JP4366673B2 - Housing for mobile communication terminals - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnesium alloy case for a portable communication terminal which improves the shield/absorption function of electromagnetic waves, prevents malfunction of electronic circuits due to electromagnetic waves, and improves the rigidity even it is thin and the productivity in manufacturing. SOLUTION: A magnesium alloy rolled blank is plastically processed to form a case with its main portions 0.2-1.0 mm thick and electric resistance of 4.3-4.80 μΩ.cm.

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a case for a mobile communication terminal used for wired or wireless communication, such as a case for a mobile phone in which a material made of a magnesium alloy is formed by plastic working such as forging.
[0002]
[Prior art]
It has been pointed out that an electromagnetic wave leaks to the outside from an electronic device such as a mobile phone, and this electromagnetic wave causes a malfunction of another electronic device or adversely affects the human body. For this reason, various measures for shielding and absorbing electromagnetic waves leaking from electronic devices have been proposed. For example, in an electronic device including a casing made of thermoplastic resin, a seal portion that shields and absorbs electromagnetic waves is provided on the inner side of the casing. In addition, an aluminum alloy or magnesium alloy that is conductive and has a low specific gravity is molded by a die casting method or a semi-melt molding method, and electromagnetic waves leaking from an electronic circuit are subjected to this conductive case. A method of shielding / absorbing is also proposed. Furthermore, a method for shielding / absorbing electromagnetic waves by a composite casing made of a thermoplastic resin and a conductive metal has also been proposed.
[0003]
[Problems to be solved by the invention]
In addition to the shielding and absorption of electromagnetic waves described above, functions required for a casing for mobile communication terminals such as mobile phones that have been rapidly spreading in recent years include design, reduction in size and weight, and impact resistance (rigidity). There is productivity at the time of manufacture. In the future, the aperture ratio of the housing (ratio of the opening to the total surface area of the housing) will increase further due to the increase in the size of liquid crystal display devices, etc., especially as mobile phones become more sophisticated. There is a strong demand for improved rigidity.
[0004]
Furthermore, with the enhancement of the functionality of mobile phones, the intensity of electromagnetic waves leaking from the electronic circuits incorporated therein becomes stronger, and there is a need for further improvement of electromagnetic shielding properties for the housing.
Furthermore, malfunction may be caused by electromagnetic waves generated from other electronic devices, and there has been a demand for an improvement in electromagnetic shielding properties against the casing.
[0005]
As a method for shielding and absorbing electromagnetic waves leaking from mobile phones and further reducing the weight and rigidity of the case, a case in which a magnesium alloy is molded by a casting method such as die casting or a semi-melting process has been proposed. ing. However, a molded body obtained by casting a molten magnesium alloy is limited to a relatively thick one due to restrictions on the manufacturing method, and when formed by the casting method, the minimum value of the thickness is 0. The limit is about 8 mm. In addition, there is a high risk that casting defects and oxides are present inside and on the surface during the manufacturing process. If these defects are present, problems occur with respect to mechanical strength and corrosion resistance.
[0006]
When a magnesium alloy casing is formed by a die casting method, ASTM standard AZ91 material is generally used as the magnesium alloy. The molded casing needs to be stamped to provide operation buttons and an opening for a liquid crystal display device. However, when the AZ91 material is used, burrs are likely to be generated on the press punched surface, and it is necessary to provide a process for removing the burrs, which causes a decrease in productivity.
The object of the present invention is to improve the drawbacks of the above prior art, improve the electromagnetic wave shielding / absorption function, prevent malfunction of electronic circuits due to electromagnetic waves, improve rigidity even if it is thinner, and produce at the time of manufacture The object is to provide a casing for a mobile phone that improves the performance.
[0007]
[Means for Solving the Problems]
In the present invention, the magnesium alloy rolled sheet material is subjected to plastic working by hot or warm forging , the thickness of the main part is 0.2 to 1.0 mm, and the electrical resistivity is 4.3 to 4. It is a casing for a portable communication terminal excellent in electromagnetic wave shielding property, characterized in that a concave and convex portion having an average roughness Ra of 0.01 to 0.2 mm is formed on the inner surface at 80 μΩ · cm. .
[0008]
As the magnesium alloy material used in the present invention, a rolled plate material is optimal in consideration of workability, structure uniformity, etc., among extruded materials considered as plastic working materials.
