Abstract:
A mold for making a workpiece is shown performing the steps of positioning a workpiece in a cavity of the mold, moving a first mold piece and a second mold piece towards a closed configuration with an extending wall of the first mold piece pressing into a peripheral portion of the workpiece so as to hold the workpiece between the first and second mold pieces, wherein a space is defined between the workpiece and a surface of the first mold piece in the closed configuration, the space receiving a molding material so as to form at least a second layer of the workpiece, and pressing the extending wall of the first mold piece into the peripheral portion of the workpiece to form a seal between the extending wall and the workpiece to prevent flow of a fluid through the seal. Moveable pieces for such a mold are shown.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is Continuation of U.S. patent application Ser. No. 14/065,745, filed 29 Oct. 2013, now pending, which is a Division of U.S. patent application Ser. No. 13/747,833, filed 23 Jan. 2013, now pending, which was a continuation-in-part of U.S. patent application Ser. No. 13/651,980 filed on 15 Oct. 2012, which is itself a continuation-in-part of U.S. patent application Ser. No. 13/277,673 filed on 20 Oct. 2011, which claims priority in Chinese Patent Application No. 2011 0037281.4, filed 1 Feb. 2011, the contents of these applications being incorporated herein in their entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method of manufacturing a workpiece, such as a plate, with multiple metal layers. For the purpose of this invention, where the context allows, the terms “metal” and “metals” shall also include “alloys of metal” and “alloys of metals” respectively. It should also be understood that, for the purpose of this invention, the metal layers may be made of a same metal or different metals. 
       BACKGROUND OF THE INVENTION 
       [0003]    With the rapid development in the communication, consumer electronic and computer industries (so called “3C industries”), consumers not only expect good performance of such products (so called “3C products”) (such as processing speed and storage capacity), but also a high class and durable cosmetic surface of such products. A metal casing with good strength and light weight will thus become more and more important to 3C products. Such characteristics have also become the consumer requirements or expectation of products in other industries, such as the household industry and automotive industry. In most products, there is a need to over-mold at least one metal layer onto another metal layer, for example to form a cover or a plate. There is therefore a need to improve the joining or bonding strength between two metal layers, which has become a significant production requirement. 
         [0004]    A conventional colorful plastic casing of an electronic product is easily broken and damaged by external impact, while a casing of a single metal layer may rust due to environmental factors, or subsequent surface treatment cannot be performed on the casing due to the limitation of material properties. Therefore, casings with multiple metal layers with thin thickness, good cosmetic performance, good strength for resisting external impact, and good corrosion resistance are needed to solve the various shortcomings of casings of a single metal layer. In the prior art, a casing for a consumer electronic apparatus which is formed of double metal layers or of a mechanical laminate of materials is usually prepared by vacuum evaporation or ion sputtering, which entails high manufacturing cost. However, as such a prior art casing is not good for receiving surface treatment involving wet process, such as plating and anodizing, it is less corrosion resistant. 
         [0005]    In conventional techniques, solid-state welding processes (such as cold welding, friction welding and ultrasonic welding) may be used for bonding a veneer to a cast metal part. However, such solid-state welding processes may significantly increase the complexity and cost of the processing flow. Therefore, persons skilled in the art are still looking for effective methods of manufacturing a workpiece with multiple metal layers which is less costly and less complex. 
         [0006]    In addition, there is an ever-increasing requirement for electronic products (such as tablet computers and smart phones) and domestic electrical appliances to be as compact and slim as possible. Consumers also make the same requirements on products in the automotive industry and household product industry. Consumers are at the same time making a higher and higher demand on the functions and capability of such products. Manufacturers are thus looking for ways to make bodies of the products as compact as possible while retaining sufficient space for housing the necessary components. Existing methods do not allow a thin layer of metal to be injected onto and bonded/engaged with a layer of metal to form a workpiece with multiple metal layers. In addition, as such products get compacter and slimmer, problems arise as regards post-treatment, such as trimming and computer numerical control (CNC) works, which are required for achieving the necessary features. 
         [0007]    As such products get more and more compact and slim, problems arise as regards post-treatment (such as trimming) of such workpieces as covers, housings, casings and chassis, because such post-treatment will exert pressure on the workpieces, which may deform the workpieces. 
         [0008]    It is thus an object of the present invention to provide a method of manufacturing a workpiece with multiple metal layers, a mold and a workpiece with multiple metal layers in which the aforesaid shortcomings are mitigated or at least to provide a useful alternative to the trade and public. 
       SUMMARY OF THE INVENTION 
       [0009]    According to a first aspect of the present invention, there is provided a method of manufacturing a workpiece with multiple metal layers, the method including the steps of (a) providing a mold with at least a runner, a gate and a cavity, (b) providing in the cavity of the mold a first metal layer made of a first metal, the first metal layer having a surface, the surface being roughened and/or including at least one engagement structure, and (c) injecting a molten second metal onto the surface of the first metal layer to form a second metal layer on the first metal layer wherein said second metal layer engages with said roughened surface of said first metal layer or with said engagement structure of said surface of said first metal layer, wherein said molten second metal enters said cavity of said mold at a speed of at least substantially 70 meters per second (m/s). 
