Sandblasting Mold Texture: Media, SPI D Grades and Gloss

Sandblasting, more accurately called abrasive blasting, is a mold finishing process that fires abrasive media at the cavity steel with compressed air to create a uniform matte surface or to adjust the gloss of an existing texture. It is the process behind the SPI D textured finishes, from fine glass bead (D-1) to coarse aluminum oxide (D-3), and it is also the final step on many etched and laser textures, where a light blast sets the gloss level of the molded part. Despite the name, mold shops do not use silica sand; they use controlled media such as glass bead and aluminum oxide. This page explains how mold blasting works, which media produce which finish, how it maps to the SPI surface finish grades, how gloss and roughness are controlled, the draft it needs, its common defects, and how it compares with chemical etching, laser texturing, and EDM texturing.

What is mold sandblasting?

Mold sandblasting is the controlled impact of small abrasive particles on the cavity steel. Each particle leaves a microscopic dent or cut, and millions of overlapping impacts produce an even matte surface with no direction and no repeating pattern. That is the defining difference from the other texturing methods: etching, laser, and EDM can create a designed texture, while blasting creates a random, uniform roughness.

Blasting has two roles in a mold shop. As a finishing process, it produces the SPI D matte finishes directly on polished or machined steel. As a post-process, it is applied lightly over a grain or pattern to remove shine, even out gloss differences between areas, and set the final appearance of the part. Most textured automotive and appliance molds receive a blast at the end, even when the texture itself was etched or lasered.

How mold blasting works, step by step

Mold blasting process step by step

Step	What happens	Why it matters
1. Surface preparation	The cavity is machined, stoned, or polished to an even base finish and degreased.	Blasting reveals base defects; scratches and tool marks show through a fine blast.
2. Masking	Areas that must stay polished or sharp, such as shut-offs and sealing faces, are masked.	Blasting a parting line or shut-off rounds it and causes flash.
3. Media selection	Media type and grit size are chosen for the target finish or gloss.	Media choice decides the character of the finish more than any other variable.
4. Blasting	The operator or a cabinet nozzle moves across the surface at a set pressure, angle, and distance.	Even passes give an even finish; dwelling in one spot creates patches.
5. Cleaning	Residual media and dust are blown off and the cavity is cleaned.	Embedded or loose media can mark parts and contaminate the molding.
6. Comparison and approval	The finish is checked against a master sample or SPI reference plate.	Matte finishes are judged visually and by gloss, not only by Ra.

Blasting media and the finish each one gives

The media is the most important choice in mold blasting. Round media peen the surface and give a soft, satin matte; angular media cut the surface and give a sharper, duller matte that holds paint and hides marks well.

Blasting media and the finish each produces

Media	Shape	Finish produced	Typical mold use
Glass bead	Round	Fine, smooth satin matte with low aggressiveness	SPI D-1, light gloss reduction over textures
Fine aluminum oxide (high grit number)	Angular	Fine dull matte	SPI D-2, uniform technical mattes
Coarse aluminum oxide (low grit number)	Angular	Coarse, deep matte	SPI D-3, rough non-slip or concealing surfaces
Ceramic bead	Round	Satin matte, more durable than glass	Repeatable bead finishes in production shops
Plastic or soft media	Varied	Little or no change to the steel	Cleaning molds and removing residue without altering the texture

Grit number works in reverse to particle size: a higher number such as #240 means finer grit, and a lower number such as #24 means coarser grit.

Sandblasting and the SPI D finishes

SPI defines its D category as textured finishes produced by dry blasting. The three grades are set by the media used, and their approximate roughness follows from it. The values below are the approximate roughness published by DME, the same reference used on our SPI surface finish page and the SPI to VDI conversion chart. For how to choose and specify each grade, see SPI D textured finish.

