Turn an empty plastic storage box over and the inside is covered in thin lines. They are not there to look technical. A moulded box is a thin shell trying to do three incompatible things: use as little material as possible, come out of a steel mould without fighting it, and hold a stack of other boxes without folding. The ribs are the answer to all three.
Thin walls, not thick ones
The obvious way to stiffen a wall is to make it thicker. In injection moulding that is the wrong move. Plastic cools from the outside in; a thick section keeps a hot core that shrinks later, pulling a visible dent into the surface and leaving internal stress. Worse, it takes longer to cool, and cooling time is most of the cycle. A thin wall with a network of ribs is stiffer, lighter, and out of the machine sooner.
A rib is therefore a stiffener with a budget. Make it too tall or too thick relative to the wall and it becomes the thick section you were avoiding, with its own sink mark on the outside. The lines you see are usually shallow, evenly spaced and rounded at the base for exactly that reason.
Everything leans a little
A moulded box is never a perfect cube. Its walls taper slightly and its corners are rounded. The taper — draft — is what lets the part release from the steel: a straight vertical wall grips the cavity and has to be forced out. Rounding does two jobs at once, letting molten plastic flow around the corner without a weld line and removing the sharp inside corner where a crack would start.
The taper has a consequence you can feel. Boxes of the same design nest into one another when empty, which is why a stack of them takes up surprisingly little room in transit. Lids use the same geometry in reverse: a shallow ring of ribs locates the lid so it seats the same way every time.
The stack decides the wall
When boxes are filled and stacked, the load does not go through the bottom alone. It travels up the side walls and through the rim, so the rim is thickened or ribbed and the corners carry more than the middle of a wall. A box that is stiff enough empty can still bulge after a summer under three full boxes, because plastic creeps slowly under a constant load. Ribs are placed where that slow movement would show first.
Drop behaviour is designed the same way. A sharp corner concentrates the impact into one point; a rounded corner spreads it. This is why the cheapest-looking box often survives a fall better than a crisp, glossy one: the softness at the edges is doing work.
What the mould leaves behind
Look for a small circle on the bottom — the gate, where the plastic entered — and a faint line where the two halves of the mould met. Then look at the ribs. Their direction tells you which way the maker expected the wall to bend, and their spacing tells you how thin the wall was allowed to be. A box is a drawing of its own mould, printed on the inside.