JP2003194282A - Electrothermal mat - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrothermal mat capable of preventing adjacent electrothermal wires from shorting even if kink is generated, and winding up the electrothermal wires at a suitable interval according to wire diameters. <P>SOLUTION: This electrothermal mat for molding an electric welding joint spirally winds electrothermal wires 14 around a through hole provided on the center portion of a synthetic resin sheet. In this electrothermal mat, assuming that the wire expansion rate of the electrothermal wire is α, the diameter of the electrothermal wire is d, the diameter of the most outer peripheral portion of the electrothermal wire spirally wound is D, a temperature rise until the electrothermal wire stars buckling is ΔT, and a temperature deducting the ΔT from a temperature with which resin of a joining part is melted by heating of the electrothermal wire is ΔT', a value H is obtained from the below expression (1). H=/2[(d/4√αΔT+DαΔT)<SP>2</SP>(1+αΔT)<SP>2</SP>-(d/4√αΔT-DαΔT')<SP>2</SP>]<SP>1/2</SP>...(1) The interval W of the electrothermal wire in the most outer peripheral portion is set in a range indicated by the below expression (2). 2H>W>H...(2). <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric heating mat, and more particularly, to an electric heating joint provided on a lower surface of a saddle portion of an electric fusion joint used for providing a branch pipe or the like to a synthetic resin pipe. The present invention relates to an electric heating mat that is integrally fused. 2. Description of the Related Art As a branch pipe joint for providing a branch pipe in a synthetic resin pipe, an electric wire buried in the lower surface of a saddle portion is energized to generate heat, and the surrounding resin is melted and fused and joined. Fusion joints are known. Such an electric fusion joint is
An electric heating mat, in which a heating wire is embedded in a predetermined shape in advance, is inserted into a joint molding die and injection-molded to form the heating wire at a predetermined position on the lower surface of the saddle portion. The electric heating mat is disclosed in, for example, Japanese Patent No. 292
As described in JP-A-9350 and JP-A-2001-82670, usually, a thermoplastic resin is injection-molded to form a substrate having a spiral groove on one surface thereof, and a heating wire is formed in the groove. Is formed by embedding. [0004] The distance between the heating wires in such an electric heating mat is set so that the surrounding resin can be uniformly melted at the time of joining. Accordingly, the distance is set in the range of 0.6 to 5.0 mm. If the distance between the heating wires is too large, the resin may not be uniformly melted and a predetermined bonding strength may not be obtained. Therefore, usually, the distance between the heating wires is often narrowed. On the other hand, the heating wire expands due to heat generated at the time of energization, and locally buckles and deforms, and a phenomenon called kink may occur. When the interval between the heating wires is reduced, the kink is adjacent to the heating wire. Heat wires are short-circuited, some of the heating wires do not generate heat, and the amount of heat for melting the resin becomes insufficient. As described above, there is a possibility that the resin cannot be melted uniformly and a predetermined bonding strength cannot be obtained. The generation of the kink tends to occur particularly on the outer heating wire. Further, when the interval between the heating wires is narrowed, there is a problem that the length of the heating wire is lengthened and the heating wire usage increases. Accordingly, the present invention provides an electric heating mat capable of preventing adjacent electric heating wires from being short-circuited even if a kink is generated, and winding the heating wires at an optimum interval according to a wire diameter. It is aimed at. In order to achieve the above object, in the electric heating mat of the present invention, a heating wire is spirally wound around a through hole provided in a central portion of a synthetic resin sheet. In the electric heating mat for forming the electric fusion joint, the coefficient of linear expansion of the heating wire is α, the diameter of the heating wire is d, the outermost diameter of the spirally wound heating wire is D, and the heating wire buckles. The temperature rise until the start of the heating is ΔT, and the temperature obtained by subtracting the ΔT from the temperature at which the resin at the bonding portion is melted by the heat generated by the heating wire is ΔT ′, and the following equation (1) is used. (1) The distance W between the heating wires at the outermost periphery is set to a range represented by the following expression (2) 2H>W> H (2) with respect to the value H obtained by (1). Features. FIG. 1 is a plan view showing an example of an electric heating mat, FIG. 2 is a partial cross-sectional side view showing an example of an electric fusion joint formed using the electric heating mat, and FIG. It is explanatory drawing which shows the generation state of the kink in a heating wire. First, as shown in FIG.
