What Surface Finish Does PVD Need on Zinc Castings?
The short answer is a pair of numbers, not one: Ra 0.5 µm and Rz 2–4 µm. Ra alone can pass while the surface still carries peaks the film cannot cover. The PVD coating copies the profile it lands on, so the pre-coat finish has to be described by both the average and the peak-to-valley distance.
Short Answer
PVD needs a surface that is clean, dense, and flat at the scale of the film. In practice that means Ra 0.5 µm with Rz 2–4 µm, no open porosity on the visible face, and an edge that has not been rounded by the polishing wheel.
The film is 0.5–3 µm thick. Any surface feature larger than that is reproduced, not hidden. That single ratio explains most of the finish requirements that follow.
What PVD Copies
PVD is a line-of-sight process. Metal vapor reaches the part, condenses on the surface, and builds a film that follows the existing geometry. There is no flow, no leveling, and no filling.
That explains why a polished surface and a coated surface look related but not identical. The coating adds color and wear resistance, and it adds contrast. A scratch that was faint before coating reads darker after, because the film sits in the groove and reflects differently from the surrounding face.
Ra and Rz as a Pair
Ra is the arithmetic average of the profile. A surface can sit at Ra 0.5 and still have isolated peaks of 6 µm, because the average is dominated by the valleys. Rz reports the peak-to-valley height, which is the number the film has to clear.
With a 0.5–3 µm film, an Rz of 2–4 µm keeps the peaks inside the coating. At Rz 6, the tallest peaks stand above the film and stay exposed. They read as scratches or as a dull patch under direct light, and no amount of top-layer thickness fixes it without changing the color.
Pre-coat finish | Film 0.5–3 µm | Result after coating |
|---|---|---|
Ra 0.5, Rz 2–4 | Peaks inside film | Even color, no exposed scratch |
Ra 0.5, Rz 6 | Peaks above film | Scratches and dull patches |
Ra 0.8, Rz 8 | Profile larger than film | Visible texture, color shift |
Open pore 20 µm | Far larger than film | Dark point, film lines the pore |
Polishing Sequence
The finish is produced in two steps: a roughing wheel removes most of the stock, a finishing wheel brings Ra down. The finishing step is also where the mating edge gets rounded, and that edge matters as much as the face.
A hard felt wheel with a small contact angle keeps the edge sharper than a soft wheel. A fixture that supports the mating face stops the wheel from rolling over it. Where the edge is already machined, the polishing step should stay away from it; where it is not, the radius has to be controlled and inspected.
Defects the Film Cannot Hide
Open porosity, cold shuts, drag marks, and rounded edges are geometry, not stains. Each one survives the coating step. A 20 µm pore is six to forty times the film thickness, so the film lines the pore wall and the pore stays visible.
The finish specification should name these as rejects before coating, not after. Once the part is coated, the only cure is stripping and rework, which costs more than holding the casting and polishing steps to the drawing.
Drawing Callouts
A usable drawing carries four lines: substrate and condition, the two roughness numbers together, layer stack with thicknesses, and masked faces. Add the acceptance tests: adhesion to ASTM D3359 at 4B–5B, and neutral salt spray to ASTM B117 for 48 hours.
For the machining step, see CNC post machining; for the coating side, see PVD on zinc die castings. The substrate question is answered in Can Zamak 5 be PVD coated, and the casting side in zinc die casting.
