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.
Peak Material Volume Vmp
Meaning
Morphological analysis of the uppermost portion of a surface texture quantifies the amount of solid material present in the peaks of the topography. This parameter, known as peak material volume vmp, is defined as the volume of material above a certain height threshold, typically at the 10 percent material ratio. In the plastics industry, it is used to assess the initial wear resistance of a mould surface and its influence on the aesthetic finish of the part.
A high Vmp indicates a surface with many high peaks that may be easily deformed or worn away during the first few cycles of production. The measurement is derived from the material ratio curve of a three dimensional surface map.
Asperity Volume
The volume of material contained within the highest points of a surface determines how the tool will interact with the polymer melt at the start of its life. When a tool is new, the peak material volume vmp provides a measure of the roughness that will be most visible on the moulded part. These peaks are the first part of the tool to make contact with the resin and the first to experience the forces of friction and abrasion.
If the Vmp is too high, the surface may feel rough or have a low gloss appearance. Over time, these peaks will be worn down or flattened, a process often referred to as run in. Monitoring this volume helps the toolmaker understand how the surface will evolve and how it will affect the quality of the parts over the long term.
Wear Resistance
Tracking the reduction in this value over the course of a production run provides a quantitative measure of the tool wear. As the abrasive flow of the resin removes the tips of the peaks, the peak material volume vmp will decrease. This change can be used to evaluate the effectiveness of different tool steels or surface treatments.
A surface with a lower Vmp that is still within the required roughness limits may be more stable and resistant to wear than a more irregular surface. By measuring this parameter, engineers can predict how quickly the surface finish of the part will change. This information is valuable for setting the maintenance intervals and the expected life of the tool.
A stable Vmp value across several thousand cycles indicates a high quality and durable surface finish.
Contact Mechanics
The distribution of material in the peaks affects the friction and adhesion between the part and the tool. A higher peak material volume vmp means that there is more material available to interlock with the polymer, which can increase the ejection force. This parameter is also important for understanding the heat transfer at the interface, as the peaks are the primary points of contact.
In some applications, a specific Vmp may be required to achieve a certain tactile feel or to improve the bonding of secondary coatings. Metrology systems provide the precision needed to measure these microscopic volumes and ensure that the tool meets the design specifications. Comparing the Vmp across different tools or cavities helps to maintain consistency in the final product.
Accurate control of this parameter is essential for high performance moulding applications.