In 2007, the first iPhone was released. In ten years, Apple released a total of twelve generations and nineteen iPhones. This year is the tenth anniversary of Apple's mobile phone, and Apple has released three iPhones in one go, the most notable of which is the iPhone X.

The three newly released iPhones all use double-sided glass + metal middle frame design

1. Why is stainless steel used
As Apple's high-end flagship, iPhone X has been replaced with a similar front and rear cover to the iPhone 4 series, using glass and supplemented by a stainless steel middle frame design. As we all know, aluminum alloy frame has many advantages, as follows:
1) Aluminum alloy has excellent comprehensive performance
2) Aluminum alloy is cheap, stainless steel is expensive
3) Aluminum alloy material has strong dyeability
4) Beautiful appearance, small density of aluminum alloy makes the equipment light and thin
5) Strong metal texture, easy to process high-grade, beautiful feeling
6) High temperature resistance, no handprint, antistatic, environmental protection and non-toxic

Aluminum alloy is known as God's gift to the 3C industry, and there are so many advantages, so why choose stainless steel?

The iPhone 6 used to be a seething curved door
Lenovo's once seething "iPhone 6 curved door", the shortcomings are obvious - the aluminum alloy is soft, the light ones bump and leave marks, and the heavy ones are bent and deformed. At the same time, double-sided glass, ceramic mobile phones, and full-screen era have put forward higher requirements for mobile phone frames, and stainless steel frames are also a trend.

Advantages and disadvantages of stainless steel material as a middle frame of a mobile phone
Although stainless steel processing is difficult, man-hours will increase and the cost is high. However, there are still some mobile phone terminal companies that choose stainless steel as the middle frame material. Among them, the iPhone 4 is indispensable.

iPhone 4 Strong, striking and avant-garde stainless steel bezel
From iPhone 4, Xiaomi 4 to iPhone X, Xiaomi 6, stainless steel in the aluminum alloy is king mobile phone frame world has also broken a remarkable and promising road.

History of stainless steel middle frame

2. Using stainless steel, it is impossible to escape the process and other material choices
The future of mobile phones will be ceramics, glass as the appearance of dominance, the requirements for the middle frame will be higher, must use high-strength aluminum alloy or stainless steel, and from the perspective of strength, more inclined to the latter.

Stainless steel makes the phone no longer afraid of bending

The mainstream of stainless steel material surface treatment process is PVD (Physical Vapor Deposition) technology. After "sandblasting, wire drawing or polishing + PVD", the metal surface can be coated with a decorative coating with high hard plating and high wear resistance.
PVD technology is currently the international high-tech and widely used ion coating technology, it has the characteristics of dense and uniform coating layer, strong adhesion, good plating, fast deposition speed, low processing temperature and a wide range of plating materials.
The selection of PVD process puts forward new requirements for the selection of nano-injection parts: taking vacuum evaporation as an example, the metal needs to be heated under high vacuum, melted, evaporated, and cooled to form a metal film on the surface of the sample to obtain a mirror-like surface.

PVD process
Because the post-treatment process of stainless steel frame involves PVD and radium carving process, the processing temperature reaches 180 degrees, which requires nano-injection molding materials to be able to withstand high temperatures. High Temperature Resistant Nylon (HPPA) came into being, especially when connected to stainless steel, clad metals and liquid metals.

The subsequent laser carving process also requires the matching of other related materials

3. Nano injection molding material selection
Since RF signals cannot penetrate metal, special processing designs are required in most cases. The commonly used method is to cut off the metal frame where there is an antenna, and make an injection molding connection at the cutting point.
Commonly used nano injection molding materials are PPS (polyphenylene sulfide), PBT (polybutylene terephthalate), PA (nylon, polyamide) and PPA (polyphthalamide). Due to the high processing temperature of the stainless steel frame post-treatment process, the nano-injection molding material actually used needs to be resistant to high temperature, and high-temperature resistant nylon HPPA is one of the emerging and cutting-edge materials that meet this requirement.
HPPA, a kind of PPA, has excellent material properties and can be called an excellent matching material for stainless steel frames.