         [0010]    According to a second aspect of the present invention, there is provided a mold including a first mold piece and a second mold piece, wherein said first mold piece and said second mold piece are movable relative to each other between an open configuration in which said first mold piece and said second mold piece are detached from each other and a closed configuration for holding a semi-finished workpiece between said first mold piece and said second mold piece, and wherein at least said first mold piece includes a wall member which, when said mold is in said closed configuration and holds a semi-finished workpiece, punches into at least part of said semi-finished workpiece to form a seal between said wall member and said semi-finished workpiece which prevents flow of a fluid through said seal. 
         [0011]    According to a third aspect of the present invention, there is provided a workpiece with multiple metal layers, said workpiece being formed by injecting at least a second metal layer onto a first metal layer, wherein each of said first metal layer and second metal layer includes at least one engagement structure. 
         [0012]    According to a fourth aspect of the present invention, there is provided a mold including a first mold piece and a second mold piece, wherein said first mold piece and said second mold piece are movable relative to each other between an open configuration in which said first mold piece and said second mold piece are detached from each other and a closed configuration in which said first mold piece and said mold piece are engaged with each other to form a cavity for containing a semi-finished workpiece, wherein said first mold piece includes a passageway allowing supply of molding material into said cavity, and wherein said mold is without a channel allowing flow of molding material out of said cavity. 
         [0013]    According to a fifth aspect of the present invention, there is provided a method of manufacturing a workpiece with multiple metal layers, said method including steps (a) providing a first metal layer made of a first metal, (b) pre-treating said first metal layer, (c) placing said pre-treated first metal layer in a mold, and (d) injecting a molten second metal onto said surface of said pre-treated first metal layer to form a second metal layer on said pre-treated first metal layer. 
         [0014]    According to a sixth aspect of the present invention, there is provided a workpiece with multiple metal layers, said workpiece being formed by injecting at least a second metal layer onto a first metal layer, wherein said second metal layer is of a thickness of not more than substantially 0.5 mm. 
         [0015]    According to a seventh aspect of the present invention, there is provided a method of manufacturing a workpiece with multiple metal layers, said method including steps (a) providing a mold with at least a runner, a gate and a cavity, (b) providing a first metal layer made of a first metal, said first metal layer having a surface, said surface including at least one engagement structure, (c) pre-treating said first metal layer, (d) placing said pre-treated first metal layer in said mold, and (e) injecting a molten second metal onto said surface of said first pre-treated metal layer to form a second metal layer on said pre-treated first metal layer, wherein said molten second metal enters said cavity of said mold at a speed of at least substantially 70 meters per second (m/s), wherein said second metal layer includes at least one engagement structure which engages with said engagement structure of said surface of said pre-treated first metal layer, wherein said mold includes a first mold piece and a second mold piece, wherein said first mold piece and said second mold piece are movable relative to each other between an open configuration in which said first mold piece and said second mold piece are detached from each other and a closed configuration in which said first mold piece and said second mold piece are engaged with each other to form said cavity for containing said pre-treated first metal layer, wherein said first mold piece includes a passageway allowing supply of said molten second metal, and wherein said mold is without a channel allowing flow of said molten second metal out of said cavity. 
         [0016]    According to an eighth aspect of the present invention, there is provided a workpiece with multiple metal layers formed of a method including steps (a) providing a mold with at least a runner, a gate and a cavity, (b) providing a first metal layer made of a first metal, said first metal layer having a surface, said surface including at least one engagement structure, (c) pre-treating said first metal layer, (d) placing said pre-treated first metal layer in said mold, and (e) injecting a molten second metal onto said surface of said first pre-treated metal layer to form a second metal layer on said pre-treated first metal layer, wherein said molten second metal enters said cavity of said mold at a speed of at least substantially 70 meters per second (m/s), wherein said second metal layer includes at least one engagement structure which engages with said engagement structure of said surface of said pre-treated first metal layer, wherein said mold includes a first mold piece and a second mold piece, wherein said first mold piece and said second mold piece are movable relative to each other between an open configuration in which said first mold piece and said second mold piece are detached from each other and a closed configuration in which said first mold piece and said second mold piece are engaged with each other to form said cavity for containing said pre-treated first metal layer, wherein said first mold piece includes a passageway allowing supply of said molten second metal, and wherein said mold is without a channel allowing flow of said molten second metal out of said cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  illustrates a method of manufacturing a workpiece with multiple metal layers according to an embodiment of the present invention; 
           [0018]      FIG. 2-1  illustrates a device of manufacturing a workpiece with multiple metal layers, for carrying out the method shown in  FIG. 1 , in which a mold in the device is in an open configuration; 
