JPH0578406B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a thermoplastic resin molding screw used in an extrusion device for thermoplastic resin molding. [Background technology and its problems] Various molded products such as sheets, films, and containers can be obtained from thermoplastic resin by extrusion molding, blow molding, injection molding, etc. In any of these molding fields, heating of the resin, The structure of the screw in the extrusion device is an important factor in achieving sufficient melting and kneading. The most common conventional screw is, for example, a full-fly metering screw, which is provided with a supply section 1, a compression section 2, and a metering section 3 in order from the resin supply side to the resin extrusion side, as shown in Fig. 1. The main purpose was to sufficiently knead the molten resin by increasing the compression ratio.
Therefore, although the resin is sufficiently kneaded, the compression ratio is large and high shear is involved, resulting in several problems. That is, when the compression ratio is large and high shear is involved, the resin pressure tends to fluctuate during high-speed extrusion molding, making it impossible to stably obtain a high-quality molded product. In addition, when performing high-speed extrusion molding, sufficient kneading means that the resin is subjected to large shear stress near the tip of the screw.
Mechanical heat generation occurs due to this high shear stress, which causes the resin temperature to rise more than necessary, resulting in low viscosity, leading to resin deterioration and decreasing the physical properties of the molded product.Furthermore, in extrusion molding, take-off and sizing are difficult. As a result, injection molding can cause nasal drip. Furthermore, in high-speed extrusion molding, a large amount of power is required to drive the screw, and this power causes an increase in resin temperature. Therefore, in order to prevent this drive energy and the resin temperature from overheating, it is necessary to cool the resin from the outside, which poses a major problem in terms of energy conservation and the need for cooling equipment. A solution is desired. These problems are especially true for thermoplastic resins such as polypropylene, linear low-density polyethylene, polycarbonate, and polystyrene, whose Newtonian shear properties (compared under high shear rates) arise from factors such as relatively narrow molecular weight distribution. This becomes a big problem in the case of thermoplastic resins that exhibit a characteristic of high melt viscosity. For this reason, various screws have been proposed that have a relatively low compression ratio, in some cases less than 1, to suppress the generation of shear heat generation, but in all cases, kneading is insufficient, and as a result, This results in non-uniform resin temperature and insufficient dispersion of various additives, making it difficult to obtain good molded products, and the range of use is extremely limited. By the way, in order to solve the problems of these conventional techniques, the present applicant has already proposed a new technology in which the compression part of the screw has a gentle compression part with a relatively small compression ratio, and has proposed a new technology in Japanese Patent Application Laid-Open No. 1986- 144324, Japanese Patent Application No. 59-266398), the present invention has the same object as the above, and is a further improvement. [Object of the Invention] An object of the present invention is to provide a screw structure that can achieve both sufficient kneading properties and high molding stability, which have been difficult to achieve in the past. [Means and effects for solving the problems] The present invention includes a compression section of a slow compression type having a compression ratio of 3 or less in the supply section, which sequentially moves from the resin supply side to the resin extrusion side, and a compression section of the resin from the compression section. A torpedo-shaped shearing section that provides resistance to flow and a stress relaxation section with a compression ratio of 2 to 0.5 are provided, and the compression section or shearing section performs sufficient compression and shearing without applying excessive stress to the resin, and the shearing section By restricting resin inflow from the compression part to the stress relaxation part, the stress relaxation part is
It enables a low compression ratio of 0.5, and
Separately from the shearing section, at least one or more gear-shaped or Dalmage-type mixing sections are provided immediately before or after the shearing section to ensure sufficient kneading, thereby ensuring sufficient kneading while suppressing the accumulation of shear stress. The aim is to achieve the above objective in this way. [Example] Hereinafter, the present invention will be explained in more detail based on the drawings. First, the thermoplastic resin used in the present invention is not particularly limited, and includes, for example, high-density polyethylene, high-pressure low-density polyethylene, linear low-density polyethylene, polypropylene, ethylene or propylene and other α-olefins. copolymer,
