1. Field of the Invention
The present invention relates to a continuous kneader for kneading a polymeric resin material such as a plastic material or rubber, as well as a material discharging method and rotor for use in the continuous kneader.
2. Description of the Related Art
In a continuous kneader, usually a material to be kneaded such as a plastic material or rubber is subjected to a strong shearing action with use of rotors which are rotating in different directions at a high speed and is thereby melted into a plasticized state in a short time. Various fillers and additives are kneaded and dispersed efficiently into the thus-plasticized resin. In this way there can be produced resin products of various qualities.
Particularly, in a continuous kneader of a both-end supported structure wherein rotors are each supported at both axial ends thereof by means of bearings, there is no fear of tip-vibration of each rotor and contact of the rotor tip with a chamber. Therefore, the rotors can be rotated at a high speed and a kneading and pelletizing equipment of a high production capacity can be realized easily.
In a twin-rotor type continuous twin-screw kneader as such a both-end supported type continuous kneader, a pair of right and left rotors each having a feed portion and a kneading portion for a material to be kneaded are inserted into a chamber rotatably while being supported at both axial ends thereof, the said chamber having a material supply port at one end thereof, the said rotors each having at one end thereof a discharge section (discharge flights) for scraping out the material radially outwards after having been kneaded by the rotors, and the said chamber having at the opposite end thereof a discharge port formed radially outwards for discharging the kneaded material to the exterior of the chamber after having been scraped out by the discharge section (see, for example, Japanese Patent Publication Nos. 58-50533 and 6-41135, all of which are hereby fully incorporated by reference).
In the both-end supported type continuous twin-screw kneader, as referred to above, the front end portion of the chamber cannot be opened and the discharge port cannot help being opened radially outwards, so the kneaded material flowing within the chamber toward the downstream side in the axial direction of the rotors is scraped out radially outwards of the rotors by the discharge sections (discharge flights) of the rotors upon arrival at the area (hereinafter referred to as the xe2x80x9cdischarge areaxe2x80x9d) corresponding to the discharge port in the chamber and its flowing direction is changed from the rotor axis direction into a direction nearly perpendicular thereto.
In the conventional continuous twin-screw kneader, since the amount of projection of each rotor discharge section is set the same for both rotors in the rotor axis direction, the kneaded material stuffed between the inner surface of the chamber and the discharge sections of the rotors become higher in temperature under a shearing force induced by rotation of the rotors, which may result in that the temperature of the kneaded material present in the discharge port becomes non-uniform in the rotor axis direction.
More particularly, when the kneaded material flowing in the rotor axis direction is discharged radially outwards at the downstream end of the chamber, there exist a flow advancing to the exterior directly from the upstream side of the discharge port and a flow going out after staying in the discharge area up to the downstream end of the chamber. For example, as shown in FIG. 15, the flow rate distribution of the kneaded material in the discharge port is smaller on the downstream side (the right-hand side in FIG. 15).
On the other hand, a resin pipe connected to the discharge area or the discharge port in the chamber is usually filled with the kneaded material, so in the case where the amount of projection of each rotor discharge section is the same for both rotors in the rotor axis direction, a shearing work done which the kneaded material present in the discharge area undergoes from the discharge flights with rotation of the rotors changes little in the rotor axis direction.
Thus, the shearing work done imposed on the kneaded material in the discharge area is almost constant despite the flow rate distribution of the material being smaller on the downstream side. It follows that the shearing work done imposed on the kneaded material per unit weight is larger on the downstream side of the discharge port. Consequently, as shown in the upper graph in FIG. 15, a relatively large temperature difference xcex94T arises between the kneaded resin passing upstream through the discharge port and the kneaded resin passing downstream through the discharge port.
Once there arises a large temperature difference xcex94T in the melted resin present in the discharge port, there may be obtained a kneaded product held at a desired temperature on the upstream side of the discharge port, but on the downstream side of the same port the temperature of the kneaded product will become too high and decomposition of part of the kneaded product and deterioration in quality of the product may result. Besides, there also arises the problem that pellets extruded from a die (a pelletizer) which follows become non-uniform in length due to a difference in viscosity caused by the difference in temperature.
