GB2175244A

GB2175244A - Screw adapted for plastic moulding

Info

Publication number: GB2175244A
Application number: GB08607731A
Authority: GB
Inventors: Shigeru Fujita; Mitsuaki Eshima; Osamu Segawa; Takashi Kamiyama
Original assignee: Toshiba Machine Co Ltd
Current assignee: Shibaura Machine Co Ltd
Priority date: 1985-04-01
Filing date: 1986-03-27
Publication date: 1986-11-26
Also published as: JPS61227003A; GB8607731D0; DE3610699A1; GB2175244B; JPH0257494B2

Abstract

The screw is generally divided into a feed section (24), a compression section (23) and a metering section (22), and comprises a main helical flight (25) and a parallel helical sub-flight 26 arranged between the main flight and having a smaller radial height than the main flight. The groove between the main flight is divided into a solid bed groove (28) between the main flight and the sub-flight on a downstream side of the sub-flight and a melt pool groove (27) between the main flight and the sub-flight on an upstream side thereof, and the solid bed groove (28) has a depth gradually decreasing from a portion at which an upstream end of the sub-flight is positioned towards a downstream end thereof. <IMAGE>

Description

SPECIFICATION Screw adapted for plastic formation This invention relates to a screw to be incorporated in an extruder, an injection molding machine, a blow molding machine and the like and, more particularly, to a screw provided with a sub-flight suitable for plastic formation.

Figure 1 shows one example of a conventional screw provided with a sub-flight for an injection molding machine, for example, which consists of a shaft portion 11 constituting a main body of the screw, a main flight 12, and a sub-flight 13. The shaft portion 11 is provided with melt pool groove 14 between the sub-flight 13 and the main flight 12 on the upstream side of the screw, i.e. righthand side of the subflight 13 as viewed in Figure 1, and a solid bed groove 15 therebetween on the downstream side of the screw, i.e. lefthand side of the sub-flight 13 as viewed in Figure 1.The axial width of the solid bed groove 15 is gradually reduced from the upstream side of the screw 10 towards the downstream side thereof to thereby attain a so-called dam-flight function between the main flight 12 and the sub-flight 13 to increase a kneading effect of a material such as resin to be kneaded by the screw rotation.

With the screw 10 of the conventional type shown in Figure 1, however, since the front (upstream) and rear (downstream) ends of the sub-flight 13 are ended at portions of the main flight 12, the threads of the sub-flight and main flight are not parallel with each other and the lead angles of both flights 12 and 13 are per se different from each other. For this reason, the sub-flight has to be worked or machined by a cutting tool of the type different from that used for cutting the main flight at a different feeding speed.

This fact makes impossible an automatic working in use of a numerical controlled (NC) screw cutting lathe and obliges the operator to use an end milling machine to be worked manually.

In order to eliminate this defect of the prior art, there is provided another example of a conventional screw provided with a sub-flight which is arranged in parallel to a main flight to obtain the same leading angles of both the flights. With this example, it is possible to use the NC cutting tool, but it is obliged to connect the front and rear ends of the sub-flight to the main flight by some means in order to attain the dam-flight function without changing the lead angles. This will require a troublesome work.

In addition, in the case where it is required to use a quenched screw, the use of an automatic screw cutting lathe is made impossible and a pre-quenching working operation is required unwillingly, which lowers the precision of the screw as a product and requires correction or amendment thereof after the quenching operation, which will require much time and additional cost.

An object of this invention is to eliminate defects encountered in the prior art and to provide a screw member provided with a sub-flight suitable for plastic formation in use of a numerically controlled thread cutting lathe, an automatic cutting tool or the like with a proper dam-flight function of the main and subflights of the screw.

This and other objects of this invention can be achieved by providing a screw adapted for polastic formation and provided with a feed section, a compression section and a metering section, the screw comprising a shaft portion constituting a main body of the screw, a main flight herically formed on the shaft portion and a sub-flight herically formed on the shaft portion and arranged in a groove formed by the main flight so as to have the same lead angle as that of the main flight and have a height in a radial direction of the shaft portion smaller than that of the main flight, the screw being characterized in that the groove between the main flight is divided into a solid bed groove between the main flight and the sub-flight on a downstream side of the sub-flight and a melt pool groove between the main flight and the sub-flight on an upstream side thereof, and the solid bed groove has a depth gradually decreasing from a portion at which an upstream end of the sub-flight is positioned towards a downstream end thereof.

