The Geometry of a Sink Mark: Why "Strong" Ribs Ruin Cosmetic Plastics

You design a perfectly flat, highly polished plastic enclosure. It needs structural stiffness to pass drop testing, so you add a grid of thick reinforcing ribs to the inside. But when the first molded parts arrive, the outside cosmetic surface is marred by shadowy depressions that perfectly mirror the rib pattern underneath.

You didn't get a bad batch of plastic. You violated the thermal mass ratio at the T-junction.

Plastics undergo significant volumetric shrinkage as they cool (often between 0.5% and 2.0%). The physics of this cooling dictate that polymer melts always shrink inward toward their thermal center of mass.

To predict if a rib will cause a sink mark on your cosmetic surface, you don't necessarily need complex FEA software. You just need to calculate the Effective Wall Thickness (T_eff) at the junction.

At a sharp 90° T-junction, the longest heat-flow path (the thermal center) is the diagonal hypotenuse connecting the two outer corners.

The Equation:

T_eff = √ (T² + t²)

Where:

• T_eff = Effective thickness at the junction
• T = Nominal wall thickness of the part
• t = Base thickness of the rib

💡 The Example

Let’s say your enclosure’s nominal wall (T) is 3.0 mm.
To maximize stiffness, you design the rib with the exact same 3.0 mm thickness (t).

Let’s calculate the thermal center at that junction:
T_eff = √ (3.0² + 3.0²) = √ (9 + 9) = √ 18 = 4.24 mm

Now, apply Fourier’s Law of thermal conduction, which states that cooling time is proportional to the square of the thickness:
(4.24)² / (3.0)² = 18 / 9 = 2.0

The Result: The intersection where your rib meets the wall has 100% more thermal mass than the rest of the part. The surrounding 3.0 mm walls will freeze solid and lock their dimensions, while that massive 4.24 mm core remains a hot liquid. When that core finally cools and shrinks, it pulls the already-frozen cosmetic surface inward, creating a permanent, visible crater.

🛠️ The Solution

To eliminate sink marks, you must mathematically starve the T-junction of thermal mass.

1. The 60% Rule: Never make a structural rib the same thickness as the nominal wall. The rib base (t) should strictly be 50% to 60% of the wall thickness (T).
   Re-running the math with a 1.5 mm rib:
   T_eff = √ (3² + 1.5²) = 3.35 mm
   The thermal mass ratio drops from 2.0 down to 1.24, effectively eliminating the sink defect.
2. Watch the Fillets: To prevent stress concentrations (as discussed in the k-factor), you must add a radius where the rib meets the wall. But a massive radius acts like a giant block of plastic, ruining your T_eff math. Keep junction fillets small—ideally 0.25 × T—to balance stress relief with thermal mass.
3. Core it Out: If your structural calculations absolutely demand a thick feature (like a heavy mounting boss), do not mold it solid. "Core out" the base of the feature from the underside, leaving only a thin ring of plastic that maintains the 60% ratio against the cosmetic face.

Great design for manufacturability (DFM) means finding mechanical strength without paying a thermodynamic penalty.