JPH0730345Y2 - Extrusion molding die for synthetic resin pipes - Google Patents

Description

TECHNICAL FIELD The present invention relates to an extrusion die for a synthetic resin pipe.

Conventional technology and problems Extrusion molding is mainly used to manufacture synthetic resin pipes. The extrusion die used here mainly comprises a mandrel as an inner die, a die ring as an outer die, and a spider located in the annular resin flow path between them and supporting the mandrel. Straight dies are common.

However, it is known that the synthetic resin pipe extruded by the die as described above generally has weak points in strength at a plurality of positions in the circumferential direction. For this reason, for example, when the pipe is used under a relatively high temperature with a large internal pressure applied, or when used as an inner surface lining material of a metal pipe, it is used after being subjected to a secondary process of expanding after heating and softening. In such cases, cracks were often formed on the peripheral surface of the pipe.

The cause of such strength problem can be found in the manufacturing process of the synthetic resin pipe. That is, the molten synthetic resin supplied from the extruder is once separated at the portion of the spider leg located in the resin flow path when passing through the die, and after passing through the spider, merges and fuses again to give an appearance. Although it is molded into a seamless or seamless pipe, the bonding force of the resin is weaker in the above fused portion than in other portions, and when external stress such as heat expansion is applied for this reason. This is because there is an internal structural defect that fractures such as cracks along the axial direction are likely to occur.

Conventionally, various proposals have been made in order to eliminate or reduce the above structural defects. For example, as one of the improvement measures by the structure of the die, the annular resin flow path in the die is
Formed in a curved path that reciprocates from the spider part to the extrusion port, the resin flow path is made sufficiently long to re-knead the resin in between, and the welding of the weld part is made more complete. Has been proposed (for example,
38-3581). However, in the case of this prior proposal, since the resin flow path in the die becomes long, the retention time of the synthetic resin in the die becomes long, and for example, when the hard vinyl chloride resin is extruded, the resin is decomposed. In addition, the die structure is complicated, and maintenance of the die itself and setting and maintaining suitable extrusion conditions are troublesome.

On the other hand, as another prior proposal example, a plurality of slant grooves inclined in opposite directions are provided on the surface of the mandrel and the inner surface of the die ring between the spider and the extrusion port to form a substantially reticulated resin flow path portion. It has also been proposed that the molten resin after passing through the spider be given a re-kneading action to reduce the cut marks of the resin by the spider leg (for example, Japanese Examined Patent Publication No. 43-20772). However, according to this die structure, it is effective for the intended purpose.However, since the kneading action is given not only to the resin separation portion by the spider legs but also to the entire portion in the die, pressure adjustment in the extruder or Not only is it difficult to control the thickness of the pipe, but there is also the drawback that the resin stagnant portion in the die may increase, leading to decomposition of the resin.

This invention is intended to prevent the re-fusion portion of the resin cut by the spider from remaining as a marked weak point portion of the strength without inducing the above-mentioned drawbacks, and an extrusion molding die for a resin pipe. The purpose is to provide.

Means for Solving the Problems The present invention is directed to the above object, and at a position downstream of the spider, on one or both of the outer peripheral surface of the mandrel and the inner peripheral surface of the die ring, the shaft on which the spider leg exists. A gist of an extrusion molding die for a synthetic resin pipe is characterized in that a groove for fusing is provided on the line in parallel with the direction of resin flow.

In order to extrude the pipe with equal wall thickness in the circumferential direction regardless of the formation of the fusing groove, the fusing groove is a resin flow from the deepest portion located between the spider and the annular extrusion port. It is desirable that the depth be gradually reduced in the direction of, and that it be completely eliminated at the extrusion port. Of course, in order to further improve the strength of the fusion-bonded portion, it is also permissible to leave the fusion-bonding groove up to the extrusion port and leave the bulging rib corresponding to the groove on the inner or outer peripheral surface of the pipe. It

The fusing groove corresponds to the cut portion of the molten resin separated by the spider leg, imparts a kneading action intensively to the portion, and disturbs the weld line to increase the fused portion area,
In addition, it is bent in a complicated manner, and as a result, the resin bonding force of the fusion-bonded portion is made sufficiently high. Therefore, in order to increase the kneading action due to further disturbance of the resin flow in the fusing groove, the cross-sectional shape of the fusing groove is changed to the center line connecting the deepest point and the center of the mandrel. On the other hand, it is also recommended as one of the preferred embodiments to form a shape that is asymmetrical on the left and right sides.