Examples include polystyrene, polyamide, polyester, polycarbonate, polyvinyl chloride, polyacrylonitrile, polyvinylidene chloride, polyphenylene oxide, polyimide, polysulfone, polyphenylene sulfide, and polyketone. Among these, polypropylene, linear low-density polyethylene, polycarbonate, polystyrene, etc. have Newtonian shear properties (characteristics that exhibit relatively high melt viscosity under high shear rates) caused by factors such as relatively narrow molecular weight distribution. It is suitably used in the case of the thermoplastic resin shown below. This is because these resins have a high viscosity in the high shear region of the extruder, and extruding this high viscosity resin requires a large amount of screw power consumption, resulting in low extrusion productivity and high manufacturing costs. It is from. Furthermore, the rise in resin temperature due to shear heat generation may cause deterioration of the resin, deterioration or decomposition of the additives. In addition, among the resins used in the present invention, the resins that can be used to improve the transparency, gloss, etc. of the extruded molten resin include polypropylene, a random copolymer of 20% by weight or less of α-olefin such as ethylene, etc. Examples include polypropylene and linear low-density polyethylene. Due to crystallization of these resins, it has been extremely difficult to produce molded sheets and films with truly excellent transparency. FIG. 2 shows an example of a screw for molding thermoplastic resin according to the present invention, and in the figure, from the resin supply side to the resin extrusion side, the screws are shown in order: a supply section 11, a compression section 12, a shearing section 13, a mixing section. 14 and a stress relaxation section 15 are provided. Resin pellets are supplied to the supply section 11 from a resin supply port of a cylinder (not shown), and while being melted and plasticized, they are forced into the compression section 12 by the driving force of the screw. For the supply section 11, a straight screw with a compression ratio of 1 is normally used. In addition, the compression ratio of the compression section 12 is increased by gradually decreasing the depth of the groove of the screw, and this compression ratio is configured as a gentle compression section of 3 or less, preferably 2.5 or less, in which the molten resin is compressed. be done. If the compression ratio exceeds 3, the shear stress at the end of the compression part 12, that is, the part before reaching the shearing part 13 will increase, which is undesirable as it will cause an increase in resin temperature, accumulation of shear stress, break-up phenomenon, etc. . Therefore, the type of screw used for the portion from the supply section 11 to the compression section 12 may be of any type as long as it is a slow compression type, such as a type with a groove depth that changes at a constant pitch, or a type with a constant groove depth. A type in which the pitch changes may also be used. In any case, the supply section 11 and the compression section 12 perform the functions of supplying and melting the resin, and it is desirable that the resin be sent to the next stage without being subjected to too vigorous kneading. However, if the compression ratio is too small, for example, 1.5 or less through the supply section 11 and the compression section 12, it is not preferable because it causes unnecessary retention of the resin and causes insufficient initial kneading. The shearing section 13 is a torpedo or a modified torpedo with axial grooves formed on the circumferential surface, where resistance is provided to the flow of the molten resin, and uniform melting of the molten resin is achieved by cutting the molten resin in a relatively short time. Kneading is done. The gap between this shearing portion 13 and a cylinder (not shown) that accommodates the screw is made extremely small to increase shearing. The length of the shearing section 13 is usually 0.1 to 2 times, preferably 0.2 to 1.5 times, the screw diameter D, and the average gap cross-sectional area between the shearing section 13 and the cylinder is the average gap cross-section of the supply section 11. The area is, for example, 1/2 to 1/20, preferably 1/3 to 1/8. however,
These values need to be optimally selected depending on the type of resin, section length, etc. At this time, the structure is simplest if the shearing section 13 is a torpedo. Next, the mixing section 14 adjacent to this shearing section 13
are single or multiple gears, dalmage (having multiple discontinuous shallow grooves with a large helical angle cut on the circumferential surface), polygonal, fluted (having several vertical grooves parallel to the screw axis) The outer diameter of the mixing section 14 is approximately equal to the outer diameter of the shearing section 13, but the troughs are made considerably smaller and the gap with the cylinder is also made larger. Therefore, here, no driving force is applied to the molten resin by the screw, the residual stress of the resin is relaxed, and only the mixing of the molten resin is promoted. In this way, if the mixing section 14 is disposed on the resin extrusion side of the shearing section 13, the shearing stress once relieved in the mixing section 14 will not be increased again in the shearing section 13, and in this respect, Efficient. The stress relaxation part 15 has a compression ratio of 2 to 0.5, preferably
