In early March 2025, Xiaomi launched its "Lightning Yellow" stainless steel drinking cup. Priced at just ￥29.9 (about $2.1/cup) and designed with an eco-friendly concept, it quickly became a hot item. This cup, which matches the "Lightning Yellow" color of Xiaomi's SU7 Ultra electric sports car, sold out soon after it was released. Its price on the second-hand market even soared to ￥198.0  (about $13.8/cup). It has become another viral hit after Xiaomi's car.

However, behind its popularity lies the high difficulty of stainless steel processing and the complex game in the industry chain. This 350ml drinking cup may look simple, but it faces many technical challenges in material selection, forming process, and surface treatment. In fact, it is a microcosm of the industry's technological upgrade.

The Xiaomi drinking cup uses food-grade 304 austenitic stainless steel (also known as 06Cr19Ni10). It has a nickel content of 8%-10.5% and a chromium content of 18%-20%. This makes it really good at resisting rust and easy to work with. However, the material's high plasticity (it can stretch a lot), low thermal conductivity (it doesn't transfer heat easily), and tendency to harden during processing make it harder to work with from the start.

For example, when 304 stainless steel is pressed and stretched, its high stretchability can cause the cup walls to be uneven in thickness. To fix this, we need very precise mold designs and multiple stretching steps to make sure the walls are even (about 0.5-1.0mm thick). Also, the amount of impurities like sulfur and phosphorus in the material must be kept below 0.03%.

Xiaomi's stainless steel cups with a paper-like coating have caught the industry's attention recently. Based on the product's features and the trend toward eco-friendliness, it is believed that Xiaomi is using food-grade spray paint instead of traditional electroplating or sandblasting. This choice fits Xiaomi's brand promise of "healthy and safe" and meets the chemical safety needs for drinking directly from the cup. Here's a closer look at the food-grade spray paint process:

(1) Carbonated Drinks: A Double Restriction from Base Material and Coating

Hidden Risks of No Inner Coating: The inner wall of Xiaomi's cup is not coated with a protective layer. When 304 stainless steel comes into direct contact with acidic liquids, such as phosphoric acid and citric acid in carbonated drinks, it can cause pitting corrosion. Over time, this may lead to rust stains.

Indirect Risks of the Outer Coating: If liquid splashes outside the cup, the residual acid can seep into the gaps between the coating and the stainless steel, worsening the corrosion.

(2)  Boiling Water Restrictions: A Thermodynamic Contradiction.

Different Thermal Expansion Rates: Stainless steel and the spray paint coating expand at different rates. Sudden temperature changes, like pouring boiling water, can create stress cracks at the interface, causing the coating to bubble or peel off.

Safety Concerns: The cup is made in one piece without an insulating layer. When boiling water is added, the rim temperature rises quickly, posing a risk of burning the mouth when drinking directly.

Xiaomi's stainless steel cup mimics the rolled rim design of disposable paper cups (with the cup's edge curled outward). While this design enhances visual appeal and holding comfort, it creates a major cleaning challenge over long-term use:

The popularity of Xiaomi's stainless steel cup proves that there is a strong market demand for "eco-friendly design." However, its rolled rim design and limitations in coating durability also show the challenge of balancing biomimetic design (imitating real-life objects) with long-term practicality. For CCY, this is both a challenge and an opportunity—we see more possibilities for innovation in stainless steel structures. By breaking through in surface treatment technology, CCY can also help the next generation of stainless steel daily products evolve toward the standard of being "eco-friendly and easy to use.