Comparison of several metal processing techniques for mobile phone casings (casting, forging, stamping, CNC)

Editor: Yang Daxin Technology

The following figure compares several metal processing techniques for mobile phone casings in terms of processing cost, CNC usage, processing cycle, yield rate, designability, and appearance texture.

Overall, the larger the area of a process radar chart, the better its overall performance; From a single dimension analysis, each dimension is divided into 10 levels, with higher scores indicating that a certain process is better in that dimension.

Casting | Casting

Casting is one of the earliest metal hot working processes mastered by humans and is one of the fundamental processes in modern mechanical manufacturing industry. The casting blank achieves the goal of no mechanical processing or minimal processing due to its almost formed shape, reducing costs and to some extent reducing time.

Metal casting is the process of injecting molten metal into a hollow mold made of high-temperature resistant material, condensing it to obtain the desired shape of the product; The resulting product is a casting.

Image: Liquid Metal - Filling - Solidification Shrinkage - Casting

Classification of Casting

1、 Gravity Casting

It refers to the process of injecting molten metal into a mold under the influence of Earth's gravity, also known as casting. The metal liquid is usually manually poured into the sprue, and the product is obtained by filling the mold cavity, exhausting, cooling, and opening the mold by the weight of the metal liquid.

Gravity casting has the advantages of simple process, low mold cost, few internal pores, and the ability to undergo heat treatment. However, it also has defects such as poor density, slightly lower strength, unsuitability for producing thin-walled parts, low surface smoothness, low production efficiency, and high cost.

2、 Die Casting

The method of filling the cavity of a die-casting mold with liquid or semi liquid metal at a high speed under high pressure, and forming and solidifying it under pressure to obtain castings.

High pressure casting can quickly fill molds, with extremely high production efficiency, good product density, high hardness, good surface smoothness, and can produce parts with relatively thin wall thickness; At the same time, due to the use of high-pressure air for filling, there is a lot of gas involved inside, which can easily form pores inside the product. Therefore, heat treatment cannot be carried out (internal gas will expand during heat treatment, causing defects such as bulging or cracking of the product) and post-processing with excessive processing volume (to avoid penetrating the surface dense layer, exposing subcutaneous pores, and causing the workpiece to be scrapped).

However, the ordinary aluminum die-casting process faces the challenge of achieving smooth aluminum oxide film treatment. The reason is that silicon was added to the raw material in order to improve its fluidity and allow it to flow through all areas of the mold. Therefore, if aluminum die castings are to be colored, they may lose their high-end feel after coating due to appearing like plastic.

Die casting technology is applied to mobile phones, but due to the inability to anodize the surface, most companies adopt spray coating treatment, such as Meizu's latest release of Meizu Blue Metal, as shown in the following picture.

Recently, OTAX, a Japanese company, announced the development of a low-cost die-casting process to achieve smooth aluminum oxide film treatment, enhancing the high-end feel of products. It has been applied to mobile phone cases and headphone components. The following picture shows the headphone components produced by Bang&Olufsen in Denmark using aluminum die-casting technology.

Forging | Forging

Forging is one of the metal pressure processing methods. A machining process that uses pressure to change the shape of metal raw materials in order to obtain forgings with certain mechanical properties, shapes, and sizes.

Classification of Forging

1. Hammer or Smith Forging

The same method is used for hammering or forging iron, which involves heating the workpiece to the forging temperature and forging it between a flat hammer and a drill plate; Small items can be manually called blacksmith; Large items require the use of a Steam Hammer, as shown in the following diagram. Among them, the workpiece is placed between the drilling plate and the flat hammer. As for the structure of the steam hammer, it depends on the forging capacity, with light ones being single frame type and heavy ones being double frame type.

