JP7612468B2 - Sheath structure - Google Patents

Description

The present invention relates to a sheath tube structure that protects a flexible resin tube.

Flexible plastic pipes for cold and hot water supply are made of cross-linked polyethylene, polybutene, etc., and have a soft surface, so they are protected by a sheath pipe structure. This prevents the surface of the flexible plastic pipe from being damaged, for example, when it rubs against the building structure during plumbing work.

The sheath pipe structure disclosed in Patent Document 1 is composed of a corrugated pipe made of rigid polyethylene. This corrugated pipe has peaks and valleys that are alternately arranged in the axial direction of the pipe and have a semicircular cross-section, and connecting portions that connect these peaks and valleys and are approximately perpendicular to the pipe axis, resulting in a corrugated cross-sectional shape. The corrugated pipe is rigid even though it is thin-walled, so it can protect flexible plastic pipes.

In addition to protecting the flexible plastic pipe, the corrugated pipe is required to be stretchable in the axial direction because, when connecting the end of the flexible plastic pipe to a joint such as a header, a water tap, or a hot water tap, the end of the corrugated pipe needs to be shifted away from the end of the flexible plastic pipe to shrink it in the axial direction and expose the end of the flexible plastic pipe.
In Patent Document 1, the thickness of the peaks (e.g., 0.25 mm) is made thinner than the thickness of the valleys and connecting portions (e.g., 0.5 mm), thereby facilitating elastic deformation of the peaks and enabling them to expand and contract in the axial direction of the tube.

The sheath pipe structure disclosed in Patent Document 2 has a double structure, comprising a corrugated pipe and an intermediate material interposed between the corrugated pipe and a flexible resin pipe. The corrugated pipe has short cylindrical peaks and valleys arranged alternately in the pipe axis direction, and a connecting portion that connects the peaks and valleys and is substantially perpendicular to the pipe axis. The corrugated pipe is made of low-density polyethylene and has a thin wall (e.g., 0.1 to 0.4 mm), which increases the flexibility in the pipe axis direction, but it has low scratch resistance and cannot protect the flexible resin pipe by itself. In the sheath pipe structure of Patent Document 2, the intermediate material ensures the protection of the flexible resin pipe. In addition, the intermediate material is made of a foam material, which also provides heat retention.

Patent No. 4190923 WO2018/123779 publication

As described above, the sheath pipe structure for the flexible resin pipe is required to have a function of protecting the flexible resin pipe and flexibility in the axial direction of the pipe. In recent years, a heat retention function (insulation function) has also been required.
The corrugated pipe of Patent Document 1 is made of hard polyethylene, so it is basically scratch-resistant and has excellent resin pipe protection function, but it has poor elasticity in the pipe axis direction. If the thickness of the crest is further reduced in an attempt to increase the elasticity, it will result in a decrease in scratch resistance. Thus, it is difficult for the corrugated pipe of Patent Document 1 to achieve both elasticity and scratch resistance. In addition, the heat retention function is also poor.

In the sheath pipe structure of Patent Document 2, the corrugated pipe has low damage resistance and is easily torn when dragged around the construction site, exposing the intermediate material from the torn area and marring the appearance. Furthermore, if the intermediate material is repeatedly damaged by the building's structural members after the corrugated pipe is torn, it may no longer be able to protect the flexible plastic pipe. There is also a limit to how much flexibility can be increased due to the resistance of the intermediate material. Furthermore, the double structure of the corrugated pipe and intermediate material increases manufacturing costs.

The present invention has been made to solve the above problems, and provides a sheath tube structure that is covered over a flexible resin tube to protect the flexible resin tube, comprising:
The corrugated pipe is provided with a corrugated pipe that is fitted onto the flexible resin pipe without an intermediate material, the corrugated pipe being a molded product whose main raw material is polyethylene and whose expansion ratio is 1.05 to 4.0 times, and is characterized in that it has annular maximum diameter portions and annular minimum diameter portions that are arranged alternately in the pipe axial direction, and a connecting portion that connects the maximum diameter portions and the minimum diameter portions, the maximum diameter portions include a short cylindrical portion, the minimum diameter portions do not include a short cylindrical portion, the connecting portion is inclined with respect to the pipe axis of the corrugated pipe, and a V-groove is defined by the minimum diameter portion and the connecting portions located on both sides of it.

