WO2010052186A1 - Dual profile extrusion die - Google Patents

Abstract

The invention relates to a die for extruding a mixture primarily composed of rubber, consisting of a first wall (1) as well as a second fixed wall (2) disposed opposite the first wall for imparting a given cross-sectional shape to the rubber profile, characterised in that the second wall (2) includes an upstream portion (21) defining, with the first wall (1), an air gap having a height H, and a downstream portion (22) defining a setback having a height h relative to the upstream portion (21) of said second wall so that the air gap between the first wall (1) and the downstream portion (22) of the second wall (2) is equal to H+h.

Description

 DOUBLE PROFILE EXTRUSION LINE

[001] The field of the invention relates to the extrusion of profiles formed mainly of a rubber mixture.

In known manner, these profiles are obtained by means of extruders comprising an upstream portion in which the rubber mixture is plasticized and pressurized, and a downstream portion in which the mixture passes through an extrusion die intended for to give the product a transverse profile whose geometrical characteristics are determined with precision. The extrusion die is generally constituted of a first fixed or movable wall, and generally flat or cylindrical and a second fixed wall, placed vis-à-vis the first wall and whose profile is adapted to obtain the transverse profile of the extruded product. The space between the first and the second wall forms the gap of the die.

[003] By modifying in known manner the parameters such as the pressure upstream of the die, the exit speed of the mixture, and the temperature of the mixture, the skilled person will seek to optimize the performance of the extrusion machine in order to obtain a product with the best geometric characteristics.

[004] However, certain mixing qualities, more commonly known as decohesive mixtures, do not lend themselves to this optimization. Indeed, the shear forces acting on the surface of the mixture in contact with the fixed wall of the extrusion die have the effect of causing tears on the surface of the profile, and come back against the efforts to obtain precise geometry.

To combat these drawbacks, solutions have been proposed to adapt the operating parameters of the extrusion machine or the profile of the extrusion die in order to reduce this decohesion phenomenon.

[006] A first solution, presented here as the closest prior art, consists of using a conventional extrusion die and decreasing the extrusion speed while maintaining a relatively high pressure upstream of the die. Although satisfactory from the point of view of the geometry of the profile obtained, this solution remains a worse for its unfavorable economic performance.

[007] The purpose of the patent is to propose an adaptation of the profile of the die making it possible to significantly improve the extrusion rate, while maintaining a quality profile surface in accordance with the expected geometrical requirements.

[008] The extrusion die according to the invention is formed of a first wall and a second fixed wall placed opposite the first wall and intended to give the rubber profile a given transverse profile, This die is characterized in that the second wall comprises an upstream portion forming an air gap of height H with the first wall and a downstream portion forming a step of height h relative to the upstream portion of said second wall.

[009] The gap between the first wall and the downstream portion of the second wall is then equal to H + h.

One then arranges to adjust the operating parameters of the extrusion device so that the point of separation Pd defines as the point beyond which the pressure exerted on the mixture is zero, is arranged on the downstream portion of the second wall, at a distance d downstream of the recess of the second wall and formed by the upstream edge of the downstream portion of said second wall.

During the extrusion of the mixture through the die thus defined, said pressurized mixture flows from the upstream portion of the die formed by the first wall and the upstream portion of the second wall towards the downstream portion of the die. . The height step ha has the effect of creating a strong decrease in shear stresses at the surface of the mixture in contact with the downstream portion of the second wall, which has the effect of releasing the internal stresses in the mixture and reducing strongly the threshold of appearance of cracks and other rips specific to so-called decohesive mixtures.

This device for producing the die makes it possible to significantly increase the flow rate and the extrusion speed, thereby making it possible to improve the efficiency of the machine, without altering the geometrical characteristics of the profile produced.

The following description is based on a preferred embodiment of the invention, and in Figures 1 to 4 in which: Figure 1 shows a schematic sectional view (AA) of a die according to the known prior art, - Figure 2 shows a front view of the die according to the known prior art, Figure 3 shows a schematic sectional view (AA) of a die according to the invention, FIG. 4 represents a front view of the die according to the invention.

