Metal forging

Once the steel plate has been pressed and cut, holes are punched into the center. These holes can be used to hold the steel plate in place during subsequent processing steps. 100-tonne impulse drawing, oil hydraulic press forging, and preliminary outline forging are used to shape the frame of the phone. Additional processes such as annealing then further harden the steel. Reforming and cleaning complete the 8-step forging process.

CNC milling

A total of 58 milling operations make up the 8-step CNC milling process. First the frame outline, antenna partition grooves and curved recess are milled, then the tiny 0.6mm-diameter speaker holes, the headphone jack, and the holes for the buttons. The extreme precision of the CNC milling process makes a big difference to the premium overall feel of Mi 4. After each CNC milling stage, the frame undergoes a “reforming – deburring – reforming” process to ensure no residues interfere with the next process step.

3 steps to the perfectly
polished surface

The first step is rough polishing, which lasts 100 seconds and removes the milling imprint from the surface of the frame. A wet medium polish with wax then smoothes any residual rough areas. Lastly, a final mirror polish gives the curved recess a glossy finish.

Ultra-fine sandblasting, perfectly smooth

To give Mi 4 the best possible feel, we fine-tuned our sandblasting process over more than 6 months. We tried 4 different suppliers, and carried out over 100 tests with 7 different types of blasting media before finally settling on the best one. Before sandblasting, the chamfer edge is polished with oil ink, which is then baked on to form a protective layer, and the side of the frame is ground and buffed to reduce surface roughness. Compressed air sandblasters are then used to blast the back and sides of the frame with zirconium microspheres until a silky smooth finish is achieved. To ensure every Mi 4 has the same beautiful feel, the blasting media are replaced every 2 days – twice as often as on most assembly lines.

PVD coloring – deep, lasting
color, ultimate fingerprint resistance

PVD coloring is the benchmark technique for achieving a rich, deep black color. Using a process called sputtering carried out in a vacuum chamber, the color is bonded to the metal of the frame at the atomic level. This resulting color is highly abrasion and fade-resistant, and the process actually strengthens the metal as well. A nano-grade Anti-Fingerprint (AF) coating process protects against fingerprint marks. The process we have chosen is the most advanced in the industry, as shown by its results in water droplet testing. The greater the contact angle of a water droplet that falls on a surface, the more hydrophobic – and therefore fingerprint-resistant – that surface is. For the AF coating on the Mi 4, the contact angle was a massive 110°.

Note: The PVD coloring and AF coating are only applied to Mi 4's black frame.

Laser cutting to complete the frame

A highly focused laser beam vaporizes stainless in seconds, separating the frame cleanly from the unneeded metal. Skilled technicians then polish the frame to remove all burrs and residues in the cut-off areas. Before the frame is passed complete, it is inspected no fewer than 3 times to ensure the quality of the finish.

Edge Coating for screen protection

Mi 4 is the first smartphone to feature Edge Coating technology. A 0.05mm-thick layer under the glass with a special Edge Coating acts as a shock absorber if the phone is dropped, reducing the chances of the screen breaking.

A chamfer this smooth
takes serious craftsmanship

Just polishing the top chamfer on Mi 4 takes 13 steps, involving 37 individual processes, and more than 6 hours. The router bit used on the Mi 4 gives a 1.35mm chamfer.
It then undergoes a 3-stage polishing process, culminating in a final mirror polish which gives the chamfer its distinctive brilliant finish.

Just 0.3mm thick

Between the CNC-milled chamfer edge and the glass of the touchscreen is a thin, flat frame that protects the screen. The flat surface of the frame is just 0.3mm across.

Antenna insulation
for optimum signal quality

To optimize signal quality, the grooves in the frame that separate the antennae from the rest of the phone are injected with plastic. This insulation means that your fingers and palm cannot act as a conductor between the WiFi and mobile antennae, which would interfere with reception.