JP2003039489A - Propeller fan and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost propeller fan with a high air blowing performance and a manufacturing method therefor. SOLUTION: In the propeller fan 1 molded by an injection molding process, a blade 3 is of a good blade surface shape free from a sink mark and as such, exhibits a high air blowing performance by setting an injection position for a molten resin on the blade surface 3b. In addition, when the propeller fan 1 is injection-molded, a gate 29 is set at a position corresponding to the blade surface 3b of the blade 3 of the propeller fan 1. Consequently, the molten resin injected into a cavity 27 from the gate 29 flows toward a region side corresponding to the tip part of the blade 3 inside the cavity 27 and is packed there. In this case, as the gate 29 is set at a position, of the cavity 27, corresponding to the blade surface 3b of the blade 3, the flowing distance of the molten resin is short and the flow resistance is minimized. Thus the blade 3 with a sink mark-free good blade surface 3b is formed and the propeller fan 1 with a high air blowing performance is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a propeller fan manufactured by injection molding of a resin material and a manufacturing method thereof.

[0002]

2. Description of the Related Art FIG. 5 shows a general injection molding machine 10 used for injection molding of a propeller fan. This injection molding machine 10 includes an injection unit 11 that melts a material resin and injects the resin, a mold 12 that molds the injection resin from the injection unit 11, and a mold that opens the mold 12 after molding. An opening / closing mechanism 13 and a projecting mechanism 14 for projecting a molded product after mold opening and releasing the molded product are provided.

Here, a molding procedure using the mold 12 will be briefly described with reference to FIGS. 6 to 8.

As shown in FIG. 6, the die 12 is a three-piece die including a fixed die 21, a movable die 22 and an intermediate die 23 located between them. Projecting pins 24, 24, ... Which are driven by the projecting mechanism 14 are arranged on the rear side of the mold 22. Then, with the fixed mold 21, the movable mold 22, and the intermediate mold 23 abutting against each other, the fixed mold is placed in a cavity (not shown) formed between the movable mold 22 and the intermediate mold 23. 21 and an intermediate mold 23 formed across the runner (not shown)
Through the injection of the molten resin from the injection unit 21 side, a molded product conforming to the inner surface shape of the cavity is formed.

FIG. 7 shows the above-mentioned molds 21 and 2 after molding.
2 and 23 are separated from each other and opened. Due to this mold opening, the molded product 16 molded in the cavity remains in a state of being adhered to the molding surface of the movable mold 22 and is solidified in the runner portion 1.
No. 7 remains attached to the fixed mold 21 side.

FIG. 8 shows a state in which the mold 12 is opened, and in this state, the projecting pins 24, 24, ... Are advanced and the molded product 16 is made by the projecting pins 24, 24 ,. Is removed from the movable die 22 side, and the runner portion 17 is removed from the fixed die 21 side.

The above-mentioned molded product 16 manufactured in this way
Is a propeller fan 1 which is a molding object, and this is shown in FIG. Further, an injection molding state of the propeller fan 1 in the mold 12 is shown in FIG.

As shown in FIG. 4, a cavity 27 for molding the propeller fan 1 is formed at an abutting portion between the movable die 22 and the intermediate die 23. The fixed mold 21 and the intermediate mold 23 have the injection unit 1
A runner 28 for supplying the molten resin injected from the first injection nozzle 11a to the cavity 27 is formed. In the cavity 27, in consideration of the releasability of the hub 2 having a bottomed cylindrical shape, the molding surface 25 on the moving die 22 side is configured to have the pressure surface 3a of the blade 3 and the intermediate die 23. The shape is set so that the side molding surface 26 constitutes the negative pressure surface 3b of the blade 3, respectively.

As shown in FIG. 3, the runner 28 has a three-pronged branch path shape corresponding to the number of blades 3 of the propeller fan 1.
The downstream end of each branch passage portion of 8 serves as a gate 29 that functions as an injection port of the molten resin into the cavity 27.

Accordingly, the molten resin is injected from the injection nozzle 11a at a predetermined injection pressure into the cavity 27 through the runner 28, and the cavity 27 is filled with the molten resin. The propeller fan 1 having a shape conforming to is molded.

