VDI 24 surface finish explained for mold texture and drawing callouts

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What VDI 24 means

A VDI 24 surface finish is a medium matte mold texture used mainly on injection mold tooling and EDM-textured cavity surfaces. In common VDI 3400 comparison charts, VDI 24 corresponds to about Ra 1.6 µm, or roughly 64 µin, with Rz often shown around 6.3 to 6.5 µm. It is not a mirror polish, and it is not a heavy leather-like grain. It sits in the dull, controlled texture range that designers use to reduce gloss, mask light cosmetic variation, and add a restrained tactile feel.

The important design point is that VDI 24 should be specified as a texture grade with a defined surface location, not only as an Ra number. Two surfaces with similar Ra values can look different when they are produced by different processes or applied to different materials.

Where VDI 24 sits in the VDI 3400 scale

VDI 3400 is widely used as a practical reference scale for mold textures, especially spark-eroded or EDM-like finishes. The scale is numerical: lower numbers are smoother and higher numbers are rougher. VDI 24 falls near the middle of the commonly used cosmetic range for plastic housings, caps, covers, and functional surfaces that need a non-glossy appearance without looking coarse.

The scale is useful because it gives designers, toolmakers, and molders a shared language. A note that simply says matte finish can be interpreted several ways. A note that says VDI 24 gives the supplier a more defined roughness target and a closer visual reference. For related terminology across finishing methods, see the Breezcats surface finishes section.

Even so, VDI grades should not be treated as exact universal conversions. Published charts often agree on the approximate Ra value for VDI 24, but they may differ slightly in Rz, draft guidance, or the equivalent SPI finish. For critical cosmetic parts, the finish should be reviewed against a physical texture plaque, a molded sample, or an approved master part.

Ra, Rz, and appearance at a glance

Ra is the arithmetic average roughness of a measured surface profile. It is useful for comparison, but it does not fully describe visual texture. A surface with directional EDM marks, a blasted surface, and a chemically etched surface can all have similar Ra values while reflecting light differently. Rz, peak-to-valley behavior, lay, waviness, tool steel condition, resin, pigment, and filler content can all change the final molded appearance.

These values are best treated as common reference values, not automatic inspection rules. If a drawing requires measurable acceptance, it should define the parameter, tolerance, measurement direction, sampling method, and whether the mold surface or the molded part is being checked. ISO profile-texture terminology such as Ra and Rz can support measurement, but it does not replace a visual texture standard for molded cosmetic parts.

When designers choose VDI 24

VDI 24 is often chosen when a part needs a controlled matte surface but should not feel aggressively rough. Common design reasons include reducing shine, hiding fingerprints, softening minor molding flow variation, and giving handheld parts a more technical or durable appearance. It can also make small scuffs less obvious than they would be on a polished surface, although it will not eliminate scratches or molding defects.

Typical applications include electronic housings, appliance bezels, interior plastic covers, tool handles, battery cases, clips, and non-optical surfaces on consumer or industrial components. It is less suitable for optical windows, sealing faces, sliding surfaces that require low friction, or parts where a highly polished brand appearance is required.

A practical way to think about VDI 24 is balance. It is more forgiving than a smooth polish, yet still refined enough for many visible molded parts. If the design needs a stronger grip, a deeper etched grain or a higher VDI number may be more appropriate. If the design needs a soft satin finish with minimal texture, VDI 18 or VDI 21 may be a safer starting point.

Process and material factors that change the result

The final appearance of VDI 24 depends on more than the number on the drawing. Tool steel, EDM settings, polishing before texturing, blasting media, etching method, cavity position, resin shrinkage, and molding conditions can all influence the visible result. A texture that looks balanced on one resin may look sharper, duller, or less uniform on another.

Plastic material has a major effect. ABS often reproduces texture cleanly and is commonly used for cosmetic housings. PC can show a crisp matte texture but may require more attention to draft and release. Nylon and other engineering plastics can be affected by moisture, shrinkage, and filler content. Glass-filled resins may produce a rougher or more speckled surface because the filler can influence how the surface replicates and reflects light.

Color matters as well. Dark colors tend to show gloss differences, weld lines, and scuffs more clearly. Light colors may hide some marks but can make flow or contamination more visible. Before freezing a cosmetic requirement, designers should review molded samples in the final material and color rather than relying only on a metal texture card.