According to the above-mentioned structure, the molded product has an expansion ratio of 1.05 to 4.0, so it has excellent elasticity, can be smoothly connected to the joint of the flexible resin pipe, and has excellent heat retention. Furthermore, since the intermediate material can be omitted, the manufacturing cost can be reduced.
The corrugated pipe made of foamed material can be thickened even though its weight per unit length is small, so that cutting allowance can be secured and breakage can be avoided even if it rubs against a building structure made of concrete during piping installation. In addition, since the foaming ratio is 4.0 or less, deterioration of scratch resistance due to excessive foaming can be avoided.
The corrugated pipe has a maximum diameter portion including a short cylindrical portion, and these short cylindrical portions are connected via a V-groove, so that the risk of the pipe getting caught on the building structure during installation can be minimized. From this viewpoint, the damage resistance can be improved.

Preferably, the corrugated pipe has an expansion ratio of 1.2 to 2.5 times.
According to the above-mentioned configuration, by setting the foaming ratio of the corrugated pipe to 1.2 or more, it is possible to improve the elasticity and heat retention. Also, by setting the foaming ratio to 2.5 or less, it is possible to improve the scratch resistance.

Preferably, the thickness of the maximum diameter portion is greater than the thickness of the minimum diameter portion.
According to the above-mentioned configuration, the maximum diameter portion of the corrugated pipe is thickened to improve the scratch resistance of the corrugated pipe. Also, since the groove located on the radially outer side of the minimum diameter portion is a V-groove, it is difficult for building structures to enter the thin-walled minimum diameter portion, and the thin-walled minimum diameter portion can be protected.

Preferably, the average thickness of the maximum diameter portion and the minimum diameter portion is 0.4 mm or more, which allows a sufficient cutting allowance to be secured and enhances scratch resistance.
More preferably, the average thickness is 0.4 to 1.0 mm.

Preferably, when the outer diameter of the maximum diameter portion is Φ (mm) and the average thickness of the maximum diameter portion is T ₁ (mm), the weight per unit length (g/m) is (3.2949×Φ−26.402)×0.3/T ₁ or more.
By complying with the above conditions, scratch resistance can be reliably obtained.

Preferably, when the flexible resin pipe is bent to a minimum bending radius and the corrugated pipe is bent to a minimum bending radius accordingly, the width of the radial outer end of the V-groove located inside the bent shape is Wm and the average thickness of the maximum diameter portion is _T1 , Wm/2< _T1 .
According to the above configuration, the depth to which the piping penetrates into the V-groove of the building structure during piping work is limited, thereby further improving scratch resistance.

Preferably, the distance in the tube radial direction from the outer surface of the minimum diameter portion to the center of the wall thickness of the maximum diameter portion is equal to or greater than half the width of the maximum diameter portion in the tube axial direction.
According to the above-mentioned configuration, since the V-groove is deep, the air layer between the flexible resin pipe and the corrugated pipe can be thickened, and the heat insulating effect can be improved. In addition, when the corrugated pipe is molded by expanding the diameter, the minimum diameter part can be made thin, and the stretchability can be further improved.

Preferably, the corrugated pipe contains up to 40% polypropylene.
According to the above configuration, it is possible to improve heat retention while maintaining the flexibility of the corrugated pipe.

The sheath tube structure of the present invention provides both scratch resistance (protection of the flexible resin tube) and flexibility in the tube axial direction, while also providing heat retention (insulation) and reducing manufacturing costs.

1 is an enlarged longitudinal cross-sectional view showing a sheath tube structure according to a first embodiment of the present invention in a state in which it is covered with a flexible resin tube. FIG. 1 is a graph showing the relationship between the wall thickness of the peaks and the wall thickness of the valleys of a corrugated pipe obtained by diameter expansion forming. 5 is an enlarged longitudinal cross-sectional view showing a sheath tube structure according to a second embodiment of the present invention in a state in which it is covered with a flexible resin tube. FIG. FIG. 2 is a schematic diagram showing a method for a scratch resistance test. 1 is a graph showing the results of a scratch resistance test. 1 is a schematic side view showing an apparatus for molding a corrugated pipe in a state in which the corrugated pipe is placed over a flexible resin pipe.