In this case, a runner portion 17 having a shape matching the inner surface shape of the runner 28 is formed in the runner 28 by the molten resin supplied thereto, and the runner portion 17 is formed in each of the gates. 29, 29, ...
The propeller fan 1 and the runner portion 17 are connected integrally with the propeller fan 1 at a portion thereof, but when the mold 12 is opened, the gates 29, 29 ,.
-The propeller fan 1 has a unitary structure by being cut and separated at a portion. Therefore, in the propeller fan 1 after molding, cutting traces with the runner portion 17, that is, gate traces 18, 18, ... (See FIG. 3) are left on the surface thereof.

By the way, in general, the blade surface shape of a propeller fan has a great influence on its air blowing performance, so it is designed based on extremely careful calculation from the viewpoint of ensuring the air blowing performance. It is necessary to pay close attention to obtain a wing surface shape that matches the shape. Therefore, for example, when the gate 29 of the runner 28 is set at a position corresponding to the blade surface of the blade 3 of the cavity 27, the gate is formed on the blade surface of each blade 3, 3, ... Of the propeller fan 1 to be molded. Since there is a possibility that the traces 18 remain and the presence of the gate traces 18 makes the wing surface uneven and the shape thereof is impaired, which in turn lowers the air blowing performance, it is generally shown in FIGS. 3 and 4. , The gates 29, 29, ... on the runner 28 side with respect to the cavity 27 are connected to the blades 3
It is customary to set it on the end face wall 6 of the hub 2 and at a position corresponding to the peripheral wall 5, avoiding being provided on the side.

[0013]

However, as described above, the arrangement position of the gate 29 of the runner 28 with respect to the cavity 27 is set to the hub 2 of the propeller fan 1.
If it is set on the peripheral wall 5 side of the
Although it is possible to prevent the problem of the change of the blade surface shape due to the presence of the air conditioner, and consequently the deterioration of the blowing performance, a new problem as described below occurs.

That is, when the gate 29 of the runner 28 is set on the hub 2 side of the propeller fan 1, the peripheral wall 5 of the hub 2 is generally of a thin structure, and the peripheral wall 5 is integral with the peripheral wall 5. Since the blades 3 are provided in the above, the injection resin from the gate 29 flows through the portion corresponding to the peripheral wall 5 of the cavity 27 to the portion side corresponding to the blades 3 and is filled therein. Become. In this case, the passage of the molten resin is narrowed at the portion corresponding to the peripheral wall 5, the flow resistance of the molten resin flowing toward the portion corresponding to the blade 3 increases, and moreover, the portion corresponding to the peripheral wall 5 and the blade. 3 from the boundary with the part corresponding to 3 (that is, the starting point of the molten resin flowing into the blade 3 side)
The distance to the portion corresponding to the tip (that is, the end point of the molten resin flowing into the blade 3 side), that is, the flow distance L of the molten resin also becomes longer. As a result, the filling property of the molten resin into the portion corresponding to the blade 3 is impaired, "shrinkage" is likely to occur on the blade surface of the blade 3 due to the cooling shrinkage of the molten resin after the filling, and a change in the blade surface shape may occur. Cause

On the other hand, as one means for improving such a problem, the injection pressure applied to the gate 29 portion is maintained as it is even after the injection is completed, and the molten resin is solidified under the load of the injection pressure, whereby the blade is formed. "Close" of the surface
It is known to suppress the occurrence of However, as described above, in the conventional manufacturing method, since the flow distance L of the molten resin in the blade 3 portion is long, the blade 3 having the injection pressure is used.
The transmission efficiency is low toward the tip side of the, and therefore, the injection pressure is set to a higher pressure in order to compensate for the reduction in the transmission efficiency. However, the increase in the injection pressure is great for the propeller fan 1 as a molded product. There is a problem that residual stress is caused to cause deformation after release from the mold, or a mold structure having a high pressure resistance is required, resulting in an increase in cost.

Therefore, the present invention has been made for the purpose of proposing an inexpensive propeller fan having a high blowing performance and a method for manufacturing the same.

[0017]

In the present invention, the following constitution is adopted as a concrete means for solving such a problem.

In the first invention of the present application, in the propeller fan having the plurality of blades 3, 3, ... On the outer periphery of the hub 2, the blade surface 3b of the blade 3 is set as the injection position of the molten resin. It is characterized by being molded by injection molding.

