Casein protein supplementation is essential for bulking since it helps provide a small trickle of protein to your muscles over a long period of time. It keeps feeding your muscle through the day or night to aid in a slower, steadier, and more efficient release of casein’s amino acids. However, I wouldn’t suggest taking casein directly after your workout. Right after your workout you need a fast-digesting protein like Whey.
Casein can be taken in the morning, first thing upon waking, or at night before bed to ensure your muscles get fed all through the night while you sleep. This will ultimately help your gains by reducing protein breakdown and amino acid oxidization.