Mold Texturing Glossary: Key Terms Defined
This glossary defines the core terms used across mold texturing, surface finish, and the SPI, VDI 3400, and Mold-Tech standards. Each entry gives a concise, practical definition for engineers, designers, and buyers specifying or interpreting a mold finish. Terms are grouped by theme and link to the detailed reference pages where they are explained in depth.
Surface roughness terms
Ra (arithmetic average roughness). The most common surface roughness parameter, equal to the arithmetic average of the absolute deviations of the surface profile from its mean line, over a defined evaluation length, expressed in micrometers (µm). Ra is the metric through which the three mold texture standards can be approximately compared, though none is defined by Ra alone.
Rz (mean roughness depth). A roughness parameter representing the average of the largest peak-to-valley height differences across several sampling lengths. Rz is more sensitive to individual peaks and valleys than Ra and is often quoted alongside it, for example in VDI 3400 grade tables.
Rmax. The maximum peak-to-valley height within the evaluation length. It captures the single deepest irregularity and is sometimes specified where a worst-case surface defect must be controlled.
Surface texture. The overall geometric character of a surface, including roughness, waviness, and lay. In molding, it is the controlled pattern or finish transferred from the mold cavity to the part.
The three standards
SPI surface finish. The American mold finish standard from the Society of the Plastics Industry, defining 12 grades from A-1 to D-3 in four categories, glossy, semi-gloss, matte, and textured, based on finishing method and appearance. See SPI surface finish.
VDI 3400. The German roughness standard from the Verein Deutscher Ingenieure, used as 12 reference grades from VDI 12 to VDI 45, produced mainly by EDM, with roughness rising logarithmically. See VDI 3400.
Mold-Tech. A proprietary texture library from Standex Engraving, organized into pattern series rather than a numeric roughness scale, produced by chemical etching, and widely used in automotive interiors. See Mold-Tech textures.
Charmilles CH scale. The underlying 0-to-45 reference scale from which VDI 3400 grades derive, and the source of the common claim that VDI 3400 has 45 grades, when in practice 12 reference grades are used in molding.
Processes and methods
EDM (Electrical Discharge Machining). A machining process that erodes mold steel using controlled electrical sparks between a shaped electrode and the workpiece in a dielectric fluid, producing the uniform, non-directional texture characteristic of VDI 3400 finishes.
Chemical etching. A texturing process in which a patterned mask is applied to the mold steel and acid removes material to a controlled depth, used to create the grains and decorative patterns of Mold-Tech textures.
Diamond buffing. A polishing method using diamond paste or compound of decreasing grit to produce the high-gloss SPI A finishes; slow, skilled, and the most costly finishing process.
Dry blasting. A texturing method using pressurized abrasive media, such as glass beads or aluminum oxide, to create matte and textured SPI D finishes; fast and economical.
Dielectric fluid. The insulating liquid used in EDM that controls the spark gap, flushes away eroded particles, and cools the work, influencing the resulting surface texture.
Design and tooling terms
Draft angle. The taper applied to the walls of a molded part, measured in degrees, that allows it to eject from the mold without dragging. Textured surfaces require more draft than smooth ones, and deeper textures require more still. See draft angle by texture.
Texture depth. The depth to which a texture is etched or machined into the mold surface, the primary driver of the required draft angle in Mold-Tech texturing.
Shrink-away. A surface condition where the molded part shrinks away from the mold steel as it cools, creating natural clearance and following the standard draft rule.
Shrink-on. A surface condition where the part shrinks onto the steel core, gripping it, and therefore requiring roughly three times more draft than a shrink-away surface to release cleanly.
Cavity. The hollow in the mold that forms the outer surface of the part; its finish transfers directly to the molded part surface.
Ejection. The stage of the molding cycle when the finished part is pushed out of the mold; insufficient draft on a textured surface causes scuffing or scratching during ejection.
Materials
Amorphous polymer. A plastic with randomly arranged molecular chains, such as PC, PMMA, or ABS, that generally achieves higher gloss and reproduces a polished cavity more faithfully because it shrinks uniformly and scatters less light.
Semi-crystalline polymer. A plastic with regions of ordered chain alignment, such as PP, PE, or POM, that shrinks more and less uniformly, tending toward duller surfaces and a higher risk of the orange-peel defect.
Glass-filled (reinforced) polymer. A resin containing glass fibers or mineral fillers that increase strength but disrupt the surface skin, raising roughness and lowering achievable gloss.
Mold steel. The tool material whose hardness and composition affect the achievable finish; high-chrome steels such as S136 or 420 polish to a higher gloss and hold a fine finish better than general-purpose P20.
Common defects
Orange peel. A rippled, pitted surface resembling citrus skin, typically caused by poor mold-surface replication at the end of the flow path or by surfaces solidifying too quickly during filling.
Blush. A localized area of dullness or cloudiness on a molded surface, usually appearing near the gate.
Gloss-transition defect. Alternating glossy and dull zones on a high-gloss surface, caused by unstable melt flow and pressure during filling, which interrupt uniform replication of the cavity finish.
Related references
For the full treatment of each standard, see the mold texture standards overview, the SPI, VDI 3400, and Mold-Tech references, and the conversion page for approximate cross-references.
Reviewed by Marcel Dias — Technical Editor. Definitions are provided for reference. Verify specifications against the primary standard and a physical sample before production. See our Disclaimer.