In the accompanying drawings: Figure 1 shows a side view of a part of a conventional screw provided with a sub-flight; Figure 2 shows a side view similar to Figure 1, of a screw according to this invention; Figure 3 shows a part of a development of the screw shown in Figure 2; Figure 4 shows a part of a screw shown in Figure 2 showing a flow of a material therein; and Figure 5 shows a part of an axial section of the screw shown in Figure 2.

One embodiment of this invention will be described hereinbelow with reference to Figures 2 through 4.

Referring to Figure 2 showing an essential part of a screw utilized for an extruder, injection molding machine and the like, the screw 20 is generally divided into a metering section 22, a compression section 23 and a feed section 24 as viewed from the downstream side towards the upstream side of the screw 20. In another meaning, the screw 20 comprises a screw shaft portion 21, a main flight 25 and a sub-flight 26, and both the flights are herically provided on the shaft portion 21. The upstream end of the sub-flight 26 is positioned approximately at the starting position of the compression section 23 and the downstream end thereof is positioned slightly beforehand the ending position of the compression section 23.The subflight 26 is arranged to have the same lead angle as that of the main flight 25 so that both the flights are parallelly arranged with each other, and the sub-flight 26 has a height slightly lower than that of the main flight 25. A shaft portion between adjacent two threads of the main flight 25, i.e. one pitch thereof, in the compression section 23 is formed to have a melt pool groove 27 and a solid bed groove 28 which are separated by a thread of the sub-flight 26, the melt pool groove 27 being located rightwardly of the sub flight as viewed in Figure 2 and the solid bed groove 28 being located leftwardly thereof.The width of the melt pool groove 27 in the axial direction of the screw 20 is shorter than that of the solid bed groove 28, and the depth of the melt pool groove 27 is substantially equal to the depth of the screw shaft portion of the metering section 22 or the feed section 24 which is provided with a full flight and no subflight.

Referring to Figure 3 representing a development of the screw 20 shown in Figure 2, the tapered angle of the bottom surface of the solid bed groove 28 is determined to be larger than that of the melt pool groove 27, so that a material, usually a resin, is forcibly compressed in a heating cylinder of an injection molding machine, for example, thereby to knead and disperse the resin. The thus melted resin is flowed from the solid bed groove 28 into the melt pool groove 27 over the sub-flight 26 as shown in Figure 4.

Since the bottom surface of the melt pool groove 27 is also gradually decreased, the kneading effect of the resin is facilitated.

More in detail, the depth of the solid bed groove at an approximately downstream end position of the sub-flight is made sufficiently shallow for feeding the melted resin into the melt pool groove, but if the depth of the solid bed groove is maintained as it is towards further downstream side of the screw, the feeding amount of the screw is limited. Accordingly, it seems necessary to make considerably large the depth of the solid bed groove towards further downstream side of the rear end portion of the sub-flight to obtain a sufficient resin drain amount. However, the resin may adversely be stayed in the solid bed groove when the depth of the solid bed groove is rapidly changed, and this results in the degradation of the resin quality, less thermal stability of the resin, inefficiency in resin changing and so on.In order to obviate these defects, it is desired to gradually reduce the depth of the solid bed groove towards the downstream of the screw from approximately the downstream end portion of the sub-flight. It is therefore desired to construct the screw 20 so that the depth of the solid bed groove 28 is gradually increased towards the downstream side of the screw 20 from the downstream end portion of the sub-flight 26, and preferably, it is desired to make the depth of the solid bed groove 28 accord with the depth of the groove of the full-flight portion of the screw provided with no sub-flight in the metering section 22 at a position apart from the downstream end portion of the sub-flight 26 in the axial direction by the length of about 0.1 - 3.0 times of the diameter of the main flight 25 of the screw 20. This was confirmed in the experiment.It is also noted that the kneading, dispersing and plasticizing of the resin are facilitated in the screw portion at which the depth of the solid bed groove is gradually increased because the solid bed groove has a depth less than that of the melt pool groove at this screw portion. These constructional features of the screw according to this invention will be more easily understood from Figure 5 which briefly illustrates the axial cross section of the screw taken for clearly showing the variation of the solid bed groove 28 and the melt pool groove 27 between the main flight and the sub-flight between the compression section 23 of the screw.