First Embodiment of the Present Invention
A first embodiment of the present invention will be described below with reference to the drawings. As shown in Fig. 1, a flexible resin pipe 1 is covered with a sheath pipe structure 2 and is arranged to be laid under the floor of a building. Hereinafter, a pipe consisting of the flexible resin pipe 1 and the sheath pipe structure 2 will be referred to as a composite pipe.
The flexible resin pipe 1 is made of a cross-linked polyethylene pipe, a polybutene pipe, a polyethylene pipe, a heat-resistant polyethylene pipe, a composite pipe of these resins, or a metal-reinforced composite pipe, and is used to transport water at 95° C. or below.

The sheath pipe structure 2 is composed of only a corrugated pipe 10, and no intermediate material is interposed between the corrugated pipe 10 and the resin pipe 1, with only an air layer interposed therebetween.
The corrugated pipe 10 is obtained by expanding the diameter of a parison made mainly of polyethylene, for example, low-density polyethylene (LDPE) having a tensile yield strength of 20 MPa or less, by blow molding or vacuum molding. The raw material contains a foaming agent, and the corrugated pipe 10 is molded with an expansion ratio of 1.05 to 4.0 times, more preferably 1.2 to 2.5 times.

The corrugated pipe 10 can be molded by adding polypropylene (PP) to the main raw material polyethylene, for example low density polyethylene (LDPE), in the following ratio (parts by weight):
LDPE:PP=100:0~60:40
More preferably,
The LDPE:PP ratio is 90:10 to 70:30.
Since polypropylene has a low thermal conductivity, the inclusion of polypropylene can improve heat retention. As the polypropylene content increases, the flexibility of the corrugated pipe 10 decreases, so the polypropylene content is set to a maximum of 40%, preferably 30% or less, as described above.

The corrugated pipe 10 has annular peaks 11 (maximum diameter portions) and annular valleys 12 (minimum diameter portions) arranged alternately at equal intervals in the pipe axial direction, and a connecting portion 13 connecting the peaks 11 and valleys 12.

The peaks 11 are short cylindrical sections concentric with the tube axis. The valleys 12 do not have short cylindrical sections and are rounded sections. The connecting sections 13 are inclined with respect to a plane perpendicular to the tube axis of the corrugated tube 10 and have a conical shape. Each valley 12 and a pair of connecting sections 13 connected to this valley 12 form a V-groove 14. The V-grooves 14 are located between the peaks 11 on the radial outside of each valley 12.

In FIG. 1 , the thickness of the peaks 11 (average thickness; the same applies below) is indicated by the symbol _T1 , the thickness of the valleys 12 is indicated by the symbol _T2 , the width of the peaks 11 is indicated by the symbol _W1 , the width of the V-groove 14 when the corrugated pipe 10 is in a straight state is indicated by the symbol Wg, and the depth of the V-groove 14 (the distance between the outer surfaces of the peaks 11 and the valleys 12) is indicated by the symbol D.

The thickness _T1 of the peak portion 11 is greater than the thickness _T2 of the valley portion 12. In this case, preferably, _T1 / _T2 =1.3 to 1.6. The thickness _T1 of the peak portion 11 may be less than or equal to the thickness _T2 of the valley portion 12. In this case, preferably, _T1 / _T2 =0.8 to 1.0. Regardless of the relationship in magnitude between the thickness _T1 and the thickness _T2 , the thickness _T1 of the peak portion 11 is preferably 0.4 to 1.0 mm, more preferably 0.4 to 0.7 mm. Similarly, the thickness _T2 of the valley portion 12 is preferably 0.4 to 1.0 mm, more preferably 0.4 to 0.7 mm.