[0009]
In the portable communication terminal phone casing of the present invention, the magnesium alloy preferably contains Al: 1 to 6%, Zn: 2.5% or less, and trace elements of 0.2% or less by mass ratio.
When manufacturing a mobile phone casing by forging a magnesium alloy, it is desirable that the magnesium alloy has excellent forgeability. If the aluminum content is low, the forgeability is good, but the rigidity deteriorates, so at least aluminum is contained at 1% or more. On the other hand, when the aluminum content increases, forgeability and corrosion resistance decrease. For this reason, aluminum content shall be 6% or less. Zinc also has the same effect as aluminum, and is contained in an amount of 2.5% or less because of the balance between forgeability and metal flow. The trace elements are rare earth elements, lithium, zirconium and the like. Examples of such alloys include wrought materials made of ASTM standard AZ31 alloy, AZ21 alloy, AM20 alloy, and the like.
[0010]
With a magnesium alloy having the above composition, it is possible to punch and form the liquid crystal display portion engagement hole, the push button engagement hole, and the like easily and accurately by plastic working, in particular, hot forging to make it ultra thin and highly rigid.
In particular, by hot forging the magnesium alloy rolled sheet material, the strength improvement from the material state is remarkable and the elongation does not change, but the tensile strength is about 1.3 times and the proof stress is about 1.5 times High rigidity. More preferably, the casing for a portable communication terminal has a tensile strength of 270 MPa or more and a proof stress of 140 MPa or more, and further, by using a rolled sheet material, there is almost no generation of defects in the material. There is little occurrence of the problem of strength reduction due to body defects. If the thickness of the main part is less than 0.2 mm, the rigidity of the housing is insufficient. On the other hand, if the thickness of the main part exceeds 1.0 mm, the volume mounted in the housing may not be ensured when the outer dimensions are defined. The main part thickness is preferably 0.3 to 0.8 mm. The main part refers to a part having no wall thickness variation, and does not include a part having a protrusion or the like.
[0011]
In the present invention, the magnesium alloy material is subjected to plastic processing such as forging, so that the electrical resistivity, which is one of the indicators of electromagnetic shielding properties, is higher than that of a casing formed by a casting method such as a die casting method. can do. The electrical resistivity is 4.3 to 4.80 μΩ · cm. The higher the electrical resistivity, the higher the effect of shielding / absorbing electromagnetic waves. A casing formed by forging a thin plate material made of a magnesium alloy as in the present invention has an improved effect of shielding and absorbing electromagnetic waves compared to a casing formed by a casting method. A preferable electrical resistivity is 4.5 to 4.80 μΩ · cm.
[0012]
Moreover, in the case for portable communication terminals of the present invention, fine irregularities are formed on the inner surface of the case. The electromagnetic wave of several GHz generated from the electronic circuit in the casing of the cellular phone can suppress reflection into the casing by the fine uneven portion. That is, electromagnetic waves are attenuated by eddy currents and attenuated by the fine irregularities on the inner surface of the casing. With this action, the cellular phone casing of the present invention also has an effect of preventing malfunction of the electronic circuit in order to prevent reflection of electromagnetic waves generated from the electronic circuit. In addition, it is preferable to form said fine uneven | corrugated | grooved part formed in the inner side part of a housing | casing in the widest possible range of the location which has a flat surface. Further, when the average roughness Ra of the fine irregularities is set to 0.01 to 0.2 mm, reflection of electromagnetic waves can be suppressed and the attenuation effect can be improved. As a method for forming fine irregularities, it may be formed on the surface of a mold used for forging and transferred to a housing, or may be formed by performing shot blasting after forging.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. 1 and 2 are exploded assembly views showing an embodiment of the present invention. In the figure, 1 is an upper housing (front case), 2 is an opening for operation buttons, 3 is an opening for a liquid crystal display device, 4 is a flip, 5 is a lower housing (rear case), 6 is an antenna, Reference numeral 7 denotes an insertion slot for inserting the antenna 6 provided in the lower housing 5, 8 denotes a circuit board, 9 denotes a battery, and 10 denotes a battery case lid. Screw holes 11, 12, 13 and the like are provided in the upper housing 1, the lower housing 5, and the circuit board 8, and the circuit board is provided between the upper housing 1 and the lower housing 5 by screws (not shown). 8 is fixed. In addition, although the upper housing 1 and the lower housing 5 are not shown in the drawing, a fitting uneven portion is provided on the mating surface to achieve an airtight fitting. Although not shown in FIGS. 1 and 2, a speaker opening is formed in the upper portion of the upper casing 1, and a microphone opening is formed in the lower portion thereof.
