GB2111397A

GB2111397A - Extrusion screw for extrusion of polymeric materials

Info

Publication number: GB2111397A
Application number: GB08228405A
Authority: GB
Inventors: Katsumi Sugano; Yoshio Ikegami; Tadashi Kouge; Shiro Kishi
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1981-11-16
Filing date: 1982-10-05
Publication date: 1983-07-06
Also published as: IN156522B; JPS6114414Y2; KR840001885A; GB2111397B; JPS5874521U

Abstract

The screw (1) is rotatably accommodated in a cylinder and comprises at least in one of compressing (E) and metering (F) zones thereof: main flights (C) of a constant height having the top faces thereof disposed in a small gap relation with the inner surface of the cylinder (2); at least one thread (A, B) of eccentric flight formed between the main flights (C) in parallel relation therewith and having a height alternately reduced and increased gradually through a predetermined angle on the screw; and at least a couple of grooves (a, b, c) formed between the main and eccentric flights, one of the grooves having a depth alternately reduced and increased gradually through an angle on the screw corresponding to that of the variation in height eccentric flight (A, B) and the other one of the grooves having a depth alternatively reduced and increased with a phase lag from the first-mentioned groove. <IMAGE>

Description

SPECIFICATION Extruding screw for extrusion of polymeric materials This invention relates to an improvement in a screw extruder for mixing and extruding polymeric materials such as synthetic resins, rubber and the like.

In order to obtain products of good quality in the extrusion of a polymeric material, it is required to mix the material to be extruded sufficiently uniformly and to keep the extruding temperature to a level which does not cause deterioration of the material and to the lowest temperature range in which moulding of the material is feasible. To effect the mixing to a sufficient degree, it is necessary for the material to undergo sufficient shearing actions while it is passed along the screw. A large shearing force, however, can cause deterioration of the material because of the phenomenon of heat generation as a result of evanescence of viscosity.It is conceivable that extrusion of a well-kneaded material at a low temperature level which does not induce the thermal deterioration of the material is possible by partially subjecting the material to large shearing forces while absorbing and dispersing the generated heat in those portions which have undergone small shearing forces. Further, in order to obtain products of good quality, the raw material (pellets) which is fed to the extruder should be melted promptly to prevent unmelted pellets from lingering in the extruded products.

To this end, there has been proposed an extrusion screw which is provided with a plural number of threads forming eccentric flights with the respective heights varied with a phase lag (Japanese Utility Model Publication No.

55-18179). This screw construction is intended to disintegrate congregations of unmelted pellets (solid beds) which are formed in the front portions of screw grooves as seen in the direction of advancement, thereby accelerating the melting of the material as well as providing uniform melting and dispersion and through localized shearing of the material portions which pass over the eccentric flights, and at the same time uniformalizing the temperature by absorbing in the screw grooves the heat resulting from the localized shearing, while homogenizing the material at a low temperature by the stirring and mixing action which divides or breaks the solid beds in the thread grooves.However, with such a screw construction, the molten material is passed through clearances of a large width which are formed between the inner surface of the cylinder and low eccentric flight portions with a reduced height. This phenomenon occurs conspicuously especially in the case of a high pressure extruding screw with a discharge pressure greater than 300 kg/cm2, resulting in a drop in the extruding rate.

Further, it is difficult to use the screw of the above described construction in a compression zone of an extruder in view of its poor compressing ability.

In addition, the screw is not provided with a mechanism for forcibly transferring the solid beds, so that it is incapable of shearing and disintegrated the solid bed at the apex points of the eccentric flights for effectively accelerating its plasticization.

The invention provides an extrusion apparatus suitable for use in the extrusion of polymeric materials comprising an extruding screw and a cylinder said screw being mounted to rotate in the cylinder and comprising in at least one of a compression and a metering zones thereof: at least one main flight having a top face disposed close to the inner surface of said cylinder to provide a small gap therebetween;; at least one eccentric flight provided between successive turns of said main flight and parallel thereto and having a height which alternately decreases and increases gradually through a predetermined phase angle, and at least two grooves formed between successive turns of said main flight, a first one of said grooves having a depth which alternately decreases and increases gradually through a phase angle corresponding to that of said eccentric flight and the second one of said grooves having a depth which alternately decreases and increases gradually with a predetermined phase lag with respect to decreases and increases of the depth of said first groove.

The above features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings which show by way of example preferred embodiments of the invention.

In the accompanying drawings: FIGURE 1 is a partly cutaway sectional view of an extruding screw according to the present invention; and, FIGURES 2 (A) to (E) are schematic fragmentary sections of the screw, showing variations in the heights of flights and depths of the screw grooves at different phase angle positions.

Referring to Figure 1, there is illustrated a preferred embodiment of the invention, comprising a screw 1, a cylinder 2 and a hopper 3.

