ISO 25178 Surface Parameters for Injection Moulding Optical Components
ISO 25178 areal parameters define 3D optical surface texture to control stray light and guide injection moulding process windows for precision lenses.
Surface Skewness Ssk
Meaning
Probability distribution analysis of height values determines the symmetry of a surface profile relative to its mean plane. This parameter, known as surface skewness ssk, indicates whether the topography is dominated by peaks or by valleys. A value of zero represents a perfectly symmetric distribution of heights.
A negative skewness indicates a surface with more valleys and flat plateaus, while a positive skewness describes a surface with more peaks and ridges. In the plastics industry, this measurement is used to evaluate the bearing capacity of a tool surface and its ability to retain lubricants or release parts. The calculation is based on the third moment of the height distribution within a defined area.
Profile Symmetry
The directional nature of the surface features determines how the tool will behave under the pressure and friction of the moulding cycle. A tool with a negative surface skewness ssk is generally preferred for many injection moulding applications because the flat plateaus provide a stable contact surface while the valleys can act as reservoirs for air or lubricants. This type of topography is typical of a surface that has been ground and then polished.
A positive skewness, on the other hand, often indicates a surface with many sharp points that can increase friction and wear. This parameter helps the toolmaker understand the fundamental character of the finish they have created. It is a more descriptive measure than average roughness because it tells the user whether the roughness is made of peaks or holes.
This distinction is critical for predicting the functional performance of the tool.
Bearing Capacity
The ability of a surface to support a load without deforming or causing excessive friction is closely related to its symmetry. Surfaces with a negative surface skewness ssk have a higher bearing area, meaning they can distribute the force of the polymer melt more effectively. This reduces the local stress on the tool steel and helps to prevent the onset of wear.
This parameter is also important for the part itself, as the texture of the tool is replicated onto the plastic. A part with a negative skewness will have more peaks, which might be desirable for a non slip grip. Conversely, a negative skewness on the part means it will have more valleys, which might be better for an aesthetic matte finish.
Engineers use this data to fine tune the texturing process and ensure it meets the mechanical requirements of the design.
Friction Control
Maintaining the correct skewness value is essential for ensuring consistent ejection and long tool life. Over time, the peaks of a positive skewness surface will be worn away, causing the surface skewness ssk to become more negative. While this might improve the bearing capacity, it can also change the appearance of the moulded parts.
Monitoring this parameter allows the moulder to detect these changes and take corrective action, such as repolishing the tool. The value is also used to evaluate the effectiveness of different surface treatments and coatings. A stable Ssk value indicates a robust and repeatable manufacturing process.
Metrology tools provide the precision needed to measure these subtle changes in surface symmetry. This information is valuable for optimizing the performance of high volume production moulds and ensuring that every part produced meets the required quality standards.
Optical Profilometry Replica Metrology Protocol for Mould Cavity Topography
Indirect optical profilometry on elastomeric cavity replicas yields sub-micron texture data while eliminating tool teardown and optical access limits.
Optical Profilometry and ISO 25178 Parameters for Cavity Wear
ISO 25178 areal parameters like Ssk, Sku, and Vvc measured via optical profilometry quantify cavity wear, predicting ejection failures before part dimensions drift.