Gloss Levels and Gloss Units for Molded Plastic Parts

Gloss is the amount of light a surface reflects in the mirror direction, and it is measured in gloss units (GU) with a gloss meter at a fixed angle of 20, 60, or 85 degrees. The scale is anchored to a polished black glass standard that reads 100 GU at each angle. In practice, a surface is measured at 60 degrees first: above about 70 GU it is re-measured at 20 degrees, and below about 10 GU at 85 degrees. For molded parts, gloss is the number that turns words such as "high gloss" or "matte" into a requirement that a toolmaker and a molder can check.

What gloss is and why it is not roughness

Gloss describes how much light a surface sends back at the mirror angle, the angle equal and opposite to the light that strikes it. A smooth, polished surface returns most of that light in one direction and looks glossy. A rough or textured surface scatters it in many directions and looks matte. Gloss is therefore an optical property, measured with light, while roughness values such as Ra are geometric properties, measured with a stylus or an optical profiler.

The two are related but not interchangeable. The same Ra can give different gloss on different resins, in different colours, and with different surface structures. A directional stoned surface and a random blasted surface at the same Ra, for example, do not reflect light the same way; see SPI C matte finish for that comparison. This is why mold finish standards such as SPI and VDI 3400 describe the cavity, while gloss describes the molded part.

How gloss units are defined

A gloss meter shines a beam of light onto the surface at a fixed angle and measures the light reflected at the mirror angle. The reading is expressed against a reference: a highly polished black glass with a defined refractive index, which is assigned 100 GU at each standard angle. A surface that reflects half as much light as that reference at the same angle reads about 50 GU.

Two consequences follow from this definition:

• 100 GU is not a maximum. Materials that reflect more than black glass, such as metals, mirror coatings, and some clear plastics, can read well above 100 GU, especially at 20°.
• Values at different angles are not comparable. The same part can read very differently at 20°, 60°, and 85°. A reading is only meaningful with its angle stated.

The three measurement angles

Each angle is suited to a different range of gloss. Steep angles separate high-gloss surfaces well; grazing angles separate matte surfaces well.

Standard gloss measurement angles and their use

Angle	Best suited to	When to use it	Typical molded surfaces
20°	High gloss	When the 60° reading is above about 70 GU	SPI A and the brightest B surfaces, piano-black trim
60°	All levels; the universal angle	First measurement on any surface; final angle between about 10 and 70 GU	Most SPI B surfaces, fine C finishes, general housings
85°	Low gloss and matte	When the 60° reading is below about 10 GU	Blasted D finishes, VDI and chemical textures, interior trim

The 70 and 10 GU switch points come from the guidance in the gloss test methods. They are a rule for choosing the angle, not a formal classification of gloss levels, and a specification may set a different angle if the customer requires it. Some plastics specifications also use a 45° geometry under ASTM D2457.

Gloss levels in practice

There is no single standard that defines "high gloss", "semi-gloss", or "matte" in gloss units for molded plastics. Each industry and each customer sets its own targets. The angle-selection rule gives a practical way to group surfaces, and the table below shows how that grouping lines up with mold finishes in general terms.

Practical gloss groups and the mold finishes that usually produce them

Gloss group	60° reading	Angle for the final value	Mold finishes usually involved
High gloss	Above about 70 GU	20°	SPI A-1 to A-3, often B-1
Medium gloss	About 10 to 70 GU	60°	SPI B-2 and B-3, C-1
Low gloss and matte	Below about 10 GU	85°	SPI C-2 and C-3, D-1 to D-3, VDI and chemical textures

The finish-to-gloss column is a general tendency, not a conversion. The resin, the colour, and the molding conditions move the result enough that a given cavity finish can land in a different group on a different part. Always set the target on the part and verify it on a sample.

What controls the gloss of a molded part

The cavity finish sets the starting point, but the part only reaches that gloss if the polymer copies the steel faithfully. Four groups of factors decide how closely it does.

Mold surface

The finer and more even the cavity surface, the higher the potential gloss. Polished SPI A and B cavities give the highest values; stoned C, blasted D, EDM, and chemically textured cavities give progressively lower ones. Wear, residue, and mold release agents on the steel lower gloss over a run, so cleaning intervals matter on gloss-critical tools. For the polished grades, see SPI A glossy finish and SPI B semi-gloss finish.

Resin and colour

Amorphous resins such as ABS, PC, PMMA, and PC/ABS copy a polished cavity closely and reach high gloss. Semi-crystalline resins such as PP, PE, and POM usually mold with lower gloss on the same cavity. Fillers, glass fibre, and some additives roughen the skin and reduce gloss. Colour does not change the physics, but gloss differences are much easier to see on dark colours, so black and dark grey parts usually get the tightest targets.

Molding conditions

A hotter mold and a well-packed part let the melt stay in contact with the steel long enough to copy it, which generally raises gloss on polished cavities. A cold mold, low packing pressure, or a slow fill can freeze the skin before it copies the surface and lower gloss. On textured cavities the effect is more complex: better replication fills the texture more completely, which can make a part look either more matte or, on some textures, more glossy at the peaks. Gloss targets should be confirmed at the production process settings, not only at sampling.

Part geometry and flow

Gloss can vary across one part. Areas far from the gate, thin sections, and areas behind ribs and bosses may fill and pack differently from areas near the gate, and show visibly different gloss. Weld lines and flow fronts can appear as gloss bands even when the cavity finish is uniform.

