JP2849349B2 - Electrofusion joint, method of manufacturing the same, and mold for injection molding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric fusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint body for fusion splicing a thermoplastic resin tube, and the electric fusion joint. More particularly, the present invention relates to a method for producing an injection-molded metal mold having a large-diameter and large-diameter electro-fusion joint, which has no internal defects and can be manufactured efficiently.

[0002]

2. Description of the Related Art Conventionally, as a pipe joint for electrically welding and connecting a thermoplastic resin, for example, a polyethylene pipe, an electrofusion-type plastic pipe joint disclosed in Japanese Patent Application Laid-Open No. 57-69010 is known. is there. As shown in a partial cross-sectional view of FIG. 8, a groove 34 is provided from one end to the other end, and an inner cylindrical member 31 having a heating wire 33 wound around the groove 34 and an outer surface of the inner cylindrical member 31 are provided. This is an electrofusion-type plastic pipe joint provided by an outer member 32 integrally formed of the same resin. This pipe joint is connected to a resin pipe inserted into the pipe joint by being melted by energizing a heating wire 33 embedded in the inner peripheral portion.

[0003] The plastic pipe joint is obtained by injection molding an outer member 32 on the outer surface of an inner cylindrical member 31 around which a heating wire 33 is wound, and integrating the inner cylindrical member 31 and the outer member 32. Since the heating wire 33 heats the boundary between the connecting resin pipe inserted into the joint and the inner peripheral surface of the joint and melts both, it is necessary to provide the heating wire as close as possible to the inner peripheral surface of the joint. For this reason, the thickness t of the bottom of the groove 34 for accommodating the heating wire 33 of the inner cylinder member 31
A thin material having a thickness of 1 mm or less is used. On the other hand, as the nominal diameter of the pipe joint increases, the thickness of the pipe joint itself also increases, so the thickness T of the outer member 32 also increases. As an example, FIG. 8 shows the thickness of the inner cylindrical member and the outer member of each of the nominal diameters of the two-layer pipe joint for connecting a polyethylene gas pipe.
Shown in

[0004]

When the outer member 32 having a large thickness is injection-molded on the outer surface of the above-mentioned conventional thin inner tube member 31, the inner tube member 31 is formed by the large injection pressure and heat capacity of the outer member 32. Is melted, the heating wire 33 wound around the groove 34 of the inner cylinder member 31 moves irregularly, and the heating wire 33 comes into contact with each other at the time of connection with the pipe, causing a short circuit, thereby preventing normal heating from being performed. There are issues that arise. In general, the thermal conductivity of a synthetic resin is very poor, and the volume shrinkage from melting to solidification is considerably large. This shrinkage may reach 4% to 10% depending on the material of the resin. For this reason, depending on the injection molding method of the mold, after the resin of the gate and runner has solidified, if there is an unsolidified resin part in a part of the molded product in the cavity, the resin injection pressure is further applied. However, the molten resin cannot be supplied to the unsolidified resin portion in the molded product, and sinks occur inside the resin layer due to volume shrinkage during solidification.

In order to prevent sink marks, the gate and the runner may be made thicker, the injection pressure of the resin may be increased, and the injection pressure may be applied for a long time. In such a case, the thin inner cylindrical member 31 may melt. As a result, there is a problem that the arrangement of the heating wire 33 is moved or the time until the thick molten resin in the thick T portion is completely solidified is long, so that the molding tact becomes long and the efficiency is remarkably deteriorated. As a result, injection molding of thick resin into a thin inner cylinder member is difficult to control in manufacturing,
Quality is not stable. This problem occurs remarkably for a joint having a nominal thickness of 75A or more where the maximum thickness T of the outer member 32 is 10 mm or more. The present invention solves the above-mentioned problems, and does not cause defects such as sink marks and the like, and has excellent quality in an electro-fusion joint, a method for manufacturing the electro-fusion joint which can efficiently manufacture the electro-fusion joint, and injection of the same. This is to provide a molding die.

