Curl Ratio Testing

Curl rate testing mainly conducts scientific testing and analysis on the paper curl determination, curl rate testing of the plastic films, weft skew and bowing detection of the textile fabrics, warpage measurement of the thin metal sheets or strips, curl performance evaluation of the printed matters, and curing deformation evaluation of the composite laminates, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Paper curl determination: Mainly evaluates the degree of the curl deformation of the paper caused by the single-side moisture absorption or drying under the specific temperature and humidity conditions. This testing is crucial for the quality control of the printing, packaging and office papers, and directly affects the smooth paper feeding of the printing, the flatness of the finished products and the subsequent processing performance.

Curl rate testing of the plastic films: For the various uniaxially oriented (BOPP, BOPET, etc.) or cast films (CPP, CPE, etc.), the edge curl phenomenon generated after slitting or under the heating and moisture absorption is measured. This is related to the registration accuracy and production efficiency of the films in the processes such as the lamination, bag making and printing.

Weft skew and bowing detection of the textile fabrics: Although it is often called the "weft skew testing", its essence is to evaluate the non-perpendicularity of the warp and weft yarns of the fabric after the dyeing and finishing processing, which can be regarded as a kind of "curl" deformation in the plane. This indicator directly affects the marking efficiency of the garment cutting and the appearance quality of the finished products.

Warpage measurement of the thin metal sheets or strips: After the metal rolling or heat treatment, the sheets may produce the three-dimensional deformations such as the centre waves, edge waves or buckling. The curl rate testing extends to the "flatness" testing here, which has the decisive influence on the stamping formability of the high-end products such as the automotive sheets and foils for the electronic components.

Curl performance evaluation of the printed matters: Specially tests the curl of the finished printed sheets produced during the ink drying process or the environmental changes. Especially for the printed matters using the large-area solid inks or the overprinting of the inks of the different properties, this testing can predict their stacking stability and the subsequent processing adaptability.

Curing deformation evaluation of the composite laminates: For the laminates made of the prepregs such as the carbon fibres and glass fibres, due to the differences in the fibre directions and thermal expansion coefficients of the layers, the inherent curl or distortion deformation will be produced after curing. The testing data is used to optimize the lay-up design to reduce the internal stress.

Testing Scope

Printing and packaging industry: Covering the offset papers, coated papers, cardboards, corrugated paperboards and the various plastic film label materials. The testing environment usually simulates the printing workshops (such as 23±1 ℃ and 50±5% RH) or the subsequent storage environments, to predict the problems in the production and use.

Textile and garment industry: Applicable to the woven fabrics, knitted fabrics and nonwovens. The testing is usually carried out under the standard atmospheric conditions (as specified in GB/T 6529), focusing on the dimensional stability and morphological changes of the fabric after washing, setting or printing.

Polymer film industry: Including the food packaging films, industrial stretch films, capacitor films and solar backsheet films. The testing not only focuses on the normal state, but also involves the curl behaviour of the materials after the thermal aging and damp-heat aging, to evaluate their long-term reliability.

Metal processing and electronic materials field: Applied to the cold-rolled steel sheets, aluminium foils and copper strips with the thickness usually below 2.0 mm. The high-precision laser flatness measuring instrument can be used for the online detection, ensuring the precision of the materials in the high-speed stamping or etching processes.

Composite material manufacturing and aerospace: For the structural parts made of the prepregs through the autoclave or moulding processes. The testing scope ranges from the small laboratory test pieces to the large skin components, and the deformation data is the key input for the stress compensation and tooling design.

Papermaking and paper product quality control: Running through the whole process from the pulp proportioning, coating process to the finished product slitting. By testing the curl directions and curvature radii under the different humidity conditions, the adjustment of the production process parameters can be reversely guided.

Testing Methods

Cantilever method: The most classic method. The rectangular specimen is placed flat, one end is fixed, and the free end sags under its own weight or a slight force. The lifting height or bending angle is measured, and the curl curvature or curl index is calculated through the formula. It is widely used in the standard testing of the papers and films (such as ASTM D5459).

Cylindrical mandrel test: The specimen is closely attached to the standard cylindrical mandrels of the different diameters, and it is observed whether it fits or springs back, to qualitatively or semi-quantitatively evaluate its curl tendency and direction. This method is simple to operate and is commonly used for the rapid judgement in the printing workshops.

Projection method: A parallel light source is used to project the side profile of the specimen onto a scale or CCD sensor, the maximum warpage height and span are directly read, and the curvature is then calculated. This method has the higher precision and is suitable for the materials with the slightly higher rigidity.

Laser triangulation method: A non-contact high-precision measurement method. The laser line scans across the specimen surface, the positions of the reflected light spots are received by the sensor, the three-dimensional morphology of the material surface is reconstructed, and the overall flatness, waviness and local curl parameters can be precisely calculated.

Damp-heat cycling treatment method: This is not a direct measurement method, but a pre-treatment method. The specimen is alternately cycled between the high-humidity environment (such as 90% RH) and the dry environment (such as 30% RH), and then measured by the above methods, to evaluate the curl stability of the material under the environmental changes.

Digital image correlation method (DIC): A speckle pattern is made on the specimen surface, the images are captured by two or more cameras before and after the deformation of the specimen, and the software is used to analyse the full-field displacements and strains, which can comprehensively and intuitively analyse the complex three-dimensional curl and distortion deformations.

Testing Standards

Domestic Standards:

1. GB/T 10739, Paper, board and pulp — Standard atmospheres for conditioning and testing

2. GB/T 22364, Paper and board — Determination of bending stiffness

3. GB/T 6529, Textiles — Standard atmospheres for conditioning and testing

International Standards:

1. ASTM D5459, Standard test method for curl of paper and paperboard

2. TAPPI T466, Curl of paper and paperboard

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Digital paper curl tester: Specially designed for the paper, usually equipped with a constant temperature and humidity testing chamber. The instrument has a precise horizontal platform and optical or laser distance measuring sensors, which can automatically measure the lifting height of the free end of the specimen and directly display the curl angle or curvature value, meeting the standards such as TAPPI T466.

Film curl testing device: The structure is similar, but pays more attention to the fixing and measurement of the light films. It usually includes an adjustable clamping device, an arc angle measuring plate with the graduations or a high-precision displacement sensor, used to evaluate the natural curl state of the film after the stress release by slitting.

Fabric weft skew and bowing measuring instrument: Usually composed of a large transparent platform with the graduations and a movable scale. During the operation, the fabric is spread flat, the offset distance of the warp and weft yarns is measured with the scale, and the weft skew rate is calculated. The advanced models use the image recognition technology for the automatic reading.

Sheet flatness laser measuring instrument: Used for the metal or composite sheets. The multi-head laser displacement sensors scan along the width direction of the sheet, generating the flatness curve diagrams (such as the "I-unit" diagrams) in real time, precisely calculating the parameters such as the wave height and wave pitch, and realizing the high-speed online detection.

Constant temperature and humidity environmental test chamber: The core auxiliary equipment of the curl rate testing. It is used to condition the specimens before the testing (such as GB/T 10739), or to create the specific temperature and humidity environments during the testing, to study the curl behaviour of the materials under the different conditions.

General-purpose material mechanical testing machine (with the custom fixtures): By designing the three-point bending or four-point bending fixtures, the flexural rigidity and bending resilience of the materials can be precisely measured. These mechanical property parameters have the direct theoretical correlation with the curl tendency of the materials, and are suitable for the in-depth mechanism research.