment between the ferrule and the rear end of the pencil could also depend on the density of the thermoplastic material of the peripheral body of the pencil, in the ultrasonic welding zone. However, it is known, in particular from document EP.0.505.262, that the wood material surrounding the mine is an expanded material. The purpose of this expansion is to obtain a plastic pencil which has substantially the same density as traditional wooden pencils. It was noted that the attachment between the shell and the expanded material of the peripheral body could be less efficient in the case of a particularly expanded material, having a density of between 0.5 and 0.6.

To overcome this drawback, the rear end of the peripheral body of the pencil comprises a constriction, produced by compression, intended to locally increase the density of the expanded material of the peripheral body.

According to the invention, the shell in which the eraser is housed can have any suitable shape, provided that it has a contact zone with the thermoplastic material of the outer body, capable of constituting an ultrasonic welding zone.

In particular, the shell can be made, in a very simple manner, with a cylindrical housing for the rubber, closed at one end by a transverse bottom. In this case, it is at the bottom that the ultrasonic welding zone is located. Thus, unlike aluminum ferrules, it does not have cylindrical walls strictly speaking surrounding the rear end of the pencil.

However, from a decorative point of view, it may be preferred that the ferrule also has a cylindrical wall, like traditional aluminum ferrules.

Advantageously, in this case, the rear end of the body of the pencil has a constriction which is arranged so that after fixing the ferrule on said end, the outer face of the ferrule is substantially in the extension of the outer face of the peripheral body pencil, whether it is cylindrical or polygonal.

This particular embodiment makes it possible to obtain an original decorative effect, in which the outside diameter of the pencil is uniform over its entire length, including at the level of the ferrule.

It is another object of the invention to provide a ferrule specially designed to serve as a housing for an eraser, in a pencil in accordance with what has been described above.

Typically this ferrule, made of thermoplastic material, has a cylindrical shape with a transverse bottom, the outer face of which has a shoulder forming an energy director for ultrasonic welding, said shoulder being situated outside a circular central zone. , intended to be opposite the graphite mine.

In fact, it has been observed by the applicant that, even in the case where the lead material is produced from a thermoplastic material loaded with graphite, the presence of the latter prevented, during ultrasonic welding, an adequate attachment with the ferrule. It is therefore imperative that the shoulder acting as energy director, according to the well-known technique of ultrasonic welding, be located outside the zone corresponding to the graphite mine.

Preferably the shoulder in question has an annular shape. This allows a uniform distribution of the weld to be obtained.

According to a preferred version, of a shoulder having an annular shape, the latter has in cross section a symmetrical shape having a rectangular portion terminated in point by a triangular portion. This particular arrangement makes it possible to obtain very good penetration of the material constituting the shell, at the level of this annular shoulder, in the material constituting the peripheral body, surrounding the mine.

According to a preferred embodiment, the height of the shoulder was of the order of 1mm, of which 0.6mm for the rectangular portion and 0.4mm for the triangular portion, the thickness of the shoulder was, in the portion rectangular, about 0.4mm.

Preferably, for a pencil having an outside diameter of the order of 8mm, the annular shoulder had an outside diameter of about 5mm. In this way, the annular shoulder, forming an energy director for ultrasonic welding, is positioned substantially, concentrically with the axis of the pencil, at equal distance between the graphite lead and the outer face of the peripheral body.

Furthermore, it has been noted that it was preferable that, for a pencil having a graphite lead of a given diameter, the bottom of the shell is provided with a central orifice whose diameter is greater than the given diameter of said lead. Indeed during ultrasonic welding, the thermoplastic materials involved can be caused to creep. The presence of the central opening makes it possible to maintain perfect alignment between the pencil and the ferrule, despite such creep, since the material in question, instead of locally creating an additional thickness in the weld zone, escapes through this opening central.

Furthermore, preferably, the shell of the invention present on the inner face of the eraser housing, a plurality of regularly distributed longitudinal grooves. The advantage of this embodiment resides in the possibility of using, for ultrasonic welding, a sonotrode comprising a cylindrical end piece having a diameter adapted to penetrate the fitting housing of the ferrule. Thus, the positioning of the ferrule on the sonotrode is facilitated by the fact that the ferrule remains locked in position on the end fitting, once it has been positioned there. These same grooves have another function of serving as means for anchoring the rubber in its housing. The rubber is forced into said housing. The grooves, which locally crush the rubber, prevent it from being easily removed from the shell. Certainly during ultrasonic welding, the longitudinal grooves can be partially crushed, due to the heating due to the vibration of the sonotrode. However, this slight crushing does not alter the fixation of the rubber in the shell.

• La figure 1 est une vue schématique en coupe selon un premier exemple de réalisation de virole,
• La figure 2 est une vue schématique en coupe de la virole du premier exemple avant application des ultra-sons,
• La figure 3 est une vue schématique en coupe d'un crayon selon un second exemple de réalisation,
• Et la figure 4 est une vue schématique en coupe de la virole du second exemple.

