Optical Profilometry Parameter Verification for Hardened Steel Tool Cavities
Verify hardened steel tool cavity topography using ISO 25178 areal parameters and filtered coherence scanning interferometry to guarantee polymer part release.
SPI Polish Standards
Meaning
Surface finish classifications established by the society of the plastics industry provide a standardized system for specifying the appearance and roughness of moulded plastic parts. Spi polish standards are the most common way for designers and moulders to communicate the required level of finish on a mould tool. They govern the choice of abrasive materials and the sequence of steps needed to achieve a specific look, from a rough sandblast to a high gloss mirror finish.
The standards stop at the point where a functional or decorative texture is required, as those are usually covered by other systems like the mt or vdi scales. Using the spi scale ensures that both the customer and the toolmaker have a clear understanding of the final part appearance.
Finish Classification
Divided into four main categories, the spi scale ranges from a to d, with each letter representing a different type of finishing process. The a grades are achieved using diamond polishing compounds and result in a high gloss surface, with a1 being the highest level of shine. B grades use grit paper and have a semi gloss appearance, while c grades are achieved with stone polishing and have a matte finish.
D grades are the result of sandblasting or glass bead blasting, which creates a textured or non reflective surface. Each grade has a corresponding range of roughness values that can be measured to verify the quality of the finish. This classification system simplifies the sourcing process by providing a clear set of options for the designer.
Mould Release
Quality of the surface finish also affects the ease with which a part can be removed from the mould at the end of the injection cycle. A very smooth, high gloss finish can sometimes cause the part to stick because of the vacuum created between the plastic and the steel. In some cases, a slightly rougher finish, like an spi b or c grade, is preferred to allow for better air release and easier ejection.
Moulders must balance the cosmetic requirements of the part with the practical needs of the manufacturing process. Choosing the wrong spi grade can lead to longer cycle times or increased scrap because of parts being damaged during ejection.
Cosmetic Requirement
Specification of the correct spi grade is essential for meeting the aesthetic expectations of the end user. For a transparent part like a car headlight or a medical vial, an spi a1 finish is necessary to ensure clarity and light transmission. For a structural part that will be hidden inside an assembly, a lower grade like spi d3 might be perfectly acceptable and much more cost effective to produce.
Sourcing teams should consider the cost implications of high grade finishes, as the labor required to achieve an a1 finish is much greater than that for a c grade. By using the spi standards, companies can ensure they are getting the finish they need without paying for unnecessary quality. The standard provides a common reference point that avoids confusion and ensures a consistent result.