Gloss on textured parts

On textured parts, gloss is often the main appearance requirement after the pattern itself. Two parts with the same Mold-Tech texture or the same VDI 3400 grade can look mismatched if their gloss differs, especially when they sit next to each other in an assembly. Automotive interiors are the clearest example: specifications commonly set low gloss targets at 60° or 85° so that adjacent parts from different tools and resins look alike and do not reflect into the windscreen.

Where a texture alone does not bring the gloss down far enough, texturing suppliers can apply a secondary low-gloss treatment over the main pattern, usually a fine blast or a fine etch on top of it. That treatment changes the micro-surface without changing the visible pattern, and it should be specified and approved together with the texture.

Measuring gloss on molded parts

A gloss reading is only as good as the way it is taken. On molded parts, a few points cause most of the disagreements between supplier and customer:

• Flat area. The meter needs a flat area larger than its measuring spot. Curved surfaces, small parts, and faces near edges give unreliable readings; agree on the measuring locations in advance, or use a small-spot meter.
• Direction. On directional finishes such as SPI B and C, gloss depends on whether the meter is aligned with or across the polishing lines. State the direction, or measure both and report both.
• Calibration. Check the meter against its calibration standard before each measuring session and keep the standard clean.
• Condition of the part. Measure clean parts, after the part has stabilized at room temperature, and away from release agent residue.
• Several readings. Take several readings per location and report the average, since a single value can be affected by a local mark.

For very high-gloss parts, gloss alone may not capture the visual quality. Haze, a milky glow around the reflection, and distinctness of image, how sharply a reflection is seen, are measured separately with dedicated instruments when the part needs a premium mirror appearance.

How to specify gloss on a drawing

A complete gloss callout gives the toolmaker, the molder, and the inspector the same target. It should state:

• Value and tolerance: for example "35 ± 5 GU", rather than a single number or a word such as "semi-gloss".
• Angle: 20°, 60°, or 85°, chosen with the selection rule above.
• Test method: ASTM D523, ISO 2813, or ASTM D2457, as agreed with the customer.
• Location and direction: the faces to measure and, on directional finishes, the meter orientation.
• Mold finish: the SPI, VDI, or texture callout for the cavity, since the gloss target and the cavity finish must be consistent.
• Reference part: an approved part in production resin and colour, kept as the appearance master.

Cavity finish and part gloss are two halves of one requirement. Specifying only the cavity leaves the gloss to chance; specifying only the gloss leaves the toolmaker without a starting point. For the full set of cavity finishes, see the SPI surface finish reference, and for how cavity standards relate to each other, the SPI, VDI and Mold-Tech conversion page.

Common gloss problems on molded parts

Common gloss problems on molded parts: cause and correction

Problem	Probable cause	Correction
Gloss lower than target on a polished tool	Cold mold or low packing; residue on the steel	Raise mold temperature and packing within the process window; clean the cavity
Gloss bands or patches across the part	Uneven fill or packing; flow fronts and weld lines	Review gate position, fill speed, and venting
Mismatch between adjacent parts	Different resins, tools, or finishes for parts seen together	Set one gloss target and angle for the assembly and approve the parts side by side
Shiny spots on a textured part	Worn texture peaks, or scuffing on ejection	Repair or re-treat the texture; check draft and ejection
Supplier and customer readings disagree	Different angle, direction, location, or meter calibration	Agree on method, angle, locations, and direction; compare on the same reference part

Scuffing on ejection is often a draft problem rather than a gloss problem; the draft angle by texture page gives the minimum draft for each finish.

Frequently asked questions

What is a gloss unit?
A gloss unit (GU) expresses specular reflection relative to a polished black glass standard, which is set at 100 GU at each measuring angle.

Which angle should I use to measure gloss?
Measure at 60° first. If the reading is above about 70 GU, use 20°; if it is below about 10 GU, use 85°. State the angle with every value.

Can a gloss reading be higher than 100 GU?
Yes. Surfaces that reflect more than the black glass standard, such as metals, mirror finishes, and some clear plastics, can read above 100 GU, especially at 20°.

Can I convert Ra to gloss units?
No. There is no fixed conversion, because gloss also depends on the resin, the colour, the surface structure, and the molding conditions. Set the gloss target on the part and verify it on a sample.

What standard is used to measure gloss on plastics?
ASTM D523 and ISO 2813 define specular gloss measurement at 20°, 60°, and 85°. ASTM D2457 covers plastic films and solid plastics, including a 45° geometry.

Why does the same mold give different gloss with different resins?
Resins copy the cavity differently. Amorphous resins such as ABS and PC replicate a polished surface closely; semi-crystalline resins and filled grades usually mold with lower gloss.

Does mold temperature affect gloss?
Yes. On polished cavities, a hotter mold and good packing generally raise gloss because the melt copies the steel more completely before the skin freezes.

How should gloss be written on a drawing?
Give the value with a tolerance, the angle, the test method, the faces and direction to measure, the cavity finish, and an approved reference part.

Normative References & Sources

• ASTM D523, Standard Test Method for Specular Gloss: astm.org
• ASTM D2457, Standard Test Method for Specular Gloss of Plastic Films and Solid Plastics: astm.org
• ISO 2813, Paints and varnishes, determination of gloss value at 20°, 60° and 85°: iso.org

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Written and technically reviewed by Marcel Dias, Technical Editor, with over 28 years in mold texturing. Gloss values and finish relationships are approximate and vary by resin, colour and process; verify against the applicable test method and an approved sample before production. See our Disclaimer. Part of the Mold Texture Standards section. Last updated: October 9, 2026.