EP0944292A2

EP0944292A2 - Loadspeaker diaphram of an injection foam molded body

Info

Publication number: EP0944292A2
Application number: EP99105479A
Authority: EP
Inventors: Masatoshi c/oTOHOKU PIONEER ELECTRONIC CORP. Sato; Kunio c/oTOHOKU PIONEER ELECTRONIC CORP. Mitobe
Original assignee: Tohoku Pioneer Electronic Corp; Pioneer Electronic Corp
Current assignee: Tohoku Pioneer Corp; Pioneer Corp
Priority date: 1998-03-19
Filing date: 1999-03-17
Publication date: 1999-09-22
Also published as: EP0944292A3; JP3602327B2; JPH11275687A

Abstract

An inorganic or organic filler in an amount of 3 to 30 % by weight is added to a resin containing a foaming agent. The resulting resin mixture material is injection molded into a loudspeaker diaphragm (1) having a three-layer structure in which a foamed layer (3) is covered with unfoamed skin layers (2). For forming the three-layer structure, a resin mixture material of polypropylene containing a foaming agent is injected into a cavity of a mold (20). Immediately after the injection and the completion of forming the skin layers (2), the mold (20) is separated so that the material may start foaming at its inside to form the foamed layer (3). Since the surfaces of the resin mixture material are in contact with the inner surfaces of the mold in the injection, the surfaces of the resin mixture material have solidified before the foaming takes place, so as to form the unfoamed skin layers (2). By adjusting the filler to the appropriate content, the loudspeaker diaphragm (1) is improved in appearance quality while keeping the other physical properties with favourable characteristics.

