Metal deforming – With 'coating' of work
Reexamination Certificate
1998-06-11
2001-02-06
Jones, David (Department: 3725)
Metal deforming
With 'coating' of work
C072S060000, C072S058000, C072S709000, C029S527100, C029S527400, C029S421100, C264S510000
Reexamination Certificate
active
06182486
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a superplastic alloy-containing conductive plastic article for shielding against electromagnetic interference (EMI), and more particularly relates to a process for manufacturing a conductive plastic article containing one continuous superplastic alloy layer in one processing stage.
2. Description of the Prior Art
In recent years, progress in technology has led to an extensive increase in the amount of sophisticated electronic equipment. However, the high-density electromagnetic waves produced from electronic equipment have the potential to damage or adversely affect the performance of other equipment or components. Also, exposure to electromagnetic waves is harmful to the human body. Therefore, an electrically conductive outer shell is needed to shield electromagnetic interference (EMI) produced from electronic equipment.
Heretofore, various methods have been used to shield electronic equipment. Metallic boxes and cans fabricated from steel, copper, aluminum, etc., were used to surround high EMI emitters as shielding. However, metallic shields with intricate shapes were difficult to be fabricated by the conventional metalworking methods. Moreover, metallic shields were cumbersome, heavy and costly. Therefore, the electronic industry has resorted to metallized plating on plastics. Unfortunately, the results obtained with metallic coatings were not always satisfactory. In addition to being relatively non-economical, once such metallic coatings were scratched through, they would lose part of their shielding efficiency. Unless such conductive coatings are continuous and free of voids, electromagnetic waves will be free to pass through. Frequently, it was difficult to obtain a dependable, 100% effective coating which was also resistant to peeling.
Further efforts by the electronics industry to develop more dependable light-weight materials for EMI shielding have led to a third approach, namely electrically conductive component-filled plastic composites. It was anticipated that intricate shapes could be molded from the composite materials by conventional means, yielding a finished part that promised to be more economic and dependable than metal or metal-coated plastics.
The principle factor influencing the performance of conductive component-filled plastic composites is the aspect ratio of the conductive fillers. The aspect ratio is defined as the ratio of the maximum dimension to the minimum dimension of the filler. For example, the aspect ratio of a fiber is the ratio of the length to the diameter of the fiber. According to the electromagnetic wave percolation theory, if the conductive filler in the plastic retains a higher aspect ratio, the filler easily forms a conductive network, thus, the critical concentration of the conductive filler required to achieve the electromagnetic shielding effect (that is, the threshold percolation concentration) is lower.
The methods for preparing conductive component-filled plastic composites can be classified into four types. The first type involves compounding the conductive fillers in the form of powders, short fibers or flakes with the plastic matrix, and then having the mixture hot-press molded or injection molded into various kinds of plastic products for shielding EMI.
For example, U.S. Pat. No. 4,474,685 discloses a process for fabricating electromagnetic shielding products by first compounding and then molding a composition including a thermosetting resin binder and an electrically conductive filler (including carbon black, graphite and conductive metal powders). However, during the compounding with the resin matrix, the conductive powders may easily cluster, and thus are not capable of dispersing in the resin matrix. Consequently, the electromagnetic shielding efficiency of the molded products can not be effectively improved. Furthermore, since the powder filler has a lower aspect ratio, according to the electromagnetic wave percolation theory as mentioned above, the amount (i.e., threshold percolation concentration) of the powder filler added must be relatively high to achieve electrical conductivity. Consequently, the mechanical properties, color and other physical and chemical properties of the molded products are adversely affected.
On the other hand, if the conductive filler is in a higher aspect ratio form such as fibers or flakes, the filler can be loaded to a lower level. However, the cluster phenomenon is still difficult to prevent. In addition, during the compounding process, in order to maintain the original aspect ratio, the conductive filler should be strong enough to prevent breakage due to compounding. However, such a strong conductive filler is very expensive, and is thus not suitable for ordinary low cost electronic equipment.
The second type of method for preparing conductive component-filled plastic composites involves binding a plastic layer to enclose the conductive continuous filler by immersion or extrusion, and then cutting the conductive long fiber-filled plastic stick to a predetermined length. For example, Japanese Patent No. 60-112854 discloses a process including continuous extruding thermoplastic plastic to enclose a copper fiber to form a copper fiber-filled plastic round stick, and then cutting the plastic round stick into pellets of a predetermined size. In order to increase the aspect ratio of the filler, the diameter of the conductive long fiber should be as small as possible. The fibrous filler must be strong enough to prevent breakage, but suitable fillers, such as stainless steel fiber, copper fiber or metal-coated carbon fiber, are very expensive.
To decrease the total cost of the production of conductive component-filled plastic composites, aluminum filler, which has the advantages of low price, low density, excellent electromagnetic shielding efficiency, and easy of color matching, has already been used. However, since aluminum has low strength, when aluminum material is utilized in the first method for preparing aluminum-filled plastic composites, the process involves compounding aluminum flakes with plastic. However, since aluminum has low strength, many aluminum flakes or fibers break during processing, resulting in a rapid decrease of the aspect ratio. Therefore, the incorporation amount (threshold percolation concentration) should be increased to a very high level (generally, as high as 30 to 40%) to achieve an acceptable electromagnetic shielding efficiency. The consequence is that the total cost is increased, and more seriously, the electromagnetic shielding plastic products obtained have poor mechanical properties. For example, elongation, tensile strength, bending strength and impact strength are all adversely affected. Also for the second method, the low strength of aluminum causes the rupture of continuous long fibers, which results in an interruption of the binding process.
In order to solve the above-mentioned problems, the inventor of the present invention with his coworker have disclosed a third type of process making metallized plastic pellets in U.S. Pat. No. 5,531,851, in which radially arrangedmetal is filled. The process involves sandwiching an electrically conductive metal foil in between two plastic films to form a metallized laminated plastic sheet; slicing the plastic sheet into plastic strips; radially arranging the metallized plastic strips into a die of an extruder to be wetted and bound by molten plastic into a metallized plastic bar; and finally cutting the plastic bar into metallized plastic pellets of a predetermined size.
In the third method, aluminum can be successfully filled in the plastic. In addition, since no conventional compounding step is needed, the breakage of aluminum can be prevented and a higher aspect ratio can be maintained. Also due to the employment of a wider material (aluminum foil) with is further reinforced with plastic, the interruption of binding process in the second method will not occur. However, the procedures and apparatus for manufacturing such radi
Clark & Elbing LLP
Jones David
National Science Council
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