On the other hand, the invention disclosed in Japanese Patent Laid Open No. 9-1630 recommends that the downstream end portion of each rotor be formed in a columnar shape free of discharge flights as means for preventing the resin temperature from becoming non-uniform in the discharge port referred to above (see claim 1 in the said laid-open print).
If the discharge flights are removed from the downstream ends of rotors into a bald state, the irregularity of the resin temperature in the discharge port will be corrected, but there no longer is the resin scraping-out function in the discharge area and therefore a certain viscosity of the kneaded material may give rise to a large pressure variation in the discharge port.
Thus, where the above means of making the discharge sections of the rotors into bald sections free of discharge flights is adopted, it is necessary that the rotors are rotated at a high speed in order to ensure a sufficient power for feeding melted resin under pressure, giving rise to a new problem that a gear pump extending from the discharge port to the downstream side is difficult to be operated in a normal condition and that operating conditions for the continuous kneader become narrower.
According to the means of making the discharge sections of rotors into bald sections free of discharge flights, the resin is no longer present uniformly in the inlet of a viscoseal disposed at the opposite end wall of the chamber, so that the resulting decrease of output makes it no longer possible to seal the resin by the viscoseal and the flow of resin to the discharge port becomes too smooth. As a result, particularly in the case of two-stage kneading (kneading is performed also on the downstream side of a gate device), the resin will flow through the second kneading section without stopping and is difficult to stay therein, so that the gel removing capacity may be deteriorated markedly.
It is an object of the present invention to effectively prevent the occurrence of a trouble in the operation of a gear pump due to a pressure variation in the discharge port, also prevent a decrease of the gel removing capacity caused by non-stop flowing of the resin through the kneading section, and at the same time minimize the temperature difference in the rotor axis direction of the kneaded material in the discharge sections of the rotors, thereby improving the product quality.
A discharge section of each rotor in a preferred embodiment of the present invention is characterized by being provided on its outer peripheral surface with twist flights inclined in a direction in which a material after kneading is extruded to an opposite side of a chamber with rotation of the rotor.
In this case, since a portion or the whole of the kneaded material present in the discharge area is discharged from the discharge port while being extruded to the opposite side (downstream side) of the chamber by the twist flights, the difference in the discharge flow rate between the upstream side and the downstream side of the discharge port decreases and an offset of the flow rate distribution in the discharge port becomes smaller, resulting in that a shearing work done per unit weight imposed on the kneaded material flowing through the discharge port is rendered almost uniform in the rotor axis direction.
In the present invention, unlike the invention disclosed in Japanese Patent Laid Open No.1630/97, it is not that the kneaded material scraping-out function is removed completely from the discharge sections of rotors, but the discharge sections still possess the said function, so that the occurrence of a large pressure variation in the discharge port is prevented and the deterioration of the gel removing capacity caused by non-stop flow of the resin from the discharge area toward the discharge port is also prevented.
In the present invention it is preferred that the twist angle of each twist flight relative to the rotor axis direction be set at a value in the range of 30xc2x0 to 70xc2x0.
The reason is that if the twist angle of each twist flight is smaller than 30xc2x0, the twist flight assumes a shape similar to a parallel flight in the conventional like kneader and there arises a fear that the temperature difference in the discharge port may not be lowered to an extent of not affording defective pellets. The reason is also because if the twist angle of each twist flight exceeds 70xc2x0, there arises a fear that the kneaded material may become easier to pass through the kneading section without stopping and the percentage gel removed may become lower.
In the present invention it is preferred that the discharge section be provided on its outer peripheral surface with parallel flights positioned on an upstream side of the twist flights and it is also preferred that the twist flights extend throughout the whole axial region of the discharge section.
In the former case, it is desirable that the ratio of length of the twist flights in the rotor axis direction relative to the discharge section be set at a value in the range of 0.2 to 0.8.
The reason is that if the above ratio of length of the twist flights is smaller than 0.2, the greater part of the discharge section is constituted by parallel flights and that therefore it may become impossible to diminish the temperature difference in the discharge port to an extent of not affording defective pellets. The reason is also because if the above ratio of the twist flights exceeds 0.8, the parallel flights which induce a flow resistance of the resin in the discharge section will become too short and therefore it becomes easier for the kneaded material to pass the kneading section without stopping, which leads to deterioration of the percentage gel removed.