As described later, the flexible plastic pipe 1 and the corrugated pipe 10 are bent together during piping work. The flexible plastic pipe 1 has a minimum bending radius that allows it to be used for a long time without buckling or other abnormalities. The minimum bending radius of the corrugated pipe 10 can be set to be approximately equal to or slightly smaller than the minimum bending radius of the flexible plastic pipe 1. When the corrugated pipe 10 is bent at the minimum bending radius, adjacent peaks 11 approach each other on the inside of the bent shape as shown by imaginary lines in Figure 1, so that the width of the V-groove 14 becomes narrower than Wg and becomes Wm. This narrowed groove width Wm and the wall thickness _T1 of the peaks 11 have the following relationship.
Wm/2<T ₁ ...(1)

Experiment on wall thickness A mold was prepared in which the width _W1 of the peaks 11 of the corrugated pipe 10 was 3.14 mm, the width Wg of the V-groove 14 was 1.256 mm, the depth D of the V-groove 14 was 0.95 mm, and the diameter R of the outer surface of the R part of the valleys 12 was 0.5 mm. Corrugated pipes were obtained by blow molding from parisons with different thicknesses and different blend ratios (parts by weight) of low-density polyethylene, polypropylene, and microcapsules. The microcapsules are polymer shells containing a volatile expansion agent (foaming agent) as a core agent.

The experimental results are shown in Table 1 below.

Table 1 can be represented graphically as shown in FIG.
The above experimental results show that, if blow molding is performed using a parison having a thickness such that the thickness of the valley portion 12 is 0.35 mm or more, the thickness of the peak portion 11 can be made larger than that of the valley portion 12 without being greatly affected by the blending ratio of low-density polyethylene and propylene. Changing the foaming ratio by changing the blending ratio of the microcapsules did not have a significant effect. The corrugated pipe 10 also has similar results when vacuum molded.

The difference in thickness between the peaks 11 and valleys 12 is due to the cross-sectional shape of the corrugated pipe 10, i.e., the mold shape. That is, the mold has a cylindrical main molding surface corresponding to the peaks 11, and an annular molding protrusion that protrudes radially inward from the main molding surface corresponding to the V-groove 14. Both sides of this molding protrusion are inclined. When the amount of resin in the parison is equal to or greater than a predetermined amount, a portion of the resin located at the tip of the molding protrusion flows radially outward along both sides of the molding protrusion and reaches the main molding surface that forms a cylindrical surface. As a result, the valleys 12 formed at the tip of the molding protrusion have a smaller amount of resin and are thin-walled, while the peaks 11 formed at the main molding surface have a larger amount of resin and are thick-walled.

Performance of the sheath tube structure The performance of the sheath tube structure 2 having the above-mentioned configuration will now be described.
(1) Scratch Resistance When a composite pipe consisting of a flexible resin pipe 1 and a sheath pipe structure 2 is installed under the floor of a building, the peaks 11 of the corrugated pipe 10 rub against the building structure (for example, a sleeve penetrating the partition foundation of a detached house). In this case, the corrugated pipe 10 has a short cylindrical shape with the peaks 11 of these short cylindrical parts connected via the V-groove 14, so that it is possible to minimize the possibility of the corrugated pipe 10 getting caught on the building structure. The thickness _T1 of the peaks 11 is thicker than the valleys 12, being 0.4 mm or more, so that it is difficult to break even if it is scratched. Even if it is shallowly scratched or worn, other materials such as intermediate materials are not exposed, and the appearance is not impaired. Even if the thickness _T2 of the valleys 12 is thin, it is not scratched because it does not rub directly against the building structure. In addition, since the groove width of the V-groove 14 narrows toward the inside in the radial direction, even if the building structure enters the V-groove 14, it is difficult to reach the valleys 12, and the thin-walled valleys 12 are protected.

(2) Elasticity in the pipe axis direction The corrugated pipe 10 is made primarily of polyethylene, for example, low-density polyethylene (LDPE) with a tensile yield strength of 20 MPa or less, and is a molded product with an expansion ratio of 1.05 to 4.0 times, so that sufficient flexibility can be ensured even if the peaks 11 are thick, and the valleys 12 are thin, so that the pipe has excellent elasticity in the pipe axis direction. Therefore, when connecting the flexible resin pipe 1 to a joint, the end of the corrugated pipe 10 can be easily shifted in the pipe axis direction to expose the flexible resin pipe 1, and the connection work can be performed smoothly.
Furthermore, since there is no intermediate material between the corrugated pipe 10 and the flexible resin pipe 1, even if the corrugated pipe 10 is repeatedly expanded and contracted, or even if the corrugated pipe 10 is once stretched by being dragged during construction, the problem of the intermediate material getting caught and making it difficult to expand and contract does not occur.