Description

The invention relates to a loudspeaker diaphragm of an injection foam molded body.
Materials for loudspeaker diaphragms generally require small density, rigidity, appropriate internal loss, and environmental resistance.
Of conventional materials for loudspeaker diaphragms, polypropylene (PP) is excellent in environmental resistance (particularly, in water resistance) and large in internal loss. Liquid crystal polymer is high in rigidity.
Besides, for realizing light-weight and high-rigidity diaphragms by structural means, those in honeycomb structure and in three layer structure in which a foamed body is sandwiched between flat skin layers have been proposed.
In the conventional examples as described above, it is difficult to select a perfect diaphragm material for satisfying all the aforementioned requirements; that is, PP is higher in specific gravity as compared to paper and lower in Young's modulus, and liquid crystal polymer is higher in specific gravity and smaller in internal loss than PP, for example.
Accordingly, there have been made some approaches in which physical properties such as density and Young's modulus are adjusted by structural means as described above, and the other requirements are satisfied by the selection of the material. However, the structural adjusting of the physical properties requires additional processes such as adhesive bonding of respective layers in the three layer structure. This gives rise to a problem in higher cost in manufacturing processes.
In order to solve the aforementioned problem, the inventors of the present invention have precedently proposed, in Japanese Patent Application No. Hei 7-14782 (Japanese Patent Application Laid-open No. Hei 8-340594), a loudspeaker diaphragm which is formed in three layer structure having a foamed layer at its inside and unfoamed layers at its surfaces by injection molding a resin containing a foaming agent. In the diaphragm, the three layer structure is formed without adhesive bonding the layers. This improves the physical properties by structural means with no increase in cost in manufacturing processes, thereby obtaining lighter weight, higher internal loss, higher rigidity, and improved environmental resistance.
An object of the present invention is to further enhance the aforesaid proposal to improve the physical properties and appearance quality, in consideration of the influence of the content of resin in an integral molded article of three layer structure having a foamed layer at its inside and unfoamed layers at its surfaces as described above.
The foregoing object and other objects of the present invention have been achieved by the provision of a loudspeaker diaphragm of an injection foam molded body which is formed in three layer structure composed of a foamed layer at the inside thereof and unfoamed layers at the surfaces thereof by injection molding a resin containing a foaming agent, characterized in that the aforesaid resin contains an inorganic or organic filler of 3-30% by weight.
According to the present invention, a resin containing a foaming agent is injection molded into an injection foam molded body in three layer structure having a foamed layer 3 at its inside and unfoamed skin layers 2 at its surfaces. This allows both low specific gravity and large thickness, thereby obtaining a light-weight and high-rigidity diaphragm. Since being covered with the unfoamed layers at the surfaces, the diaphragm has excellent environmental resistance. Besides, no adhesive bonding is needed unlike conventional three layer structure, so that the cost in manufacturing can be lowered. Moreover, an inorganic or organic filler of 3-30% by weight is contained in the resin to be injection molded, which improves the appearance quality with keeping the aforesaid physical properties favorable. In this connection, it should be noted that too small content of the filler causes the easy shrinking of the surface unfoamed layers, which deteriorates the appearance quality; too large content of the filler affects the foaming state, resulting in low rigidity.
Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings in which:
Fig. 1 is an explanatory diagram showing an embodiment of a loudspeaker diaphragm of an injection foam molded body according to the present invention;
Fig. 2 is an explanatory diagram showing an injection molding machine for producing the loudspeaker diaphragm of an injection foam molded body shown in Fig. 1;
Figs 3(a), 3(b), and 3 (c) are explanatory diagrams showing a method for producing the loudspeaker diaphragm by the injection molding machine shown in Fig. 2;
Fig. 4 is an explanatory diagram showing molding characteristics of the injection molding machine in Fig. 2;
Fig. 5 is a table showing the changes in the properties of the speaker diaphragm of an injection foam molded body shown in Fig. 1, depending on foaming magnification;
Fig. 6 is an explanatory diagram showing the change in Young's modulus of the loudspeaker diaphragm of an injection foam molded body shown in Fig. 1, depending on foaming magnification;
Fig. 7 is an explanatory diagram showing the change in internal loss of the loudspeaker diaphragm of an injection foam molded body shown in Fig. 1, depending on foaming magnification; and
Fig. 8 is an explanatory diagram showing the change in modulus of rigidity of the loudspeaker diaphragm of an injection foam molded body shown in Fig. 1, depending on foaming magnification.
Fig. 1 shows an embodiment of a loudspeaker diaphragm of an injection foam molded body according to the present invention. The shown loudspeaker diaphragm 1 of an injection foam molded body has three layer structure composed of a foamed layer 3 at its inside and skin layers (unfoamed layer) 2 at its surfaces. In forming the three layer structure, a resin mixture material of polypropylene (PP) containing a foaming agent is injected into a mold. Immediately after the injection, the mold is separated so that the material starts to foam at its inside to form the foamed layer 3. Since being in contact with the inner surfaces of the mold in the injection, the surfaces of the resin mixture material have solidified before the foaming, so as to form the unfoamed skin layers 2. It should be noted that the resin mixture material also contains an inorganic or organic filler of 3-30% by weight. Here, too small content of the filler causes the easy shrinking of the surface unfoamed layers, which deteriorates the appearance quality; too large content of the filler affects the foaming state, resulting in low rigidity. Through experience, the most appropriate content of 3-30% by weight is determined.
Examples of the inorganic filler to be contained in the resin include: silica; diatomaceous earth; oxides such as alumina, titanium oxide, iron oxide, zinc oxide, and magnesium oxide; hydroxides such as aluminum hydroxide, calcium hydroxide, and basic magnesium carbonate; carbonates such as calcium carbonate, magnesium carbonate, and dolomite; sulphates and sulphites such as calcium sulphate, barium sulphate, and calcium sulphite; talc; clay; mica(phlogopite, biotite, muscovite, and the like); asbestos; glass fibers; glass balloon; glass beads; silicates such as calcium silicate, montmorillonite, and bentonite; and silicon carbide monocrystal. Of organic fillers, crystalline cellulose fine powders and urea resin fine powders may be cited. Each of the fillers may be used by itself, or a plurality of the fillers may be mixed for use. In addition, appropriate coupling materials may be applied to the fillers so as to improve the interaction with the resin.
The loudspeaker diaphragm 1 is 0.17mm to 1.8mm in thickness, and the skin layer 2 is 0.05mm to 0.20mm in thickness. These dimensions are required for well-balanced properties of the loudspeaker diaphragm 1. This will be described for details, later.
Fig. 2 shows an injection molding machine for producing the loudspeaker diaphragm 1 of an injection foam molded body in Fig. 1. The injection molding machine has the molding characteristics as shown in Fig. 4.
In the shown injection molding machine, a mold 20 comprises a moving-side mold 21 held with a moving platen 24 and a stationary-side mold 22 held with a stationary platen 25. The clamping pressure between the moving-side mold 21 and the stationary-side mold 22 is controlled with a clamping cylinder 10 which is controlled by a mold clamping pressure control unit 30.
Into an injection opening of the stationary-side mold 22 is inserted a nozzle of an injection apparatus 40 for injecting a resin mixture material of polypropylene (PP) containing a foaming agent. An injection process control unit 31 controls injection conditions to control the injection apparatus 40. In the meantime, the injection apparatus 40 outputs information on the molding process. According to the information and other information such as on the distance of the moving platen 24, the mold clamping pressure control unit 30 controls the clamping pressure.
Next, a method for producing a loudspeaker diaphragm by the injection molding machine in the above-mentioned configuration will be described below.
