SPI D Finish: Textured Mold Surfaces D-1, D-2, D-3
SPI D finish is the textured family of the American SPI mold finish standard: three dry-blasted grades, D-1, D-2, and D-3, that give a molded part a uniform matte surface. D-1 is blasted with fine glass bead and gives a soft satin; D-2 and D-3 are blasted with aluminum oxide, from a fine dull matte to a coarse rough texture. In the reference values published by DME, roughness runs from about Ra 0.25 µm at D-1 to about Ra 5.8 µm at D-3. SPI D is the lowest-cost family in the standard and the usual choice for grip surfaces, housings, and any part that has to hide flow lines, sink, or handling marks.
SPI D finish at a glance
- Grades: D-1 (satin), D-2 (dull), D-3 (rough).
- Method: dry blasting; glass bead #11 for D-1, aluminum oxide #240 for D-2 and #24 for D-3.
- Roughness (DME reference): about 0.25 to 0.30 µm (D-1), 0.66 to 0.81 µm (D-2), 4.8 to 5.8 µm (D-3).
- Best for: grip areas, housings, internal and industrial parts, surfaces that must hide molding marks.
- Watch out for: D-3 is deep enough to need real draft, and every blasted finish polishes back where the part rubs the steel.
What is an SPI D finish?
The SPI system, published by the Society of the Plastics Industry and now maintained by the Plastics Industry Association, sorts mold finishes into four families by the way the steel is finished: diamond buffing for A, abrasive paper for B, abrasive stone for C, and dry blasting for D. The D family is the only one in which the final surface is created by impact rather than by a polishing stroke, so it is also the only one with no direction or lay. A blasted cavity is covered in small, randomly placed craters and fractures, and the molded part copies that random matte.
Like every SPI grade, a D callout names a method and a visual result. The grade tells the toolmaker which medium to blast with, and the roughness that results falls inside a range rather than on a single number. For the complete standard and all twelve grades, see the SPI surface finish reference; this page covers only the D family, where the decisions are about texture depth, draft, and how long the finish will last in production.
The three D grades: D-1, D-2, D-3
The three grades differ in the blasting medium, its size, and therefore in how deep and how sharp the texture is. DME's mold finish guide also publishes the blasting conditions it uses as a reference for each grade, which makes the D family one of the few SPI groups with process settings attached.
| Grade | Character | Blast medium | DME reference conditions | Ra (µin) | Ra (µm, approx.) | Approx. VDI 3400 |
|---|---|---|---|---|---|---|
| D-1 | Satin | Glass bead #11 | 8 in distance, 100 psi, 5 s | 10 to 12 | 0.25 to 0.30 | About VDI 8 to 10 |
| D-2 | Dull | Aluminum oxide #240 | 5 in distance, 100 psi, 6 s | 26 to 32 | 0.66 to 0.81 | About VDI 16 to 18 |
| D-3 | Rough | Aluminum oxide #24 | 6 in distance, 100 psi, 5 s | 190 to 230 | 4.8 to 5.8 | About VDI 34 to 35 |
The micrometer values are converted from DME's microinch figures, and the VDI positions are calculated with the VDI 3400 relation between grade and Ra explained on the VDI 3400 page. All of them are approximate. Published SPI tables do not agree on D-grade roughness: some list noticeably higher values for D-1 and D-2 and a much wider range for D-3. The method and the medium are the stable part of the standard; the roughness is a consequence of them, so the final check is always a physical reference plaque, not a number.
How an SPI D finish is produced
A D finish is the last operation on the cavity, after machining, heat treatment, and any repair are complete. The usual sequence is:
- Prepare the steel. The cavity is machined to size and cleaned. Tool marks deeper than the target texture will still show through a fine D-1, so they are removed first.
- Mask what must stay smooth. Shut-off faces, parting lines, and any polished areas are masked, because blasting attacks every surface the stream reaches.
- Blast with the specified medium. The operator works the nozzle across the surface at a controlled distance and angle, overlapping passes so the coverage is even.
- Clean and inspect. Loose medium and dust are blown and washed out, and the surface is compared with a reference plaque under even light.
The equipment, media handling, and the role of blasting as a gloss adjustment on etched and laser textures are covered in the sandblasting process page. What matters for the specification is that the same grade, blasted with the same medium under the same conditions, gives a repeatable result.