[0006]

The gist of the present invention is to provide a thin circle.
It has a tube socket for receiving the connecting resin tube on the inner surface and has an inner surface.
Inner tubular member made of thermoplastic resin provided with a groove for accommodating a heating wire
Layer, a heating wire wound in a groove of the inner cylinder member layer,
On the outer surface of the inner cylinder member layer around which the heating wire is wound, the inner cylinder member layer and
An intermediate member layer provided by injecting the same thermoplastic molten resin,
On the outer surface of the intermediate member layer, the same thermoplastic as the intermediate member layer
An outer cylindrical member having an outer shape of a joint provided by injecting molten resin
Layer, and connected to both ends of the heating wire on the outer surface of the outer cylindrical member layer.
The inner cylinder part
For joints with a thickness of 10 mm or more, excluding the material layer
About three to five layers including the inner cylinder member layer.
The intermediate member layer provided with a resin layer and excluding the inner cylinder member layer
The thickness per outer cylinder member layer was set to 15 mm or less.
It is an electrofusion joint characterized by the above-mentioned.

A method for obtaining an electric fusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint main body, the method comprising :
Heat friendly forming the inner peripheral surface of the joint body formed by winding the heating wire in the groove
An inner cylinder member made of a plastic resin is provided, the inner cylinder member is mounted in a cavity of a molding die, and a thermoplastic resin is injection- molded on an outer surface of the inner cylinder member to form an intermediate member integrated with the inner cylinder member. The intermediate member molded below is installed in the cavity of the molding die, and a thermoplastic resin is injected onto the outer surface of the intermediate member.
An outer cylindrical part that forms and forms the outer shape of the joint integral with the intermediate member
A method for manufacturing an electro-fusion joint, comprising providing a material and obtaining an electro-fusion joint having a final target shape.

The method of manufacturing the electro-fusion joint includes one or more sets of the molding dies, a step of obtaining an inner cylindrical member and a step of obtaining an intermediate member in each of the cavities of the mold. The step of obtaining the molded member is performed in synchronization with the step of obtaining an electro-fusion joint having a final target shape.

The present invention also provides an injection molding die for obtaining an electrofusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint main body, wherein a plurality of cavities capable of injection molding a molten resin using the same mandrel. has a section, the cavity portion has a Ruki Yabiti portion to form a Uchitsutsu member provided with grooves for accommodating the heating wire forms an inner peripheral surface of the joint, slightly larger the inner than the cavity portion A cavity portion for forming an intermediate member by injection molding a thermoplastic resin on the outer surface of the cylindrical member,
And injection molding a thermoplastic resin layer on the molded outer surface
And a cavity portion for forming the outer cylindrical member, each key
An injection-molding mold for an electro-fusion joint, wherein the thickness of one layer of the cavity portion is 15 mm or less.

[0010]

According to the present invention, the cylindrical member layer of the entire joint is formed by injection molding the conventional thick outer member layer on the outer surface of the thin inner cylindrical member layer in a plurality of times. This is an electrofusion joint having a multilayer structure of three to five layers including the inner cylinder member layer. Especially effective for large-diameter joints with a nominal diameter of 75A or more , excluding the inner cylinder member layer around which the heating wire is wound.
Is a joint having a size of 10 mm or more, the thickness of the cylindrical portion of the entire joint is made into three to five multilayer resin layers including the inner cylinder member layer, and the intermediate member layer excluding the inner cylinder member layer
By reducing the thickness per outer cylinder member layer to 15 mm or less, it is possible to improve quality and significantly reduce molding time.

[0011] To this end, it is common to form the inner peripheral surface of the joint.
An inner cylinder member containing a heating wire is provided in an outer circumferential groove, and this is mounted in a cavity of a molding die, and a thermoplastic molten resin is injection-molded on an outer surface of the inner cylinder member with a heating wire. And an intermediate member integrated with it, and resin is sequentially injection-molded on this outer surface
And finally provide an outer cylinder member that forms the outer shape of the joint,
An object of the present invention is to obtain an electrofusion joint in which the entire resin layer of the joint has a multilayer structure of three to five layers.