First, as shown in Fig. 3(a), the clamping cylinder 10 closes the moving- and stationary- side molds 21 and 22 into the mold 20. Into a cavity of the mold 20, a resin mixture material of PP containing a foaming agent and an inorganic or organic filler is injected by the injection apparatus 40.
The resin mixture material in the injection apparatus 40 is kept at a temperature of approximately 230°C. The cavity surfaces of the mold 20 are kept at a temperature of approximately 90°C. The mold clamping pressure control unit 30 controls the clamping cylinder 10 to keep a clamping pressure of approximately 100 t. The cavity formed between the moving- and stationary- side molds 21 and 22 in the mold 20 is approximately 0.3 mm in general thickness.
Here, as shown in Fig. 3(b), the resin mixture material filled into the cavity between the moving- and stationary- side molds 21 and 22 starts to solidify at the portions in contact with the mold 20, thereby forming skin layers 2. In the other molten portions, a gas released by the decomposition of the foaming agent is constricted under an extrusive pressure of a screw in the injection apparatus 40 and the clamping pressure between the moving- and stationary- side molds 21 and 22, so that the foaming is confined in the course of solidification.
Then, as shown in Fig. 3(c), the mold clamping pressure control unit 30 controls the clamping cylinder 10 to instantly reduce the clamping pressure down to approximately 0 t immediately after the completion of the filling of the resin mixture material. At this point, the foaming agent in the molten portions still holds a foaming pressure enough to extend the surrounding skin layers (solidified portions) 2. This allows the constricted decomposed gas in the molten portions to expand with extending the resin around so that the foaming starts.
Hereinafter, the timing of the mold opening in the moving-side mold 21 will be described.
When the mold opening is done before the filling of the resin mixture material is completed, the resin mixture material is excessively injected into the cavity between the moving- and stationary- side molds 21 and 22 of the mold 20, thereby increasing the weight of the resultant product. Conversely, the mold opening in belated timings advances the solidification of the resin excessively, so that the solidification is completed before the foaming agent foams. In this embodiment, the mold opening is favorably done in 0.3-0.4 second after the beginning of the injection. It should be noted that the appropriate time depends on such conditions as the resin temperature of the resin mixture material, the temperature of the mold 20, the thickness of the product, and the amount of the foaming agent content.
The mold 20 is opened by approximately 0.1-1.5 mm. Since the mold 20 needs to be opened in a short time of 0.04-0.05 second, the foaming agent and the clamping pressure are controlled so as to open the mold 20 at a speed of approximately 0.0020-0.0375 mm/ms. In molding a foam molded diaphragm of thin-type, the mold is satisfactorily opened at speeds above approximately 0.001 mm/ms.
By mounting a spring between the moving-side mold 21 and the stationary-side mold 22, the opening force of the moving-side mold 21 on reducing the clamping pressure can be increased to further increase foaming magnification.
To take a concrete example of the injection molding machines and the foaming agent employed in this embodiment, polypropylene (PP) is MA06, a trade name of MITSUBISHI CHEMICAL CORPORATION, containing 7% carbon fibers, and the foaming agent is EE-205, a trade name of EIWA CHEMICAL IND. CO., LTD., which was used in compounding ratio of 0.1 parts by weight. For the injection molding machine, Ultra 220, a trade name of Sumitomo Heavy Industries, Ltd., is used.
Properties of the product obtained by the aforementioned producing method of a foam molded body are shown in Figs. 5-8.
More specifically, Fig. 5 shows the measurements in specific gravity, Young's modulus, internal loss, total thickness, and modulus of rigidity under a constant product weight and varied foaming magnifications. Fig. 6 shows the change in Young's modulus depending on the foaming magnifications, Fig. 7 the change in internal loss depending on the same, and Fig. 8 the change in the modulus of rigidity depending on the same, respectively.
As shown in the drawings, a rise in foaming magnification decreases Young's modulus and specific gravity, but increases total thickness. Since rigidity is in inverse proportion to Young's modulus and in proportion to the cube of total thickness, a rise in foaming magnification increases rigidity.
A foaming magnification of approximately 1.1 provides rigidity equivalent to existing PP cones (Young's modulus of 6.4E+9 N/m² and thickness of 0.3 mm) and increased internal loss. An additional increase in foaming magnification further raises rigidity.
At foaming magnifications above approximately 3.0, however, excessively grown foam cells produce irregular foaming, and increase the dispersion in properties of the diaphragm. Therefore, foaming magnifications of 1.1-3.0 are appropriate.
Besides, at foaming magnifications above 1.5, the foam cells in the foamed layer 3 are oriented along the total thickness direction as shown in Fig. 1 to reinforce the skin layers 2, thereby slowing the decrease of Young's modulus and boosting the increasing rate of rigidity. This partly results from that the foam molding is performed with separating the mold 20 at higher speeds.
However, at foaming magnifications above 2.5, the resin in the foamed layer 3, which reinforces the skin layers 2, becomes too small in density. This increases the decreasing rate of Young's modulus, and thereby the dispersion in the rigidity of the products rises gradually. Hence, foaming magnifications of 1.5-2.5 are appropriate to utilize the structural increase of rigidity by the foam molding to obtain stable products.
For light-weight and high-rigidity structural body of a sandwich structure of the skin layers 2 and the foamed layer 3, the skin layers 2 are favorably made as thin as possible within the limits of the strength. However, in injection foam molding, excessive thinning produces problems such as in deformation of the skin layers 2 in separating the mold 20 for foaming, and in easy breakage.
However, too thick skin layers decrease the amount of the resin for forming the foamed layer 3, preventing effective foaming magnifications (in other words, decreasing the foaming magnifications). Accordingly, the skin layers 2 are best balanced in a thickness of approximately 1/3 of the total thickness before foaming. Unfoamed PP plates of a thickness of 0.15-0.6 mm are generally used for loudspeaker diaphragms; therefore, favorable skin layers have a thickness of 0.05-0.20 mm.
As described above, in this embodiment, a resin containing a foaming agent is injection molded into an injection foam molded body in a three layer structure in which a foamed layer 3 is covered with unfoamed skin layers 2. This allows the resultant articles to be low in specific gravity and thick in total thickness, obtaining a light-weighted and high-rigiditied diaphragm. The diaphragm has excellent environmental resistance since the skin layers 2 cover its surface. In addition, no adhesive bonding is needed unlike conventional three layer structures, thereby lowering the cost in production.
Besides, the whole loudspeaker diaphragm 1 including the unfoamed skin layers 2 is provided within average foaming magnifications of approximately 1.1-3.0 so that the dispersion in the properties of the loudspeaker diaphragm 1 can be diminished by effectively using the features of higher rigidity and higher internal loss resulting from the foaming.
Moreover, as mentioned above, the skin layers 2 are provided in a thickness of approximately 0.05-0.20 mm to make the properties of the loudspeaker diaphragm 1 well balanced.
Furthermore, immediately after the resin mixture material containing the foaming agent is injected into the cavity of the mold 20, the mold 20 is separated at a high speed so that the foam cells in the foamed layer 3 are oriented along the total thickness direction to reinforce the skin layers 2, thereby slowing the decrease of Young's modulus and boosting the increasing rate of rigidity.
This embodiment has been described the case of a producing method in which the loudspeaker diaphragm 1 is formed in a sandwich structure of the skin layers 2 and the foamed layer 3 by the mold opening immediately after the resin mixture material of PP containing the foaming agent is injected into the cavity of the mold 20. However, the present invention is not limited thereto, but may be embodied using a producing method in which, for example, the resin mixture material is injection molded at temperatures for keeping the foaming agent unfoamed before the resin mixture material is heated to foam at temperatures above the decomposing temperature of the foaming agent in a mold such as a hot press mold or a vacuum forming mold.
While there has been described what are at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.