The second invention of the present application is characterized in that, in the propeller fan according to the first invention, the injection position of the molten resin is set in the vicinity of the hub 2 on the blade surface 3b.

In the third invention of the present application, the above-mentioned first or second
In the propeller fan according to the invention of claim 1, the blade surface 3b is
Is the negative pressure surface 3b of the blade 3.

In the fourth invention of the present application, a propeller fan 1 having a plurality of blades 3, 3, ...
Is a cavity 27 and a runner 28 connected to the cavity 27 through a gate 29 and serving as a molten resin injection passage.
In the method of manufacturing a propeller fan, in which a molten resin is injected into the cavity 27 through the runner 28 to form the mold, the mold 12 including
The gate 29 is set at a position corresponding to the blade surface 3b of the blade 3 of the propeller fan 1.

According to a fifth invention of the present application, in the method for manufacturing a propeller fan according to the fourth invention, the gate 29 is set at a position corresponding to the vicinity of the hub 2 on the blade surface 3b of the blade 3. It is characterized by having done.

In the sixth invention of the present application, the above-mentioned fourth or fifth invention is provided.
In the method for manufacturing a propeller fan according to the invention, the blade surface 3b is the negative pressure surface 3b of the blade 3.

[0024]

According to the present invention, the following effects can be obtained by adopting such a configuration.

In the first invention of the present application, in the propeller fan including the plurality of blades 3, 3, ... On the outer circumference of the hub 2, the blade surface 3 b of the blade 3 is set as the injection position of the molten resin. It is molded by injection molding. Therefore, according to the propeller fan of the present invention, at the time of molding, the distance from the injection position of the molten resin to the tip of the blade 3, that is, the flow distance of the molten resin, is the same as the conventional one. Compared with the case where the injection position is set to the hub 2 side, the "shrink" of the molten resin is suppressed more effectively, and the blade surface shape of the blade 3 can be changed to the design shape by that much. It will match as much as possible and will exhibit higher air blowing performance.

According to the propeller fan of the second invention of the present application, in the propeller fan of the first invention, the injection position of the molten resin is set in the vicinity of the hub 2 on the blade surface 3b. The gate mark 18 remains in the vicinity of the hub 2 and the blade surface becomes uneven, which may be a cause of lowering the air blowing performance. However, due to the air blowing characteristics of the propeller fan 1, the air blowing work on the blade surface of the blade 3 is a blade. The hub 2 is larger on the side closer to the tip and smaller on the side closer to the hub.
Even if the blade surface in the vicinity of is roughened by the gate mark 18, the effect on the air blowing performance is extremely small. Therefore, when viewed as a whole of the propeller fan 1, the propeller fan 1 has the gate mark 18 on its blade surface. Despite this, it will exhibit high air blowing performance.

According to the propeller fan of the third invention of the present application, in the propeller fan of the first or second invention, since the blade surface 3b is the negative pressure surface 3b of the blade 3, the negative pressure surface Gate traces 18 are present on 3b, which causes the blade surface to become uneven, but since the suction surface 3b originally does not participate in air blow work, the gate traces 18 are present on the suction surface 3b. Even if the propeller fan 1 does exist, it has almost no effect on the blowing performance of the propeller fan as a whole, and therefore the propeller fan 1 exhibits high blowing performance.

According to the method of manufacturing a propeller fan according to the fourth aspect of the present application, the propeller fan 1 including a plurality of blades 3, 3, ... In a method for manufacturing a propeller fan, a mold 12 having a runner 28 which is connected to a gate 29 via a gate 29 and serves as an injection passage for the molten resin is used to inject and mold the molten resin through the runner 28 in the cavity 27. Since the gate 29 is set at the position corresponding to the blade surface 3b of the blade 3 of the propeller fan 1 in the cavity 27, the molten resin supplied through the runner 28 is the blade 3 of the propeller fan 1 in the cavity 27. Is injected into the cavity 27 from the gate 29 set at a position corresponding to the wing surface 3b of the It flows toward the tip portion, that is, the portion side corresponding to the tip portion of the blade 3, and is filled therein.
In this case, since the gate 29 is set at a position corresponding to the blade surface 3b of the blade 3 of the cavity 27, the gate 29 is more likely to be located than the hub 2 side, for example. The distance from the portion 29 to the portion corresponding to the tip of the blade 3, that is, the flow distance of the molten resin is short, and the molten resin is transferred to the hub 2 as in the case where the gate 29 is arranged on the hub 2 side. The flow resistance of the molten resin can also be suppressed to a small level because it does not undergo a throttling action in the course of flowing through the corresponding narrow passage to the portion corresponding to the blade 3.