Is a spirally wound heating wire 14 around a through hole 13 provided at the center of a substrate 12 made of a synthetic resin sheet. A spiral groove for embedding heat rays is provided. Further, terminals (not shown) are connected to both ends of the heating wire 14. By setting such an electrothermal mat 11 in a mold for electrofusion joint molding and insert molding, the electrothermal mat 11 is integrally formed on the lower surface of the curved saddle portion 15 as shown in FIG. Is obtained. In the present example, the shape of the substrate 12 is circular, but may be square or the like. Generally, the substrate 12 is formed of a polyolefin resin such as polyethylene or polybutene, and usually has a thickness of about 1 to 3 mm. The heating wire 1
4, a nichrome wire or a copper nickel wire having a wire diameter of 0.1 to 1.5 mm is used, and is wound at substantially uniform intervals from the inner peripheral side to the outer peripheral side. Furthermore, the heat generation temperature of the heating wire 14 at the time of fusion bonding is about 350 ° C., and the temperature at which buckling deformation starts due to melting of the resin is usually 120 to 150 ° C.
° C. In such an electric heating mat 11, FIG.
When the kink 17 as shown in FIG.
7, the buckling length (Lk [mm]), the length of the buckling portion (Ls [mm]), the amount of deflection (h [m
m]) can be obtained as follows. First, the compressive stress (σ) generated by the thermal expansion of the heating wire 14 is determined by the elastic modulus (E [N · m ⁻² ]) of the heating wire,
Linear expansion coefficient (α [/ ° C]), temperature rise of heating wire (ΔT
[° C.]), it can be obtained by the equation of σ = E · α · ΔT. Further, from Euler's buckling theoretical formula, the buckling strength (σk) of the heating wire is a constant (Dk) obtained from the buckling length (Lk) and the wire diameter (diameter: d [mm]). λ), that is, σ k = π ² · E / λ ² from the constant (λ) of λ = 4Lk / d and the pi (π). It becomes. At this time, it is assumed that the heating wire rises by the temperature (ΔT) from heat generation to the start of buckling, and the heating wire buckles at the moment when the compressive stress (σ) exceeds the buckling strength (σk) of the heating wire. If σ = σk, then E · α · ΔT = π ² · E (d / 4Lk) ^2, and the buckling length (Lk) is given by: It is obtained by the formula. The length (Ls) of the buckled portion at this time is as follows, assuming that both ends of the buckled heating wire are fixed. Diameter of spiral wire (D [mm]) and pi (π)
Therefore, taking into account the extension (π · D · α · ΔT) of the heating wire in this part due to thermal expansion, it can be obtained by the following formula: Ls = (Lk + π · D · α · ΔT) / 2. After the occurrence of buckling, the temperature of the heating wire is further increased by the temperature (Δ
T ′) When the temperature rises to the fusion bonding temperature (maximum temperature), that is, the temperature rise from the temperature at which the resin at the bonding portion melts due to the heat generated by the heating wire to the time when the buckling starts (Δ)
T), the buckling length (Lk '), the length of the buckling portion (Ls'), and the amount of deflection (maximum deflection: H [mm]) when the temperature rises by the temperature (ΔT') minus Lk are Lk '= Lk-π · D · α · ΔT' Ls '= Ls (1 + α · ΔT') Can be obtained by the following equations. Therefore, since the maximum height of the kink generated at the time of fusion bonding is obtained as the maximum deflection (H), the distance (W) between the heating wires is larger than the maximum deflection (H).
