There is a slight difference between laminitis and founder.
Founder is elevated laminitis. In a broad sense, you could liken it to a human catching a horrible cold/bronchitis that did not get tended to and it elevated into pneumonia
Here are two credible articles that may help you The Two Phases of Laminitis: News: Kentucky Equine Research Laminitis vs. Founder – America’s Horse Daily
The are also different types of founder:
1. Road founder is caused by riding a horse beyond a walk for lengthy periods of time on a hard surface such as a paved road.
2. Grain founder generally happens when the horse manages to get into the grain bin and eat at least 20 pounds of feed.
3. Grass founder generally happens when a horse is insulin sensitive (think Type II Diabetes in humans) and the grass fructans & starches from too many hours of grazing cause the horse's insulin to rise. The result is often inflammation inside the hoof.
It is this type of founder that requires a horse to be put on a grain-free diet and have limited or no pasture time. Much depends on the horse and its living conditions