[0014]
In the present invention, the upper casing 1, the lower casing 5, and the battery case lid 10 are formed by combining a thin plate material (stretched material) made of magnesium alloy and having a thickness of 3 mm or less by a combination of forging and pressing. There are features. This magnesium alloy is preferably a wrought material having good forgeability. As described above, the magnesium alloy used in the present invention has a weight ratio of Al: 1 to 6%, Mn: 0.5% or less, trace elements of 0.2% or less, and the balance consisting of Mg and inevitable impurities. It is good to use a stretched material. As such a wrought material, for example, ASTM standard AZ31 alloy and AM20 alloy are suitable. The reason for using a thin plate material having a thickness of 3 mm or less is as follows. When using a magnesium alloy thin plate material exceeding 3 mm in thickness and trying to forge a thin casing having a thickness of 0.2 to 1.0 mm in the thinnest part, the reduction rate of forging must be increased, The metal flow of the magnesium alloy during forging becomes remarkable, and there is a possibility that recrystallized fine particles may appear along the metal flow or segregate. For this reason, it is preferable to use a thin plate material made of a magnesium alloy having a thickness of 3 mm or less.
[0015]
In the present invention, fine uneven portions are formed on the surfaces of the bottom portions 1 a and 5 a on the inner side of the upper housing 1, the lower housing 5 and the battery case lid 10. The range in which the uneven portion is formed is preferably as wide as possible in the bottom portions 1a and 5a. For example, a concavo-convex portion is also formed in a flat portion between the operation button openings 2 of the upper housing 1. Moreover, it is preferable to shape | mold so that the average roughness Ra of the surface of this uneven | corrugated | grooved part may be set to 0.01-0.2 mm. By forming such a fine uneven portion, electromagnetic waves generated from the electronic circuit inside the housing can be absorbed by the uneven portion to prevent reflection, and are attenuated by loss due to generation of eddy current. By this action, electromagnetic waves generated from the circuit board 8 of the mobile phone can reduce leakage to the outside of the mobile phone as much as possible, and also prevent malfunction of the electronic circuit. Furthermore, in the present invention, the electrical resistivity of magnesium is higher in the rolled (forged) state than in the cast state. Therefore, compared with a case in which a magnesium alloy is formed by casting, the case for mobile phone of the present invention formed by forging has a greater effect of preventing leakage of electromagnetic waves to the outside.
[0016]
Then, the manufacturing method of the housing | casing for mobile phones of this invention is demonstrated. The cellular phone casing of the present invention can be manufactured by the following procedure.
1) A thin plate material made of a magnesium alloy having a plate thickness of 3 mm or less is heated to 300 to 500 ° C. (preheating step). As the thin plate material, it is preferable to use a wrought material made of ASTM standard AZ31 alloy or AM20 alloy, which is a magnesium alloy excellent in forgeability as described above.
2) While holding this preheated thin plate material at 300 to 500 ° C., it is placed in a mold heated and held at 300 to 450 ° C. and 1 to 500 mm while applying a molding load of 100 to 3000 MPa. A sheet material is stretched at a forging rate of 75% / sec and a reduction rate of 75% or less, and forged into a bottomed molded body having a peripheral wall (rough forging step). The reason why the forging speed is set as described above is as follows. That is, when the forging speed exceeds 500 mm / second, the metal flow cannot smoothly follow the forging speed, and the metal flow is disturbed and a desired shape cannot be obtained. On the other hand, if the forging speed is less than 1 mm / second, the temperature of the thin plate material is lowered and good forging accuracy cannot be obtained, and the productivity is also lowered. The reason for rough forging with a rolling reduction of 75% or less in the rough forging process is that if it exceeds 75%, heat generated by forging is added, and there is a risk of ignition and combustion. Considering safety, it is preferable to perform rough forging at a rolling reduction of 50% or less. Further, the reduction ratio can be processed by one forging, but forging may be performed in a plurality of times.
