haze meterBy measuring the scattering and transmission characteristics of the sample towards light, the transparency and optical uniformity of the material are evaluated, and the reliability of the detection results highly depends on scientific sampling standards and reasonable selection of testing areas. The safety optical performance of car windshields, the barrier and light transmission requirements of food packaging films, and differentiated standards for haze detection are proposed. Based on the specifications, the sampling and testing area requirements for two typical samples will be dismantled to provide a basis for accurate detection.

The core of the sampling standard for car windshields is to balance representativeness and integrity, and to match their curved structure and usage scenarios. As a critical component for vehicle safety, sampling should follow the principle of "non-destructive priority, destructive sampling supplement": for finished windshields, in-situ testing is preferred. If cutting is required, the sample should be taken from the non visible area of the glass (such as the edge reserved processing area), and the sampling size should meet the testing diameter of the haze meter (usually ≥ 50mm × 50mm). During the cutting process, glass edge cracking and surface scratches should be avoided. The sampling quantity should be determined according to the testing purpose: 3-5 pieces of glass are randomly selected from each batch for factory inspection, and 2 samples are taken from each piece of glass; Fault analysis requires sampling separately from the defect area and the surrounding normal area to form a comparison. Special attention should be paid to allowing tempered or laminated windshields to stand for 24 hours after sampling to eliminate internal stresses generated during the cutting process and avoid haze measurement deviations caused by stress.

The selection of its testing area should closely follow the "visual safety core area" and avoid interference from edges and defects. According to the automotive glass inspection standards, the testing area should focus on the driver's visual main area - the area extending forward from the center of the steering wheel to the glass (about 30% of the glass area). Three testing points should be evenly distributed in this area, forming an isosceles triangle with a spacing of not less than 50mm. At the same time, it should be clarified that no testing points should be set within 20mm of the edge of the glass, as the edge is prone to processing defects; Areas with heating wires and rain sensors should be avoided to prevent metal components or coatings from affecting the transmission and scattering of light. If there are defects such as scratches and bubbles on the glass, point tests should be set at the center of the defect and 5mm and 10mm away from the defect to evaluate the impact range of the defect on the overall optical performance.

The sampling standard for food packaging film focuses on "uniformity and typicality", adapting to its mass production characteristics. The sampling of thin films requires the use of the "random multi-point sampling method", sampling from different rolls (3-6 rolls per batch) of the same batch of products. Each roll of thin film is cut from 5m away from the roll head to avoid wrinkles and contamination of the roll head. The sampling size needs to be adjusted according to the type of film: cut a sample of 100mm × 100mm from the flexible film, and ensure that the sample is free of wrinkles and tensile deformation; The sampling size of hard packaging film (such as PET sheet) can be reduced to 50mm × 50mm, but the edge must be ensured to be flat. For composite films, it is necessary to ensure the integrity of the composite layer during sampling to avoid delamination. Each sample should be labeled with the sampling roll number and sampling position (such as roll head, roll middle, and roll tail) for subsequent quality traceability.

The selection of the testing area should cover the "production variation points" to ensure that the test results reflect the overall quality. Each thin film sample needs to be evenly distributed with 5 testing points, using the "cross cross method" distribution -1 point in the center and 4 points 20mm away from the edge, covering the entire sample area. Special attention should be paid to the special areas of the film: the heat sealed edge area needs to be tested separately, as the heat sealing process may cause changes in the crystallinity of the film, affecting the haze value; The printing area should avoid ink coverage and be tested in the blank area. If it is a fully printed film, the thinnest part of the ink should be selected as the test point. For films that are prone to moisture absorption (such as PE and PP), after sampling, they need to be equilibrated in a standard environment (temperature 23 ± 2 ℃, humidity 50 ± 5%) for more than 4 hours before selecting and testing the testing area to avoid haze deviation caused by environmental humidity.

The testing of both types of samples must follow general requirements: the sampling tool must use a dedicated cutting knife to ensure a smooth incision; The testing area needs to be cleaned with a dust-free cloth dipped in anhydrous ethanol to remove surface oil stains and dust; During testing, the sample should be tightly attached to the testing port of the haze meter to avoid light leakage. At the same time, it is necessary to regularly calibrate the instrument with a standard haze film to ensure detection accuracy.