The present invention will be better understood on reading the description which will be given of exemplary embodiments of a graphite pencil provided with a ferrule for eraser, fixed by ultrasonic welding on the rear end of the body of the pencil surrounding the mine, illustrated by the attached drawing in which:

• FIG. 1 is a schematic sectional view according to a first embodiment of a ferrule,
• FIG. 2 is a schematic sectional view of the shell of the first example before application of the ultrasound,
• FIG. 3 is a schematic sectional view of a pencil according to a second embodiment,
• And Figure 4 is a schematic sectional view of the shell of the second example.

The object of the present invention is a pencil 1 with graphite lead 2 which is provided towards its rear end 3, opposite the cut part 4 of a ferrule 5 serving as a housing for an eraser 6.

Typically, the body 7 of the pencil, surrounding the graphite lead 2 and the ferrule 5 are made of compatible thermoplastic materials. It may in particular be the same thermoplastic material, such as ABS or also two thermoplastic materials of good compatibility, in particular a polystyrene ferrule and the external body 7 of the pencil in ABS.

In addition, the ferrule 5 is fixed to the rear end of the pencil itself 8 is obtained by localized interpenetration of said thermoplastic materials in at least one zone 9 of ultrasonic welding.

In the first example illustrated in Figures 1 and 2, the shell 5 consists of a hollow and cylindrical part 10 with a rear end 11 open and a front end 12 closed by a bottom 13 whose outer face 13 a is provided with an energy director 14 capable of promoting ultrasonic welding with the thermoplastic material of the outer body 7 of the rear end 3 of the pencil 8.

In the example illustrated in FIG. 2, the bottom 13 of the shell 5 is pierced with a circular opening 15, the diameter D1 of which is greater than the diameter d1 of the graphite lead 2. In addition, the energy director 14 has an annular shape whose diameter D2 is greater than the diameter D1 of the orifice 15.

During the application of the ultrasound, the energy director 14 is applied under constant pressure against the rear face 16 of the pencil 8 so that the longitudinal axis of the graphite lead 2 coincides with the axis of symmetry of the ferrule 5. In this position, the energy director 14 is located opposite the external body 7 of the pencil 8. The action of the ultrasounds causes a localized fusion of the thermoplastic materials constituting the energy director 14 and the zone of the outer body 7, so that there is an interpenetration of the corresponding thermoplastic materials. After a cooling time, the ferrule is attached to the rear face 16 of the pencil 8.

The presence of the orifice 15 allows possible creep of the thermoplastic materials through said orifice 15, creep which prevents a localized excess of material and therefore a misalignment of the ferrule 5 relative to the pencil 8.

In the example illustrated in FIG. 1, the peripheral face of the rod 8 is aligned with the external face of the ferrule 5.

It should be noted that the energy director cannot be positioned in a zone which is opposite the graphite mine 2. Indeed, even if the graphite mine would itself be based on a thermoplastic material compatible with that of the ferrule, it has been observed that the presence of graphite prevents the attachment which is necessary to have a good fixation after ultrasonic welding.

Likewise, it has been noted that the best bonding results are obtained with thermoplastic materials which have substantially the same proportion of dyes both in the shell 5 and in the outer body 7 of the pencil 8.

The second example illustrated in Figures 3 and 4 presents compared to the first example above several significant differences.

A first difference resides in the constriction 17 situated at the rear end 3 of the rod 8. This constriction 17 is obtained by the localized compression of this rear end 3, compression which is carried out for example in a heated metallic conformator. The shape of the neck 17 is here frustoconical.

The advantage of this constriction is to increase the attachment between the ferrule 18 and the outer body 7 of the pencil 8 when the thermoplastic material of said body 7 is particularly expanded. The compression generated by the formation of the constriction 17 locally increases the density of said thermoplastic material in the zone which is intended to form the ultrasonic weld.

The second difference lies in the presence of a skirt 19 which extends the annular upright 20 forming, in the ferrule 18 the housing for the rubber 6. This annular skirt 19 fulfills several functions. It has an aesthetic role since it makes it possible to hide the connection zone between the bottom 21 of the shell 18 and the rear end 3 of the rod 8, in particular any burrs liable to be formed by creep of the thermoplastic materials in the zones of ultrasonic welding.

The skirt 19 also has a function of centering and aligning the pencil 8 and the ferrule 18 in the ultrasonic welding installation.

It may possibly have a third complementary securing function, in the case where the outside diameter of the pencil, towards its rear end 3 is slightly greater than the inside diameter of the skirt 19. In fact during the application of ultrasound, under constant pressure between the ferrule 18 and the pencil 8, there is also a localized fusion of the thermoplastic material constituting the ferrule and the external body 7 of the pencil 8 in certain contact zones, for example in zone 22 illustrated in the figure 3.

In this same example in FIG. 3, the rear end 3 of the pencil 8 had a second constriction 23, allowing perfect alignment of the external faces of the ferrule 18 and of the pencil 8.