(3) Heat retention The corrugated pipe 10 is foam-molded, and therefore has excellent heat insulation and heat retention properties. In addition, the valleys 12 close to the flexible resin pipe 1 are narrow, while the peaks 13 far from the flexible resin pipe 1 are wide, so that the air space can be increased. In addition, by making the valleys 12 thin-walled, the air space between the flexible resin pipe 1 can be increased. As a result, the heat retention can be further improved.
In addition, due to the shapes of the valleys 12 and peaks 13 described above, they are less likely to get caught on corners at the construction site.
(4) Manufacturing Costs Since the sheath pipe structure 2 does not require an intermediate material between the corrugated pipe 10 and the flexible resin pipe 1, manufacturing costs can be reduced.

As described above, the corrugated pipe 10 has an expansion ratio of 1.05 or more, so that it can ensure elasticity and heat retention. Furthermore, by making the expansion ratio 1.2 or more, it is possible to sufficiently reduce the thermal conductivity, and it is possible to further increase elasticity and heat retention.
When the expansion ratio is 4.0 or less, scratch resistance can be ensured, but when it exceeds 4.0, scratch resistance decreases. When the expansion ratio is 2.5 or less, scratch resistance can be further improved.

Regarding the wall thickness of the crests (0.4 to 1.0 mm), the wall thickness _T1 of the crests 11 is 0.4 mm or more, so that the crests will not break even if they rub against the building structure and are scratched shallowly. If the wall thickness _T1 is less than 0.4 mm, the scratch resistance decreases. The wall thickness _T1 is set to 1.0 mm or less in order to suppress an increase in the outer diameter of the corrugated pipe 10 and save material. More preferably, the wall thickness _T1 is set to 0.7 mm or less.

Regarding the thickness of the valley portion (0.4 to 1.0 mm), the thickness _T2 of the valley portion 12 is set to 0.4 mm or more in order to obtain strength. The thickness T2 is set to 1.0 mm or less in order to increase the clearance with the flexible resin pipe 1 to improve heat insulation and to save material. More preferably, the thickness _T2 is set to 0.7 _mm or less.

The meaning of the above-mentioned formula (1) will be explained. Usually, many pipes are passed through the sleeve penetrating the partition foundation. When the last pipe is passed through, the flexible resin pipe 1 is pulled in a state where it is bent at the minimum bending radius, and there is a possibility that the corrugated pipe 10 hits the corner of the sleeve. Even in such a worst-case situation, if the above-mentioned formula (1) is satisfied, it is possible to reliably prevent the corrugated pipe 10 from being damaged and broken. That is, when the corrugated pipe 10 is at the minimum bending radius as described above, if the corner of the sleeve that forms a right angle in cross section bites into the V-groove 14 narrowed to the groove width Wm, the bite depth of this corner is at most half the groove width Wm. Therefore, by making the bite depth (Wm/2) of the corner of the sleeve into the V-groove 14 smaller than the wall thickness _T1 of the peak portion 11, it is possible to reliably prevent the corrugated pipe 10 from breaking.

Second Embodiment of the Present Invention
3 shows a second embodiment of the present invention. Since the basic structure of this embodiment is similar to that of the first embodiment, the same reference numerals as those in the first embodiment are used for the respective components in the figure, and detailed description thereof will be omitted.

In the second embodiment, a distance Dx in the tube diameter direction from the outer surface of the valley portion 12 to the center of the wall thickness of the peak portion 11 is equal to or greater than half of a width _W1 in the tube axis direction of the peak portion 11. See the following formula: Da≧ _W1 /2 (2)
According to the above configuration, the V-groove 14 is deep, so that the air layer between the flexible resin pipe 1 and the corrugated pipe 10 can be thickened, and the heat insulating effect can be improved. Also, during molding by expanding the diameter, the amount of resin flowing from the valley portion 12 to the peak portion 11 increases, making it easier to create a thickness difference. In other words, the valley portion 12 can be made thin, and the stretchability can be improved.

<Conditions for weight per unit length of corrugated pipe to ensure scratch resistance>
In order to determine the condition of the weight per unit length of the corrugated pipe to ensure the scratch resistance, a scratch resistance test was conducted using a composite pipe A consisting of a flexible resin pipe 1 similar to that of the first embodiment and a sheath pipe structure 2 (corrugated pipe 10) as shown in FIG.
The composite pipe A was passed through one of the holes 50a in the concrete block 50, rubbed against the upper edge of the hole 50a, and pulled up with a force of 15 kgf while checking with a hand scale 51. After this, the appearance of the composite pipe A was visually checked, and if no tears were found in the corrugated pipe, it was judged to have passed, and if tears were found, it was judged to have failed.