           [0019]      FIG. 2-2  illustrates the device shown in  FIG. 2-1  in which the mold is in a closed configuration; 
           [0020]      FIG. 2-3  is a partially enlarged view of the mold shown in  FIG. 2-2 ; 
           [0021]      FIG. 3  illustrates a method of manufacturing a workpiece with multiple metal layers, according to a further embodiment of the present invention; 
           [0022]      FIG. 4  illustrates a mold of manufacturing a workpiece with multiple metal layers, for carrying out the method shown in  FIG. 3 ; 
           [0023]      FIG. 5  illustrates a method of manufacturing a workpiece with multiple metal layers, according to a yet further embodiment of the present invention; 
           [0024]      FIG. 6  illustrates a workpiece with multiple metal layers manufactured by the method shown in  FIG. 5 ; 
           [0025]      FIG. 7  illustrates a method of manufacturing a workpiece with multiple metal layers, according to a still further embodiment of the present invention; 
           [0026]      FIG. 8  illustrates a first workpiece with multiple metal layers manufactured by the method shown in  FIG. 7 ; 
           [0027]      FIG. 9A  illustrates a second workpiece with multiple metal layers manufactured by the method shown in  FIG. 7 ; 
           [0028]      FIG. 9B  illustrates a third workpiece with multiple metal layers manufactured by the method shown in  FIG. 7 ; 
           [0029]      FIG. 10A to 100  illustrate the process whereby a second metal in molten form is injected into a mold to bond or engage with a first metal layer, in a method according to a further embodiment of the present invention; 
           [0030]      FIG. 11  is a sectional view of an alternative mold suitable for use in a method according to the present invention; 
           [0031]      FIG. 12  is a partial sectional view of the first metal layer of  FIG. 11  after engagement with a second metal layer; 
           [0032]      FIG. 13  is a side view of a first metal layer, after pre-treatment, and ready for injection of a molten second metal, according to a still further embodiment of the present invention; 
           [0033]      FIG. 14A  is a side view of a cover of multiple metal layers, including the first metal layer of  FIG. 13 ; 
           [0034]      FIG. 14B  is a partial enlarged view of  FIG. 14A ; and 
           [0035]      FIG. 15  is a top partial view of a cover of multiple metal layers with a thin bay covered in part by a second metal layer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]      FIG. 1  illustrates a method of manufacturing a workpiece with multiple metal layers according to an embodiment of the present invention. Generally speaking, in this method, a first metal layer in the form of a semi-finished plate formed of a first metal is disposed in a mold. A second metal (which is different from the first metal) in liquid (molten) form is then injected into the mold and onto the plate, so as to form a second metal layer on the first metal layer (S 101 ). The second metal layer in the mold is then pressed by a pressure in the mold (S 102 ) to facilitate bonding of the second metal layer to and with the first metal layer. It should of course be understood that a third metal (which may be the same as or different from the first and second metals) in liquid form may be injected onto the first metal layer or the second metal layer to form a workpiece with three metal layers by repeating the above operation. 
         [0037]    The injection operation includes different aspects, such as high-pressure and high speed injection molding, pouring and/or flowing. The pressure, the speed at which the second metal is injected into the mold and the short time duration in which the second metal covers the first metal layer all assist in improving the adhesiveness and strength of bonding between the first metal layer and the second metal layer, removing bubbles in the second metal when in liquid state, and improving the compactness of the second metal layer, so that only very few pores are left after cooling and curing of the second metal layer, thereby achieving the strength of a composite metal. Such may also prevent the formation of a liquid flow mark due to flowing of the second metal in liquid form during injection. Pressing the second metal layer may also enable a surplus of the second metal in liquid form to overflow. 
         [0038]      FIG. 2-1  and  FIG. 2-2  illustrate a device  201  for manufacturing a workpiece with multiple metal layers, for carrying out the method shown in  FIG. 1 . The device  201  includes a mold  202  with a front mold  208  and a rear mold  207 . The mold  202  is shown in  FIG. 2-1  in an open configuration in which the front mold  208  and the rear mold  207  are detached from each other. A semi-finished metal cover  203  (constituting a first metal layer) formed of a first metal is disposed on the rear mold  207  of the mold  202 .  FIG. 2-2  shows the mold  202  in a closed configuration in which the semi-finished cover  203  is held between the front mold  208  and the rear mold  207 . When the mold  202  is in the closed configuration, a second metal in liquid form  2041  (which is a different metal from that of the first metal layer  203 ) is injected onto the semi-finished cover  203  in the mold  202 , so as to form a second metal layer  204  on the cover  203 . 
         [0039]      FIG. 2-3  is a partially enlarged view of  FIG. 2-2 , and shows that the mold  202  includes a pressing component  205  for applying a pressure on the second metal layer  204  in the mold  202 . 
         [0040]    In a further embodiment, a space is provided between the rear mold  207  and the pressing component  205 , so that the second metal in liquid form  2041  may be injected into the space. Moreover, the mold  202  additionally includes an overflow port  206 , so that a surplus of the second metal in liquid form  2041  overflows through the overflow port  206  when the pressing component  205  presses the second metal layer  204 . 
         [0041]    Although, in the foregoing discussions, it is mentioned that the first metal is different from the second metal, it is of course envisaged that the first metal layer and the second metal layer may be made of a same metal. 