Glass bead versus aluminum oxide
The split inside the D family comes from the medium. Glass bead is round; aluminum oxide is angular. That single difference decides how the surface looks and how long it lasts.
| Factor | Glass bead (D-1) | Aluminum oxide (D-2, D-3) |
|---|---|---|
| Particle shape | Spherical | Angular, sharp edged |
| Action on the steel | Peens and dimples the surface, removes very little metal | Cuts and fractures the surface, removes metal |
| Resulting surface | Smooth-edged dimples, soft satin with a slight sheen | Sharp micro-relief, flatter and duller matte |
| Depth | Shallow | From fine (#240) to deep (#24) |
| Effect on edges and fine detail | Slight | Rounds sharp edges and softens fine detail, more so with coarse grit |
In practice, D-1 is chosen when a part needs a quiet, even satin with little texture to the touch. D-2 gives a clearly matte, non-reflective part that still feels smooth. D-3 is a real texture: it can be felt, it holds the hand on a grip, and it hides almost every molding mark, at the cost of draft and cleaning effort.
Appearance and gloss on the molded part
The D family produces the lowest gloss in the SPI system, but the part is never as rough as the steel. How faithfully the polymer copies the blasted cavity depends on the material and the process. Amorphous resins such as ABS and PC, molded with enough mold temperature and pressure, reproduce the texture closely. Semi-crystalline resins such as PP and PE, and filled grades, read differently, and the same D-2 cavity can produce visibly different sheen on two materials. Colour matters too: dark parts show gloss differences far more than light ones.
For that reason a D grade sets the cavity, not the part appearance. Where the look of the part matters, approve a first-shot sample in the production resin and colour, and keep it as the reference for later production.
Draft angle for SPI D finishes
A blasted cavity grips the part like any other texture, and the deeper the texture, the more draft the wall needs. Using the approximate VDI positions above and the per-grade values on the draft angle by texture page gives these starting points:
| Grade | Approx. VDI position | Indicative minimum draft |
|---|---|---|
| D-1 | About VDI 8 to 10 | About 1.0 degree |
| D-2 | About VDI 16 to 18 | 1.0 to 2.0 degrees |
| D-3 | About VDI 34 to 35 | 3.0 to 5.0 degrees |
These are starting points on a shrink-away wall. Walls that shrink onto a core, high-shrink or glass-filled resins, and deep draws all need more. If the part cannot take the draft a D-3 needs, specify D-2 on the steep walls and keep D-3 for the faces that have the draft.
Steel and polymer considerations
Blasting works on the usual mold steels, from pre-hardened P20 to hardened and stainless grades, because it does not depend on the steel taking a polish. Harder steels hold a blasted texture longer in production, which matters most on D-1, where the shallow dimples are the first thing to wear. On softer steel, coarse angular grit can leave fragments lodged in the surface, so the cavity is cleaned thoroughly after blasting.
On the polymer side, a D finish is forgiving: it hides sink marks, flow lines, weld lines, and scratches that would be obvious on a B or A surface. That is why D grades are often specified to rescue the appearance of parts in difficult materials or geometries. Glass-filled resins are abrasive, though, and wear a blasted cavity faster than unfilled ones.
SPI D compared with VDI 3400 and Mold-Tech
SPI D, VDI 3400, and Mold-Tech can all produce a matte part, but they describe different things.
| System | How the texture is made | How it is specified | Character |
|---|---|---|---|
| SPI D | Dry blasting with glass bead or aluminum oxide | Grade D-1 to D-3 | Random matte, from satin to rough |
| VDI 3400 | EDM spark erosion | VDI number tied to Ra | Random matte, graded on a logarithmic Ra scale |
| Mold-Tech | Mainly chemical etching or laser, often finished with a light blast | Pattern code from the Standex library | Designed pattern such as grain, leather, or geometric |
A blasted D finish and an EDM VDI finish of similar Ra can look alike from a distance but are not interchangeable: the crater shapes differ, and so do sheen and wear behavior. To move a callout between systems, use the SPI to VDI 3400 to Mold-Tech conversion chart as a guide and confirm with a sample. For how EDM matte is made, see EDM surface finish; for designed patterns, see Mold-Tech textures.
How to specify an SPI D finish on a drawing
Write the grade on each surface that receives it, for example "SPI D-2", rather than a single note for the whole part, because most parts mix finishes. Three additions prevent most disputes:
- Name the reference. Because published roughness for D grades varies by source, state the reference plaque or the measured Ra the finish will be checked against.