Embodiments In FIGS. 1 to 3, (1) is an inflow side die ring attached to the extruder (15) side, (2) is an extrusion side die ring, and (3) is a spider. The (1), (2) and the spider (3) are fastened to each other with bolts (4). (5) is die ring (1)
A mandrel that forms an annular resin flow path (6) with (2), and the spider leg (3a) of the spider (3) is integrally connected to the outer peripheral surface of the mandrel. (7) is a die lip that regulates the outer peripheral surface of the extrusion port (8), and (9) is a band heater.

In the annular resin flow path (6) formed between the die rings (1) and (2) and the mandrel (5), the spider legs (3a) are present at a plurality of positions in the circumferential direction. Therefore,
The molten resin flowing from the extruder (15) side through the breaker plate (16) into the die is the spider leg (3a).
After being cut and separated by the above, it passes through the spider (3) portion, then merges and fuses in the resin flow channel (6), and is extruded as a pipe from the extrusion port (8).

Here, the extrusion molding die of the illustrated embodiment is parallel to the resin flow direction on the axis where the spider leg (3a) exists on the outer peripheral surface of the mandrel (5) at a position downstream of the spider (3). A plurality of fusing grooves (10) are provided. The groove (10) has an expanded U-shaped cross section as shown in FIG.
It is made deepest at a position immediately after passing, and thereafter the depth is gradually reduced toward the extrusion port (8), and completely disappears in the vicinity of the extrusion port (6) forming the die land. It has been done.

Therefore, the molten resin once cut and separated by the spider leg (3a) passes through the spider (3), and then the resin mainly in the vicinity of the cut portion is flown into the fusing groove (10) in a rolled-up state, and the inflow is caused. In the portion, the resin flow is disturbed and subjected to the kneading action, the cut surfaces are fused with each other, and the cut mark disappears. Then, as the groove depth is gradually reduced, the cross-sectional area is reduced to an annular shape in which the back pressure is increased and the wall thickness is made equal in the circumferential direction, and the resin pipe seamless from the extrusion port (8). Is extruded as.

[Strength test of molded pipe]

A resin pipe extruded using the extrusion die shown in the above example, and a resin pipe molded using a conventional extrusion die having no fusing groove, a comparison test of their strength is performed. As a result, it was as follows.

In the extrusion die according to the example, the fusing groove had a depth of 15 mm at its deepest portion, and the groove width was about 20 mm at the opening. Further, in any of the dies, the extrusion molded pipe was made of a circular hard vinyl chloride resin having an outer diameter of 127 mm and a wall thickness of 3.0 mm.

The above-mentioned pipe (P) extruded by the die of the example and the conventional die respectively was inserted into two kinds of metal test tubes (M) having inner diameters of 150 mm and 185 mm as shown in FIG.
When both ends are closed with stoppers (R), the temperature is
After injecting steam (S) at 0.5 ° C and pressure of 0.5 kg / cm ² to expand the pipe (P) and bring its outer peripheral surface into close contact with the inner surface of the test tube (M), keep the above pressure for 2 hours. Held

Then, after the lapse of the above time, the pipe (P) was pulled out from the test pipe (M), and the occurrence of cracks in the expanded portion was examined. The results are shown in Table 1 below.

As shown in the results of Table 1 above, the pipe extruded using the die according to the embodiment of the present invention is clearly separated from the resin by the spider as compared with the pipe extruded using the conventional die. The strength improvement effect was recognized in the re-fusion portion.

