Problem Definition
Dry-electrode processing forms a film by rolling powder without solvent. News confirmed in August that Naintek had begun performance verification of dry-electrode process equipment on a domestic all-solid-state battery maker’s actual mass-production line. Wet slurry coating secures film support strength during solvent-evaporation drying, but dry rolling carries a mechanical contradiction: the initial film has no self-supporting strength and must be transported to the next nip in that state. What conditions must be satisfied for the film to be transported in close contact to the next rolling stage without departing from the roll?
Kinematic Analysis
A multi-stage calender consists of a horizontal array of 3, or 6–7, rolling rolls. Each roll is driven by an individual servo motor, with the downstream roll’s surface speed set higher than the upstream roll’s. This speed differential induces shear in the film passing through the nip, adhering the film to the downstream roll’s surface. As long as this adhesion force is maintained, the film is transported along the roll without a separate idle roll. Nip gap and roll surface temperature are individually controlled targets, and the final roll assists current-collector lamination through temperature control.
Calculation / Formula Verification
Roll surface speed is defined as $v_i = \omega_i \cdot r_i$, where $\omega_i$ is the roll’s angular velocity (rad/s) and $r_i$ is the roll radius (mm). Shear strain rate through the nip is approximated as $\dot{\gamma} = \dfrac{v_2 – v_1}{h}$, where $h$ is the nip gap (mm). If shear strain rate is too low, the film remains on the upstream roll and adhesion transfer fails; if too high, film cracking occurs. The actual $v_2/v_1$ ratio and target shear strain rate vary with the material’s cohesive strength and should be confirmed after field measurement.
Roll deflection under nip load is approximated with a simple-beam model as $\delta = \dfrac{F L^3}{48EI}$, where $F$ is nip load (N), $L$ is roll effective length (mm), $E$ is elastic modulus (N/mm²), and $I$ is the second moment of area (mm⁴). Deflection $\delta$ must be managed to no more than half of the film-thickness design tolerance $\pm t_{tol}$ (mm) to secure width-direction thickness uniformity — meaning deflection directly translates into film thickness deviation.
The safety factor is dually verified. The structural-stiffness safety factor must satisfy $SF_{static} = \sigma_{allow} / \sigma_{max} \geq 2$, and, separately, the repeated load from continuous rolling must satisfy $SF_{fatigue} = \sigma_{endurance} / \sigma_{max} \geq 2$. A design that satisfies only one of the two safety factors while omitting the other carries fatigue-failure risk.
Shop-notes
- An alternative mechanism of belt-supported scatter coating could be considered: the belt supports the powder as it enters the nip, lowering the film-self-support requirement, but belt-tracking error management and belt surface-wear replacement cycles add extra management items, so it was not adopted for this verification.
- Machinability note: chrome plating or ceramic coating used to control roll surface hardness has plating-thickness deviation that directly translates into roll roundness error, so roundness tolerance must be separately managed in the post-plating grinding process.
- Motor capacity and roll surface-hardness specifications should be confirmed after field measurement.
Design-Reflection Checklist
- Measure per-roll surface-speed ratio and calculate shear strain rate
- Compare nip-load-based roll deflection against film-thickness tolerance
- Calculate static and fatigue safety factors separately and report both
- Include a roundness re-inspection item after roll surface coating
One-Line Summary
A dry-electrode multi-stage calender achieves film-adhesion transport through roll speed differential, but nip deflection must be kept within thickness tolerance while dually confirming static and fatigue safety factors.