In the other experiments, it was found that it is desirable that the depth of the solid bed groove 28 at the downstream end portion of the sub-flight is of 1/2 - 1/5 time of the depth of the same at the upstream end portion of the sub-flight, and that the axial width of the solid bed groove 28 is of 1/2 - 7/8 times of one pitch of the main flight. With regard to these, Table 1 shows resin kneading effect and resin drain amount by a five (5) point evaluation method in the case where the depths of the solid bed groove and the melt pool groove at the downstream end portion of the sub-flight are changed with respect to the upstream end portion thereof, while the ratio of the axial widths of both the grooves being maintained to be 4:1.As is found from Table 1, when the ratio is over about 60%, the good resin kneading efficiency is not obtained and not practical, but when the ratio is below about 50%, relatively good kneading efficiency is attained. Table 2 also shows the resin kneading effect and the resin drain amount by the five (5) point evaluation method in the case where the width of the solid bed groove is changed while the depth of the solid bed groove at the downstream end portion thereof being 1/5 time thereof at the upstream end poriton. As is found from Table 2, the kneading efficiency lowers extremely when the axial width of the solid bed groove is below about 3/8 of the full width (i.e., one pitch of the main flight) thereof, whereas when that ratio is within 1/2 - 7/8, relatively good results are obtained.

TABLE 1 A*f%) Kneading Effect Drain Amount 70 1 5 60 2 5 50 3 5 40 3 5 30 4 5 20 4 5 10 5 4 0 5 3 *A: Depth of the solid bed groove at the downstream end thereof with respect to the depth of the same at the upstream end portion.

TABLE 2 B* Kneading Effect Drain Amount 7/8 5 5 6/6 5 5 5/8 5 4 4/8 4 3 3/8 2 2 2/8 1 1 1/8 1 1 *B: Width of the solid bed groove with respect to the full width of the groove (i.e. one pitch of the main flight).

As described hereinbefore, according to this invention, the screw is provided with a sub-flight having the same lead angle as that of a main flight, so that an NC cutting lathe or automatic cutting machine can be used for preparing the screw. Moreover, since the depth of the solid bed groove is gradually decreased downwardly from the upstream end of the sub-flight, the dam-flight function is endowed to the screw to thereby evenly knead and melt a material to be treated and also obtain a good color dispersion of the material.

Claims

1. A screw adapted for plastic formation and provided with a feed section, a compression section and a metering section, the screw comprising a shaft portion constituting a main body of the screw, a main flight helically formed on the shaft portion and a sub-flight helically formed on the shaft portion and arranged in a groove formed by the main flight so as to have the same lead angle as that of the main flight and have a height in a radial direction of the shaft portion smaller than that of the main flight, said screw being characterized in that said groove between said main flight is divided into a solid bed groove between said main flight and said sub-flight on a downstream side of said sub-flight and a melt pool groove between said main flight and said sub-flight on an upstream side thereof, and said solid bed groove has a depth gradually decreasing from a portion at which an upstream end of said sub-flight is positioned towards a downstream end thereof.

2. The screw according to Claim 1, wherein the depth of the solid bed groove at the downstream end of said sub-flight is smaller than about 50% of the depth of the solid bed groove at the upstream end of said sub-flight.

3. The screw according to Claim 1 or Claim 2, wherein the width of said solid bed groove in an axial direction of the screw is within 1/2 to 7/8 of the width of said groove formed between one pitch of said main flight.

4. The screw according to any of Claims 1 to 3, wherein the depth of the melt pool groove is substantially the same as that of a groove at an area of the screw beyond said upstream and downstream end portions of said sub-flight.

5. The screw according to any of Claims 1 to 4, wherein the depth of the solid bed groove gradually increases downwardly from said downstream end of said sub-flight and accords with the depth of a groove at a portion of the screw apart from said downstream end of said sub-flight by a distance of about 0.1 to 3.0 times the diameter of the main flight of the screw in an axial direction thereof.

6. The screw according to Claim 5, wherein said upstream end portion of said sub-flight and a portion at which the depth of said solid bed groove accords with the depth of a groove formed on the downstream side of said downstream end of said sub-flight substantially correspond to an upstream end and a downstream end of a compression section of the screw.

7. A screw adapted for plastic formation substantially as hereinbefore described with reference to any of Figures 2 to 5 of the accompanying drawings.