The number of the three to five layers can be set to an optimum number in consideration of the shape and thickness of the electrofusion joint, the time required for forming work of each layer, and the like. If there are three or more layers, the thick outer member layer of the conventional joint having an outer member layer having a thickness exceeding 10 mm will be divided into a plurality of layers, and the difference in thickness from the inner cylindrical member will be reduced. In addition, since the thickness and resin capacity per layer are reduced, the injection pressure can be reduced. Therefore, there is a problem that the thin inner cylinder member is melted by the resin capacity of the outer member at the time of injection molding , and a heating wire attached to the inner cylinder member may melt the inner cylinder member.
There is no problem such as moving to a short circuit caused by melting, 1 sweeping
Since the thickness of the shrinkage is reduced, the cooling rate after injection is high, and the cooling time is short, so that internal defects such as sink marks do not occur. In this case, since the injection molding operation of each layer can be performed in synchronization, the longest molding time per layer becomes the entire molding tact, and the thick portion of the joint having the thick outer member layer of 10 mm or more is required. By dividing into multiple layers, the thickness per layer is 15m
m or less, the resin capacity is reduced, and the cooling speed is increased due to the reduced wall thickness, and the cooling time is greatly reduced. For this reason, the tact of forming the whole electrofusion joint is greatly reduced.

Generally, the cooling time of the molten resin in the mold is
Although it depends on the temperature at which the molded product is taken out of the mold, it is theoretically proportional to the square of the thickness of the molded product. Therefore, if the thickness per layer in one cavity is halved, the cooling time is reduced to １ ／. Further, since the capacity of the injected molten resin per one layer in one cavity portion is greatly reduced, the cooling rate of the injected molten resin is increased, and the injection-molded resin portions are uniformly cooled. For this reason, the cooling time is significantly reduced, and no defects such as sink marks are generated inside.

Therefore, it is no longer necessary to continuously apply a large injection pressure or a long injection pressure in order to prevent sink marks, and even if the resin molding machine has a smaller injection capacity and a smaller injection pressure than the conventional resin molding machine, a large-diameter electric machine can be used. A fusion joint can be formed. In general, when the injection resin capacity of one layer per one cavity part is large, the injection molding conditions such as the resin temperature during molding and the temperature of the mold greatly affect the quality after molding. By doing so, the thickness per layer becomes 15 mm or less, and the injection resin capacity is small and the thickness becomes uniform. Therefore, the cooling time is shortened, the molding operation is not much affected by the injection molding conditions, the molding operation is easy, and a good quality electrofusion joint without internal defects can be obtained.

[0015]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 to FIG. 3 are cross-sectional views of an injection-molding mold for electric fusion joints and an electric fusion joint showing an embodiment of the present invention. FIGS. 4A and 4B are views of a molding state for explaining an embodiment of the present invention in comparison with the prior art. FIG. 4A shows a molding state of a conventional two-layer structure, and FIG. It is a figure showing a molding state. In FIG. 1, three cavities A, B, and C for forming a molten resin in a set of molds are provided.
Each cavity portion is sequentially larger in the radial direction. In addition, a common mandrel 10 is attached to each cavity portion so that molding can be performed. The molten resin injected from the mold inlet 11 passes through the runner 12 and the gate 13 and is filled in the respective cavities A, B, and C.

When the mandrel 10 is mounted on the cavity portion A and the molten resin is injected, FIG. 2, FIG. 3 or FIG.
The thin inner cylindrical member 21 of the electric fusion joint shown by the symbol is formed.
After the resin injected into the cavity A is solidified, the inner cylindrical member 21 with the mandrel 10 is taken out of the mold, and the heating wire 22 is inserted into the outer peripheral groove 23 of the thin inner cylindrical member 21 with the mandrel 10 attached. Is wound. The heating wire 22 is fixed by inserting connector pins 24 into the end of the inner cylindrical member 21, and is connected to the connector pin 24.

The inner cylindrical member 21 with the heating wire 22 forming the inner surface of the joint thus formed is mounted on the cavities of B with the mandrel 10 attached thereto. The inner cylindrical member 21 is formed by injection molding the molten resin.
To obtain the intermediate member 27 attached. While the intermediate member layer 25 is being injection-molded, the above-described thin inner cylinder member 21 is formed in the cavity portion A. After injection molding of the intermediate member 27 with the inner cylinder member in the cavity portion B, the intermediate member 27 molded with the mandrel 10 is taken out of the mold in the same manner as described above. The resin of the final outer cylinder member layer 26 is injection-molded on the outer surface of the intermediate member 27 by being mounted on the portion. After the final outer cylinder member layer 26 is formed on the outer surface of the intermediate member layer 25, the molded product is taken out of the mold with the mandrel 10, and the mandrel 10 is removed from the molded product as shown in FIG. 2, FIG. 3 or FIG. ) Is formed.