As a result, the gate 2 is operated at a predetermined injection pressure.
When molten resin is injected from 9 into the cavity 27,
The molten resin smoothly flows in the cavity 27 toward the site corresponding to the tip of the blade 3 by the injection pressure and is sufficiently filled therein, and when the injection pressure is maintained after the filling, This injection pressure is the cavity 27
Is efficiently transmitted to the tip side of the molten resin, and the molten resin is solidified under the load of this injection pressure, so that the blade 3 having a good blade surface without "withdrawal" is formed, and, in turn, high blowing performance is achieved. It will be possible to provide the propeller fan 1 having it.

Further, the injection pressure itself can be set to a low pressure as much as the transferability of the injection pressure to the side of the cavity 27 corresponding to the blade 3 is good, and as a result, the propeller as a molded product. In the fan 1, there is little residual stress, deformation after releasing due to residual stress is suppressed as much as possible, and it is possible to provide a propeller fan with a high air blowing performance having a blade surface shape that matches the design shape as much as possible. At the same time, the strength performance of the mold 12 can be set lower due to the lower injection pressure, and the cost of the mold can be reduced.

In a fifth invention of the present application, in the method for manufacturing a propeller fan according to the fourth invention, the gate 29 is set at a position corresponding to the vicinity of the hub 2 on the blade surface 3 b of the blade 3. Therefore, in the molded propeller fan 1, the gate mark 1 is formed near the hub 2.
8 may be a cause of deterioration of the air blowing performance due to the unevenness of the blade surface, but due to the air blowing characteristics of the propeller fan 1, the blade 3
Since the air blow work on the blade surface of the blade is large on the blade tip side and small on the hub side, the effect of the unevenness of the blade surface due to the gate marks 18 on the air blowing performance is extremely small, and therefore, the blade surface is not affected by the gate. Despite the presence of the marks 18, the propeller fan 1 having high air blowing performance can be provided.

In the sixth invention of the present application, in the propeller fan manufacturing method according to the fourth or fifth invention, since the blade surface 3b is the negative pressure surface 3b of the blade 3, the negative pressure surface 3b is Although the gate mark 18 exists and the blade surface becomes uneven due to this, since the suction surface 3b originally does not participate in the blow work, even if the gate mark 18 exists on the suction surface 3b. Therefore, it is possible to provide the propeller fan 1 that exerts a high air blowing performance without being affected by the existence of the gate mark 18 at all, which hardly affects the air blowing performance of the entire propeller fan.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below based on preferred embodiments.

FIG. 1 shows a propeller fan 1 manufactured by the manufacturing method according to the present invention. This propeller fan 1 is a resin-integrally molded product, has a peripheral wall 5 and an end face wall 6, and a hub 2 having a substantially bottomed cylindrical shape in which a boss 4 is provided at the center of the end face wall 6, Three blades 3, 3, ... Integrally mounted on the peripheral wall 5 of the hub 2.
・ Composed of and. As shown in FIG. 2, the propeller fan 1 is a mold 1 including a fixed mold 21, a movable mold 22, and an intermediate mold 23 located between them.
It is manufactured by injection molding using an injection molding machine 10 (see FIG. 5) equipped with 2.

That is, as shown in FIG.
In the abutting part of the intermediate mold 23 and the intermediate mold 23, the above propeller fan 1
A cavity 27 for molding the is formed. Further, the fixed mold 21 and the intermediate mold 23 are formed with a runner 28 for supplying the molten resin injected from the injection nozzle 11a of the injection unit 11 (see FIG. 5) to the cavity 27. In addition, the cavity 27
The shape is set such that the molding surface 25 on the movable die 22 side constitutes the pressure surface 3a of the blade 3 of the propeller fan 1 and the molding surface 26 on the intermediate die 23 side constitutes the suction surface 3b. Has been done.

