Introduction
The VDI surface quality is a designation for the roughness of a surface. This article shows how VDI structure can be simulated in Speos on metallized and opaque surfaces.
The VDI guideline 3400 is a standard of the Association of German Engineers, which defines the surface texture of tool and molded parts with a scale of roughness values. This scale usually ranges from 0 to VDI 45. Each VDI value is assigned to a specific arithmetic mean of roughness Ra (in µm). A higher VDI value means a rougher surface, while a lower VDI value indicates a smoother surface.
Overview
Originally, these VDI textures were mainly created by electrical discharge machining (EDM). In EDM machining, electrical discharges create craters on the surface. Their size and density determine the roughness. Spark-eroded surfaces have a characteristic crater-shaped structure that differs from the surfaces of other machining processes. The VDI values are specially tailored to these EDM-typical textures.
Another manufacturing process is chemical etching, in which material is selectively removed by a controlled chemical reaction.
In our free Ansys library we provide *.unpolished files to model surface roughness of transparent materials for EDM and chemical Etching VDI structures.
Unpolished file (*.unpolished)
An unpolished file simulates the behaviour of an unpolished surface in transmission or reflection. The transmission/ reflection of a ray will depend on the underlying material refraction indices.
These unpolished VDI structure files are normally used as Surface Optical Property (SOP) on transparent materials. However, there is also the possibility to use these unpolished files for opaque and metallized materials.
VDI structure on opaque material
To model VDI structure on an opaque material the opaque part must have a *.material applied as Volume Optical Property (VOP). The *.material must be built following the characteristics of the material itself for index and absorption variation.
*.material file applied as VOP for the opaque material
VDI.unpolished applied as SOP
The *.material can be created with the User Material Editor. Under downloads are two examples available that can be used and adjusted by the specific needs.
First example is a black opaque PC. On the left you see the Refractive Index and Abbe Number and, on the right, the high absorption coefficient because the material is black.
Tabs related to Scattering can be filled as suggested here:
Second example is a white opaque PMMA with index of refraction and Abbe Number shown in the left picture. Because the material is white there is no absorption coefficient (right picture).
The Scattering tab must be filled in the same way as for a black, opaque material. If your part is truly opaque (meaning it should not allow any light transmission), consider increasing the diffusion coefficient (mm⁻¹) in the Scattering Variation tab. A low value such as 2 may cause the material to appear slightly translucent. Increasing it to around 50 will prevent light from passing through and ensure the material behaves as fully opaque.
Note: If you have more data about the absorption spectrum, you can use them for a more precise analysis.
If you have another coloured material as an opaque material (red, green, etc.) you should use its correct absorption spectrum.
After definition of the opaque material, the VDI.unpolished can be added as surface property.
Below you can see a simulation result with a VDI 35 EDM manufactured structure on black opaque PC.
VDI structure on metallized surface
There is also a process to use the VDI.unpolished structure files for metallized surfaces.
- Define metallic VOP material with optical polished as SOP
- Using VDI.unpolished structure as FOP
Define metallic VOP material with optical polished as SOP
In a first step we must create a metallic material for the metallized surface. Therefore, launch the user material editor and choose “Metallic” as Material Type.
For metallic material the index values must be set directly in the table.
The Refractive Index here is composed of two parts: the real part and the imaginary part. The index of refraction corresponds to n-i*k, n being the real part and k the imaginary part. You can select the Real and Imaginary check boxes in Spectrum to respectively display real and imaginary parts' spectra.
The values for real part and imaginary part can be used for example from refractiveindex.info
The data for n and k can be downloaded there as CSV and then inserted into the material file via Notepad.
Once you have the values for n and k, open the created metallic material file with a text editor and insert the values as described below.
In the fifth row put the number of lines you have in the csv file.
The first column should contain the values for wavelength, second column the values for n (real part) and third column the values for k (imag. part).
After inserting all values save the file and open it again in the material editor to check the spectrum for real and imaginary part.
Next create a VOP material with this metallic.material file and set SOP to optical polished like shown below.
Using VDI.unpolished structure as FOP
For the VDI structure the VDI.unpolished files from the Ansys library can be used. They must be applied as Face Optical Property (FOP) on the surfaces.
Below is a result example for silver material, on the left with optical polished property and on the right with VDI12_chemical etching.