The screw 1 comprises, successively from the hopper 3, a feed zone D, a compression zone E and a metering zone F. In the region G which extends into both the compression and metering zones E and F, the screw is provided with eccentric flights A and B formed parallel to and between adjacent turns of flight 1 1 (also indicated by letter C in region G) the height of the eccentric flights A and B gradually increasing or decreasing at predetermined intervals along the length thereof whilst the flight C remains a uniform height.The height of the eccentric flight A gradually decreases from a level close to the inner surface of the cylinder 2 to a level close to the surface 10 of the core of the screw 1 through a phase angle of 180 , and then gradually increases again to a level close to the inner surface of the cylinder 2 in a succeeding phase angle of 1800.

repeating the same gradual increase and decrease in height at every phase angle of 1800. More specifically in the position of Figure 1, the eccentric flight A has its minimum height on the upper side of the screw and its maximum height on the lower side with gradual decreases and increases therebetween. On the other hand, the height of the flight B is a maximum at the phase angles where the flight A is the minimum height, and is a minimum at the phase angles where the flight A is the maximum height, so that the heights of the flights A and B vary with a phase lag of 1800. Main flight C is formed similarly to the ordinary screw flight 1 1, and has a uniform height with its outer face disposed close to the inner surface of the cylinder 2.The grooves a, b and c which are formed between two successive turns of the main flight portions C have respective depths which vary over a predetermined length corresponding to the phase angle of the eccentric flights A and B. Thus, the depth of the intermediate groove b varies gradually such that it becomes shallowest on the upper side of Figure 1, approaching the inner surface of the cylinder 2 and reaching almost the level of the minimum height of the flight A, and it becomes deepest on the lower side. The depth of outer grooves a and c vary such that they become shallowest at the phase angle where the intermediate groove b is deepest and vice versa, that is to say, they vary similarily to groove b but with a phase lag of 1800 with respect to the intermediate groove b.

Now, the mechanism of melting a polymeric material by the above described screw construction is explained with reference to Figure 2. We refer first to Figure 2(A) which is a longitudinal section of a front end portion of the region G which extends to both the aforementioned compression and metering zones E and F. A solid bed 5 and a melt pool 6 are formed in the front and rear portions, respectively, (as seen in the direction of advancement) of each one of the grooves a to c. This is because the solid bed 5 is melted in the portions of the grooves a to c which are located close to the cylinder 2 by the heat transmitted from the cylinder 2 and by the heat generated by shearing the melted material forming filmy streams which flow into the melt pool 6 over the solid bed 5 as indicated by arrows in the figure.In this phase, the eccentric flight B and grooves a and c are a maximum height and depth, respectively, while the flight A and groove b are a minimum height and depth, respectively, the height of the flight A being almost as low as the level of the bottom surface of the groove b. The eccentric flight B and grooves a and c gradually decrease in height and depth, respectively, in proportion to increase of phase. On the other hand, the eccentric flight A and groove c gradually reduce in height and depth, respectively, in proportion to increase of phase, as seen in Figure 2 (B) which shows the respective flights and grooves at a position which is 900 advanced in phase with respect to Figure 2(A). Accordingly, the solid beds 5 in the, now, shallower grooves a and c c are subjected to a shearing action.Since the groove b is, now, deeper, the solid beds 5 are forcibly transferred into the groove b over the eccentric flights A and B, accelerating the uniformalization of the material temperature, coupled with the effective melting and mixing to which the solid bed 5 is subjected by the large shearing force as it passes over the flight. As seen in the section of Figure 2(C) at a position which is further advanced by 900, the groove c becomes shallower so that a large shearing force acts on the solid bed 5 there, accelerating its melting, and the molten material flows in the direction of the arrow. As a result, the solid bed 5 gathers again in the rear portions as seen in the direction of advancement.In the position of Figure 2(D) which is again further advanced by 900 in phase, the depth of the groove b gradually reduces and instead the groove c gradually deepens, so that the solid bed 5 in the groove b is transferred into the groove c passing over the eccentric flight B.

Similarly to the situation in Figure 2!B), this causes melting, dispersion and mixing of the solid bed 5 as well as uniformalization of temperature.

As can be seen, Figure 2(E), which shows the screw in a position a further 900 advanced in phase, exhibits the same situation as Figure 2(A) which is 3600 earlier in phase.