Further, as shown in FIG. 1, the runner 28 has a three-pronged branch path shape corresponding to the number of the blades 3 of the propeller fan 1.
The downstream end of each branch passage portion of 8 serves as a gate 29 that functions as an injection port of the molten resin into the cavity 27.

The manufacturing method of this embodiment has the greatest feature in the construction of the runner 28. That is,
In the conventional type, as shown in FIG.
The gates 29, 29, ... Located at the respective downstream ends of 7 are set at the portions corresponding to the peripheral wall 5 of the cavity 27, whereas in the present embodiment, the gates 29, 29 ,. .. are set on the negative pressure surface 3b of the blade 3 in the cavity 27 and in the vicinity of the hub 2.

Using the mold 12 thus constructed, the molten resin is injected from the injection nozzle 11a at a predetermined injection pressure through the runner 28 into the cavity 27 from the gates 29, 29 ,. Then, the cavity 27 is filled with the molten resin, and by solidifying the resin, the propeller fan 1 having a shape matching the inner surface shape of the cavity 27 is molded.

As described above, when the propeller fan 1 is injection-molded by using the mold 12, according to the manufacturing method of this embodiment, the gate 29 serving as the injection port of the molten resin into the cavity 27 is provided. Since the cavity 27 is set at a position corresponding to the blade surface 3b of the blade 3 of the propeller fan 1 and in the vicinity of the hub 2, the following peculiar operational effects can be obtained.

That is, the gate 29 serves as the suction surface 3 of the blade 3.
Since the gate 29 is set at a position corresponding to b, compared with the case where the gate 29 is set at a position corresponding to the peripheral wall 5 of the hub 2 as in the prior art, for example, the gate 29 portion to the tip of the blade 3 is compared. The distance to the portion corresponding to the portion, that is, the flow distance L of the molten resin is short. Further, molten resin flows from the gate 29 to the blade 3 of the cavity 27.
Since the molten resin is directly injected to the portion corresponding to, the molten resin moves to a portion corresponding to the vane 3 through a narrow passage corresponding to the hub 2 of the cavity 27 as in the conventional case, that is, the molten resin However, the flow resistance of the molten resin can be suppressed to such a small level that it does not undergo a throttling action during its flow. The effect of reducing the flow resistance of the molten resin is remarkable in a structure in which the flow resistance of the molten resin to the blade 3 side is high when the blade 3 is a thin blade, for example. As a result, when the molten resin is injected from the gate 29 into the cavity 27 with a predetermined injection pressure, the molten resin directs the inside of the cavity 27 toward the portion corresponding to the tip of the blade 3 by the injection pressure. It smoothly flows and is sufficiently filled there, so that the deterioration of the shape accuracy due to the defective filling of the molten resin is surely prevented.

After the molten resin is filled in the cavity 27, the injection pressure is maintained as it is, and the filled resin is solidified under the injection pressure, so that the molten resin is cooled and contracted. However, in this embodiment, as described above, the flow distance L from the gate 29 to the tip of the blade 3 is short, and the distance between them is short. With the straight path, the efficiency of transmitting the injection pressure applied to the gate 29 to the tip end side of the blades 3 is high, and the "closed" prevention of the blade surface due to the load of the injection pressure is more reliable. It is tightened.

As a synergistic effect of these, according to the manufacturing method of this embodiment, it is possible to provide the propeller fan 1 having a good blade surface free from "withdrawal" on the blade surface of the blade 3 and exhibiting high blowing performance. become. Such an effect is obtained, for example, as in the case where the blade 3 is a thick blade.
This is remarkable in a structure in which the amount of molten resin filled in the side is large.

Further, in this embodiment,
As described above, the injection pressure itself can be set to a low pressure as much as the transferability of the injection pressure to the side of the cavity 27 corresponding to the blade 3 is good. When the injection pressure is set to be low, the residual stress inside the propeller fan 1 as a molded product is small,
And that there is little deformation after mold release due to residual stress, and as a result, a propeller fan having a blade surface shape that matches the design shape as much as possible, that is, a propeller fan 1 having a high air blowing property. Can be provided. In addition, since the injection pressure of the molten resin can be set to a low pressure, the strength performance of the mold 12 can be set correspondingly lower, and the cost of the mold can be reduced.