2. Drop Hammer Forging

The difference between drop hammer forging and flat forging lies in the fact that the hammer used in drop hammer forging has a mold cavity, where the workpiece is subjected to pressure or impact forces from both sides, and then undergoes plastic deformation according to the shape of the mold cavity. As shown in the figure below, in order to ensure the sufficient flow energy of the metal, forging is often divided into several stages, with each stage changing gradually to control the flow direction. As for the number of stages, it depends on the shape and size of the forging, the malleability of the metal, and the dimensional accuracy requirements of the workpiece.

3. Press Forging

Pressure forging is a forging method that uses slow pressure to extrude metal in a mold. Due to the long duration of force on the metal, the extrusion effect is not only on the surface of the forging, but also on the central part of the workpiece. Therefore, it can achieve a uniform effect inside and outside, and the quality of its products is also better than that of hammer forging.

Forging used in the manufacturing process of mobile phone casings can effectively reduce CNC time, resulting in relatively lower costs; And aluminum alloys with aluminum content exceeding 95% can be selected for anodizing. Process: First, obtain thicker mobile phone structural components through forging and pressing; CNC mill off the unnecessary parts again; NMT obtains metal+plastic integrated structural components; Anodizing surface treatment; Finally, glue the antenna cover together.

Like OPPO R7/R7 PLUS, as shown in the following figure

4. Upset Forging

Rough forging or end pressure forging is usually performed on one end of a uniformly long rod to achieve rough forging or shaping, as shown in the following figure. Clamp the long rod inside the mold, heat one end to high temperature, and apply pressure along its axis to make it rough or shaped.

5. Roll Forging

The principle of rolling forging is shown in the following figure.

Using two non 100% circular rollers (with a diameter reduction rate of 25-75%, the rest can be cut into grooves as needed), feed the rod material between them and clamp them, then continue to rotate to compress the rod material, reduce its diameter, and push it forward; When the roller rotates to the open position again, the rod material can be pulled back to its original position for the next cycle of rolling, or sent to another groove for another construction.

Stamping | Punching

Metal stamping is a metal cold treatment processing method, also known as cold stamping or sheet metal stamping. With the power of stamping equipment, the metal sheet is directly pressed and formed in the mold. Stamped parts are widely used in the manufacturing of automotive parts and household appliances.

Applicable materials: Suitable for most metal plates, especially suitable for carbon steel plates, stainless steel plates, aluminum plates, magnesium plates, copper plates, and zinc plates

Process flow:

1. Fix the metal sheet on the mold table

2. The upper punch falls vertically, causing the metal sheet to be subjected to force and formed inside the mold

3. The punch rises and the parts are taken out, waiting for the next trimming and polishing process, which takes about 1s-1 minutes

The recently launched Redmi Note 3 uses a metal back cover made by stamping technology.

One direct benefit of stamping is cost savings, as it does not require nanoinjection molding and can be directly anodized after polishing, resulting in a fast ramp up to mass production; But the disadvantage of this technology is that the upper and lower ends of the body must be made of plastic splicing. Unable to create complex internal structures, can only be used for making back covers.

CNC | Numerical Control Machine Tool

CNC, commonly known as "numerical control machine tool", refers to automated machine tools controlled by programs. This control system is capable of logically processing programs with control codes or other symbolic instructions, decoding them through a computer to enable the machine tool to perform predetermined actions, and finally shaping a raw metal sheet into the desired shape through long-term precision machining.

CNC metal integrated molding, also known as Unibody integrated molding body process. It was first used in Apple iPod, iPad, MacBook, and finally realized in the iPhone 5 generation, leading the craze for all metal phones.

iPhone 5、 6 adopts aluminum alloy integrated molding, that is, the body and frame are both CNC machined from a single piece of aluminum alloy. However, considering the RF signal problem of the mobile phone, the body will be divided into several sections, and injection molding or other partitions are generally selected at the upper and lower ends.

In order to achieve a higher quality appearance, surface treatments such as anodizing, sandblasting, and polishing will also be carried out to achieve unique colors and smooth touch.