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FIELD OF THE INVENTION

The present invention regards a welding process of parts (also herein referred as "articles") in thermoplastic material, for example cellulose acetate or mixtures thereof.

KNOWN PRIOR ART

The technical field refers to the sector that utilizes parts produced in plastic materials, particularly in cellulose acetate.

This material is versatile and widely used in, for example, the production of fabrics, the stationary-office supply sector and optics sector such as the production of frames for eyeglasses.

In fact, cellulose acetate is a very precious plastic material, which well lends itself to machining due to its elasticity and strength. The ductility of this material leaves a large margin to designer creativity whose expertise in this material allows the design of creative and colourful parts. For example, regarding the textile sector, cellulose acetate fibres are often utilized since the products obtained have optimal characteristics of transpiration, easy handling and absorption. They may also be tinted in many different colours and connected with numerous other fibres such as rayon, cotton, wool, silk etc.

Another interesting application is that relative to the sector of eyeglass frames. Initially, eyeglass frames were cut out of cellulose acetate sheets. Today, this material has partly been substituted by modern thermoplastic materials that allow the production of frames by a process of injection moulding. However, still now, when particular frames are desirable, for example, with varied colours mixed together according to design plans, cellulose acetate is still widely used. A typical end product example is the so called tortoise-shell effect.

Cellulose acetate is very comfortable and smooth to the touch even when used for the production of moulded parts after possible polishing. Despite the numerous advantages described above, it is necessary to also take note of certam disadvantages concerning the use of this material in the above mentioned sectors.

In fact, in the preparation of the final product, it is often necessary to connect together different or same parts of the same material, or connect the cellulose acetate part with parts and elements produced in other materials. In this case, the actual method mostly used provides for gluing these materials together. Despite highly trustworthy new generation glues that guarantee strong bonding of glued parts, the connection of two or more parts by gluing is always subject to certain limits, the most significant being the strength of the resulting material, that cannot withstand a too intense load or tension. Furthermore, combining two or more parts together by gluing, requires removal of residue corresponding to the gluing position and often does not provide satisfactory results from an aesthetic point of view.

A particularly important sector for the production of cellulose acetate parts often of small size is that of costume jewellery. Particularly link articles such as chains, like those used as shoulder chains or handles for women's handbags or light weight travel cases, are often made of cellulose acetate due to material versatility obtaining products of various single and blended colours including the "tortoise-shell" effect with very interesting aesthetic features. Furthermore, these materials are light-weight which do not weigh down the article, a handbag in this case, they are connected thereto.

The limit imposed by the use of this material for the production of the above mentioned products, as is generally, the limit that the use of cellulose acetate has in application other than those mentioned above, is that of employing the gluing of same or different parts together as the only bonding technique. In fact, in the particular case of chains used as handles and shoulder straps for handbags and light weight travel bags, the primary problem is weakness in load resistance typical of cellulose acetate chains which necessarily require the gluing of one link out of two along the whole chain length. Obviously, a chain produced in this fashion, being able to withstand only very limited loads, is poorly adapt to be used as a handle or shoulder strap for women's handbags or light weight small sized travel cases that obviously must be filled and require an handle that can bear the load. For this reason, shoulder straps and handles for handbags and travel cases when produced in a "chain-like" way, traditionally are produced with metallic materials that permit welding and have a higher grade of load resistance. Unfortunately, metal is not adapted to particular and creative colouring since, if enamelled, is not cost effective and is prone, in any case, to rapid deterioration. It is often used for these purposes in its raw state underlining a very limited availability of colour scale (silver, gold, bronze, palladium, ruthenium, metallic oxide paints and their variations).

OBJECTS OF THE INVENTION

The aim of the present invention therefore, is to provide a process for the production of parts produced in cellulose acetate or mixtures thereof that are welded together and therefore support intense loads without breakage.

Another object of the present invention is to provide a product made in cellulose acetate or mixtures thereof that, if composed by multiple parts bonded together, it is the result of welding said parts and not of their gluing. Another object of the invention is to provide a product made in cellulose acetate or its compounds that is aesthetically pleasing, and that is optimally resistant regarding specific loads to be carried.

