Surface Finishing of Precision Castings: From Blasting to Zinc Plating and Polishing
Investment casting (the lost-wax method) gives a part very good geometric accuracy and low surface roughness compared to sand casting — typically around Ra 6.3 (N9 per DIN) as cast. Even so, the raw casting is usually just a semi-finished part — it's the machining and surface operations that follow which determine the final functional dimensions, appearance, and durability of the part. In this article, we look at what processes follow casting and what determines the right combination for a given part.
Removing the Gating System and Initial Cleaning
The first step after the casting has cooled is removing the gating system and the residual ceramic shell. This is followed by blasting, which evens out the surface, removes the oxide layer formed during solidification, and prepares the part for further processing — whether that's machining or a direct surface treatment.
Machining to Final Dimensions
As cast, dimensional tolerances typically meet VDG P690 D1, which isn't sufficient for many functional surfaces, threads, or sealing diameters. That's why machining follows — milling, turning, drilling, grinding, or multi-axis CNC machining — to bring critical surfaces to the precise dimension and a lower roughness than casting alone can achieve.
Zinc and Nickel Plating
For parts exposed to corrosive environments, electroplating is used. Zinc plating is a common corrosion protection for steel castings, while nickel plating adds a harder, more durable surface as well as an improved appearance. The choice between the two depends on the casting material, the operating environment, and the customer's requirements for service life.
Painting
Where an aesthetic or additional protective layer is needed, the casting is painted. The coating protects the part from environmental effects while also allowing the part's appearance to be matched to customer or end-application requirements.
Polishing
For applications with higher demands on smoothness or appearance, the casting is polished. Polishing removes residual unevenness left by blasting or machining, giving the surface lower roughness and a glossy finish — typically used on visible or functionally demanding surfaces.
Heat Treatment
In addition to surface finishing, castings may also undergo heat treatment, which affects the mechanical properties of the material — normalizing annealing, soft annealing, solution annealing, quenching and tempering, or tempering, depending on the requirements for the specific material and application.
Choosing the Right Combination
The final specification of machining and surface treatment always depends on the casting material (carbon, low- and medium-alloy, corrosion-resistant austenitic and martensitic, tool, or heat-resistant steels), the part's function, and the environment in which it will operate. It's advisable to specify the required roughness, treatment type, and tolerance already in the inquiry documentation — this saves time and avoids additional costs later in production.