[0017]
3) After heating the bottomed molded body molded in the rough forging step of 2) above to 300 to 500 ° C, the bottomed molded body is heated in a mold for finishing forging held at 300 to 400 ° C. It is placed and forged with a rolling reduction of 30% or less at a forging rate of 1 to 200 mm / sec while applying a molding load of 100 to 2000 MPa while maintaining at 300 to 500 ° C. By this processing, the operation button opening 2 and the liquid crystal display device opening 3 have not been punched yet, but the upper housing 1 as shown in FIG. 2 can be formed. Since it is forging molding, the thinnest part among the thicknesses of the upper casing 1, the lower mold casing 5 and the battery case lid 10 can be 0.2 to 1.0 mm. The reason why the reduction ratio in the finishing forging process is smaller than that in the rough forging process is that the formability is more important than the processing amount in the finishing forging process, and is preferably 30% or less in order to minimize work hardening. Because. Further, the reduction ratio can be processed by one forging, but forging may be performed in a plurality of times.
4) Subsequently, press working is performed in order to set the average roughness Ra of the bottom surface of the inner side of the casing to 0.01 to 0.2 mm. This press working can be performed using an upper mold provided with minute uneven portions corresponding to an average roughness Ra of 0.01 to 0.2 mm on the entire surface. At this time, the housing does not necessarily have to be heated.
[0018]
5) Subsequently, openings such as the operation button opening 2 and the liquid crystal display device opening 3 are punched out by punching. Even when this punching press process is performed, the casing does not necessarily have to be heated. In the present invention, when a wrought material made of ASTM standard AZ31 alloy or AM20 alloy having excellent forgeability is used as the magnesium alloy material for the casing, the state of the cross section punched out by the above pressing becomes extremely smooth, There is no generation of burrs on the surface, and when assembled as a product, there is an effect that the contact between the operation button and this cross section becomes very smooth.
6) The antenna hole 7 and the screw holes 11 and 12 are machined. The magnesium alloy case obtained by the forging process or the like may be oxidized and lose metallic luster. In order to prevent this, surface treatment such as provisional anti-corrosion and coating is performed. In particular, by forming an oxide film by anodic oxide film treatment, excellent corrosion resistance and alloy that cannot be obtained by usual coating A magnesium alloy casing having a metallic luster utilizing the substrate can be obtained.
[0019]
In the above description of the embodiment of the present invention, a mobile phone using a flip has been described in order to protect the operation buttons. However, the present invention can be applied to a mobile phone without a flip. In the present invention, the following secondary effects also occur. It is known that a magnesium alloy has a high vibration damping capability, and in particular, a rolled material is higher than a cast material. Therefore, when a casing made of a magnesium alloy thin plate material is formed by forging as in the present invention, the sound quality of the sound output from the speaker opening is further improved.
[0020]
【The invention's effect】
The present invention described above has the following effects.
That is, because the magnesium alloy rolled thin plate material is a casing formed by plastic processing such as forging, the electrical resistivity of the casing itself is higher than that of a casing formed by casting such as die casting. Shielding and absorption of electromagnetic waves are improved. Furthermore, since the electromagnetic wave can be absorbed and attenuated by the minute uneven portion provided at the bottom of the inner side of the housing, malfunction of the electronic circuit can be prevented. Further, among the thicknesses of the casing, it is possible to maintain sufficient rigidity even if the thickness of the main part is 0.2 to 1.0 mm, which can contribute to weight reduction of the mobile communication terminal. .
[Brief description of the drawings]
FIG. 1 is an exploded view of a mobile phone for explaining an embodiment of the present invention, and shows a state seen from the front side.
FIG. 2 is an exploded assembly view of a mobile phone for explaining an embodiment of the present invention, and shows a state seen from the back side.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Upper housing | casing 2 Opening part for operation buttons 3 Opening part for liquid crystal display 5 Lower housing | casing 8 Circuit board 10 Battery case cover

Claims (1)

The magnesium alloy rolled sheet material is subjected to plastic working by hot or warm forging , the thickness of the main part is 0.2 to 1.0 mm, and the electrical resistivity is 4.3 to 4.80 μΩ · cm. A casing for a portable communication terminal excellent in electromagnetic wave shielding properties , wherein an uneven portion having an average roughness Ra of 0.01 to 0.2 mm is formed on the surface of the inner portion .

Images