Claims

A loudspeaker diaphragm (1) of an injection foam molded body, wherein the diaphragm is formed in a three layer structure composed of a foamed layer (3) at the inside thereof and unfoamed layers (2) at the surfaces thereof by injection molding a resin containing a foaming agent, characterized in that the resin contains an inorganic or organic filler.
The diaphragm (1) according to claim 1,
wherein the inorganic or organic filler is contained in an amount of 3 to 30 % by weight.
The diaphragm (1) according to claim 1 or 2,
wherein a foaming magnification of the foamed layer (3) is 1.1 to 3.0.
The diaphragm (1) according to any of claims 1 to 3,
wherein each of the unfoamed layers (2) has a thickness of approximately 1/3 of the total thickness of the molded body before foaming.
The diaphragm (1) according to any of claims 1 to 4,
wherein the thickness of the respective unfoamed layer (2) is 0.05 to 0.20 mm.
The diaphragm (1) according to any of claims 1 to 5,
wherein the resin used for making the diaphragm is polypropylene.
The diaphragm (1) according to any of claims 1 to 6,
wherein the filler is an inorganic filler selected from the group comprising silica; diatomaceous earth; oxides such as alumina, titanium oxide, iron oxide, zinc oxide, and magnesium oxide; hydroxides such as aluminum hydroxide, calcium hydroxide, and basic magnesium carbonate; carbonates such as calcium carbonate, magnesium carbonate, and dolomite; sulphates and sulphites auch as calcium sulphate, barium sulphate, and calcium sulphite; talc; clay; mica (phlogopite, biotite, muscovite, and the like); asbestos; glass fibers; glass balloon; glass beads; silicates such as calcium silicate, montmorillonite, and bentonite; and silicon carbide monocrystal, or an inorganic filler selected from crystalline cellulose fine powders and urea resin fine powders or a mixture thereof.