Smooth or wavy

10.01.23 Surface quality

Surface Roughness what is the meaning in Moulds?

To improve mould performance, reduce wear and produce high-quality moulded parts, mould builders must understand surface texture and how to properly measure, analyse and control it.

Surface texture is separated into roughness, waviness and form.

Surface roughness is frequently considered a number measured on a gage or the number of a sample on a comparator chart. However, describing surface texture with a number is a lot like describing a concert in terms of decibels: loudness is only part of the story. The Rolling Stones, Tchaikovsky and an angle grinder can all produce 100 decibels, but the whole picture is much more complex and interesting.

More than a number, roughness or more generally “surface texture,” describes the shape of the surface. Texture affects not just the appearance of the moulded components but also grip, wear, vibration, noise, sealing, coating adhesion, etc. It also impacts the performance and wear of the tooling itself.

Aspects of surface texture are essential to create durable surfaces that perform well. For example, the finish of a moulded gear will have different requirements than that of a sealing surface, knob or sensor component. Here we’ll look at the nature of surface texture, how it’s measured and analysed, and what we need to control to improve mould performance, reduce wear and produce high-performing moulded components.

It’s critical to understand that changing the roughness may have little or no effect on waviness and vice versa.

Surface Texture Includes Roughness, Waviness and Form

Over the past several decades, measurement equipment and software advances have shaped our understanding of surface texture. Rather than thinking of a surface as having a generalized “roughness,” we now describe surface texture in terms of as various features with varying sizes. The size of those features are described in terms of “wavelength” — ranging from short wavelength roughness to longer wavelength “waviness” and “form error.”

Surface texture is measured with a variety of instruments. The most prevalent is the roughness gage, which measures texture by moving a stylus across the surface. The result is a 2D “profile” . Noncontact optical techniques are also sometimes used to obtain 3D texture measurements. In both measurements , we can detect fine roughness peaks but also a larger, periodic shape remaining from a machining operation. Both of these aspects of the texture may affect the final performance of the component, and both may need to be controlled independently.

Comparing a surface to a cosmetic finish card gives a general sense of the texture, though this method is subjective and provides only limited information about the surface. Measuring a surface with a stylus gage or other instrument provides numerical values that can be specified and tracked over time. With modern instruments, the measured surface points can be stored to retain the actual “shape” of the surface for comparison over time.
Surface roughness is often specified using parameters such as average roughness (Ra), which is the average deviation of the surface from the mean height. A “rougher” surface generally has a larger Ra value.
The challenge, however, is that very different surfaces can produce the same Ra value. All of these surfaces might pass a specification for Ra. However, will the component leak or seal? Will it appear matte, dimpled, streaked or smooth?

Another issue is that larger shapes in the texture (waviness) may have as significant an impact on the part’s function as roughness. We can separate surface texture into its component wavelengths through “filtering” to examine its components. A filter attenuates texture above or below a set wavelength. For example, the “roughness cutoff” separates the data into shorter wavelength roughness and longer wavelength waviness .

Early measurement systems only provided a handful of roughness parameters, such as average roughness, RMS roughness, maximum peak-to-valley height and possibly waviness. As measurement technology and processing capability have advanced, it’s become possible to calculate dozens of parameters to look at many additional aspects of the texture, such as spacings between peaks, ratios of peaks versus valleys, etc. (see Figure 5). All of these parameters were developed to track aspects of texture that affect part performance, but could not be discerned by more basic parameters.