VDI 24 surface finish guide for mold texture, Ra value, and design callouts

What VDI 24 surface finish means

A VDI 24 surface finish is a medium matte mold texture on the VDI 3400 scale. It is commonly specified for injection molded plastic parts and EDM-textured tooling surfaces. In widely used VDI 3400 comparison charts, VDI 24 is usually listed at about Ra 1.6 µm, or roughly 63 µin. Some charts also show an associated Rz or Rmax value around 6.3 to 6.5 µm, but those figures should be treated as approximate conversions, not complete surface specifications.

In practical terms, VDI 24 gives a controlled dull texture. It is rougher than fine matte finishes such as VDI 18 or VDI 21, but smoother than stronger grain textures such as VDI 30 or VDI 33.

For designers, buyers, and toolmakers, VDI 24 is not just a number on a chart. It can affect appearance, touch, mold release, draft angle, and inspection. It should be specified together with the surface location, reference standard, acceptance method, and any required visual sample.

For related manufacturing finish topics, see the Surface Finishes section.

VDI 24 in the VDI 3400 scale

VDI 3400 is commonly used in mold making to describe spark-eroded, EDM, and textured mold surfaces. The scale is often presented as numbered grades, with higher numbers representing rougher textures. Many shop charts list grades from VDI 12 to VDI 45 for practical injection molding reference, although some conversion tables include additional intermediate values.

VDI 24 sits near the middle of the practical mold-texture range. It is often described as a dull or medium matte finish. This makes it suitable for visible plastic housings, consumer product shells, automotive interior parts, and industrial components where a polished gloss would show fingerprints, flow marks, or minor scratches too easily.

Reference item	Typical value or meaning	Important note
VDI grade	VDI 24	A mold texture grade, not a full metrology instruction by itself
Approximate Ra	About 1.6 µm, or about 63 µin	Values vary slightly between published comparison charts
Approximate Rz or Rmax	Often shown around 6.3 to 6.5 µm	Do not assume Rz and Rmax are identical unless the drawing defines the parameter
Visual character	Controlled medium matte or dull texture	The final molded part can look different from the mold surface
Nearby grades	VDI 21 is smoother; VDI 27 is rougher	Use physical plaques or agreed samples when appearance matters

The Ra number helps engineers compare VDI 24 with machined or polished surfaces, but it does not fully define the visual texture. Two surfaces can have the same Ra and still look different because peak shape, spacing, lay, directionality, gloss, and replication quality are different. For cosmetic surfaces, a VDI callout is stronger when it refers to a recognized plaque, texture card, or approved molded sample.

How VDI 24 compares with Ra, Rz, and ISO surface texture terms

Ra is the arithmetic mean height of a roughness profile. It is useful because it gives a single roughness number, but it reduces the profile to one average. Rz, Rmax, and similar height parameters describe different peak-to-valley characteristics. They are not interchangeable unless the drawing clearly defines the parameter, filtering method, and evaluation length.

Current ISO catalogue information shows that ISO 21920 is the modern profile surface texture standard series. ISO 21920-1:2021 covers the indication of surface texture in technical product documentation, while ISO 21920-2:2021 covers terms, definitions, and parameters. Older drawings may still refer to ISO 4287, but ISO catalogue listings show ISO 4287:1997 as withdrawn and replaced by ISO 21920-2:2021.

This matters because a drawing that says only VDI 24, or only Ra 1.6 µm, may leave too much room for interpretation. If the goal is functional roughness, an ISO profile parameter may be appropriate. If the goal is a molded cosmetic texture, a VDI 24 callout with a reference sample is usually more practical. If both appearance and measurable roughness are important, the drawing should state which requirement controls if the two do not perfectly match.

Where VDI 24 is commonly used

VDI 24 is a balanced choice when a part needs a visible matte finish without an aggressive grain. Compared with a glossy surface, it can reduce the perception of minor scratches and fingerprints. It can also help hide small molding imperfections better than a highly polished surface, although it will not fix root causes such as poor venting, weld lines, sink marks, or poor gate placement.

Typical use cases include plastic electronic housings, appliance covers, hand-held device shells, automotive trim, switch panels, tool handles, and general industrial enclosures. In these applications, VDI 24 often provides enough texture to create a non-glossy appearance while remaining fine enough for moderate detail and manageable mold release.

The finish is not suitable for every surface. Avoid VDI 24 on sealing lands, sliding fits, precision shutoff areas, optical surfaces, or surfaces that must be very easy to clean. On textured vertical walls, the finish can increase friction during ejection. The texture decision should therefore be made with the parting line, draw direction, ribs, bosses, side actions, and material shrinkage in mind.

Draft angle and molding considerations

VDI 24 usually needs more draft than a polished surface. Published mold texture reference charts commonly show draft guidance for VDI 24 in the approximate range of 0.5° to 1.5°, depending on resin, wall geometry, texture depth, and the side of the mold. Some tables list different values for ABS, PC, and nylon because materials release and shrink differently around cores.

These numbers are starting guidance, not universal rules. A deep rib, tall side wall, glass-filled resin, soft material, or under-vented cavity may need additional draft. A shallow cosmetic surface with a low draw depth may be easier to mold. Mold temperature, packing, cooling balance, ejector layout, and tool steel condition also influence whether the part releases cleanly.