The inner cylinder member layer 21 thus formed into three layers
And each resin layer of the intermediate member layer 25 and the final outer cylinder member layer 26,
The contact surfaces of the respective layers are thermally fused by the heat of the molten resin injected onto the outer surfaces of the respective layers, and the electric fusion joints 28 are integrally joined.
Is molded. Note that the inner cylinder member layer 21 and the intermediate member layer 25 can be provided by a pure polyethylene resin material, and the final outer cylinder member layer 26 can be provided by a resin material containing carbon black. While this final outer member layer 26 is being injection-molded in the cavity portion C, the intermediate member layer 25 is formed in synchronization with the thinner inner cylindrical member 21 and the cavity portion B in the another cavity portion A. I have. Therefore, as the entire injection molding tact, the longest molding time of any of the cavities A, B, and C is the entire tact, and if the injection molding time of each layer is shortened, the entire molding tact is greatly reduced. .

FIG. 5 shows a socket type electrofusion joint (outer diameter 207 mm, length 248 mm) having a nominal size of 150 A in FIG.
The cooling time in the mold between the outer member 32 having a thickness of 22 mm and the final outer cylinder member layer 26 having a thickness of 11 mm of the present embodiment shown in FIG. FIG. 4 is a cooling diagram showing the relationship between the temperature and the cooling temperature. The cooling line A in the figure represents the cooling line of the conventional outer member 32 having a thickness of 22 mm,
B represents a cooling line of the final outer cylinder member 26 of this embodiment having a thickness of 11 mm. As can be seen from this cooling diagram, for example, when the take-out temperature from the mold is 95 ° C., the time from injection of the molten resin to cooling to 95 ° C. in the mold in the conventional method of (a) is 450 seconds, In this embodiment, the time required for cooling to 95 ° C. is 150 seconds. Therefore, if the temperature at which the mold is taken out from the mold is set to 95 ° C., the molding tact can be simply reduced to で は in the present embodiment of FIG.

FIG. 6 is a temperature diagram taken out from the mold obtained by examining the relationship between the thickness of the joint and the cooling temperature as described above.
It is a figure explaining the relationship between resin molding wall thickness and injection molding time (cooling time). In the case of a socket-type electrofusion joint having a nominal diameter of 150A, in the conventional method shown in FIG. 4A, the thickness t (the minimum thickness portion) at the bottom of the groove of the inner cylinder member layer 31 as shown in FIG. Is 0.6 mm, and the maximum thickness at the peak of the groove is 5.
5 mm. The maximum thickness T of the outer member layer 32 formed on the outer surface of the inner cylinder member layer 31 is 22 mm. The cooling time for injection molding of the inner cylinder member layer 31 was 90 seconds, and the cooling time for injection molding of the outer member layer 32 was 360 seconds. At this time, the removal temperature from the mold is 135 ° C. The state of molding of the outer member layer 32 is plotted at point E in FIG. In the present embodiment shown in FIG. 4 (b), the inner cylinder member layer 21 has the same dimensions as the conventional inner cylinder member 31, and the outer member layer 32 of the conventional method has the same thickness as the intermediate member layer 25.
And the final outer cylinder member layer 26, each having a thickness of 11 mm, which is half of the conventional thickness. As a result, the intermediate member layer 2
Injection molding time of each of No. 5 and the final outer cylinder member layer 26 is 100
Seconds, and the temperature of removal from the mold at this time is 130 ° C.
Met. Further, an electro-fusion joint having no defects such as sink marks was obtained. This state is plotted at point F in FIG.

