Automotive Plastic Dog-Tooth Scratch Resistance Testing: Methods, Standards and Evaluation Criteria

Automotive plastic dog-tooth scratch resistance testing evaluates how well interior and exterior plastic components withstand scratching from a hard, tooth-shaped stylus under controlled loads, simulating damage from fingernails, keys, rings and routine cleaning. This article explains the test principle, sample requirements, measured parameters, reference standards such as ISO 1518 and ASTM D7027, and typical acceptance criteria applied by vehicle manufacturers and accredited third-party laboratories.

Technical Overview

The dog-tooth scratch test is a laboratory method used to assess the scratch and mar resistance of automotive plastic parts. A rigid stylus whose tip is machined into a canine-tooth (fang-like) geometry—typically hardened tool steel, tungsten carbide or diamond with a defined cone angle and tip radius—is pressed against the plastic surface at a specified normal force and drawn across it at a constant speed. The resulting damage is then evaluated visually and instrumentally to determine whether the material, grain and surface finish satisfy the appearance-durability requirements of the vehicle program.

Scratches on visible plastic surfaces are among the most frequent drivers of customer complaints in modern vehicles. Semi-crystalline materials such as polypropylene (PP) and talc-filled PP compounds are prone to stress whitening, in which light-scattering microvoids and crazes form around the scratch groove, while amorphous materials such as ABS and PC/ABS tend to show glossy or polished scratch traces. The pointed dog-tooth geometry concentrates contact stress in a manner similar to a fingernail edge or a key tip, making it more severe than a hemispherical indenter and therefore a realistic worst-case representation of in-vehicle abrasion.

Typical test parameters include normal loads of 2 N to 20 N, scratch speeds of approximately 10 mm/s to 100 mm/s, single-pass or cross-hatch scratch patterns, and specimen conditioning at 23 °C ± 2 °C and 50 % ± 5 % relative humidity in accordance with ISO 291 before testing.

Testing Samples

Interior Trim Components

Instrument panels, door trim boards, center consoles, glove boxes, A/B/C pillar trims, grab handles, air vents, switch bezels and seat trim parts are the most commonly tested articles. These components are frequently produced from PP/TPO, ABS, PC/ABS, PA, POM or PVC/TPO skins, and mold-in-color as well as painted or soft-touch coated versions can be evaluated.

Exterior Plastic Components

Bumper claddings, wheel-arch moldings, side sill scuff plates, mirror housings and grille trims are tested because they are exposed to keys, luggage, boots, car-wash brushes and road debris. Common materials include PP-EPDM, PA/PPE blends, PBT and PMMA.

Flat Plaques and Mold-in-Color Panels

Injection-molded flat plaques with the same grain pattern, grain depth and gloss level as the production part are widely used for material grade qualification. Testing grained plaques rather than smooth coupons ensures that the measured scratch visibility reflects the actual component appearance.

Coated and Soft-Touch Surfaces

Top-coated parts, soft-touch painted surfaces, laminated foils and skins can be tested to verify that the coating improves scratch behavior without detaching or showing delamination along the scratch groove.

Testing Items

Depending on the specification invoked, the following results are typically reported:

• Visual scratch rating on a multi-level scale, commonly rated from 1 (severe, clearly visible damage) to 5 (no visible scratch), assessed under defined illumination.
• Scratch width and scratch depth, measured by optical microscopy or profilometry at specified magnification.
• Gloss change, expressed as the difference in gloss units (ΔGU) before and after scratching, usually measured with a 60° geometry.
• Whitening and color difference, quantified as ΔL* and ΔE* with a spectrophotometer, since stress whitening is the dominant failure mode for PP-based trim.
• Critical scratch load, defined as the load at which a visible scratch, cohesive failure or abrupt friction increase first occurs during progressive-load scratching.
• Mar resistance after multi-pass or repeated scratching at low loads.
• Post-aging scratch resistance, evaluated after UV weathering or heat aging preconditioning to reflect long-term service conditions.

Testing Methods

Apparatus and Styli

A scratch tester consists of a force-controlled arm holding an interchangeable stylus, a linear or planar motion stage, and fixtures that keep the specimen rigidly supported. Styli include the dog-tooth tip with its characteristic pointed profile, 90° or 120° diamond cones and hemispherical tips with radii of 0.5 mm to 1.0 mm, allowing the severity of the test to be matched to the intended specification.

Constant-Load Dog-Tooth Scratch Procedure

After conditioning, the specimen is cleaned and clamped with the test area flat and free of sink marks or gate marks. The stylus is lowered onto the surface at the specified load—commonly 5 N, 10 N or 15 N depending on the part zone—and drawn across the surface over a scratch length of typically 50 mm to 100 mm at a constant speed. A single-pass scratch, a cross-hatch pattern of perpendicular scratches, or several parallel scratches at increasing loads may be specified.