DESCRIPTION OF THE INVENTION

These and further objects and advantages that will be better understood from the following description, can be obtained by a production process of parts in cellulose acetate or mixtures thereof connected together and/or connected with different materials, characterized in that said connections are obtained by vibration welding. More in particular, said process comprises the following steps:

- arranging the base material in cellulose acetate

- cutting the material in desired form

- positioning at least one part to be connected with said material - connecting said part and said material by vibration welding.

According to the present invention, said at least one part to be connected to said material of cellulose acetate, can be produced in various plastic materials, for example a material other than cellulose acetate (but to which it can be welded), or otherwise, it could also be produced in cellulose acetate. As has been already mentioned, the innovating step of the process according to the invention consists in accomplishing the connection of a base material of cellulose acetate to another material, that can be produced either in cellulose acetate or another material able to withstand the process of vibration welding.

Vibration welding can be carried out according to different principles which depend upon the movement generated. For example, known techniques are linear vibration welding, orbital vibration welding and circular vibration welding.

In the case of the present invention, the step of welding the base material and the connection part is by way of circular vibration welding, exploiting the melting of contacting surfaces. The increase of local temperature necessary to the welding process is obtained by frictioning one of the two parts to be welded, upon which is applied a thrust pressure in such a way to maintain contact between elements during the process. In the case of circular vibration welding, the movement is obtained by an electric motor which activates the rotation of a shaft capable of generating an eccentricity and therefore a vibrating motion. In the circular vibration the performance is constant. The movement, which occurs without directional changes, allows transmission of melting energy in a constant way. Such energy is not subject to fluctuation and consequentially a rapid and uniform melting of the material on the contact surface between the elements is achieved. In the circular vibration welding cycle it is necessary to establish a series of parameters to prepare the instrument to be used in order to obtain a reliable procedure that gives consistent results over time. In particular, it is necessary to determine and prepare: - the width, that depends on the material to be submitted to processing,

- the frequency, generally included between 0 and 200Hz,

- the delay time, generally indicated with T1 , that indicates the lapse time from the cycle start of the instrument to the moment the vibration begins.

- the vibration time, generally indicated with T2, that determines the duration of vibration produced by the system.

- the cooling time, generally indicated with T3, also called pause time, that concludes the welding cycle,

- thrust pressures, that define the load with which the element to be assembled is pressed.

The present invention particularly refers to, and is particularly advantageous, when applied to the production of cellulose acetate chains destined to be used as shoulder straps or handles for small and medium sized handbags, also small sized sport and travel bags.

In this case, the process according to the invention, requires that the base material and the parts to be connected one another are both made with cellulose acetate. The process can be described in full detail as follows:

• arrangement of a first sheet in cellulose acetate (chosen among various colours and particular effects) approximately between 6 and

8mm thick (according to the models and desired product characteristics to be implemented)

• possible levelling/gauging (for example by a gauger with sand paper of various grains) to obtain specific thickness

· laser cutting of links - links of type A produced in a closed form with final size, links of type B to be transformed into open links

• possible turning of links (if rounded)

• rough shaping (for example, by way of ceramic pumice tumbler)

• blade cutting of each link of type B into two half links

· positioning in a mould adapt to each whole link alternated with each couple of half links • vibration welding of said two half links

• possible blade trimming of welding burrs

• rough shaping of welded chain parts (for example, by way of ceramic pumice tumbler)

· polishing of welded chain parts (wood chip tumbler).

Always according to the present invention and with particular reference to the above mentioned process, said cutting step of each link of type B into 2 half links is advantageously performed by blade cutting and not by laser cutting, to avoid possible problems of combustion/burning of the two surfaces that will need to be successively welded together.