SPI D grades: blasting media, DME reference roughness and approximate VDI position

SPI grade	Blasting media	Ra (µin, DME)	Approx. Ra (µm)	Approx. VDI 3400 position
D-1	#11 glass bead	10 to 12	0.25 to 0.30	About VDI 8 to 10
D-2	#240 aluminum oxide	26 to 32	0.66 to 0.81	About VDI 16 to 18
D-3	#24 aluminum oxide	190 to 230	4.8 to 5.8	About VDI 34 to 35

Approximate, not official. SPI Ra values differ between published charts, and a blasted surface and an EDM surface with the same Ra will not look identical. Confirm with a physical sample.

This is why coarse matte parts are often specified in VDI 3400 instead: SPI has only three blasted grades, and on DME's values there is no SPI grade between about VDI 21 and VDI 33 or above VDI 36, while VDI covers that whole range in distinct steps. Published D-3 roughness also varies widely between sources, which makes a coarse SPI callout hard to pin down without a reference plaque.

How roughness and gloss are controlled

How blasting variables change roughness and gloss

Variable	Effect when increased	Practical note
Grit size (coarser media)	Rougher, duller surface	The main lever for moving between D-1, D-2, and D-3
Air pressure	Deeper impacts and faster action	Too high on round media can fracture beads and change the finish
Blast angle	Near 90° gives the most uniform matte	Low angles can leave a slight directional sheen
Nozzle distance	Wider, softer pattern	Must stay constant across the cavity for even results
Dwell time	More coverage, then gradual dulling	Uneven dwell is the main cause of patchy finishes
Media condition	Worn media gives a finer, inconsistent result	Recycled media should be screened and refreshed

For gloss adjustment over a grain, shops use fine media at low pressure and work in light, overlapping passes. The goal is to change the micro-roughness on top of the grain without wearing down its peaks. The result is checked against a master sample, because the same grain can mold glossy or flat depending on this final step.

Which mold materials can be blasted?

Almost every mold material can be blasted, but harder steels hold the finish longer in production. Softer materials take the finish faster and wear it faster.

How common mold materials respond to blasting

Material	Blasting behavior	Notes
P20 (1.2311 / 1.2738)	Blasts evenly	Common for matte housings and grained tools
H13 (1.2344), hardened	Takes the finish more slowly, holds it longer	Good for long production runs and abrasive resins
Stainless mold steels (S136 / 420, 1.2083)	Blasts evenly	Media must be clean to avoid iron contamination and staining
Aluminum mold alloys	Blasts quickly	Use lower pressure; the matte wears faster in production

Sandblasting versus etching, laser, and EDM

Blasting is the simplest and lowest-cost way to put a matte on a mold, but it cannot create a pattern. The table compares it with the three methods that can.

Sandblasting compared with chemical etching, laser and EDM texturing

Factor	Sandblasting	Chemical etching	Laser texturing	EDM texturing
Texture type	Random uniform matte	Decorative grains, leathers, wood	Any pattern from a digital file	Uniform technical matte
Specified by	SPI D grade or media and grit	Pattern number and depth (MT series)	File, depth, and gloss	VDI 3400 grade
Pattern possible	No	Yes	Yes	No
Cost and speed	Lowest cost, fastest	Moderate, fast on large areas	Highest, slowest on large areas	Moderate
Repeatability	Operator-dependent unless automated	Good, depends on steel	Highest, fully digital	High, machine-controlled
Role on other textures	Final gloss adjustment	Primary texture	Primary texture	Primary texture

Sandblasting and draft angle

A blasted finish needs draft like any texture, but how much depends on how rough it is. The fine D-1 and D-2 finishes are shallow, so they add little to the draft the part already needs. A coarse D-3 blast can be as rough as the coarse VDI grades and needs draft accordingly. The common rule of thumb is about 1.5 degrees of additional draft for every 0.001 in (0.025 mm) of texture depth.

Because published D-3 roughness varies between sources, the draft for a coarse blast should be set from the actual texture depth or VDI equivalent agreed with the toolmaker, not from the SPI grade alone. For the full method and tables, see draft angle by texture.

