Meaning
Standardized surface finish scale established by the Verein Deutscher Ingenieure defines texture roughness grades generated by electrical discharge machining on tool steel cavities. VDI 3400 designates thirty-five discrete texture finish classes ranging from VDI twelve to VDI forty-five, corresponding to specific arithmetic mean surface roughness values from sub-micron to high-roughness finishes. In plastic tool manufacturing, this standard provides a universal reference scale for specifying mold spark erosion finishes without custom chemical etching samples.
Tool geometry limits and spark erosion electrode wear constrain finish uniformity, where worn copper or graphite electrodes drift surface roughness beyond specified VDI class tolerances.
Roughness Scale
Each integer step on the scale corresponds to a defined Ra micro-meter value, where VDI twelve measures zero-point-four microns and VDI thirty-six measures six-point-three microns. Higher VDI numbers indicate rougher tool finishes featuring deeper spark erosion craters and lower surface gloss on molded parts. Specifying VDI values on tooling drawings eliminates ambiguity between toolmakers and injection molding shops.
Electrical Processing
Spark erosion spark gaps, pulse duration, and discharge current parameters govern the resulting surface crater size and VDI classification. Uniform electrical discharge across copper or graphite electrodes ensures consistent texture depth across complex mold geometries. Fine spark finishing steps produce low VDI numbers suitable for clear or high-gloss plastic components.
Molding Fidelity
Molten polymer compliance under holding pressure dictates how accurately the resin fills VDI spark craters before skinning over. High viscosity resins require elevated mold temperatures to replicate coarse VDI textures without air entrapment pits. Required draft angles scale directly with VDI numbers to prevent drag marks during part ejection.