         [0042]    In a yet further embodiment of the present invention, and as shown in  FIG. 3 , a method of manufacturing a workpiece with multiple metal layers includes steps of injecting a first metal in liquid form into a space between a rear mold and a first front mold in cooperation with each other, so as to form a first metal layer on the rear mold (S 301 ), and when the rear mold operates in cooperation with a second front mold and when the first metal layer on the rear mold is in a semi-solid molten state, injecting a second metal in liquid form onto the first metal layer so as to form a second metal layer on the first metal layer (S 302 ). Again, the first metal layer and the second metal layer may be made of the same or different metals. 
         [0043]    The difference between the method shown in  FIG. 3  and the method shown in  FIG. 1  lies mainly in that, in the method shown in  FIG. 3 , the second metal in liquid form is injected onto the first metal layer and the second metal layer is formed when the first metal layer is still in a semi-solid molten state. This not only improves the adhesiveness between the first metal layer and the second metal layer, but also reduces cost and saves time, thereby improving the yield. 
         [0044]    A device  401  for implementing the method shown in  FIG. 3  is shown in  FIG. 4 . As shown in  FIG. 4 , the device  401  includes a first front mold  402  which can operate in cooperation with a rear mold  407 , and inject a first metal in liquid form  4031  onto the rear mold  407 , so as to form a first metal layer  403  on the rear mold  407 . The device  401  also includes a second front mold  408 , which is co-operable with the rear mold  407  to inject a second metal in liquid form  4041  onto the first metal layer  403  when the first metal layer  403  is still in a semi-solid molten state, so as to form a second metal layer  404  on the first metal layer  403 . 
         [0045]    When the second front mold  408  operates in cooperation with the rear mold  407 , a space is provided between the rear mold  407  and a pressing component  405 , so that the second metal in liquid form  4041  may be injected into the space. 
         [0046]    Moreover, the pressing component  405  in the second front mold  408  may be used for applying a pressure on the second metal layer  404 . Furthermore, the rear mold  407  further includes an overflow port  406 , so that a surplus of the second metal in liquid form  4031  may overflow through the overflow port  406  when the pressing component  405  applies a pressure on the second metal layer  404 . 
         [0047]    The device  401  further includes a movement component, for relatively moving the rear mold  407  between the first front mold  402  and the second front mold  408 . For example, the rear mold  407  may be moved from the first front mold  402  to the second front mold  408  after the first metal in liquid form  4031  is injected; or the first front mold  402  is moved away after the first metal in liquid form  4031  is injected, and the second front mold  408  is moved to a position operating in cooperation with the rear mold  407 , so as to inject the second metal in liquid form  4041  onto the first metal layer  403 . Through this arrangement, the injection of both the first metal in liquid form  4031  and the second metal in liquid form  4041  is performed in the same device  401 , thus simplifying the production process. 
         [0048]    A workpiece with three or more metal layers may be formed by repeating the above steps. 
         [0049]    Each of the first metal layer and the second metal layer may be formed of stainless steel, iron, zinc, aluminum, magnesium, chromium, titanium, copper, beryllium, nickel and alloy of these metals. A first metal layer with a smaller specific weight may first be formed, and then a second metal layer with a larger specific weight is formed. Alternatively, a first metal layer with a larger specific weight may first be formed, and then a second metal layer with a smaller specific weight is formed. For example, if the first metal layer is formed of a zinc alloy, and the second metal layer is formed of an aluminum alloy, the strength of a composite metal may be achieved, and subsequent anodizing surface treatment may be performed on the second metal (aluminum alloy) layer. In another example, the first metal layer is formed of an aluminum alloy or a magnesium alloy, and the second metal layer is formed of stainless steel, so that subsequent treatment such as direct current electroplating or vacuum evaporation may be conveniently performed on a surface of the second metal layer, thereby further forming a subsequent metal or non-metal layer. 
         [0050]    Not only does the workpiece with multiple metal layers manufactured according to a method of the present invention have the strength and elasticity of a composite metal, but also subsequent surface treatment (such as heat treatment, anodizing surface treatment, Galvanic plating, vacuum coating/film treatment, coating treatment, painting treatment, and corrosion resistant treatment) may be performed on the metal workpiece, as required, to further improve the adhesiveness between the metal layers and the strength and corrosion resistance of the workpiece, and to make the design of covers made of such workpieces more flexible. 
         [0051]    A method of manufacturing a workpiece with multiple metal layers, according to a yet further embodiment of the present invention is shown in  FIG. 5 . The method shown in  FIG. 5  includes steps of disposing a semi-finished metal workpiece (such as a plate formed of a first metal layer) in a mold, in which a surface of the semi-finished workpiece is roughened (S 501 ). Subsequently, a second metal in liquid form is injected onto the roughened surface of the semi-finished workpiece, so as to form a second metal layer on the semi-finished workpiece, in which the second metal in liquid form covers and fills the roughened surface of the semi-finished plate (S 502 ). The roughened surface of the semi-finished plate can be formed on either a cosmetic surface or an inner surface of the semi-finished plate. A third metal in liquid form may be injected onto the second metal layer to form a three-layer metal plate by repeating the above operation. 
