At present, the prior art foresees that the dyeing of plastic material be carried out before moulding, by mixing the raw material, which is usually colourless, with a coloured matter called MASTER. The so-called"MASTERS"are mixtures made up of dyes generally combined with a support made of EVA, and they are mainly supplied by industries that prepare this mixture according to the required colour.

Accordingly, a plastic-material moulding industry which produces colour objects by moulding, must place orders to third parties for the masters of the desired colour in the amount needed for meeting the production of objects with the fixed colour.

If the company which moulds plastic material wants to have a certain degree of freedom in scheduling its work, it must have in stock several masters corresponding to different colours for different parts, and in such quantity as to arrange the programming of the production according to the needs. Of course, this implies a stock increase, both in terms of space and in terms of cost, unless the company uses the so-called"just in time"procedure, which consists in receiving all of the necessary materials for the production just before starting the production itself, what requires a special organisation both of the company and of the suppliers that cannot be always provided.

This is the reason why a method is being developed for machine-dyeing moulded plastic objects by preparing the colour desired for the objects in the exact amount, and only for the necessary production.

In other words, the aim is to avoid buying and/or preparing the traditional masters, by directly introducing the suitable colour in the plastic material.

The present finding relates to one of the machines suitable for carrying out the dyeing method summarily described, and which is object of another patent filing.

The mixing device of the present finding is a device that, due to its special design and shape, is thus adapted to mix different dyes according to a

predetermined recipe so as to obtain a specific colour that can be then fed into the moulding machine of the plastic material during the part moulding.

One of the purposes of the mixing device of the finding is that the mixing obtained with said device should be homogeneous, without clots and uninterruptedly in the mixed dye obtained.

Another purpose is that of realising an almost immediate passage from one recipe to the other by making the mixing device capable of detecting the changes of colour to be mixed, thus preventing the contamination of the mixing of the successive colours by the residues of the previous dye.

It is evident that this can be achieved if the container wherein the mixing is carried out is a container having such structural characteristics as to not facilitate the deposit of the previously mixed powders, and allowing also an almost immediate and easy cleaning of the surfaces of the same container.

The purposes mentioned above and others that will be described better hereafter, are obtained by a mixing device of dyeing powders that, according to the first claim, comprises a body defining a closed volume, provided with a hatch for loading the dyeing powders, and a discharge pipe provided with opening and closing means, placed in rotation according to a horizontal axis through motor means, and which is characterised in that said body has plane surfaces defining a polyhedron, and in that the axis of rotation is different from the axis of symmetry and passes through two opposed vertices of said polyhedron, the mixing of the powders being carried out through successive detachments of the material, due to gravity, from the upper walls to the lower walls, with a violent impact.

According to a feature of the finding, the mixing device has a polyhedral geometry, preferably in the shape of a cube, and its inner surfaces are completely made smooth through specific treatments of surface finishing, so that during the rotation the powders easily fall from top downwards with a violent impact; such impact breaks the powder agglomerates that tend to remain bonded for effect of statical electricity which accumulates in any case : With an arrangement according to the finding, it is possible to prevent the powders from sliding down slowly the one upon the other, thus mixing in a very superficial way and substantially remaining laid the ones upon the others without obtaining an effective and efficient mixing.

On the contrary, it has been noted that the mixing by fall for gravity and by

violent impact from top downwards guarantees this close mixing and the crushing of possible clots so as to obtain a homogeneous mixing that guarantees a constant quality of the colour.

Brief description of the drawings Further features and advantages of the finding will appear more clearly from the following detailed description, made by way of not limitative example only, of a preferred embodiment of the finding and illustrated in the attached drawing. In such drawing: -Figure 1 shows the mixing device of the finding.

With reference to said figure, it can be seen that the mixing device, as a whole referred to with reference numeral 1, is-in the exemplified case-a cube which is articulated in rotation, rotating between the opposed vertices 2 and 3.

These vertices belong to three convergent edges, and during the rotation of the upper surfaces they ensure that the powder detaches towards the lower surfaces abruptly and with a certain distance of fall from top downwards, which increases the effect of the impact and of the breaking of the clots in the lower powders.

Each inner surface of the cube, which in the exemplified case is made of stainless steel, has been superficially treated with smoothing actions both by means of suitable machines and also with galvanic weld materials so as to guarantee the closure of the micro-porosities of the surface which hold the particles of dye. In this way, when the process continues from the mixing of a recipe to the preparation of another colour with a different recipe, it is possible to rapidly obtain the cleaning of the inner surfaces both manually, with normal cloths, and with a vacuum cleaner. The access to the inner surfaces of the cubic body 1 is realised through hatch 4, which is almost as large as a side of the cube, and which is fixed by means of clamps as at 5 to the rest of the polyhedral body 1.

It can be noted that the pivoting shaft X of the mixing device ends on edges 2 and 3, and that it is not passing through the inside of cube 1. This is to avoid the provision of further surfaces to be cleaned whenever the change of recipe is effected.

Cube 1 is provided with a discharge pipe 6 of the mixed charge, and said discharge pipe is provided with a valve 7 for opening and closing the same.

Also this valve is made in such way that a blowing with compressed air or the

like can completely clean the inner sides of the valve from any residues of dye.

It can also be noted that no mixing rods or, in any case, elements that facilitate the mixing of the powders, have been placed inside the cube, which elements are typically present in tumblers/sifters or in the mixers in general to facilitate as much as possible the mixing of the solids with one another.

The presence of these elements that would facilitate the mixing has been avoided in the construction of the finding, amplifying to the utmost the effect of fall for gravity, and avoiding any irregularity inside the mixing compartment. All of this for the purpose of preventing the necessary cleaning of the same when passing from one mixture of dyes to a different mixture.