Why Do Pits Appear on Polished Zamak Bezels?

A pit is a casting pore that the wheel has opened. Polishing removes 0.02–0.05 mm of skin from a Zamak 5 ring cast to ASTM B86 AC41A, and where the skin was thin over a pore the pore becomes a visible crater at the tumbling and polishing stage.

The ring is 1.8 mm high with a wall of 1.5–1.8 mm, so there is little metal between the surface and the casting's interior. A pore that sits just below the skin is invisible after casting and obvious after the first polish.

Why do pits show up only after polishing?

Because the pore has a lid before polishing. The die skin is denser than the metal behind it, and a pore that stops a few hundredths below the surface is covered by that skin.

The wheel cuts the lid off. Once the dense layer is gone, the pore is an open crater with a dark bottom, and the crater catches light differently from the surrounding polish.

The same pore appears again after plating. Plating bridges a small opening, but a crater that is deeper than the coating stays visible, and a blister can form over a pore that holds acid from the plating line.

Where do those pores come from in the casting?

From gas and shrinkage that the casting could not push out before the metal froze. In a thin ring the last metal to freeze is at the far side of the fill, and that is where the pores collect.

Gate and overflow position decide where that last metal lands. This ring uses a ring gate with overflows spread around the outer diameter, so the fill arrives from the inside and the cold metal leaves outward.

The melt and die temperatures decide how much gas stays dissolved. A melt of 400–420 °C and a die of 150–180 °C are the typical window for Zamak 5; outside it, gas has more chance to stay in the metal.

How are pits judged, and how many are too many?

Visually, with a 10× magnifier, on the polished ring under a defined light. The judgment is not a count of every pore; it is whether a pit is visible at the agreed magnification on the visible face.

The practical rule is a first-article standard: the customer signs a ring, and later lots are compared against it under the same light and the same magnifier. Without that signed standard, two inspectors will disagree on the same ring.

The visible face matters more than the hidden face. A ring has one face toward the wearer and one toward the case, and the drawing should say which face the acceptance covers.

Can polishing remove a pit?

No. Polishing removes metal from the surface; a pit is a hole below it, and taking more metal makes the hole wider rather than shallower.

Extra polishing also spends the 0.03 mm flatness budget. A ring re-polished to chase a pit loses flatness on the same face, and the flatness check on a surface plate rejects it.

The recovery, if any, is at the casting stage: change the fill so the pore does not sit under the visible face. Once the ring is polished and on the bench, the pit is a scrap decision rather than a repair.

Which casting settings keep the skin dense enough to polish?

Four settings and one layout: melt temperature, die temperature, intensification pressure, cycle time, and where the gate and overflows sit. These are the variables a foundry can move without touching the ring's geometry.

Pressure does the work on skin density. A higher intensification packs the metal against the die wall before the gate freezes, and a denser skin means fewer opened pores after polishing.

The layout matters as much. Moving an overflow changes where the last metal freezes, and moving the gate changes where the hot metal enters; both can move a pore away from the visible face without changing any tolerance.

Send the visible-face standard and the magnification with the enquiry. The casting settings can then be quoted against a polish result, and the die casting and polishing steps can be priced on the same acceptance.