         [0052]      FIG. 6  shows a plate with multiple metal layers manufactured by the method shown in  FIG. 5 . As shown in  FIG. 6 , a semi-finished plate  612  formed of a first metal is disposed in a mold  610 . A surface  613  of the semi-finished plate  612  is roughened. A second metal in liquid form is injected onto the roughened surface  613  of the semi-finished plate  612 , to form a second metal layer  614  on the semi-finished plate  612  which sufficiently covers and fills the roughened surface  613  of the semi-finished plate  612 . Such an arrangement increases the adhesive strength between the semi-finished plate  612  and the second metal layer  614 . The semi-finished plate  612  and the second metal layer  614  may be made of the same metal or different metals. The roughened surface  613  of the semi-finished plate  612  may be formed of a plurality of recesses, holes, grooves, balls or protrusions or a combination of these. The roughened surface  613  of the semi-finished plate  612  may be formed mechanically and/or chemically. By way of such an arrangement, detachment of the semi-finished plate  612  and the second metal layer  614  from each other is at least hindered. 
         [0053]      FIG. 7  shows a method according to a still further embodiment of the present invention. This method includes disposing a semi-finished plate formed of a first metal in a mold, in which a surface of the semi-finished plate is provided with at least one engaging structure (S 701 ), and injecting a second metal in liquid form onto the surface of the semi-finished plate, so as to form a second metal layer on the semi-finished plate, in which the second metal in liquid form caps, fills and engages with the engaging structure on the surface of the semi-finished plate (S 702 ). The engaging structure of the semi-finished plate can be formed on either a cosmetic surface or an inner surface of the semi-finished plate. A third metal in liquid form may be injected onto the second metal layer to form a three-layer metal plate by repeating the operation. 
         [0054]      FIG. 8  illustrates a plate with multiple metal layers manufactured according to the method shown in  FIG. 7 . As shown in  FIG. 8 , a semi-finished metal plate  812  formed of a first metal is disposed in a mold  810 . A surface of the semi-finished plate  812  is formed with at least one engaging structure  813 . The engaging structure  813  may be a hook, a buckle, a trench, a protrusion, a groove or a combination of these structures. A second metal in liquid form is injected onto the surface of the semi-finished plate  812 , to form a second metal layer  814  on the semi-finished plate  812  which sufficiently caps, fills and engages with the engaging structure  813  on the surface of the semi-finished plate  812 . By way of such an arrangement, at least part of the second metal layer  814  is confined to a space defined by the engaging structure  813 , so as to fix the semi-finished plate  812  with the second metal layer  814 . Such an arrangement at least hinders detachment of the semi-finished plate  812  and the second metal layer  814  from each other. 
         [0055]    In molding, the molten molding material (such as a molten metal) is injected from an injector nozzle of a molding machine to flow through a sprue, then a runner, then a gate, through which the molten molding material enters the cavity of the mold. More particularly, a sprue is a channel allowing flow of the molten molding material from the injector nozzle towards the mold cavity. A runner is a channel in fluid communication with the sprue and guides the molten molding material to flow from the sprue towards the mold cavity. The runner is joined with the gate and the gate acts as an entrance through which molten molding material in the runner enters the mold cavity. 
         [0056]    To further enhance the strength of bonding/engagement between the first metal layer and second metal layer, in an embodiment of the present invention, the second metal in liquid form is injected from the injector nozzle at such a speed that the second metal in liquid form exits the runner and enters the cavity of the mold via the gate of the mold at a speed of at least 70 meters per second (m/s). This speed will hereafter be called the “ex-gate speed”. In one embodiment, to achieve an ex-gate speed of 70 m/s, it is arranged such that the second metal in liquid form exits the sprue and enters the runner at a speed of at least 3.5 m/s. This latter speed will hereafter be called the “ex-sprue speed”. 
         [0057]      FIG. 9A  illustrates a plate with multiple metal layers according to an embodiment of the present invention. A semi-finished plate  912  formed of a first metal is first disposed in a mold  910 . A surface  913   b  of the semi-finished plate  912  is roughened to form a plurality of recesses, holes, grooves, balls and/or protrusions, and at least one engaging structure  913   a  in the form of a hook, buckle, trench, protrusion and/or groove is also formed on the surface  913   b . A second metal in liquid form is injected onto the roughened surface  913   b  and the at least one engaging structure  913   a  of the semi-finished plate  912  at an ex-gate speed of at least 70 m/s, to form a second metal layer  914  on the semi-finished plate  912  which sufficiently covers and fills the roughened surface  913   b  and engages the at least one engaging structure  913   a  of the semi-finished plate  912 , so as to increase the joining and bonding or engagement strength between the semi-finished plate  912  and the second metal layer  914  and to confine part of the second metal layer  914  within a space defined by the engaging structure(s)  913   a.    