The composite pipes A to be tested are broadly classified into the following three types.
(A) Composite pipe using foamed resin corrugated pipe (ridge thickness 0.5 mm) (B) Composite pipe using foamed resin corrugated pipe (ridge thickness 0.4 mm) (C) Composite pipe using non-foamed corrugated pipe (ridge thickness 0.3 mm) More specifically, the corrugated pipes (A) and (B) are according to the present invention, and the raw material is a mixture of 80% polyethylene (M1820 manufactured by Asahi Kasei Corporation) and 20% polypropylene (EX6000 manufactured by Japan Polypropylene Corporation) as the main material, with 1 to 8.5 parts of foaming agent (MB5885 manufactured by Sankyo Kasei Corporation) (corresponding to an expansion ratio of 1.16 to 2.20 times) relative to the main material.
The corrugated pipe (C) above was a test subject for comparison and was made of non-foamed high-density polyethylene (B470 manufactured by Asahi Kasei Corporation).

For the corrugated pipe (A) (with foaming, ridge thickness 0.5 mm), a scratch resistance test was carried out by changing the weight per unit length of the corrugated pipe for each different outer diameter (ridge outer diameter). Table 2 below shows some of the test results.

As shown in Table 2 above as sample group 1, when tested with an outer diameter of 28.1 mm, weights per unit length of 39.7 (g/m) or more passed, but anything less than that failed. Similarly, as shown in sample group 2, when tested with an outer diameter of 30.0 mm, weights per unit length of 43.5 (g/m) or more passed, but anything less than that failed. Similarly, as shown in sample group 3, when tested with an outer diameter of 30.5 mm, weights per unit length of 44.5 (g/m) or more passed, but anything less than that failed.

When further tests were conducted with a wall thickness of 0.5 mm and different outer diameters, it was found that the lower limit of the weight per unit length of a passing product had a linear relationship with the outer diameter, as shown by line A in Figure 5.

Figure 5 shows an example of the test results, with passed products indicated by black circles and failed products indicated by crosses. In simple terms, a product with an outer diameter of 28.1 mm and a weight per unit length of 42.5 g/m (expansion ratio 2.2 times) passes the test. A product with an outer diameter of 30.0 mm and a weight per unit length of 48.0 g/m (expansion ratio 2.2 times) passes the test. A product with an outer diameter of 30.5 mm and a weight per unit length of 42.5 g/m fails the test, but a product with 48.5 g/m (expansion ratio 1.6 times) passes the test.

Similarly, for the corrugated pipe (B) (with foaming, ridge thickness 0.4 mm), a scratch resistance test was conducted by changing the weight per unit length of the corrugated pipe for each different outer diameter (ridge outer diameter). Table 2 above shows the test results for sample group 4 with an outer diameter of 30.9 mm (foaming ratio 1.4 times). In this case, a weight per unit length of 56.6 g/m or more was considered to be acceptable. When further tests were conducted for each different outer diameter, it was found that the lower limit of the weight per unit length of an acceptable product had a linear relationship with the outer diameter, as shown by line B in Figure 5.

Similarly, for the corrugated pipe (C) (non-foamed, crest thickness 0.3 mm), a scratch resistance test was performed by changing the weight per unit length of the corrugated pipe for each different outer diameter. That is, when tests were performed for outer diameters of 19.0 mm, 23.5 mm, 29.0 mm, and 35.0 mm, the lower limit values of the weight per unit length of the acceptable products were 36.0 g/m, 50.0 g/m, 68.0 g/m, and 88.0 g, respectively (see the open circles in Figure 5). It was found that the lower limit value of the weight per unit length of the acceptable products has a linear relationship with the outer diameter, as shown by line C in Figure 5.

From lines A, B, and C in Figure 5, it was found that the lower limit of the weight per unit length of an acceptable product is less related to the expansion ratio and is smaller as the wall thickness increases. More specifically, it was found that the lower limit of the weight per unit length of an acceptable product is inversely proportional to the wall thickness.

