What Is Bend?
Veneer bending test, in which the veneered panel bends around a cylindrical or conical mandrel of defined diameter; It is a test that evaluates whether there is cracking in the coating or separation from the lower surface.
Purpose of Bend
The aim is to verify that the coating can flex without cracking under forming and service deformations.
Which Products Is Bend Applied To?
- Coated sheet and strip products
- Painted panels to be shaped
- Powder coated parts
- Coated pipe and profile
In Which Sectors Is Bend Used?
Standards Related to Bend
- ISO 1519 (silindirik mandrel)
- ISO 6860 (konik mandrel)
- ASTM D522
- ASTM D4145
Bend Conditions
- Mandrel diameter
- Diameters defined in the standard; The smallest crack-free diameter is determined
- Bending
- At defined speed and angle; coating on the outer surface
- Conditioning
- The coating is fully cured and conditioned in the environment defined in the standard.
- Assessment
- Crack inspection under magnification; tape test if necessary
Equipment for Bend
- Cylindrical and conical mandrel testing devices
- Coating-thickness gauge
- Magnifier or stereo microscope
- Conditioning room
Sample Requirements for Bend
- Panel bottom material thickness must comply with the specification
- Coating thickness must be measured
- Panel edges must be free of notches
- The coating must be fully cured
Duration of Bend
Typically 1–2 days.
Bend Procedure
- 1The panel is conditioned and the thickness is measured.
- 2Mandrel diameter is selected.
- 3The panel covering is bent to the outside.
- 4The bending region is examined with magnification.
- 5If necessary, it is repeated with smaller diameters.
- 6The smallest mandrel diameter without cracking is reported.
Bend Acceptance Criteria
The acceptance criterion is the absence of cracks and separation within the mandrel diameter given in the specification.
Evaluation of Results
The thickness of the substrate directly affects the result; Thicker sheet metal cracks earlier in the same coating. At low temperature, coatings become brittle, so the specification may call for low temperature warping.
Bend Report
- Standard applied
- Substrate material and coating thickness
- Mandrel diameters
- Conditioning and temperature
- Crack findings
Accreditation
EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.
Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Regulations
- Paint and coating specifications
Paint and Coating Testing — What Does This Testing Field Cover?
The section below describes the field that the Bend test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.
Paint and coating testing measures the adhesion of a coating to the surface, its mechanical durability and its resistance to environmental effects. Because a coating is a thin layer, the results depend both on the material and on the QUALITY OF APPLICATION; the same paint gives an entirely different result over a different surface preparation.
This dual dependence is the most important knowledge in the field. A coating failure usually arises not from the paint itself but from the surface preparation: oil residue, insufficient roughening, an oxide layer or moisture. Coating tests therefore assess not only the product but also the process, and the results are used for process control.
COATING THICKNESS is a recurring variable in the field. Many durability results relate directly to thickness; a thin area becomes the starting point of corrosion. Thickness measurement is therefore reported routinely alongside the other tests and is usually taken at several points — thickness falls at edges and corners, especially on complex geometries.
Three approaches are used on the adhesion side, each giving different information. The tape test is fast and suitable for field use but is a coarse scale. The cross-cut test shows behaviour at the intersections of layers. The pull-off method produces a numerical strength value and is the most suitable for comparison.
On the mechanical side, flexibility (mandrel bending), impact resistance, hardness, abrasion and stone chip resistance are measured. Stone chipping has a special place in automotive: gravel striking the coating under real road conditions is simulated, and the result is assessed according to whether the coating continues to protect the base metal.
On the appearance side, gloss and colour difference are monitored NUMERICALLY. Colour is assessed with a colour measuring instrument rather than by eye and the colour difference is expressed as a number; this makes it possible to compare different lots and different suppliers.
On the durability side, accelerated ageing (xenon arc, fluorescent UV), humidity, condensation and salt spray are applied. The result is assessed with scales for blistering, rusting, creep from a scribe and loss of adhesion; these scales are defined in separate standards and the report must state which was used.
In industrial processes such as powder coating and electrophoretic coating, the curing cycle (temperature and time) is what determines the result. An under-cured coating looks good but has poor chemical resistance and adhesion; oven profile measurement is therefore part of coating quality control.
Method groups in this field
Thickness
Dry film thickness by magnetic and non-magnetic methods; multi-point measurement and assessment of the distribution.
Adhesion
Tape test, cross-cut test and pull-off adhesion strength.
Mechanical strength
Mandrel bend flexibility, rapid deformation (impact), pencil and scratch hardness, Taber abrasion, stone chipping.
Appearance
Gloss measurement, colour difference calculation, orange peel and surface defect assessment.
Environmental durability
Salt spray, humidity, condensation, xenon arc and fluorescent UV ageing, chemical and stain resistance.
Assessment scales
Scales for blistering, degree of rusting, creep from a scribe and loss of adhesion.
Commonly confused in this field
- A coating result depends on the surface preparation; even if the paint itself is good, a poor preparation gives a poor result.
- A durability result without a reported thickness cannot be compared.
- The tape test is a coarse scale; numerical comparison requires the pull-off method.
- Colour cannot be assessed by eye; numerical colour difference measurement is required.
- An ageing result without a stated assessment scale cannot be interpreted.
- If the curing cycle is incomplete the coating looks good but its durability is poor; the oven profile is part of quality control.
Which decisions these results support
- Paint system approval and supplier selection
- Process control of the paint line and lot acceptance
- Supporting outdoor and automotive durability claims
- Detecting defects arising from surface preparation
- Monitoring colour and gloss consistency lot by lot
Related Tests
Frequently Asked Questions About Bend
According to which standard is Bend made?
Bend is executed in scope ISO 1519 (silindirik mandrel), ISO 6860 (konik mandrel), ASTM D522 and ASTM D4145. The limit values and acceptance criteria to be applied are defined in the product standard; The version valid for your product is confirmed in writing at the offer stage.
Which products does Bend apply to?
Mainly applied to Coated sheet and strip products, Painted panels to be shaped, Powder coated parts and Coated pipe and profile. Whether your product is included in the scope is determined by examining its technical specifications and intended use.
Under what conditions is Bend done?
The determining parameters are: Mandrel diameter — Diameters defined in the standard; The smallest crack-free diameter is determined, Bending — At defined speed and angle; coating on the outer surface and Conditioning — The coating is fully cured and conditioned in the environment defined in the standard.. The full terms are in the "Test Conditions" section on the page and vary depending on the product standard.
What kind of sample is needed for Bend?
Panel bottom material thickness must comply with the specification, Coating thickness must be measured and Panel edges must be free of notches. The number of samples is determined according to the product standard and the scope to be reported.
How long does Bend take?
Typically 1–2 days. This duration belongs to the experiment itself; sample preparation, queuing and reporting are also evaluated. A realistic schedule for your product is given in the offer.
What is the Bend result called "appropriate"?
The acceptance criterion is the absence of cracks and separation within the mandrel diameter given in the specification.