In the case of a socket type electric fusion joint (outer diameter 278 mm, length 321 mm) having a nominal diameter of 200 A,
In the conventional method shown in FIG. 4A, the minimum thickness t of the inner cylinder member layer 31 is 0.8 mm, and the maximum thickness is 5.5 mm. The thickness T of the outer member layer 32 injection-molded on the outer surface of the inner cylinder member layer 31 is 28 mm. The injection molding time of the inner cylinder member layer 31 was 100 seconds, and the injection molding time of the outer member layer 32 was 900 seconds as indicated by the point J in FIG. At this time, the removal temperature from the mold is 120 ° C. On the other hand, FIG.
In this embodiment shown in (b), the inner cylinder member layer 2 is formed in the same manner as described above.
1 is the same size as the conventional inner cylinder member layer 31 and the thickness T of the conventional outer member layer 32 is set to the intermediate member layer 25 and the final outer cylinder member layer 2.
6 divided into two layers, each having a thickness of 14 mm, half of the conventional thickness.
And As a result, as shown by a point K in FIG. 6, the injection molding time of each of the intermediate member layer 25 and the final outer cylinder member layer 26 is one.
It was greatly reduced to 50 seconds. At this time, the removal temperature from the mold is 115 ° C., which is lower than the conventional method. In addition, an electro-fusion joint having no defects on the inside and outside of sink marks was obtained.

Next, a description will be given of an embodiment in which the thickness of the joint is divided into four layers in order to further shorten the molding time for the socket-type electrofusion joint having a nominal diameter of 200 A. FIG. 7 is a cross-sectional view of each layer of a joint in which a joint having a nominal diameter of 200 A is formed into four layers. The first layer 41 has a heating wire 22 wound in a groove like the inner cylinder member layer 21 of the above embodiment. On the outer surface, as described above, the second layer 42, the third layer 4
The third and fourth layers 44 are sequentially injection-molded to obtain an integrated electrofusion joint 45. Also in this case, the thickness of each of the second to fourth layers is 1 mm of the conventional outer member layer 32 having a thickness of 28 mm.
/ 3, about 9.5 mm.

In this embodiment, irregularities are also formed on the outer surfaces of the second layer 42 and the third layer 43 to increase the area in contact with the inner surface of the mold cavity at the time of injection molding in the cavity to be injected. The cooling time is shortened by increasing the cooling efficiency of the resin. Further, the contact area between the injected resin layers is increased to increase the melt adhesion strength for integrally melting. The time taken out of the mold after injection molding in this example was 115 seconds for the cooling time of the second layer 42 having the longest cooling time, and the temperature at this time was 100 ° C. Therefore, in the case of the four-layer molding of this embodiment, the molding time was significantly reduced from 900 seconds to 115 seconds, as compared with the conventional molding time of 900 seconds, and the temperature for taking out from the mold was also lowered. In addition, compared to the case of the embodiment having the above three layers, 15
The time was reduced from 0 to 115 seconds for 35 seconds.

In the above embodiment, the thickness of the joint is three layers or four layers.
Although the embodiment of forming into a layer has been described, the joint may be formed into five layers depending on the shape and size of the joint and the specifications of the forming machine. The plurality of layers may be formed of the same resin material, or may be formed of resins having different resin materials and colors.
Any number of layers is possible, but in practice, three to five layers are most suitable in consideration of the shape of the joint and the processing time for forming each layer. Also, as in the electrofusion joint shown in FIGS. 3 and 7, the joint may be divided into three or five layers in the longitudinal direction of the joint as well as in the radial direction. Also, a mold having a plurality of cavity portions A, B, and C provided in one mold has been described, but the cavity portions A for injection molding each layer are described.
It goes without saying that multi-stage molding may be performed sequentially using a plurality of sets of dies each having B and C individually.

[0025]

According to the present invention, the thickness of the resin injected into the joint body can be made sufficiently small, and the volume of the molten resin injected at one time can be reduced. The cooling speed of the mold becomes faster, the molding tact time is greatly reduced, and the molding work efficiency is improved. In addition, since each part is uniformly cooled, internal defects such as conventional sink marks do not occur.
For this reason, an electrofusion joint having excellent quality can be efficiently obtained.

[Brief description of the drawings]

FIG. 1 is a plan view of an injection-molding mold for an electro-fusion joint according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of an electric fusion joint according to one embodiment of the present invention.

FIG. 3 is a sectional view of an electrofusion joint according to another embodiment of the present invention.

