As a mechanical engineer, you may encounter various surface treatment specifications on product 2D drawings, some requiring VDI, some requiring MT11010, and others labeled SPI-B1, which can be quite confusing. When you ask the on-site technicians about it, they might have only partial knowledge. Eventually, they explain that these are different standards for surface roughness, and then they advise you to look up the details yourself. But isn't surface roughness described by Ra values? What is the relationship between these standards and Ra values?
This article will provide you with identification and usage guidelines for common gating specifications, helping you avoid confusion caused by technicians who may have partial knowledge and pretend to understand it all on-site!
Surface treatment encompasses two meanings: one is surface roughness, and the other refers to the type of surface pattern. Therefore, different specifications represent different types of patterns, while different grades in the specifications represent the depth of roughness.
Surface roughness
The common standards for surface roughness include Ra (arithmetic mean roughness), Ry (maximum height), and Rz (ten-point mean roughness), defined as follows:
In addition to Ra, there are also Sa, Sy, and Sz. The difference lies in that R refers to a single line, while S refers to an entire surface.
Remember, the unit of surface roughness is micrometers (μm)! Many people may have been working for a long time without fully understanding this.
There are many types of instruments for measuring surface roughness, typically categorized into contact and non-contact types. Contact instruments include common ones like the UA3P stylus profiler, while non-contact ones usually rely on laser-based distance measurement, such as those found in conventional profilometers. However, when dealing with transparent products, laser-based methods may not be accurate. In such cases, instruments like the white-light interferometer ZYGO can be used to measure surface roughness.
Electric Discharge VDI 3400 specification
The VDI 3400 standard is a classification index formulated in Germany for the degree of spark erosion, used to classify the surface roughness of electrical discharge machining (EDM). Spark erosion refers to the mist-like appearance commonly known as a matte finish. VDI starts from VDI-0, representing the finest finish (mirror-like), and progresses upwards indicating increasing roughness. Below is a comparison table of VDI and Ra:
Mold Tech / Nihon Etching specification
Chemical etching patterns come in many varieties, hence the various names you may hear, such as snake skin pattern or sunburst pattern. Different chemicals used in chemical etching result in different texture effects. There are generally two types of specifications for chemical etching. One is the well-known MT texture, which is adopted by Western clients, while the Japanese clients use the Nihon etching. Unlike the VDI standard, etching textures specify the draft angle of the etched surface and the etching depth, rather than correlating with Ra values. However, the specified etching depth also affects the surface roughness, and the draft angle indicates how much angle the mold should be designed to release properly at that etching depth.
MT pattern reference :
SPI standard
The standard established by the Society of the Plastics Industry (SPI) is used to define the types of surface finishes for plastic products.
The SPI standard is divided into four categories: A, B, C, and D, each further subdivided into 12 grades. There are many methods for mold polishing, including diamond paste polishing, wet sanding, oil stones, buffing wheels, and sandblasting, all of which fall within the scope of polishing and can adopt this standard.
Please refer to the following:
Surface treatments certainly come in many forms, such as laser etching, for example. However, laser etching does not have specified standards because it's typically done by drawing patterns and depths manually. Then, the profiles are measured with a contour gauge to ensure they meet the specified roughness requirements of the customer. This method offers more flexibility, with each company having its own parameters and know-how.
Surface treatments are common processes in both the metalworking and mold-making industries. However, many people have only a vague understanding of the standards, and even most customers often refer to past designs when specifying their requirements.
Understanding surface treatment standards can significantly speed up communication. If you're in a product design-related role, it's essential to have a comprehensive understanding of this knowledge.