Designers should confirm VDI 24 early. Changing from a smooth surface to a matte texture late in the project can create hidden problems. A wall that was acceptable with a polished finish may drag, scuff, or stick after texturing. If the part cannot accept more draft for styling reasons, the team may need to reduce the texture grade, change the parting strategy, add a side action, or limit the texture to selected cosmetic zones.

How to call out VDI 24 on a drawing

A good callout removes ambiguity for the toolmaker, molding supplier, and inspector. It should identify the grade, the exact surface area, the standard or reference plaque, and the acceptance method. If the texture must match an existing part, the drawing should identify the approved sample or master plaque rather than relying only on a roughness number.

Useful callout elements include: See also: Buying Guides.

• VDI 24 surface finish or VDI 3400 grade 24.
• The exact surfaces to be textured, using drawing zones, CAD surface names, or marked cosmetic areas.
• Required texture direction, or a note that the texture is nondirectional, if applicable.
• Surfaces excluded from texture, such as shutoffs, parting-line pads, datum surfaces, labels, and sealing areas.
• Minimum draft angle agreed with the molder for each textured wall.
• Inspection method, such as visual comparison to an agreed plaque, roughness measurement, or approved first-article sample.
• A hierarchy note if both VDI grade and Ra value are shown.

An example callout could be: Apply VDI 3400 grade 24 matte texture to exterior cosmetic surfaces only. Match approved texture plaque. Maintain agreed minimum draft on textured draw surfaces. No texture on sealing lands, shutoff faces, or datum pads.

This wording is more useful than writing only Ra 1.6 µm because it tells the supplier what visual and functional result is expected. It also avoids the common mistake of treating a conversion chart as a complete manufacturing instruction.

VDI 24 versus SPI and other finish systems

VDI and SPI finish references are both common in plastic part discussions, but they describe finish families differently. SPI finishes are often associated with polishing, paper, stone, and blasting categories. VDI 3400 is commonly associated with EDM or mold texture grades. A rough comparison can be useful during supplier discussions, but there is no perfect one-to-one conversion between VDI 24 and an SPI grade.

The reason is that appearance depends on more than roughness average. A blasted SPI-style surface, an EDM-textured surface, and a chemically etched texture can share similar Ra values but show different gloss, grain shape, and tactile response. Mold-Tech and Yick Sang texture references add another layer because they may define specific decorative patterns rather than only roughness.

If a drawing lists both VDI 24 and an SPI finish without saying which one controls, a supplier may choose the easier interpretation. A better approach is to use one primary system and add supporting information only where it helps. For example, a drawing might state VDI 24 as the controlling finish and use Ra 1.6 µm as an approximate reference, not as an independent acceptance limit.

Common mistakes to avoid

The first mistake is assuming that the molded part will always match the mold plaque exactly. Plastic material, color, fillers, mold temperature, gloss level, and processing conditions all affect final appearance. Black ABS, natural nylon, glass-filled PC, and painted parts can make the same VDI 24 tooling surface look different.

The second mistake is adding VDI 24 after the mold design is complete. Texture affects draft, ejection, steel-safe polishing decisions, and masking areas. It should be included before tool design is frozen, especially for tall walls and visible housings.

The third mistake is using Ra alone to buy a cosmetic texture. Ra is measurable, but it does not define pattern, gloss, or visual uniformity. If the product has important appearance requirements, a texture plaque or approved sample is essential.

The fourth mistake is texturing functional interfaces. A matte surface may look good on a housing, but it can create wear, friction, contamination traps, or sealing problems on functional faces. Keep VDI 24 away from critical interfaces unless the engineering team has confirmed the effect.

Frequently asked questions

What is the Ra value of VDI 24 surface finish?

VDI 24 is commonly listed at about Ra 1.6 µm, or roughly 63 µin. The value is an approximate conversion from VDI 3400 reference charts, not a complete surface texture specification by itself.

Is VDI 24 a smooth or rough finish?

VDI 24 is best described as a medium matte or dull mold texture. It is rougher than fine matte grades such as VDI 18 or VDI 21, but smoother than coarser grades such as VDI 30, VDI 33, or VDI 36.

Can VDI 24 be replaced by Ra 1.6 µm on a drawing?

Not safely for cosmetic mold texture. Ra 1.6 µm gives a roughness average, while VDI 24 implies a recognized texture grade. If visual appearance matters, specify VDI 24 with a plaque or approved sample and use Ra only as supporting information.

How much draft is needed for VDI 24?

Published reference charts often place VDI 24 draft guidance around 0.5° to 1.5°, but the real requirement depends on resin, wall depth, mold side, part geometry, and processing conditions. The molder and toolmaker should confirm the final draft requirement before tooling release.

Does VDI 24 apply to metal parts or only plastic molds?

VDI 24 is most often used in injection mold and plastic part texture discussions, especially for EDM or textured mold cavities. Similar Ra values can be specified for metal surfaces, but the VDI 24 callout should not be assumed to define a general metal machining finish unless the drawing and supplier agree on the same reference system.