On the other hand, in this embodiment, since the gate 29 is set at a position corresponding to the vicinity of the hub 2 on the blade surface 3b of the blade 3, the molded propeller fan 1 has the above-mentioned configuration. Gate mark 18 near the hub 2
However, there is a concern that this may cause the blade surface to become uneven, which may be one of the causes of lowering the air blowing performance. However, because of the air blowing characteristics of the propeller fan 1, the air blowing work on the blade surface of the blade 3 is large on the blade tip side and is small on the hub side. Has a very small effect on
The gate mark 18 is formed on the negative pressure surface 3b side of the blade 3, that is, on the surface side that hardly participates in the air blow work, and from these facts, the propeller fan 1 manufactured.
Even if the gate mark 18 is present on the blade surface, it is not affected by this and has high air blowing performance.

In the above embodiment, the propeller fan 1 provided with the blades 3 having a thin blade structure has been described as an example, but the present invention is not limited to such a configuration, and a blade having a thick blade structure is used. Of course, it can be widely applied to the propeller fan provided.

Further, particularly in the production of a propeller fan having a thick blade structure, a foaming agent may be added to the molten resin for foam molding. In this case, by setting the injection pressure to a low pressure as described above, post-deformation of the blades 3 caused by foaming after release from the mold is prevented as much as possible, and a high-performance propeller fan having a high blade surface shape accuracy. In addition to being able to obtain the above, it is possible to reduce the weight as an effect peculiar to foam molding.

[Brief description of drawings]

FIG. 1 is a perspective view of a propeller fan manufactured by a manufacturing method of the present invention.

FIG. 2 is a cross-sectional view showing an injection molding state of a propeller fan.

FIG. 3 is a perspective view of a propeller fan manufactured by a conventional manufacturing method.

FIG. 4 is a cross-sectional view showing an injection-molded state of a propeller fan according to a conventional manufacturing method.

FIG. 5 is an overall view of an injection molding machine.

FIG. 6 is an explanatory view of an injection molding state of a mold unit.

FIG. 7 is an explanatory diagram of a mold opening state of the mold unit.

FIG. 8 is an explanatory diagram of a mold release state of a molded product from a mold unit.

[Explanation of symbols]

1 is a propeller fan, 2 is a hub, 3 is a blade, 4 is a boss,
5 is a peripheral wall, 6 is an end face wall, 10 is an injection molding machine, 11 is an injection unit, 12 is a mold, 13 is a mold opening / closing mechanism, 14 is a projecting mechanism, 16 is a molded product, 17 is a runner portion, 18 is a gate mark. , 21 is a fixed mold, 22 is a movable mold, 23 is an intermediate mold, 2
4 is a protruding pin, 25 and 26 are molding surfaces, 27 is a cavity, 28 is a runner, and 29 is a gate.

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Claims (6)

[Claims] 1. A propeller fan comprising a plurality of blades (3), (3), ... On an outer circumference of a hub (2), the blade surface (3b) of the blade (3) being melted. A propeller fan characterized by being molded by injection molding at a resin injection position. 2. The propeller fan according to claim 1, wherein an injection position of the molten resin is set in the vicinity of the hub (2) of the blade surface (3b). 3. The propeller fan according to claim 1 or 2, wherein the blade surface (3b) is a suction surface (3b) of the blade (3). 4. A propeller fan (1) comprising a plurality of blades (3), (3), ... On an outer periphery of a hub (2), a cavity (27) and the cavity (27).
A mold (12) having a runner (28) which is connected to the gate via a gate (29) and serves as an injection passage for the molten resin is used, and the runner (28) is placed in the cavity (27).
A method of manufacturing a propeller fan, which comprises injecting a molten resin through a mold to form the propeller fan, wherein the gate (29) is provided at a position corresponding to the blade surface (3b) of the blade (3) of the propeller fan (1) in the cavity (27). ) Is set, a method for manufacturing a propeller fan. 5. The wing surface (3b) according to claim 4, wherein the gate (29) is the blade surface (3b) of the blade (3).
A method for manufacturing a propeller fan, wherein the propeller fan is set at a position corresponding to the vicinity of the above hub (2). 6. The method of manufacturing a propeller fan according to claim 4, wherein the blade surface (3b) is a negative pressure surface (3b) of the blade (3).