TL;DR
Surface finish in investment casting refers to the texture characteristics of cast parts, including roughness (microscopic peaks and valleys), waviness (larger undulations), and lay (directional patterns). The quality of these surfaces is measured using three key parameters: Ra (average roughness), Rz (peak-to-valley height), and RMS (root mean square roughness), with typical investment castings achieving Ra values between 1.5 to 6.3 micrometers.
Surface Texture
Surface finish in investment casting refers to the texture and smoothness of the cast part’s surface after the manufacturing process is complete. More precisely, surface finish is a component of the broader term “surface texture,” which describes the overall topography of a surface. Surface texture is generally understood to comprise three distinct elements :
Roughness
Roughness refers to the fine irregularities on a casting’s surface that result from the investment casting process itself. These microscopic peaks and valleys typically measure between 0.1 to 25 micrometers in height.
Waviness
Waviness describes the larger, more widely spaced irregularities that appear as gentle undulations across a cast surface. These waves typically have spacing greater than the roughness sampling length
Lay
Lay indicates the predominant direction of surface texture patterns. In machined parts, lay follows the cutting tool’s path, but investment castings typically show random or non-directional lay patterns.
Surface Roughness Parameters
Measuring surface texture requires standardized parameters that quantify what our fingers feel as smooth or rough. The investment casting industry uses three primary measurements to specify and verify surface quality.
Ra
Ra represents the arithmetic average of absolute values of roughness profile deviations from the mean line. Think of it as the average height of all the microscopic peaks and valleys across a measured length.
Rz
Rz measures the average distance between the highest peaks and lowest valleys within sampling lengths. Unlike Ra, which averages everything, Rz focuses on the extremes that often matter most for sealing surfaces or wear resistance. The measurement divides the evaluation length into equal sections, finds the maximum peak-to-valley height in each section, then averages these values.
RMS
RMS roughness, also called Rq, calculates the root mean square average of roughness profile deviations. This parameter weighs larger deviations more heavily than smaller ones, making it sensitive to occasional deep valleys or high peaks.
Comparative Surface Finish Capabilities of Casting and Other Processes
- Investment Casting: Typically achieves Ra values in the range of 1.5 to 3.2μm (60 to 125μin), although it can extend up to 5μm or even 6.3μm (200 to 250μin) for some alloys or less controlled processes.
- Sand Casting: Produces significantly rougher surfaces. Typical Ra values range from 12.5μm to 200μm (500 to 7900μin). Investment casting offers a vastly superior surface finish compared to sand casting.
- Die Casting: Generally achieves finer surface finishes than investment casting. Typical Ra values for die casting are in the range of 0.8 to 1.6μm (32 to 63μin). This is because the molten metal solidifies against a smooth, polished metal die surface.
- Machining Processes:
- Milling and Turning: Standard milling or turning operations typically produce Ra values from 0.8μm to 6.3μm (32 to 250μin). Investment casting can produce surfaces comparable to a good milled surface where machining lines are just visible (around 3.2μm Ra or 125μin Ra).
- Grinding: Achieves much finer finishes, typically Ra 0.1μm to 1.6μm (4 to 63μin). This is generally smoother than as-cast investment casting finishes.
- Polishing, Lapping, Honing: These are precision finishing operations that yield very smooth surfaces, often Ra<0.4μm (<16μin), with polishing and lapping capable of achieving Ra 0.025μm (1μin) or better. These are significantly finer than as-cast surfaces.
| Process | Typical Ra Range (μm) | Typical Ra Range (μin) | Key Characteristics/Notes |
|---|---|---|---|
| Sand Casting | 12.5 – 200 | 500 – 7900 | Coarsest finish; significant machining usually required for smooth surfaces. |
| Investment Casting | 1.5 – 6.3 | 60 – 250 | Good as-cast finish, superior to sand casting; can be comparable to standard milling. |
| Die Casting | 0.8 – 1.6 | 32 – 63 | Generally finer finish than investment casting due to metal molds. |
| Milling / Turning | 0.8 – 6.3 | 32 – 250 | Standard machining finish; investment casting can match this. |
| Grinding | 0.1 – 1.6 | 4 – 63 | Finer finish than typical investment casting or standard machining. |
| Polishing / Lapping | 0.025 – 0.4 | 1 – 16 | Very fine, mirror-like finishes; significantly smoother than as-cast methods. |
Typical As-Cast Surface Finish Values
| Alloy Type | Typical As-Cast RMS Range (μin) | Approx. As-Cast Ra Range (μin) | Approx. As-Cast Ra Range (μin) |
|---|---|---|---|
| Carbon Steel | 90 – 125 | 81 – 113 | 2.0 – 2.8 |
| Stainless Steel (300 Series) | 90 – 125 | 81 – 113 | 2.0 – 2.8 |
| Stainless Steel (400 Series) | 100 – 125 | 90 – 113 | 2.2 – 2.8 |
| Tool Steel | ~120 | ~108 | ~2.7 |
| Aluminum Alloys | 60 – 100 | 54 – 90 | 1.4 – 2.2 |
| Titanium Alloys (e.g. Ti-6Al-4V) | 60 – 200 (underlying) | 54 – 180 (underlying) | 1.4 – 4.5 (underlying) |
| Copper Alloys | 60 – 100 | 54 – 90 | 1.4 – 2.2 |
| Cobalt Chrome Alloys | 80 – 100 | 72 – 90 | 1.8 – 2.2 |