         [0058]      FIG. 9B  illustrates a plate with multiple metal layers according to another embodiment of the present invention. A semi-finished plate  912 ′ formed of a first metal is first disposed in a mold  910 ′. A surface of the semi-finished plate  912 ′ is formed with at least one engaging structure  913   a ′ in the form of a hook, a buckle, a trench, a protrusion and/or a groove. A second metal in liquid form is injected onto the engaging structure  913   a ′ of the semi-finished plate  912 ′ at an ex-gate speed of at least 70 m/s, to form a second metal layer  914 ′ on the semi-finished plate  912 ′ which sufficiently covers, fills and engages with the engaging structure  913   a ′ of the semi-finished plate  912 ′. Such an arrangement increases the joining and bonding or engagement strength between the semi-finished plate  912 ′ and the second metal layer  914 ′ and confines part of the second metal layer  914 ′ within a space defined by the engaging structure(s)  913   a ′. More particularly, bonding or engagement between the semi-finished plate  912 ′ and the second metal layer  914 ′ is enhanced because the inter-engagement and/or interlocking between the second metal layer  914 ′ and the engaging structure  913   a ′ hinders detachment of the semi-finished plate  912 ′ and the second metal layer  914 ′ from each other. More particularly, it can be said that each of the semi-finished plate  912 ′ and the second metal layer  914 ′ has at least one engagement structure which engage with each other. 
         [0059]    The semi-finished plate (or the first metal layer) and the second metal layer may be made of the same metal or different metals, and the metal may be stainless steel, iron, zinc, aluminum, magnesium, chromium, titanium, copper, beryllium, nickel or an alloy thereof. 
         [0060]    The roughened surface of the semi-finished plate (i.e. first metal layer) in the above embodiments may be formed chemically and/or mechanically. For example, if the first metal layer is formed of aluminum (Al), anodizing process may be used for forming pores on the surface of first metal layer for joining with the molten second metal. In particular, the second metal in molten state may be trapped in the pores, so that the second metal will be fastened onto the first metal layer after cooling and curing thereof. 
         [0061]    The plate may be used as a cover or an insert of an electronic device, or any other kinds of products/devices in other industries in which the devices require better joining, bonding or engagement strength on multi-metal construction. 
         [0062]    The first metal layer and the second metal layer may be engaged together by bonding or confining a part of the second metal layer in a space defined by the engaging structure. 
         [0063]    In above-mentioned methods, the second metal  614 ,  814 ,  914  and  914 ′ may be injected onto the surface of the semi-finished plate (first metal layer) at an ex-gate speed of at least 70 m/s, and with an ex-sprue speed higher than 3 m/s, 3.5 m/s, 4.0 m/s, 4.5 m/s, 5.5 m/s, 6.0 m/s, 6.5 m/s or above. In this way, the second metal layer can be of an extremely thin dimension, so that the recesses, holes, grooves, balls or protrusions of the roughened surface and hook(s), buckle(s), trench(es), protrusion(s) or groove(s) of the engaging structure can be well capped (or covered) and filled by the second metal. In a preferred embodiment, the thickness of the second metal layer may be not more than 0.5 mm (such as 0.5 mm, 0.3 mm, 0.2 mm or 0.1 mm) by adjusting the ex-gate speed (e.g. by adjusting the speed at which the second molten metal is ejected from the injection nozzle), or depending on the 3D-design of product, which may become important in future. 
         [0064]    In view of the above, high speed of flow of the second molten metal is a critical parameter in minimizing the fall in temperature of the second molten metal during its flow from the injection nozzle to the mold cavity. Localized melting on the surfaces of the two metals which are going to be joined or bonded together can only result in a weak bonding. As such, post-processes (for examples, laser welding, resistance welding and some other welding processes which are known in the market) are needed to enhance the joining or bonding strength between the two metal layers. In the present invention, a bolted locking mechanism (or bolted locking space) is provided on the first metal layer for guiding the second metal in molten form to be trapped by the designated space defined by the bolted locking mechanism, as the engaging structures  913   a  and  913   a ′ depicted in  FIGS. 9A-9B  and discussed above. 
         [0065]    In particular, a purpose of injecting the second metal in liquid form into the mold at a high speed is to ensure that the second metal fills up the cavity in a very short time, and thus the second metal is still in the molten stage when it fills up the cavity of the mold to form the second metal layer. As shown in the example illustrated in  FIGS. 10A to 10C , the total time duration starting from that shown in  FIG. 10A  (when the molten second metal exits the sprue and enters the runner, at point A), through that shown in  FIG. 10B  (when the molten second metal has passed through the runner and is about to enter the gate, at point B), until that shown in  FIG. 10C  (when the molten second metal fills up the cavity of the mold, at point C) is not more than 0.02 s, with a total displacement of 130 mm. Of this time duration of 0.02 s, the time duration which the molten second metal takes to fill up the cavity only is not more than 0.005 s after it enters the cavity of the mold. In this example, the speed at which the molten second metal exits the sprue and enters the runner is 3.5 m/s, and the speed at which the molten second metal exits the gate and enters the cavity of the mold is 70 m/s. 
