Foldable display device

A foldable display device, comprising:

• a shell having an expanding state and a folding state, and comprising a first shell and a second shell, wherein the second shell is pivoted on the first shell for rotating around a rotating axis relative to the first shell so as to have an expanding position corresponding to the expanding state and a folding position corresponding to the folding state, and the first shell has an accommodating slot, and
• a flexible display having a main body section and an expandable section which are connected to each other, the first shell having a first surface covered by the main body section and a second surface opposite to the first surface, and the accommodating slot being located at the second surface;
• wherein, a first direction and a second direction are defined to be orthogonal, and the second direction is parallel to the rotating axis;
• when the shell is at the expanding state, the shell has a first length L1 along the first direction and a second length L2 along the second direction, the ratio of the first length L1 to the second length L2 is between 1.3 and 2;
• when the shell is at the folding state, the shell has a third length L3 along the second direction and a fourth length L4 along the first direction, and the ratio of the third length L3 to the fourth length L4 is also between 1.3 and 2;
• wherein when the second shell is located at the expanding position, the expandable section is adapted to move relative to the first shell and cover parts of the first surface;
• the main body section is assembled with the second shell, the expandable section is movably assembled with the first shell;
• when the second shell is located at the folding position, the expandable section is disposed in the accommodating slot;
• when the second shell is located at the expanding position, the flexible display is stretched by the first shell and the second shell, such that the expandable section moves outward from the accommodating slot;
• and
• when the second shell is located at the expanding position, the flexible display has a fifth length L5 along the second direction, the main body section has a sixth length L6 and the expandable section has a seventh length L7 both along the first direction, the foldable display device has a thickness T, and the folding display device satisfies the following condition:
• if L5:(L6+L7)=1:α, then L7≧(α−2/α)−T.