Where blasted mold finishes are used

• Housings and enclosures: matte surfaces that hide fingerprints, flow lines, and minor sink marks.
• Industrial and technical parts: non-reflective surfaces where appearance matters less than function.
• Grip areas: coarse blasted zones on handles and tool bodies.
• Parts to be painted or printed: a light matte improves adhesion of paint and pad printing.
• Textured molds: final gloss setting on etched and laser grains, especially in automotive interiors.

Advantages and limitations

Advantages and limitations of blasted mold finishes

Advantages	Limitations
Lowest-cost and fastest way to produce a matte	Cannot create patterns or designed textures
Hides minor surface defects and flow marks on parts	Results depend on operator skill unless automated
Easy to repeat on repaired areas	Fine blasted finishes wear faster in long production runs
Essential tool for gloss control on other textures	Rounds sharp edges and shut-offs if not masked
Works on almost any mold material	Coarse finishes are hard to specify by SPI grade alone

Common mold blasting defects

Common blasting defects: cause and prevention

Defect	Likely cause	Prevention
Patchy or uneven matte	Uneven dwell, distance, or angle	Consistent overlapping passes; automated cabinets for large tools
Rounded shut-offs and parting lines	Sealing faces not masked	Mask all sealing and shut-off surfaces before blasting
Embedded media	Excess pressure or fractured media	Correct pressure, clean media, thorough cleaning
Base scratches showing through	Poor surface preparation	Stone or polish evenly before blasting
Staining or rust on stainless steel	Media contaminated with iron particles	Dedicated, clean media for stainless tools
Gloss mismatch between areas	Different media condition or operators	Refresh media and check against a master sample

Why mold shops do not use silica sand

The word sandblasting comes from the original use of silica sand, but silica sand is not appropriate for mold work. Blasting silica creates respirable crystalline silica dust, a recognized occupational health hazard regulated in the United States by OSHA, and its uneven, fracturing grains give inconsistent finishes. Mold shops use glass bead, aluminum oxide, ceramic, and other engineered media in enclosed cabinets with dust collection.

Frequently asked questions

What is sandblasting in mold making?
It is abrasive blasting of the mold cavity with controlled media to produce a uniform matte finish or to adjust the gloss of an existing texture.

What SPI finish is sandblasted?
The SPI D grades: D-1 with #11 glass bead, D-2 with #240 aluminum oxide, and D-3 with #24 aluminum oxide.

What is the difference between bead blasting and grit blasting?
Bead blasting uses round media that peen the surface into a soft satin matte; grit blasting uses angular media such as aluminum oxide that cut the surface into a duller, rougher matte.

Can sandblasting create a leather or wood grain?
No. Blasting only produces random matte finishes. Grains and patterns are made by chemical etching or laser texturing, and blasting is then used to set their gloss.

Is a blasted finish the same as a VDI finish?
Not exactly. They can share the same Ra, but blasted and EDM surfaces look different, so match by physical sample rather than by number.

How much draft does a blasted finish need?
Fine D-1 and D-2 finishes add little; coarse D-3 needs draft based on its real depth, using about 1.5 degrees per 0.001 in of texture depth.

Do mold shops really use sand?
No. Silica sand is avoided for health and consistency reasons; glass bead and aluminum oxide are the standard media.

Related processes and standards

For the other texturing methods, see chemical etching, laser texturing, and EDM texturing. For how finishes are specified, see SPI surface finish, VDI 3400, and the SPI to VDI conversion chart. Terms used on this page are defined in the glossary.

Normative References & Sources

• DME, Micro Finish Mold Tool Finish Guide, SPI D blasting conditions and approximate Ra: dme.net
• Plastics Industry Association (formerly SPI), mold finish guide: plasticsindustry.org
• Verein Deutscher Ingenieure (VDI), VDI 3400 guideline: vdi.de
• ISO 1302, indication of surface texture in technical product documentation: iso.org
• OSHA respirable crystalline silica standards, 29 CFR 1910.1053 and 1926.1153: osha.gov

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Reviewed by Marcel Dias, Technical Editor. Process data on this page is general guidance; media, pressures, roughness, and draft vary by equipment, material, and part design. Confirm with your toolmaker before production. See our Disclaimer. Last updated: October 9, 2026.