The straight line C in FIG.
Y'=(3.2949×Φ-26.402)...(3)
Here, Y' is the lower limit of the weight per unit length of an acceptable product when a non-foamed corrugated pipe (ridge thickness 0.3 mm) is used, and Φ is the outer diameter of the corrugated pipe.

The weight Y (g/m) per unit length of an acceptable product when using a foamed corrugated pipe can be obtained by multiplying the value of the above formula (3) by the reciprocal (0.3/ _T1 ) of the ratio of the wall thickness _T1 to the wall thickness 0.3 mm as a coefficient. See the formula below.
Y≧(3.2949×Φ-26.402)×0.3/T ₁ ...(4)

<Method of forming corrugated pipe>
Next, we will verify whether a composite pipe having a corrugated pipe that satisfies formula (4) can be manufactured at low cost using the corrugated pipe molding method described below. Briefly, this method is a method of forming a corrugated pipe by extruding a resin material into a tubular shape so as to enclose a flexible resin pipe, and then expanding the diameter of the extruding resin material. The method will be described in detail below.

6 shows a manufacturing apparatus 100 for the composite pipe A. The manufacturing apparatus 100 includes a foaming resin supply section 110, an extrusion nozzle 120, and a corrugated pipe forming section 130.
The foaming resin supplying section 110 includes a hopper for receiving the resin that is the raw material for the corrugated pipe 3, a heater for heating and melting the resin, a foaming agent adding section, and a cylinder and a screw for kneading and extruding the resin and the foaming agent. The raw resin before being fed into the hopper may contain a foaming agent. The foaming resin supplying section 110 may be disposed vertically as shown in the figure, or horizontally.

The extrusion nozzle 120 has a circular passage opening through which the flexible resin tube 1 passes, and an annular extrusion opening formed so as to surround this passage opening.
The corrugated pipe forming section 130 (corrugator) is disposed downstream in the extrusion direction of the extrusion nozzle 120 (right side in FIG. 6). The corrugated pipe forming section 130 is equipped with two upper and lower elliptical annular tracks 131, 132, and a large number of split dies 133, 134 that are aligned along the annular tracks 131, 132, respectively, and circulate as indicated by the arrows. The axis of the extrusion nozzle 120 passes between the two annular tracks 131, 132.

In the figure, split molds 133 and 134 come together near extrusion nozzle 120 to form a cylindrical mold pair, and their molding surfaces can mold ridges and valleys at multiple pitches. In the molding surfaces of each of split molds 133 and 134, a suction port (not shown) is formed in the molding section for molding the ridges.

The composite pipe A is manufactured as follows.
The flexible resin tube 1 is prepared in advance by molding and hardening it, obtaining it, etc. This flexible resin tube 1 is passed through a passage port 121 of the extrusion nozzle 120, and is sent out from the extrusion nozzle 120 to the corrugated pipe forming section 130 at a constant speed, and is sent between split dies 133 and 134 arranged on circulation tracks 131 and 132 and moving at the same speed.

In the foaming resin supply section 110, the resin is heated and melted, and a foaming agent is added in a predetermined compounding ratio to foam at a predetermined expansion ratio. The resin is then supplied to the extrusion nozzle 120, and extruded from the annular extrusion port of the extrusion nozzle 120 toward the corrugated pipe molding section 130 at the same speed as the flexible resin pipe 1.

Because of the high pressure inside the extrusion nozzle 120, the resin has not yet started to foam. Because the pressure applied to the resin by extrusion decreases, foaming begins after the resin has been extruded about 100 mm. The resin extruded in a tubular shape is expanded in diameter by the vacuum from the suction ports of the split dies 133, 134, and reaches the molding surfaces of the split dies 133, 134 to be molded into the corrugated pipe 3. The corrugated pipe 3 foams as it passes through the corrugated pipe molding section 130, increasing in thickness.

The above molding method can significantly reduce the manufacturing costs of composite pipes compared to molding a corrugated pipe separately and then covering it with a flexible resin pipe. However, because the resin is extruded upstream of where the split dies 133, 134 overlap, increasing the amount of resin extruded will cause the resin to expand in diameter due to the die swell or ballast effect, and it will get caught in the split dies 133, 134, causing resin clogging. For this reason, there is a limit to the weight per unit length of the corrugated pipe.