- Mark the exclusions. Identify shut-offs, sealing faces, and any surfaces that must stay polished, so they are masked before blasting.
- Tie it to an approved part. Record the first-shot sample in production resin and colour as the appearance standard.
Wear and maintenance in production
A blasted finish changes during a production run. Areas that rub the part on ejection, sit near the gate, or see glass-filled resin flow polish back first and start producing shiny patches. Aggressive cleaning with abrasive pads does the same. The fine dimples of a D-1 lose their satin sooner than the deep relief of a D-3. Plan for it: clean with non-abrasive methods, inspect the cavity against the reference plaque at maintenance intervals, and re-blast worn areas before the difference shows on parts. Re-blasting the whole face, rather than a patch, keeps the appearance even.
Common problems with blasted mold finishes
| Problem | Probable cause | Prevention |
|---|---|---|
| Uneven or blotchy matte | Inconsistent nozzle distance, angle, or overlap; worn or contaminated media | Controlled passes, fresh media, inspection against a plaque |
| Shiny spots on parts | Wear at ejection contact, gate area, or from filled resin | Harder steel, adequate draft, scheduled re-blasting |
| Drag marks on walls | Too little draft for the texture depth | Draft set from the texture depth, finer grade on steep walls |
| Rounded edges and lost detail | Coarse grit blasting sharp features or lettering | Mask fine features, use a finer grade near detail |
| Gloss differs between cavities | Different media condition or blasting conditions per cavity | Blast all cavities in one session with the same media and settings |
Choosing between D-1, D-2, and D-3
Pick the finest grade that does the job. Use D-1 for a uniform satin on visible covers and consumer parts that should look clean but not glossy. Use D-2 for a clearly matte, low-reflection part that has to hide fingerprints and minor flow marks while keeping draft modest. Use D-3 for grips, handles, internal and industrial parts, and any surface where hiding defects and adding friction matter more than appearance, and only where the walls can take three degrees of draft or more.
Frequently asked questions
What is an SPI D finish?
The textured family of the SPI mold finish standard: three dry-blasted grades, D-1, D-2, and D-3, that give the part a uniform matte surface, from satin to rough.
What is the difference between D-1, D-2, and D-3?
The blasting medium. D-1 uses #11 glass bead for a satin, D-2 uses #240 aluminum oxide for a dull matte, and D-3 uses #24 aluminum oxide for a rough texture.
What Ra is an SPI D-3 finish?
DME's guide lists about 190 to 230 microinches, roughly 4.8 to 5.8 µm. Other published tables give different ranges, so confirm against a reference plaque.
Is SPI D the same as sandblasting?
SPI D is the specification; dry blasting is the process that produces it. Mold shops use controlled media such as glass bead and aluminum oxide, not silica sand.
How much draft does an SPI D finish need?
As a starting point, about 1 degree for D-1, 1 to 2 degrees for D-2, and 3 to 5 degrees for D-3 on shrink-away walls, more on cores and with filled resins.
What is the VDI 3400 equivalent of SPI D?
Approximately VDI 8 to 10 for D-1, VDI 16 to 18 for D-2, and VDI 34 to 35 for D-3, calculated from Ra. The textures are not identical, because blasting and EDM leave different surfaces.
Why do blasted molds develop shiny spots?
The texture wears where the part rubs the steel, near the gate, and under glass-filled resin. Harder steel, enough draft, and scheduled re-blasting keep it even.
Is SPI D the cheapest mold finish?
It is the lowest-cost SPI family, because blasting is fast compared with stoning or polishing and it hides defects that would otherwise need rework.
Normative References & Sources
- Plastics Industry Association (formerly the Society of the Plastics Industry), SPI mold finish designations: plasticsindustry.org
- DME, Micro Finish Mold Tool Finish Guide, SPI finish designations with blasting conditions and approximate Ra: dme.net
- ISO 1302, indication of surface texture on drawings: iso.org
Written and technically reviewed by Marcel Dias, Technical Editor, with over 28 years in mold texturing. Roughness and draft values are approximate and vary by source and process; verify against the primary standard and a physical sample before production. See our Disclaimer. Part of the Mold Texture Standards section. Last updated: October 9, 2026.