         [0066]    To further enhance the engagement and bonding between the two metal layers, as shown in  FIG. 11 , a mold  1100  for manufacturing a plate with multiple metal layers according to this invention has an upper mold  1102  with a barrier in the form of an endless wall  1104  which extends away from a surface of the upper mold  1102  directly facing a lower mold  1106 . When the upper mold  1102  is in the configuration shown in  FIG. 11 , in which the upper mold  1102  is aligned with the lower mold  1106  and a semi-finished metal plate  1108  (being a first metal layer) ready for injection of the second metal in molten form is held between the upper mold  1102  and the lower mold  1106 , the wall  1104  contacts and is pressed to cut into the semi-finished plate  1108  to form a seal which prevents flow of a fluid (including a gas and a liquid) through the seal. The gas may be air and the liquid may be a liquid molding material, such as the second metal in molten form. A space  1110  is also formed between the upper mold  1102  and the semi-finished plate  1108 . The space  1110  is in a fluid-communicable relationship with the injector nozzle via the sprue, runner and gate of the mold. The space  1110  reduces further oxidation of the molten second metal during its flow in the mold. Because of the high speed at which the second metal in molten form exits the gate and enters the cavity, and with the help of the space  1110  (which reduces further oxidation of the second metal in molten form), the molten second metal can engage with and/or penetrate the roughened surface and/or the engagement element on the semi-finished plate  1108  in a very short period of time, say of no more than 0.005 s, after it has entered the space  1110 , to thereby enhance the strength of engagement between the two layers of metal (meaning the semi-finished plate  1108  and the metal layer formed of the cooled-down second metal). On the other hand, in the absence of the wall  1104  and, thus, the fluid-proof seal between the wall  1104  and the semi-finished plate  1108 , due to the connection of the outside atmosphere and the cavity through the traditional air venting system, the molten second metal will further be further cooled down and oxidized during the injection process. The surface of the molten second metal will become oxidized and will be extended to the coming molten second metal. The surface tension of the oxidized molten second metal and/or semi-solid second metal will be higher, causing higher viscosity of the molten second metal, which will slow down the flow of the molten second metal. It will then be difficult for the molten second metal to penetrate or engage with the engagement elements of the semi-finished plate  1108 , in particular if such engagement elements are of a height of less than 0.5 mm and a width of less than 0.5 mm, or engagement elements of a depth of at least 0.5 mm. 
         [0067]    Although the engagement elements may be of a height of at least 0.5 mm, the second metal layer may be of a lesser thickness. As shown in  FIG. 12 , the semi-finished plate  1108  is schematically shown with two engagement elements, each being a hook  1112 , which are spaced apart from each other. The hooks  1112  extend from an upper surface  1114  of the plate  1108  by a height of 0.5 mm. A volume of molten second metal is injected into the space between the hooks  1112  to form a second metal layer  1116  engaged with the plate  1108 . Depending on the structural and design requirements, the thickness of the second metal layer  1116  may be more than, equal to, or less than the height of the hooks  1112 . In particular, in  FIG. 12 , the second metal layer  1116  is shown as being of a thickness (e.g. 0.4 mm, 0.3 mm or less) which is less than the height of the hooks  1112 . 
         [0068]    The mold  1100  includes a passageway through which molding materials (such as molten metals) may be supplied to the cavity of the mold when the mold  1100  is in the closed configuration. As distinct from existing practice, however, there is no channel in the mold  1100  through which excess molding material (i.e. the molten metal) exits the cavity of the mold  1100  to become burrs and flash, which have to be trimmed off after the molding process. On the other hand, when the mold  1100  is used, any excess molten second metal will flow over the first metal layer/plate  1108  and will still form part of the product. It is thus not necessary to carry out any trimming step after the method according to this invention, because there is no “over-flow material” to be trimmed off. 
         [0069]    Although  FIG. 11  shows the wall  1104  as being provided by the upper mold  1102 , it is envisaged that, depending on the designs of the products, the wall  1104  may be provided by the lower mold  1106 , e.g. on a surface facing directly the upper mold  1102 . 
         [0070]    A method according to this invention possesses at least the following advantages:
   (a) the molded product can be ejected after the injection process, which is different from the ordinary casting process in which the product has to be cooled down before it can be ejected from the cavity,   (b) the molten second metal covers the first metal layer when the molten second metal is still in liquid form,   (c) further oxidation of the molten second metal before it is cooled down is reduced, thus allowing the molten second metal to fully engage with or penetrate into different parts of the roughened surface and/or engagement elements (such as grooves, pores, recesses) of the semi-finished plate (being a first metal layer). It provides the opportunity to form the interior features in net shape and to reduce a lot of post-treatment processes and CNC works, thus saving further cost,   (d) as all the molten metal is trapped, with no overflow of such metal, the edges around the first and second metal layers become dense and sealed. There is thus no gap between the metal layers, in particular between the boundaries or between the joining lines of the metal layers, which is observable by end users, thus ensuring cosmetic quality. In addition to being a cosmetic treatment for the product, such also prevents liquid (such as water, DI water, acidic solutions, alkaline solutions or the like) from seeping between the metal layers. This at least reduces the potential problem of galvanic corrosion of the product,   (e) in cases where the workpiece is to form the outer casing of a finished product, the surface which will form the outside surface of the finished product will have no trace of the injected material, thus presenting a more aesthetically pleasing outlook, and   (f) as the second metal layer can be very thin (of not more than 0.5 mm), if a workpiece is to form a casing of a product, space of the interior of the product is saved, thus allowing more freedom to the designers.   