It has a screw structure of 1.6 to 0.8, where the molten resin is measured (adjusted the discharge amount) and
The shear stress still remaining in the molten resin is alleviated,
Furthermore, the cooling of the molten resin is also made uniform. The purpose here is to remove residual shear stress, and the structure is such that high kneading and high shear do not occur. The compression ratio in this specification is defined as "(groove depth at the resin inflow end)/(groove depth at the resin extrusion end)" when the screw pitch is constant in each structural part. means. Further, the resin extrusion side of the shearing part 13, that is, the mixing part 14, more precisely, the part between the shearing part 13 and the mixing part 14 shown in the figure, similarly to the stress relaxation part 15, releases or releases the shear stress of the resin. This is the relaxing part. The length of these stress-relieving regions varies depending on the structure thereof, but is a length at which sufficient relaxation is achieved, usually 4 to 15 times the screw diameter D. In the extrusion apparatus of the present invention, the ratio L/D between the total length L and the diameter D of the screw is designed to be 15 to 40, preferably 20 to 34. Further, when using the present invention, the inner wall of the cylinder of the supply section 11 (particularly the liner section) can be grooved in the axial direction, thereby making it possible to supply pellets smoothly. In this example of the present invention, the shear stress of the molten resin on the resin extrusion side (tip side or nozzle side) is sufficiently reduced, so there is little variation in resin pressure, and even when high-speed extrusion is performed, It is possible to perform molding with good stability and excellent thickness unevenness accuracy. Moreover, it is therefore possible to stably obtain a high-quality molded product that is homogeneous, has good surface properties, and depending on the resin, has good transparency. Moreover, since the kneading itself is sufficiently performed,
The additives are also dispersed uniformly, and from this point of view as well, it is easy to obtain homogeneous molded products. In addition, high-speed extrusion molding is possible because shear heat generation is suppressed, resin deterioration is less likely to occur, the resin temperature is low and uniform, and there is no need for cooling means to prevent overheating. Suitable for energy saving. FIG. 3 shows a second example of a screw for molding thermoplastic resin according to the present invention. In this example, mixing sections 16 and 1 are provided on both sides of the shearing section 13, respectively.
7 is provided. The length of the mixing sections 16 and 17 in the screw axis direction is, for example, two to several times longer in the mixing section 17 on the resin extrusion side than on the mixing section 16 on the resin supply side. may be promoted, and the mixing may be further sufficiently performed in the mixing section 17. FIG. 4 shows a further example of the invention. In this example, the stress relaxation section 18 has a multi-start thread structure. Further, although not shown, the stress relaxation parts 15 and 18 may be partially provided with unthreaded parts, or a mixing part may be added near the tips of the stress relaxation parts 15 and 18. Furthermore, a multi-threaded threaded portion may be added to the compression portion 12, etc.
The present invention includes various modifications. Examples and Comparative Examples Table 2 shows the results of extrusion molding under various conditions using screw A according to the present invention shown in Table 1 and screw B different from the present invention. The resin used was Idemitsu Polypropylene, F700N, MI (melt index) 7 g/10 min, manufactured by Idemitsu Petrochemical Co., Ltd. for the polypropylene, and Idemitsu Polycarbonate, A3000, molecular weight 29000, Idemitsu Polycarbonate, for the polycarbonate. Manufactured by Petrochemical Co., Ltd.

【table】

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide a screw structure that can achieve both sufficient kneading properties and high molding stability, which have been difficult to achieve in the past.

[Brief explanation of the drawing]

Fig. 1 is a side view showing a conventional general screw, Figs. 2 to 4 are side views showing mutually different examples of the screw for molding thermoplastic resin according to the present invention, and Fig. 5 is an example of a mixing section. It is an enlarged side view which shows a part. 11... Supply section, 12... Compression section, 13... Shearing section, 14, 16, 17... Mixing section, 15, 18
...Stress relaxation part.

Claims (1)

[Claims] 1. Sequentially from the resin supply side to the resin extrusion side,
A supply section, a gentle compression type compression section with a compression ratio of 3 or less, a torpedo-type shearing section that provides resistance to the flow of resin from the compression section, and a stress relaxation section with a compression ratio of 2 to 0.5 are provided. 1. A screw for molding a thermoplastic resin, characterized in that a single or plural gear-shaped or dalmage-type mixing section is provided at least either immediately before or after the screw. 2. The screw for molding thermoplastic resin according to claim 1, wherein the mixing section is provided on the resin supply side of the shearing section.