The extrusion die according to the known prior art shown in Figure 1 comprises a first wall 1. This wall can be fixed in the case of a so-called flat nose extrusion means. This wall may also be of cylindrical shape and rotatable in the direction of the arrow R about an axis (not shown). The second wall 2 of the die is a fixed wall disposed vis-à-vis the wall 1.

The second wall 2 defines the transverse profile of the profile to be extruded according to a geometry as shown for illustrative purposes in Figure 2. This second wall is fixed. The space between the first wall 1 and the second wall 2 forms the gap of the die. The distance between the first wall and the second wall of the die, which varies transversely as a function of the geometry of the profile, is denoted H.

The extrusion die is disposed in the downstream portion of the extrusion device (not shown). This extrusion device can be formed by way of example of a screw disposed in a sheath opening into an arch whose wall 3 is connected to the second wall of the die. For this purpose, the rear portion of the tool supporting the second wall of the die forms an angle W ₀ with the second wall 2.

The mixture under pressure arrives upstream of the die and flows in the direction of the arrow F in the die to acquire the desired transverse profile.

Those skilled in the art will act on the parameters of the extrusion device to adjust the parameters of pressure and extrusion speed so as to obtain the optimum flow.

In known manner, it arranges for the separation point Pd defines as being the point downstream of which the pressure exerted on the mixture is zero, is placed in a zone ideally located upstream the exit of the die and disposed approximately between three quarters and two thirds of the total length of the die. For a given extrusion speed, the pressure level in the vault is adjusted so that the separation point Pd is situated in this zone, otherwise being managed so as not to exceed the maximum temperature tolerated by the non-rubber mixture. vulcanized.

When the point of separation Pd is located in this area, there is then a better stability of the extrusion conditions. Indeed, an increase in the pressure in the vault will have the effect of moving downstream the point of separation Pd. AT Conversely, any decrease in pressure will have the effect of moving this point upstream. In the case of a so-called roller extruder, whose first wall is of substantially cylindrical shape, it is arranged that the point Pd is arranged in line with the generatrix of the cylinder located closest to the second wall, so that to minimize the effects of moving this point forward or backward from this ideal position. Indeed, the variation of the diameter of the roll can have an effect on the thickness of the profile.

This constraint is also a limiting factor in the development of the extrusion die in that it does not increase the pressure in an unlimited manner which would have the effect of placing the point of separation Pd to the downstream end of the die. This would then result in strong transverse geometry variations because of the well-known effects of foaming rubber materials.

The die according to the invention is shown in Figures 3 and 4. This die has substantially the same characteristics as the die described in the preceding paragraphs. However, it differs in that the second wall comprises an upstream portion 21 forming an air gap of height H with the first wall, and a downstream portion 22 forming a step of height h relative to the upstream portion 21 of said second wall . The gap between the first wall and the downstream portion of the second wall is then substantially equal to H + h.

Figure 3 allows to highlight the fact that the recess between the upstream and downstream of the second wall has substantially the same value, equal to h, over the entire transverse length of the die.

The stall height, experimentally fixed, depends on the nature of the decohesive material and the geometry of the profile to obtain. This height can be valid between 0.08 and 0.2 times the average value of the air gap H. It will be observed that the value H varies transversely so as to determine the geometric profile of the rubber profile. This is why, as a general rule, we will choose a value H close to the value of the air gap in the center of the die.

In practice, the height of the air gap, which corresponds substantially to the thickness of the profile, has a value between 0.5 mm and 25 mm for the profiles commonly used for the manufacture of tires intended to equip vehicles of tourism.

In a manner similar to what has been developed in the paragraphs above, it is arranged, during the adjustment of the extrusion parameters, so that the separation point Pd is located on the downstream part of the second wall. Ideally, this point of separation is located at a distance d of the detachment. The distance d is usefully between 5 and 25 times the value of the height h. The length of the downstream portion of the second wall 21 is adjusted so that the separation point Pd is located upstream of the downstream end of said downstream portion of the second wall 21.

