JPH0237760B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a molding method for obtaining a cone-shaped diaphragm from a thermoplastic resin or a foam thereof.

Conventionally, thermoplastic resins such as polyethylene,
In order to mold a cone-shaped diaphragm from polypropylene, polystyrene, etc. or foamed materials thereof, the thermoplastic resin or foamed materials thereof are molded into a film shape, and then molded into a diaphragm shape using vacuum, compressed air, or mold press. The method is capable of mass production,
It is widely used because the price of the molding equipment is relatively low.

This will be explained in more detail in the case where polypropylene is used as the thermoplastic resin and vacuum forming is used as the molding method.

In FIG. 1-a, a polypropylene film 1 is uniformly preheated by a heater 2 in a clamped state so that sufficient elongation can be obtained during vacuum forming.

The preheating temperature is raised to a rubbery temperature range approximately 50° C. above the glass transition point.

Next, as shown in Fig. 1-b, when the air inside the mold 3 is exhausted by a vacuum pump (not shown), the softened polypropylene film 1 is pressed by atmospheric pressure and tightly adheres to the surface of the cone-shaped concave mold 3. do.

Then, as shown in FIG. 1-c, the polypropylene film 1 formed into the shape of a diaphragm is peeled from the mold by blowing air into the mold 3, and unnecessary portions are cut off to obtain a diaphragm.

The thickness of the diaphragm obtained in this way is the thinnest at the bottom part of the mold 3, that is, the part corresponding to the top of the diaphragm (Fig. -c The closer it gets to Figure B), the thicker it tends to be.

This is because the amount of displacement in this part is large during molding, and as a result, the elongation is the largest.

However, in a diaphragm for a speaker, it is known that the stiffness is greatest at the top of the diaphragm and decreases toward the outer periphery, which is important in providing a speaker with high efficiency and excellent frequency characteristics.

That is, in order to transmit the vibration of the voice coil to the diaphragm without loss, it is desirable that the rigidity of the top part, which is the joint part with the coil bobbin, be large, and also to match with the edge with high compliance for the purpose of short-circuiting the vibration energy. It is desirable for the rigidity to be smaller (higher compliance) toward the outer periphery.

Therefore, the diaphragm obtained by the above-mentioned molding method has a lower rigidity (smaller thickness) at the top, and a lower rigidity at the outer periphery, which is contrary to the above-mentioned requirements.

Furthermore, a diaphragm made of a foam film molded using the above-mentioned molding method not only exhibits the above-mentioned tendency in thickness change, but also has internal loss caused by the pores as a result of the pores being crushed the most in the top portion. In a speaker incorporating such a diaphragm, resonance occurs in the high-frequency reproduction range where the root of the diaphragm mainly vibrates, and this causes harmful peaks and dips in the frequency characteristics.

Therefore, in the present invention, when the thermoplastic resin or foamed film thereof is subjected to vacuum, pressure air or mold press molding, the film is heated in the preheating step so that the temperature is lower in the middle part of the film, and This is a molding method that obtains a diaphragm that is thicker toward the top by suppressing elongation, and a case in which a polypropylene film is used as the thermoplastic resin film and vacuum forming is used as the molding method will be described in detail below.

In FIG. 2, 1 is a clamped polypropylene film, 2 is a heater, and 3 is a mold, which is the same as the conventional one.

Reference numeral 21 denotes a heat shielding plate disposed between the heater 2 and the clamped polypropylene film at the center of the polypropylene film, that is, at the portion that is to become the top portion of the diaphragm.

This heat shielding plate 21 shields the heat radiated from the heater 2 to some extent, so that the shielded central portion of the film is heated at a lower temperature than the outer peripheral portion.

After such preheating, when the conventional molding process (Fig. 1-b) is performed, the center part of the film 1, which corresponds to the root part of the diaphragm, has a lower elongation than the outer peripheral part due to the lower temperature. It's small.

Therefore, the diaphragm obtained by molding in this way is thick at the top part because it has little stretch.
The outer periphery is thin because it stretches a lot.

The foamed film also has a similar thickness distribution, and the collapse rate of the pores in the top portion is much smaller than in conventional molding methods, so the reduction in internal loss due to the presence of pores is extremely small.

FIG. 3 shows another shielding plate 22 used in this molding method, in which small holes 22a, 22b, 22c are formed in the plate 22 so that the shielding ratio decreases toward the outer periphery.
According to this, film 1
Since the temperature gradient becomes smoother, it is possible to obtain a diaphragm whose thickness decreases smoothly from the top portion toward the outer portion.

The shielding plates 21 and 22 have the advantage that the purpose can be achieved by simply adding them without any modification to the conventional device, but the purpose can also be achieved by changing the heating temperature distribution of the heater 2.

Although the above embodiment has been described with respect to polypropylene film, it is obvious that it can be applied to other thermoplastic resins or foamed films of these resins, and the forming method is not limited to vacuum forming, but also pressure forming using pressure higher than atmospheric pressure. , it can be similarly adopted in mold press molding in which pressing is performed using a mold.

As explained above, when a thermoplastic resin or a foamed resin film thereof is vacuum-formed, pressure-formed, or mold press-molded, when the film is formed on a diaphragm, the film corresponds to the area near the base of the diaphragm. After obtaining a preheating step in which the temperature of the outer circumferential portion is higher than that of the outer circumferential portion, vacuum, pressure or mold press molding is performed to form the portion corresponding to the top portion compared to the outer circumferential portion. This makes it possible to suppress the elongation of the diaphragm, thereby making it possible to obtain a diaphragm that is thicker toward the top and thinner toward the outer periphery.

Furthermore, in the case of a foam film, the proportion of pores in the root portion of the diaphragm being crushed is significantly reduced, making it possible to provide a diaphragm having a root portion with high rigidity and large internal loss.

Furthermore, it is an extremely excellent invention in practical terms, as it can be achieved with no modification or slight improvement of conventional devices.

[Brief explanation of drawings]

1-a, 1-b, and 1-c are cross-sectional views showing a conventional vacuum forming method, FIG. 2 is a cross-sectional view of an apparatus showing a forming method of the present invention, and FIG. FIG. 3 is a perspective view of an embodiment shielding plate.

Claims (1)

[Claims] 1. A thermoplastic resin or a foamed film thereof is preheated to a higher temperature at the outer circumferential portion of the film than at a portion corresponding to the top of the cone-shaped diaphragm, and then subjected to vacuum, compressed air, or mold press molding. A method for forming a cone-shaped diaphragm.