Furthermore, said laser cutting step of the links of type A and Type B is advantageously carried out in a way that the type B link size is slightly different in respect to type A link. This is made necessary because type B links are destined to be cut, alternated with the closed type A links and successively welded, insofar as to create a chain. The size of type B links must take into account the fact that, during the cutting operation and successive welding, an infinitesimal part of the material will be lost and so type B links must be produced at a size that takes into account the fact that it will be necessary to cut out a side portion (taking into account the blade thickness and the thickness (confirmed) of welding loss). This, naturally, will not be necessary for type A links, that remain unaltered throughout the preparation of the chain.

Naturally, the process according to the present invention is advantageously applied also to types of chains of type A and B links prepared of equal dimensions and sizes. In this case, after the welding operation of the type B links to form a chain, the latter will result in a slightly different size with respect to type A links, but the result of the welding operation and the relative above described advantages will remain unaltered.

With reference to the above description, according to the present invention melting steps have been carried out for the production of cellulose acetate chains, using the following parameters, where T indicates, in general, vibration time, P indicates pressure, while the terms "relative measure" and

"absolute measure" indicate the measure relative to thickness loss of the material.

EXAMPLE 1

FREQUENCY 180 Hz

Insert width 1 ,25 mm

Absolute measure

T1 1.00 sec

P1 1 ,0 Bar

measure 158,50 Mm

P2 1 ,0 Bar

T3 2,00 sec

P3 1 ,0 Bar

EXAMPLE 2

FREQUENCY 180 Hz

Insert width 2,00 mm

Relative measure

Q melting 1 ,00 mm

P1 3,0 Bar

P2 5,0 Bar

T3 5,00 sec

P3 5,0 Bar

EXAMPLE 3

FREQUENCY 180 Hz

Insert width 1 ,25 mm

Absolute measure

T1 0,50 sec

P1 0,5 Bar

measure 159,40 Mm

P2 1 ,0 Bar

T3 2,00 sec P3 1 ,0 Bar

In this case, the cellulose acetate sheets were levelled at a maximum of 7,8 mm and the cut measure (half link) was recognized as 12,10/12,50 mm.

The chains produced according to the present invention, show improved characteristics over those produced, in the same material, according to the known technique. In fact, as has been already mentioned, load resistance, tensile strength and tearing strength, which are fundamental concerns for guaranteeing a quality product, are obtained by producing cellulose acetate chains according to the invention.

Usually, in fact the consumer requires from the supplier precise parameters of resistance, in accordance to the model of handbag on which a chain is applied, resistance that must proportionally increase with the weight of the bag and its specific use. For example, a heavy sports bag requires a much more resistant chain than a small sized evening handbag.

In this case for example, 90% of the material used is cellulose acetate owing to its greater elasticity in respect to acrylic materials and owing to its greater tensile breaking strength.

Normally, small sized chains produced by the known technique, must withstand a tensile stress of 12 to 15 kg, while the larger ones, always produced according to the known technique, are tested at 22 to 24 kg.

The cellulose acetate welded chains according the present invention, surprisingly withstand up to 35 kg.

Obviously, the present invention refers to all materials with a cellulose acetate base and mixtures thereof, or for at least all the materials (meaning base materials) in which there is a percentage (in weight) of cellulose acetate greater than 70%.

To better understand the advantages of the present invention, it should be noted that, with reference to the traditionally used methods, the glues used for acetate are not commercially available but are the result of years of testing and experience by craftsmen of this sector. Usually such glues are obtained by melting the material in various solvents obtaining a paste, that once in contact with the material to be bonded, causes melting, amalgamating the- two parts into a sort of melting/welding.

The above described is currently the only method known for gluing a base material such as cellulose acetate or mixtures thereof to an element composed of the same material or a different but compatible material. Such a method according to the known technique is not able to offer any guarantee concerning possible ungluing of links once the chain is subjected to a certain relevant tensile stress. The above is demonstrated by the fact that producers of glued chains are very careful when asked to accept liability for possible damage deriving from accidental opening of glued links of chains produced according to known means.

On the contrary, the chains produced according to the present invention, guarantee a greater tightness and therefore offer many advantages, already described above, in respect to those produced according to the known technique.