The table below shows the extrusion results obtained with a mixture said experimental decohesive. The die used is placed downstream of a screw extruder and comprises a first cylindrical wall rotatable relative to the second wall.

In the five experimental plans, the value of the offset h (column h in mm) was varied from a zero value, assimilated to a bushing according to the known prior art, up to a value of 1. , 5 mm. For each of the values of h, and for a given pressure (column P Mpa), the effects of an extrusion speed variation (line V m / min) and thus of the output flow (line Kg / min) on the temperature of the mixture (line Temp ⁰ C) at the outlet of the die, and it was noted qualitatively (line A) the appearance of the upper surface of the profile obtained. The aspect ratio is related to the number and depth of cracks observed. The values of A vary from 1 (very deteriorated appearance) to 5 (excellent appearance). The threshold of acceptability is set in Note 4.

It is observed that, for a die having no step (experiment No. 1), the best extrusion conditions are obtained for a relatively low pressure value and for a very low exit speed of between 1 and 2 m. / min, which corresponds, in this case, to a flow rate of between 2.8 Kg / min and 5.35 Kg / min.

The experiment No. 2 was made with an extrusion die whose second wall had a recess of a height of 0.5 mm. The improvement observed is sensitive for the high speed values for which a limiting rise in temperature is nevertheless observed.

The experiments No. 3 and No. 4 were made with an extrusion die whose second wall had a recess with a height of 1 mm. For a low pressure value (experiment N ° 4), equivalent aspect notes are obtained, but it is observed that the temperature level is lowered for high speeds. This observation allows to increase the pressure (experiment N ° 3). This results in a significant improvement in the appearance of the profile in high speeds, without the temperature of the mixture at the outlet of the die does not rise beyond the allowable threshold.

Experiment No. 5 was made by raising to 1, 5 mm the height of the recess. In the latter case it has been observed that whatever the speed conditions, the separation point Pd was located at the step between the upstream portion and the downstream portion of the second wall. This resulted in a geometric instability of the profile that was considered unacceptable, although the aspect notes were all considered satisfactory or good.

This experimental design makes it possible to conclude that, in the case of the mixture used for the test, and in the case of the geometry of the transverse profile chosen, the optimum height h of the recess is in the vicinity of the millimeter. It allows especially to note that for a mixture and for a given geometry the effect of the recess is sensitive and allows to significantly increase the speed and therefore the extrusion rate without exceeding the thermal limits imposed by the mixture.

The description above relates to a preferred embodiment of the invention. It is nevertheless quite possible to apply the teachings of the invention to extrusion configurations in which the first wall is fixed (in the case of extruders called flat nose).

In this case, it is also possible to provide a setback of value h on the downstream part of the first wall of the die so that the gap between the upstream portions of the first and second wall is equal to H, and that the gap between the downstream portions of the first and second walls is equal to H + 2h.

Claims

1) extrusion die of a mixture composed mainly of rubber, formed of a first wall (1) and a second fixed wall (2) placed vis-à-vis the first wall and intended to give the rubber profile a given transverse profile, in which the first wall (1) is of substantially cylindrical shape, and movable in rotation with respect to the second wall., characterized in that the second wall (2) comprises an upstream part ( 21) forming an air gap of height H with the first (1) wall and a downstream part (22) forming a step of height h relative to the upstream portion (21) of said second wall, so that the gap between the first wall (1) and the downstream portion of the second wall (21) is equal to H + h. 2) extrusion die according to claim 1, wherein the ratio h / H is between 0.08 and 0.2. 3) extrusion die according to claim 1 or 2 wherein the recess has a height substantially equal to h over the entire transverse length of the die. 4) Process for extruding a mixture composed mainly of rubber in which an extrusion die according to one of Claims 1 to 3 is used, and in which, when the die is in operation, a separation point is defined. Pd downstream of which the pressure exerted on the mixture is zero, and in which the values of pressure and extrusion speed are adjusted so that said point of separation Pd is disposed on the downstream part of the second wall (21) at a distance d from the recess of the downstream portion of the second wall relative to the upstream portion of said second wall of between 5 and 25 times the value of the height of the step h.