Sink marks, and the trouble with thick sections
Why uneven wall thickness leaves a dimple on the visible face.
Sink marks appear on a moulded part because plastic shrinks as it cools from a molten fill temperature down to room temperature, and a thick section stays hot and shrinking for far longer than a thin one next to it, so the thick section keeps contracting after the quickly cooled surface around it has already set, pulling that surface inward into a shallow dimple.
A candle that sinks in the middle
A thick pillar candle, poured as hot wax into a mould and left to cool, shows the effect in a material almost anyone has watched solidify. The flat top, perfectly level the moment the wax is poured, often sinks into a shallow crater once the candle has fully set, and the crater is deepest right in the middle, where the wax is thickest and stays liquid longest while the edges have already firmed up. Nobody poured the candle unevenly, and nothing went wrong with the wax. The dip is the visible record of where the slowest-cooling material was still shrinking after the surface around it had stopped moving.
A skin that sets before the core has finished shrinking
Plastic cools from the outside in, because the mould wall touching it is comparatively cold and pulls heat away fast, while the plastic deeper inside a thick section is insulated by the material around it and cools much more slowly. A thin wall finishes cooling and locks into its final shape quickly, with little shrinkage left to happen by the time it is solid all the way through.
A thick section, or a rib or boss added to the back of an otherwise thin wall, behaves differently. Its outer skin solidifies early, like the thin wall around it, while the material still molten underneath goes on shrinking as it slowly cools. Once the skin has locked in place, that ongoing shrinkage can only pull the solid skin gently inward. The result is a shallow dimple directly opposite wherever the extra thickness sits. Plastics are especially prone to this because their thermal contraction is large and, in a thick section, slow to finish, which is why the same shape cast or machined in metal rarely shows it.
Ribs at two thirds of the wall
The standard fix is to avoid creating the uneven thickness in the first place, keeping walls as close to constant thickness as the part's function allows. Where a rib or boss is needed, the common rule is to make it no thicker than about two thirds of the wall it attaches to, often less on glossy parts, so that it cools at nearly the same rate as everything around it. A rib at full wall thickness looks reassuring on a drawing, but where it meets the wall the material is thicker still, and a designer who draws it that way often creates a visible dimple on the opposite face for a strength gain the thinner rib would have delivered almost as well.
Ribs, bosses and mounting features also go on the hidden side of a part wherever the design allows, keeping the cosmetic surface and the structural surface apart. A sink mark on a face nobody sees costs nothing, while the same dimple on a customer-facing panel gets the part rejected on a line that cares about appearance as much as function.
Not every thickness change produces a visible mark. A gentle, gradual transition between a thin wall and a thicker feature spreads the shrinkage mismatch over enough distance that no single point on the surface has to absorb it all, much as a smoothly tapered structural section avoids concentrating stress at a sharp step. A rib blended into its wall with a generous fillet can sometimes be made thicker than the rule suggests without leaving a mark, because the abruptness of the step between the two thicknesses decides whether the shrinkage shows as a sharp dimple or fades into a curve too gentle for the eye to find. The same fillet also eases the flow of molten plastic into the rib as the tool fills and removes a sharp inside corner where stress would otherwise gather, so the change tends to improve the part in more than one way.