By the way, although the fusing groove (10) is formed on the mandrel (5) in the above embodiment, the fusing groove (20) is formed on the mandrel (5) as shown in FIGS. 5 and 6. Instead of 5), it may be formed on the inner peripheral surface of the die ring (2) as an outer mold, and may be formed on both of them (not shown). In any case, the molten resin that was cut and separated by the spider legs (3a) at first glance flows into the fusing groove (20), causing a disturbance in the resin flow, which causes a substantial kneading action. Complete elimination of the above cutting marks is achieved, which in turn improves the strength of the re-fusion portion.

Further, the fusing grooves (10) and (20) may be formed to have different cross-sections as shown in FIG. 7 in order to more effectively perform the kneading action of the resin there. Seventh example
The figure illustrates the case where a fusing groove (30) having an irregular cross-section is formed in the mandrel (5), which groove has the deepest point (B) in the cross section and the mandrel (5).
It is formed so as to be left-right asymmetric with respect to the diameter line (l) connecting the center (O) of the resin, and can cause uneven flow in the resin flow in the groove. By forming such a fusing groove (30), the strength of the resin separating and re-fusing portion by the spider can be made even more comparable to other portions.

Effect of the Invention The extrusion molding die according to the present invention re-kneads the resin cutting portion by the spider leg with the fusing groove in the course of the flow to reach the extrusion port as described above, and merges them together. You can Therefore, it is possible to prevent the so-called weld line portion corresponding to the spider leg from becoming a weak point in strength, and thus it is possible to obtain a high-quality resin pipe in which the strength distribution is approximately evenly balanced in the circumferential direction. Therefore, the molded pipe thus obtained can be used more safely without any problem even in the case where it is used after being subjected to a secondary process of softening and expanding by heating. In addition, since the mandrel and / or the die ring are formed with the fusion groove in the axial direction only in the corresponding portion of the spider leg, the process at the time of manufacturing the die can be performed relatively easily and the resin flow can be resisted. As the back pressure is not excessively large, there is no difficulty in adjusting the extrusion pressure and the wall thickness of the molded pipe. In addition, since the retention time of the resin in the die does not become long, there is no risk of the resin being decomposed in the die even when hard vinyl chloride resin is extruded, and a pipe of excellent quality Can be extruded.

Further, according to claim (2), it is possible to mold a straight pipe having no ribs on the inner and outer surfaces which are based on the fusing grooves, and according to claim (3). The strength of the cutting and re-fusing part is further improved,
It is possible to extrude a pipe having a more uniform strength distribution in the circumferential direction.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view of an extrusion molding die showing an embodiment of the present invention, and FIGS. 2 and 3 are FIG. 1 II-II line and III-I.
A sectional view taken along line II, FIG. 4 is a longitudinal sectional view showing a state of strength test of the molded pipe, FIG. 5 is a longitudinal sectional view showing another embodiment of the present invention, and FIG. 6 is FIG. FIG. 7 is a cross-sectional view taken along line VI, and FIG. 7 is an enlarged cross-sectional view of main parts showing a modified example of the fusing groove. (2) ... Diling, (3) ... Spider, (3a) ... Spider leg, (5) ... Mandrel, (6) ... Resin flow path,
(10) (20) (30) ... Fusing groove.

Claims (3)

[Scope of utility model registration request] 1. At a position on the downstream side of the spider, on one or both of the outer peripheral surface of the mandrel and the inner peripheral surface of the die ring, a fusing groove parallel to the resin flow direction on the axis where the spider leg exists. An extrusion die for a synthetic resin pipe, which is provided. 2. The fusing groove has a depth that gradually decreases in the direction of resin flow from the deepest portion located between the spider and the annular extrusion port, and disappears at the extrusion port. The extrusion molding die for a synthetic resin pipe according to claim 1. 3. The fusing groove is formed in a modified cross-sectional shape in which the left and right sides are asymmetric with respect to the center line connecting the deepest point of the cross section and the center of the mandrel.
Alternatively, a die for extrusion molding of the synthetic resin pipe according to (2).