If [mm]) is increased, short-circuiting between the heating wires due to generation of kink can be prevented. And
Since the occurrence of kink occurs more frequently in the outer peripheral portion where the spiral diameter is large, at least the interval (W) between the heating wires at the outermost peripheral portion is made larger than the deflection amount (H).
If the interval (W) between the heating wires is too wide, it becomes difficult to sufficiently melt the resin on the bonding surface. Therefore, the interval (W) between the heating wires is less than twice the amount of deflection (H), that is, the interval. If the interval (W) between the heating wires is set so that (W) is in the range of 2H>W> H, preferably 1.5H>W> H with respect to the deflection amount (H), kink Since the resin can be melted in a necessary and sufficient state while preventing a short circuit, joining of the joint using the electric heating mat can be surely performed. Further, since the required length of the heating wire can be minimized, the cost required for the heating wire can be reduced. [Embodiment] The size of the electrofusion joint is 50 × 30 m.
In order to injection-mold a polyethylene resin branch saddle joint (see FIG. 2), a heating wire having a wire diameter (d) of 0.36 mm was spirally wound around a polyethylene resin substrate having a thickness of 1.5 mm. An electric heating mat (see FIG. 1) was formed.
As the design conditions of the heating mat, the linear expansion coefficient (α) of the heating wire is 1.5 × 10 ⁻⁵ / ° C., the outermost diameter (D) of the spirally wound heating wire is 84 mm, and the heating wire is The temperature rise (ΔT) from the heat generation to the start of buckling is 110 ° C., and the temperature (ΔT ′) obtained by subtracting the temperature rise (ΔT) from the temperature at which the resin at the joint is melted by the heat generated by the heating wire is 20.
Assuming that the temperature was 0 ° C., the maximum deflection (H) of the heating wire was calculated using the above equation (1), and as a result, it was 2.06 mm. The branch saddle joint was formed using an electric heating mat in which the distance (W) between the heating wires was set to 3.0 mm with respect to the maximum deflection amount of 2.06 mm. This branch saddle joint
During the fusion work with the polyethylene tube, the heating wire did not short-circuit and poor fusion did not occur. On the other hand, when the interval (W) was set to 1.5 mm, defective fusion occurred at some branch saddle joints, which could be caused by a short circuit of the heating wire. As described above, according to the electric heating mat of the present invention, it is possible to prevent the adjacent electric heating wires from being short-circuited even if a kink occurs, and to provide an optimum heating wire according to the wire diameter. The heating wire can be wound at intervals. Further, the use length of the heating wire can be optimized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view showing an example of an electric heating mat. FIG. 2 is a partial cross-sectional side view showing an example of an electro-fusion joint formed using an electric heating mat. FIG. 3 is an explanatory diagram showing a state of occurrence of kink in a heating wire. [Description of Signs] 11 ... Electrothermal mat, 12 ... Substrate, 13 ... Through hole, 14 ...
Heating wire, 15 ... saddle part, 16 ... electric fusion joint, 17 ...
Kink, D: outer diameter of spiral of heating wire, H: maximum deflection,
h: Deflection amount, Lk, Lk '... Buckling length, Ls, Ls'
… Length of buckling part, W… Space of heating wire

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Claims (1)

Claims: 1. An electric heating mat for forming an electric fusion joint in which an electric heating wire is spirally wound around a through hole provided in a central portion of a synthetic resin sheet. The expansion rate is α,
The diameter of the heating wire is d, the diameter of the outermost peripheral portion of the spirally wound heating wire is D, the temperature rise until the heating wire starts to buckle is ΔT, and the resin at the joining portion is heated by the heating wire. The temperature obtained by subtracting the above ΔT from the melting temperature is defined as ΔT ′, and the following equation (1) is used. (1) The distance W between the heating wires at the outermost periphery is set to the range shown by the following expression (2) 2H>W> H (2) with respect to the value H obtained by (1). Characteristic electric heating mat.