Progressive (Variable) Load Testing

In instrumented progressive-load testing consistent with the principles of ASTM D7027 and ISO 1518-2, the normal load is increased linearly along the scratch track while the tangential force and acoustic or displacement signals are recorded. This allows the critical scratch load and the failure mechanism—elastic deformation, plastic grooving, crazing or material removal—to be identified as a function of load.

Evaluation and Acceptance Criteria

Visual assessment is performed under standardized illumination (commonly D65 light source at approximately 1000 lx to 2000 lx) at a viewing distance of about 300 mm to 500 mm. Instrumented measurements of gloss, color and scratch geometry supplement the visual rating. Typical acceptance criteria include no visible scratch under defined viewing conditions, a rating of at least 4 on the visual scale, ΔL* below a specified limit such as 1.0 to 2.0, and scratch width below a defined maximum for the relevant surface zone.

Reference Standards

• ISO 1518-1 and ISO 1518-2 — scratch resistance testing using a scratching stylus with constant and variable loading.
• ASTM D7027 — evaluation of scratch resistance of polymeric coatings and plastics using an instrumented scratch machine.
• GB/T 9279 — the Chinese national standard for scratch testing of paints and varnishes with a scratching stylus.
• ISO 291 — conditioning of test specimens prior to testing.
• ISO 4892 — artificial weathering exposure used for preconditioning when post-aging scratch behavior is required.
• ISO/IEC 17025 — general competence requirements under which accredited third-party laboratories perform these tests.
• Vehicle manufacturer enterprise standards, which define the exact dog-tooth tip geometry, loads, scratch pattern and rating scale for each component.

Applications

Dog-tooth scratch resistance testing supports the entire development cycle of automotive plastic components:

• Material grade selection and qualification of PP, ABS, PC/ABS and engineered thermoplastic compounds for visible interior and exterior surfaces.
• Validation of anti-scratch additive packages, such as silicone-based or slip-agent masterbatches that suppress stress whitening in polypropylene.
• Verification of grain design, grain depth and gloss level, since finer textures and lower gloss tend to conceal scratch damage more effectively.
• Release and periodic production testing of supplied components as part of quality assurance programs.
• Comparative benchmarking of candidate materials, coatings and surface treatments against competitive references.
• Diagnostic analysis of field complaints and warranty returns to determine whether failures stem from material, process or usage factors.
• Long-term durability studies in which scratch resistance is re-measured after thermal aging or UV weathering.

FAQ

What is the dog-tooth scratch test for automotive plastics?

It is a surface durability test in which a stylus with a pointed, canine-tooth-shaped tip is pressed onto a plastic surface at a defined load and drawn across it. The scratch that is produced simulates damage from fingernails, keys and similar sharp objects, and its visibility is rated to judge whether the part meets appearance requirements.

Which loads and speeds are typically used?

Loads usually range from 2 N to 20 N, with 5 N, 10 N and 15 N being the most common steps, and scratch speeds typically lie between 10 mm/s and 100 mm/s. The exact combination is defined by the applicable vehicle manufacturer specification and by the sensitivity of the surface zone being tested.

How are test results rated?

Results are rated by trained assessors on a visual scale, commonly 1 to 5 with 5 indicating no visible damage, and are supported by instrumental values such as scratch width, scratch depth, gloss loss in gloss units and color difference ΔL*. Many specifications set limits such as a minimum rating of 4 and ΔL* not exceeding 1.0 to 2.0.

Do grained and textured surfaces affect the outcome?

Yes. Grain depth and pattern strongly influence scratch visibility: coarser grains and lower gloss scatter light and hide scratches, while smooth high-gloss surfaces reveal even shallow damage. For this reason, specimens should carry the same grain and gloss as the production component.

Why do polypropylene parts turn white when scratched?

Scratching PP and filled PP compounds generates crazes and microvoids around the groove that scatter light, producing a whitened appearance. Anti-scratch masterbatches, optimized talc grades and controlled molding conditions are used to reduce this stress whitening.

What sample preparation and conditioning are required?

Flat specimens or whole components are conditioned at 23 °C ± 2 °C and 50 % ± 5 % relative humidity for at least 24 hours in accordance with ISO 291. Test areas must be free of sink marks, gate marks, ejector marks and parting lines, and surfaces must be cleaned of dust, release agents and fingerprints before testing.

Can the test be performed after aging exposure?

Yes. Scratch resistance is frequently re-evaluated after thermal aging or artificial weathering per ISO 4892, because embrittlement, additive migration or surface degradation during aging can significantly worsen scratch visibility compared with the unaged condition.