Draft angle and part design implications

Texture increases friction during ejection, so VDI 24 needs appropriate draft. Many mold-texture charts list starting draft guidance around 0.5° for PA or nylon, about 1° for ABS, and about 1.5° for PC at the VDI 24 level. These are practical starting values, not universal rules. Wall height, pull direction, resin shrinkage, ribs, bosses, undercuts, tool steel, and ejection layout can all require more draft. See also: Buying Guides.

Insufficient draft can lead to drag marks, glossy streaks, scuffed texture, sticking, or inconsistent appearance from shot to shot. The risk increases on deep vertical walls and on surfaces parallel to the mold opening direction. If the visible face is tall, the design team should discuss draft early instead of adding VDI 24 late in tooling.

• Apply texture only to surfaces that can release cleanly in the mold opening direction.
• Increase draft on taller walls, textured ribs, and shutoff-adjacent surfaces.
• Avoid placing VDI 24 on sealing lands, datum pads, or moving contact faces unless the function has been tested.
• Confirm whether the texture is applied before or after tool trials, because changing texture after approval can affect both appearance and release.

How to specify VDI 24 on a drawing

A clear drawing callout should define the texture grade, the surfaces affected, any excluded areas, and the acceptance reference. A note such as Finish: VDI 24 on all visible exterior surfaces, excluding shutoffs, seals, and engraved text is much clearer than a general matte finish note. If only certain faces receive the texture, the drawing should use surface labels, shaded views, or zone references.

For higher-risk cosmetic parts, add an approval method. A practical note may state that VDI 24 should match an approved texture plaque or first-article molded sample. If measurement is required, define whether the target is Ra 1.6 µm on the tool surface, on the molded part, or both. Molded plastic measurements can be affected by replication, resin, and measurement setup, so the acceptance method should be agreed before production.

Avoid stacking too many standards in one note. Specifying VDI 24, SPI D-2, a separate Ra tolerance, and a brand texture name can create conflict if the references do not match. If a conversion is needed, identify the primary requirement and treat the others as approximate references only.

VDI 24 versus SPI and other finish references

VDI and SPI are often compared, but they are not identical systems. VDI 3400 is commonly used as a numbered roughness or texture reference. SPI finish classes describe preparation methods such as diamond buffing, paper, stone, glass bead, or aluminum oxide blasting. SPI D-2, for example, is commonly associated with a dull blasted finish using #240 aluminum oxide. That general region can overlap with VDI 24 in appearance discussions, but it should not be treated as a guaranteed one-to-one match.

The safest approach is to use one primary standard on the drawing and provide comparison language only when needed for supplier communication. If a customer, tool shop, and molder use different texture books, align them with a sample plaque or trial part before final texture work. This avoids a common problem: everyone agrees on the words matte texture, but each party imagines a slightly different surface.

Common mistakes to avoid

• Using Ra alone for appearance. Ra 1.6 µm helps define roughness, but it does not fully define gloss, lay, or tactile character.
• Adding texture after the part is fully designed. VDI 24 affects draft, ejection, and cosmetic risk, so it should be considered during design for manufacturability.
• Assuming every resin reproduces the same texture. Filled materials, dark colors, and high-gloss polymers can change the visible result.
• Texturing functional contact areas without testing. Rougher surfaces can change friction, sealing, cleaning, and wear behavior.
• Mixing VDI and SPI as exact equivalents. Use conversions as communication aids, not as uncontrolled dual requirements.

Frequently asked questions

Is VDI 24 the same as Ra 1.6 µm?

It is commonly listed at about Ra 1.6 µm, but the texture grade and the Ra value are not the same thing. Ra is a measured roughness parameter; VDI 24 is a practical mold texture reference. For cosmetic parts, visual approval is often just as important as the roughness number.

Is VDI 24 glossy or matte?

VDI 24 is generally considered a medium matte or dull texture. It should reduce gloss compared with polished mold finishes, but the final sheen depends on resin, color, processing, and the exact tool texturing method.

Can VDI 24 hide sink marks and flow lines?

It can make light cosmetic variation less noticeable, but it does not fix molding defects. Sink marks, weld lines, burn marks, and flow imbalance should be addressed through part design, gating, tooling, and process control before texture is used as a cosmetic aid.

What draft angle should be used with VDI 24?

Common starting guidance is around 1° for ABS and around 1.5° for PC, with some nylon applications using less. The correct value depends on geometry, wall height, material, pull direction, and ejection design, so the moldmaker should confirm it before tooling is finalized.

Should a drawing call out both VDI 24 and SPI D-2?

Usually, one primary finish standard is clearer. If both are mentioned, the drawing should state which one controls and which one is only an approximate comparison. Otherwise, inspection and supplier interpretation can become inconsistent.