Thus, the solid beds 5 are melted and mixed by repeating the operations in successive 1800 phases, the described operations being performed between the main flights C which are a constant height providing only a small gap with respect to the inner surface of the cylinder 2, so that there is no possibility of a larger amount of molten material passing through the gap between the main flights C and the inner surface of the cylinder 2. Consequently, reductions in the extrusion rate do not occur even in a high pressure screw extruder having a discharge pressure higher than 300 kg/cm2. The apparatus is especially effective for melting the solid beds without causing drops in the extrusion rate in the extrusion of chemifoaming DEF and in the foam and skin double layer extrusion where the material is extruded under a high pressure of 600 to 800 kg/cm2.Since the provision of the main flights C prevents decreases in the compressive force. a similar flight construction can be applied to the compressing zone E if desired.

Although a couple of eccentric flights A and B are provided between successive turns of the main flight C in the foregoing embodiment, there may be employed a construction which has a single eccentric flight between the main flights C, with two grooves 6 and c being formed on opposite sides of the eccentric flight each having respective depths which gradually increase and decrease with a predetermined phase lag. Alternatively, there may be provided three eccentric flights between successive turns of the main flight C, in this case the heights of the respective eccentric flights and the depths of the grooves gradually decreasing and increasing with a phase lag of 1200.Further, the pitch of the gradual reductions and increases in the height of the eccentric flights may be made greater than 1800 to form eccentric flights and grooves with more gentle variations in their heights or depths.

In the above-described embodiment, the minimum height of the eccentric flight is shown to be at a level close to the bottom surface of the shallowest portion of the groove, but is may be at a higher level or a lower level cut into the bottom surface if desired.

Further, the combination of the eccentric flights and grooves of a varying depth is not limited to a region which extends into both the compression and metering zones and may of course be provided in the compression or metering zones alone.

As is clear from the foregoing description, the screw according to the preferred embodiment of the present invention is provided, in at least in its compression or metering zone, with a main flight of a constant height, an eccentric flight or flights of a varying height formed between successive turns of the main flight, and grooves formed between the main and eccentric flights and having depths varying with a predetermined phase lag from each other, thereby effectively melting and mixing the extruding material without reducing the extruding rate to obtain products of good quality.

The variations in the height of the flights and the depth of grooves have the effect of forcibly transferring the solid beds and applying local shearing forces so that the material undergoes effective melting, dispersion and mixing in addition to uniformalization of temperature. These actions on the material are performed between successive turns of the constant height main flight so that a drop in the extruding rate can be prevented as mentioned hereinbefore.

Claims

1. An extruding screw suitable for use in the extrusion of polymeric materials, said screw being mounted to rotate in a cylinder and comprising in at least one of a compression and a metering zone thereof: at least one main flight of a constant height having a top face disposed close to the inner surface of said cylinder to provide a small gap therebetween; at least one eccentric flight provided between successive turns of said main flight and parallel relation thereto and having a height which alternately decreases and increases gradually through a predetermined phase angle; and, at least two grooves formed between successive turns of said main flight, a first one of said grooves having a depth which alternately decreases and increases gradually through a phase angle corresponding to that of said eccentric flight and the second one of said grooves having a depth which alternately decreases and increases gradually with a predetermined phase lag with respect to decreases and increases of the depth of said first groove.

2. The extruding screw as claimed in Claim 1, wherein said main and eccentric flights and grooves are provided in a region extending into both the compression and metering zones of said screw.

3. The extruding screw as claimed in Claim 1, wherein said screw is provided with two eccentric flights between successive turns of said main flight in parallel relation with each other, said eccentric flights having respective heights which alternately decrease and increase with a phase lag of 1800 from each other, and three grooves of varying depths, formed between said main and eccentric flights, the intermediate one of said grooves having a depth thereof which decreases and increases with a phase lag of 1 800 with respect to the grooves on opposite sides thereof.

4. The extruding screw as claimed in Claim 1, wherein said screw is provided with three eccentric flights between successive turns of said main flights in parallel relation with each other, said eccentric flights having respective heights which alternately decrease and increase with a phase lag of 1 200 with respect to each other.

5. The extruding screw as claimed in Claim 1, wherein said eccentric flight has a height which alternately decreases and increases gradually through a phase angle greater than 1800.

6. An extrusion apparatus suitable for use in the extrusion of polymeric materials comprising an extruding screw and a cylinder said screw being mounted to rotate in the cylinder and comprising in at least one of a compression and a metering zones thereof: at least one main flight having a top face disposed close to the inner surface of said cylinder to provide a small gap therebetween; at least one eccentric flight provided between successive turns of said main flight and parallel thereto and having a height which alternately decreases and increases gradually through a predetermined phase angle; and at least two grooves formed between successive turns of said main flight, a first one of said grooves having a depth which alternately decreases and increases gradually through a phase angle corresponding to that of said eccentric flight and the second one of said grooves having a depth which alternately decreases and increases gradually with a predetermined phase lag with respect to decreases and increases of the depth of said first groove.

7. An extruding screw substantially as hereinbefore described with reference to the accompanying drawings.