Meaning
Surface roughness grading defines the depth of a profile on a steel mould cavity according to German industrial standards. A vdi 27 texture represents an arithmetic mean deviation of 6.3 micrometres measured across the abraded surface. This designation provides a uniform language for mould makers to communicate finish requirements to tool shops during the build phase.
Operators use this value to ensure consistent release properties and appearance for moulded polymer components.
Moulding Performance
Controlled surface geometry governs the friction coefficient between a cooling part and the steel walls of a cavity. When vdi 27 texture occurs on an insert, the resulting micro-cavities allow air to escape during the injection stage while providing mechanical interlock for the polymer skin. High-pressure cycles require this specific roughness to prevent parts from sticking or tearing upon ejection.
Variations in tool temperature or injection velocity alter how effectively the melt replicates the finish, which often dictates the scrap rate for high-volume production.
Process Variation
Production consistency depends on the physical state of the cavity steel rather than just the initial grit blasting parameters. Tooling wear during long runs smooths the asperities of a vdi 27 texture, which increases the risk of vacuum lock during demoulding. Regrind usage or high filler content accelerates this degradation by abrasive action on the textured peaks of the steel.
Plating or coating the mould surface changes the final depth, requiring compensation during the initial electrical discharge machining or etching process.
Dimensional Influence
Cavity finishing impacts the apparent size of a part because the depth of the vdi 27 texture occupies volume previously occupied by the resin. Measurements taken on the steel reflect the nominal dimensions, yet the moulded product exhibits a slight difference due to the replication of these microscopic peaks and valleys. Precise control of the etch depth maintains the integrity of the part envelope within the tolerance stack.
A well-specified finish ensures the part geometry remains stable despite fluctuations in processing conditions.