FIG. 4 is a view showing a molded product in each step for comparing and explaining a conventional method (a) and a method (b) of this embodiment.

FIG. 5 is a diagram illustrating a relationship between a cooling time and a cooling temperature in a mold according to a conventional method and a mold according to the present embodiment.

FIG. 6 is a diagram illustrating a relationship between a resin thickness and an injection molding time (cooling time) with respect to a removal time from a mold, which is described in comparison between an example of the present invention and a conventional technique.

FIG. 7 is a cross-sectional view of a four-layer molded electrofusion joint according to an embodiment of the present invention.

FIG. 8 is a partial cross-sectional view of a conventional electrofusion joint and a diagram showing thicknesses T and t dimensions of respective layers.

[Explanation of symbols]

A, B, C Each cavity portion in the mold 10 Mandrel 11 Inlet 12 Runner 13 Gate 21 Inner cylinder member layer 22 Heating wire 23 Groove 24 Connector pin 25 Intermediate member layer 26 Final outer cylinder member layer 27 Intermediate with inner cylinder member Member 28, 45 Electrofusion joint after molding 41 First layer 42 Second layer 43 Third layer 44 Fourth layer

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI B29L 31:24

Claims (4)

(57) [Claims] 1. A thin cylindrical shape which receives a connecting resin tube on the inner surface.
A heat sink with a tube receiving port and a groove on the outer surface to accommodate the heating wire
An inner cylinder member layer made of a plastic resin, a heating wire wound in a groove of the inner cylinder member layer, and an inner cylinder member on an outer surface of the inner cylinder member layer wound with the heating wire.
Intermediate member layer provided by injecting the same thermoplastic molten resin as the layer
And the same thermoplastic as the intermediate member layer on the outer surface of the intermediate member layer
An outer cylindrical member having an outer shape of a joint provided by injecting molten resin
Layer and both ends of the heating wire and project from the outer surface of the outer cylinder member layer.
The joint thickness excluding the inner cylinder member layer is 10 mm or more.
For the joint of the above, three or more layers including the inner cylinder member layer
Provided with five multilayer resin layers and excluding the inner cylinder member layer
The thickness per layer of the intermediate member layer and the outer cylinder member layer is 15 mm or less.
An electrofusion joint provided below. 2. A method for obtaining an electric fusion joint in which a heating wire is buried in an inner peripheral portion of a thermoplastic resin joint body, wherein an inner peripheral surface of the joint body in which a heating wire is wound around a groove on an outer peripheral surface is provided. Providing an inner cylinder member made of thermoplastic resin to be formed, mounting this inner cylinder member in the cavity of the molding die,
A thermoplastic resin is injection- molded on the outer surface of the inner cylinder member to provide an intermediate member integral with the inner cylinder member. Thereafter, the molded intermediate member is mounted in a cavity of a molding die, and heat is applied to the outer surface of the intermediate member. Manufacturing an electro-fusion joint characterized by providing an outer cylinder member that forms an outer shape of a joint integral with an intermediate member by injection molding a plastic resin to obtain an electro-fusion joint of a final target shape Method. 3. The molding die has one or more sets,
Performing the steps of obtaining the inner cylindrical member, obtaining the intermediate member layer, and obtaining the outer cylindrical member layer in the respective cavity portions of the mold in synchronization with each other to obtain an electro-fusion joint having a final target shape. The method for producing an electro-fusion joint according to claim 2, characterized in that: 4. An injection molding die for obtaining an electrofusion joint in which a heating wire is embedded in an inner peripheral portion of a thermoplastic resin joint body, wherein a plurality of cavities capable of injection molding a molten resin using the same mandrel. has a section, the cavity portion has a Ruki Yabiti portion to form a Uchitsutsu member provided with grooves for accommodating the heating wire forms an inner peripheral surface of the joint, slightly larger the inner than the cavity portion A cavity portion that forms an intermediate member by injection molding a thermoplastic resin on the outer surface of the cylindrical member, and a thermoplastic resin layer is formed by injection molding a thermoplastic resin layer on the molded outer surface.
A cavity portion to be formed, wherein each of the cavities is
An injection-molding mold for an electro-fusion joint, wherein the thickness of each portion of the portion is 15 mm or less.