 
         [0077]    In a further embodiment of the present invention, and as shown in  FIGS. 13 to 14B , a first metal layer (e.g. a semi-finished plate  1200 ) is pre-treated before molding. The plate  1200 , which is made of a first metal, is originally of a generally rectangular cross-section. The semi-finished plate  1200  conforms generally with the shape and contour (in particular the outer contour) of the component which it is intended to form. Some of the first metal is removed from the plate  1200  to form one or more recesses, e.g. thin bays  1202 , on an upper surface  1204  of the plate  1200 . These bays  1202  are of a depth d of 0.3 mm or less, while the thickness D of the first metal layer  1200  is around 0.8 mm. 
         [0078]    The pre-treated semi-finished plate  1200  is then placed within the cavity of a mold. A molten second metal is then injected onto the upper surface  1204  of the pre-treated plate  1200  to form a second metal layer  1206 , and to engage with the pre-treated plate  1200  to form a bi-layer metal workpiece. Some of the second metal is received within the bays  1202  of the plate  1200 , so as to engage the pre-treated plate  1200  with the second metal layer  1206 . It is of course possible to form a workpiece with more layers of metal by repeating the above steps. It should be noted that the second metal layer  1206  may cover only part of the bays  1202 . 
         [0079]    In addition, and as shown in  FIG. 14A , because of the high speed at which the molten second metal is injected onto the upper surface  1204  of the plate  1200 , the second metal layer  1206  so formed by the second metal can form structures which extend away from a major surface of the second metal layer  1206  of the plate  1200 . Such structures may be screw boss  1208  and other mechanical, structural components  1210 . 
         [0080]    It is found in practice that this arrangement of pre-treating the semi-finished plate  1200  (in particular the removal of some of the first metal from the plate  1200  to form two recesses in the form of thin bays  1202  on the upper surface  1204  of the plate  1200 ) before molding may be advantageously combined with the use of mold  1100  with the endless wall  1104  discussed above. With such a combined method, there will be no “waste material”, as any excess molten second metal (i.e. molten second metal beyond the minimum amount necessary for molding onto the first metal layer) will be kept within the mold  1100  to form at least part of the second metal layer  1206 , which forms useful parts of the final workpiece/product. 
         [0081]    An advantage associated with adopting such a combined method is that all excess or surplus molten second metal (if any) will become part of the final workpiece/product in a planned manner, which could assist in strengthening the features formed by the second metal. In addition, as it is not necessary to post-treat any overflow material, the combined method is both environmentally-friendly and cost-saving. 
         [0082]    As mentioned above, the second metal layer  1206  may cover only part of the bays formed on the first metal layer. As shown in  FIG. 15 , a cover  1302  formed of a first metal layer is formed with a shallow bay  1304  along the periphery. Molten second metal is then molded on the first metal layer to form a second metal layer, in such a way that part of the bay  1304  is covered by the molten second metal. Hashed areas  1306  shown in  FIG. 15  are areas of the thin  1304  not covered by the molten second metal. During the molding process, the bay  1304  receives the molding material (i.e. molten second metal) and performs air-venting function for leaking air generated during the molding process. 
         [0083]    The present invention seeks to at least mitigate the shortcomings associated with the prior art, and to manufacture a workpiece with multiple metal layers at a lower cost and with a higher yield, by preparing materials according to actual material consumption, thus being more environmentally friendly and cost efficient than the technology currently available. Meanwhile, different metals of double layers or multiple layers may be designed to completely or partially cover a substrate, so as to meet the requirements for appearance and mechanical performance at the same time, which will save a large amount of work in developing different alloy materials and save global resources. 
         [0084]    The method of the present invention achieves good adhesiveness between multiple metal layers and improves the metal compactness and the surface smoothness, and facilitates subsequent metal surface treatment. 
         [0085]    It should also be understood that, for the purpose of this invention, a “workpiece with multiple metal layers” does not mean that the workpiece is formed exclusively of metal(s). It is envisaged that a “workpiece with multiple metal layers” may be formed additionally of other materials, e.g. plastics material. As an example, such a workpiece may be formed of two metal layers which are bonded/engaged with each other as discussed above and a plastic layer which is bonded/engaged with one of the two metal layers. There is thus no limitation on the number of layers of materials involved or the number of materials involved, so long as the workpiece includes two metal layers which are bonded/engaged with each other as discussed above. 
         [0086]    Although the technical contents and features of the present invention are described above, various variations and modifications can be made by persons of ordinary skill in the art without departing from the teaching and disclosure of the present invention. Therefore, the scope of the present invention is not limited to the disclosed embodiments, but encompasses other variations and modifications that do not depart from the present invention as defined by the appended claims.