A third difference between the first example and the second illustrated in FIG. 3 resides in the presence of longitudinal internal grooves 24 along the annular upright 20 of the shell 18. These longitudinal grooves 24 regularly distributed, for example six grooves spaced apart by a 60 ° angle, have a double function. They allow the perfect adaptation of the ferrule 18 on the sonotrode 25 of the ultrasonic welding installation. In the case in point this sonotrode 25 has a cylindrical end piece 26 whose dimensioning is such that the ferrule 18 can be positioned perfectly around this end piece 26, without play. Admittedly during the application of ultrasound, it heating or even localized melting of the thermoplastic material takes place according to one or other of these longitudinal grooves 24, which can cause a certain degradation of the profile of this groove, for example a certain flattening. However, this deformation is not prohibitive and does not prevent the longitudinal grooves 24 from fulfilling their second function which is to ensure the blocking in position of the rubber 6 when it is forced into the internal housing 11 of the shell. 18.

The face 27 a of the bottom 27 of the shell 18 is provided with an energy director 28, having the same shape as that of the first example. It is a ring-shaped shoulder having a cross-section a symmetrical shape with a portion adjacent to the face 27 a of the bottom 27, rectangular tapering by a triangular portion. The best results have been obtained with a shoulder of approximately 1mm in height, 0.6mm for the rectangular portion and 0.4mm for the triangular portion, the thickness of the shoulder being 0.4mm. This result was obtained for a pencil of approximately 8mm in diameter, with an annular shoulder, forming energy director 28, with an external diameter of approximately 5mm. Under these conditions, the annular shoulder was located substantially outside of the graphite mine 2 in a concentric median zone between the graphite mine and the outside face of the pencil 8.

It should be noted that in the case of a pencil having a section of non-circular shape, but for example hexagonal, it is preferred that the internal diameter of the skirt 19 is slightly less than the highest external dimension of the pencil. In this case it is the longitudinal edges of the pencil which act as an additional area 22 for ultrasonic welding.

The results obtained, with an application of ultrasound at 35,000 Hz, made it possible to produce pencils according to the invention for which the breakout forces between the ferrule and the pencil proper were at least 7 kg, generally a value admitted for the connection between the ferrules crimped in aluminum and the wooden pencils.

Of course it is up to the person skilled in the art to determine the optimal conditions for the application of ultrasound, in particular their frequency and the pressure to be exerted between the shell and the pencil to obtain the interpenetration. required thermoplastic materials constituting the shell and the outer body of the pencil.

Furthermore, the invention is not limited to the embodiments which have been described by way of non-exhaustive examples. In particular, the shape of the possible shrinkage (s), the shape and the color of the shell, the shape of the energy director (s) may vary within the scope of the present invention.

Claims (12)

Pencil with central lead, provided at its rear end with an eraser mounted in a ferrule fixed to the body of said pencil, characterized in that the peripheral body, surrounding the lead, and the ferrule are in compatible thermoplastic materials, the bottom of the ferrule is fixed to the rear end of the pencil by localized interpenetration of said thermoplastic materials in at least one ultrasonic welding zone. Pencil according to claim 1 characterized in that the thermoplastic material of the shell and of the peripheral body of the pencil is the same, in particular in ABS. Pencil according to claim 1 characterized in that it has the same proportion of dye in the thermoplastic materials constituting the shell and in the peripheral body of the pencil. Pencil according to one of claims 1 to 3 characterized in that the thermoplastic material of the peripheral body of the pencil is made of expanded material, having a density of the order of 0.5 to 0.6, and in that the end rear of said body has a constriction, having locally increased said density. Pencil according to one of claims 1 to 4, characterized in that the ferrule comprises a peripheral skirt extending, beyond the bottom, its annular upright, and in that the rear end of the pencil body has a constriction, arranged so that after fixing the ferrule on said end by welding the outer face of the skirt is substantially in the extension of the outer face of the body. Ferrule capable of serving as a housing for an eraser for a pencil according to claim 1, characterized in that it comprises on the external face of the bottom a shoulder forming an energy director for ultrasonic welding. Ferrule according to claim 6, characterized in that the shoulder has an annular shape. Ferrule according to claim 7, characterized in that in section transverse it has a symmetrical shape having a rectangular portion ended in point by a triangular portion. Ferrule according to claim 8 characterized in that the height of the shoulder is approximately 1mm including 0.6mm for the rectangular part and 0.4mm for the triangular tip, and a thickness of approximately 0.4mm. Ferrule according to one of claims 7 to 9, characterized in that for a pencil of approximately 8mm in diameter, the annular shoulder has an external diameter of approximately 5mm. Ferrule according to one of claims 6 to 10, characterized in that the bottom is provided with a central orifice whose diameter is greater than the diameter of the mine. Ferrule according to one of Claims 6 to 11, characterized in that it has longitudinal centering grooves on the inner face of the peripheral upright.

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