Folding mobile phones will represent 1.1% of the market in 2022, says TrendForce.
Folding phones have been launched by Samsung, Xiaomi, and Motorola.
TrendForce divides them into three classes: into easy to carry, vertical flip-top folding, and horizontal folding.
An inward fold requires an extra secondary screen and the outward fold is lighter in weight but exposes the screen to the risk of scratches.
Although the outward folding radius is larger, the mechanical performance of the module and hinge design is very challenging and, since the screen is exposed, it must be kept in close contact with the hinge at every angle.
The radius for inward folding screens is smaller, though compressive stress is greater, which promotes creases and related reliability issues.
However if an inward fold can effectively reduce weight and eliminate creases, coupled with the advantage of the screen being protected by the mobile phone case, this type of device would be more likely to be popular with consumers.
In terms of specifications, the size of vertical folding screens is approximately 6.5~7 inches and the size of horizontal folding screens is 7.5~8 inches.
The refresh rate of the two primary screens is 120Hz, while horizontal secondary screens have advanced from 60Hz to 120Hz and vertical secondary screens are limited by screen size, thus, maintaining a 60Hz refresh rate.
The introduction of LTPO delivers a low refresh rate to conserve power and the follow-up development of next-generation LTPO2.0 dynamic intelligent control technology maintains smooth frequency conversion.
The Xiaomi Mix fold 2 adopts the same design methodology as the Samsung fold 4. It uses low-temperature CF as a polarizer replacement to increase light output efficiency coupled with an AR (anti-reflection) coated UTG (ultra-thin glass) to reduce the impact of polarizer removal.
Overall thickness is greatly reduced which is beneficial to folding. However, in order to perfectly present the effect of a flexible folding screen, it is necessary to simulate and optimize film material, stacking, hinge selection, and mid-frame design, and to find the most suitable combination based on reliability and cost considerations.
For example, the hinges of key components are U-shaped and teardrop-shaped. A U-shaped bending radius is small and stress is concentrated within a smaller area while the teardrop-shaped bending radius is larger but its composition is more complex and can easily require hundreds of parts.
In addition, teardrop shaped material options include liquid metal, carbon fiber, Ti alloy, stainless steel, plastic, etc., and can cost at least 3 to 4 times more than the U-shaped hinge.
In this iteration, the Xiaomi hinge is made of carbon fiber material which reduces weight by 35%. With its self-developed micro-teardrop shaped hinge, average thickness is reduced by 18% compared with other models, making the thickness of the entire folded device very close to the thickness of an iPhone 13.