To find the length of the arc formed by a central angle in a circle, you can use the formula:
\[ \text{Arc Length} = r \theta \]
where \( r \) is the radius of the circle and \( \theta \) is the central angle in radians.
Given:
- Radius \( r = 27 \) mm
- Central angle \( \theta = \frac{2\pi}{3} \) radians
Substituting these values into the arc length formula:
\[ \text{Arc Length} = 27 \times \frac{2\pi}{3} \]
Calculating:
\[ \text{Arc Length} = 27 \times \frac{2\pi}{3} = \frac{54\pi}{3} = 18\pi \text{ mm} \]
Thus, the length of the arc is:
\[ \boxed{18\pi \text{ mm}} \]