Using the above molding method, the relationship between weight per unit length and resin clogging was experimentally determined. Note that in this experiment, a resin raw material of 100% non-foamed polyethylene was used, but similar results should be obtained with the foamed resin of the present invention. This is because the foamed resin is not foamed at the position where the split molds 133 and 134 overlap.

As a result of the experiment, it was confirmed that when the outer diameter of the corrugated pipe is 30.5 mm, the upper limit of the weight per unit length at which resin clogging does not occur stably is 60 g/m.
Returning to Figure 5, in the case of a non-foamed corrugated pipe with an outer diameter of 30.5 mm, the weight per unit length that passes the test from the standpoint of scratch resistance is 75.1 g/m or more (see line C in Figure 5). Therefore, in the case of a corrugated pipe made of non-foamed resin, if one tries to satisfy the scratch resistance requirement, the above molding method will cause resin clogging, and therefore the above molding method cannot be adopted.

In contrast, the corrugated pipe made of foamed resin of the present invention can satisfy the scratch resistance at a weight per unit length of 44.5 g/m or more when the outer diameter is 30.5 mm and the crest thickness is 0.5 mm, and can satisfy the scratch resistance at a weight per unit length of 56 g/m or more when the outer diameter is 30.5 mm and the crest thickness is 0.4 mm. These values are smaller than the upper limit of 60 g/m to avoid resin clogging as mentioned above, so the above molding method can be used.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.
The outer surface of the valley (the bottom surface of the V-groove) may have substantially no R portion. In this case, in the cross-sectional shape of the V-groove, both side surfaces remain substantially straight and intersect at the bottom.

The present invention can be applied to a sheath pipe structure that protects flexible plastic pipes for supplying cold and hot water.

1 Flexible resin tube 2 Sheath tube structure 10 Corrugated tube 11 Mountain portion (maximum diameter portion)
12 Valley portion (minimum diameter portion)
13 Connection portion 14 V-groove

Claims (7)

A sheath tube structure that is placed over a flexible resin tube to protect the flexible resin tube,
a corrugated pipe that is covered with the flexible resin pipe without an intermediate material and is exposed to the outside ;
The corrugated pipe is a molded product made mainly of polyethylene and has an expansion ratio of 1.05 to 4.0 times, and has annular maximum diameter portions and annular minimum diameter portions alternately arranged in the pipe axial direction, and a connecting portion connecting the maximum diameter portion and the minimum diameter portion,
the maximum diameter portion includes a short cylindrical portion, the minimum diameter portion does not include a short cylindrical portion, the connecting portion is inclined with respect to a pipe axis of the corrugated pipe, and a V-groove is defined by the minimum diameter portion and the connecting portions located on both sides thereof,
A sheath tube structure, characterized in that the wall thickness of the maximum diameter portion is greater than the wall thickness of the minimum diameter portion . The sheath tube structure according to claim 1, characterized in that the foaming ratio of the corrugated tube is 1.2 to 2.5 times. 3. The sheath tube structure according to claim 1, wherein the wall thickness of the connecting portion is gradually decreased from the maximum diameter portion toward the minimum diameter portion. The sheath tube structure according to any one of claims 1 to 3, characterized in that, when the outer diameter of the maximum diameter portion is Φ (mm) and the average thickness of the maximum diameter portion is T1 (mm), the weight per unit length (g/m) is (3.2949 x Φ-26.402 ) x 0.3/T1 or more. The sheath tube structure according to any one of claims 1 to 3, characterized in that when the flexible resin tube is bent to a minimum bending radius and the corrugated tube is bent accordingly to a minimum bending radius, the width of the radial outer end of the V-groove located inside the bent shape is Wm and the average thickness of the maximum diameter portion is T1, Wm/ 2 <T1. The sheath tube structure according to any one of claims 1 to 3, characterized in that the distance in the tube diameter direction from the outer surface of the minimum diameter portion to the center of the wall thickness of the maximum diameter portion is equal to or greater than half the width in the tube axial direction of the maximum diameter portion. The sheath tube structure according to any one of claims 1 to 6 , characterized in that the corrugated tube contains 40% or less of polypropylene.

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