Causes of Automotive Parts Label Curling: SHIMAI Uses Data to Reveal the Truth!

In the automotive production and manufacturing process, part labels play a crucial role. They are not only used to identify the name, specifications, and batch number of the parts but also involve quality traceability and production management, among other aspects. However, in actual application, problems such as label curling and detachment often occur. These issues not only affect the appearance quality of the parts but may also lead to errors in information recognition and difficulties in quality traceability, bringing many inconveniences to automotive production and after-sales management.

Targeting this issue, the company where the author of this article works has delved into the key problems causing automotive parts label curling from aspects such as label materials, environmental factors, processing techniques, and the surface treatment of the parts to which the labels are applied. The company has conducted test verifications on these problems and proposed effective technical implementation solutions.

Factors Affecting Label Curling
1. Surface Energy of Different Surface-Treated Materials

The selection of labels is related to the surface energy strength of the product's appearance parts because adhesion occurs at the interface between the two contacting surfaces, and the adhesion strength is mainly influenced by both wettability and adhesive force. Wettability is primarily determined by the surface energy of the adhesive and the material to be bonded (surface tension for liquids and surface energy for solids). Currently, the appearance parts of automotive power sources and electronic control modules mainly use surface treatment processes such as shot blasting, bare color passivation, blue film passivation, and powder coating. In actual application scenarios, labels applied to appearance parts without surface treatment are the least likely to curl. In contrast, labels applied to appearance parts treated with powder coating are the "hotspot" for curling and detachment problems. Therefore, in addition to analyzing the surface energy of different surface treatment processes, this study also synchronously analyzed whether the surface energy of products from different powder coating suppliers is consistent to rule out the impact of different surface treatment processes used by different suppliers on the labels.

2. Application Scenarios of Different Label Materials

At present, our company mainly uses two brands of labels. Label Brand A is mainly used for printed labels in our company and is applied to appearance parts without surface treatment. However, it is not clear whether it can be applied to other appearance surfaces. Label Brand B has a wide range of applications, good adhesion strength, and a higher price. It can be applied to various surfaces, but its application scope and principles are not confirmed. Therefore, this study needs to clarify the application scenarios of the two label materials and investigate the application principles of the label materials in different scenarios.

3. Label Test Standards and Peel Force Judgment Standards

According to the national standard GB/T 25978-2018 "Road Vehicles - Plates and Labels," the "180° peel strength test" method requires that the label samples be placed in a standard environment for 24 hours before testing. However, in actual incoming material inspection, IQC will directly test the label test samples that have been adhered to the test board for only 20 minutes. Therefore, this study will also further investigate initial adhesion (20-minute adhesion) and holding adhesion (24-hour adhesion) to establish standards for IQC to conduct routine inspection work. According to the performance requirements in the national standard GB/T 25978-2018 "Road Vehicles - Plates and Labels," after the 180° peel strength test, the 180° peel strength should be greater than or equal to 0.44 N/mm. To improve the adhesion strength of the labels on the products and ensure appearance quality, this study will use different surface-treated sheet metal parts as the actual adhesion surface for our company's labels. In addition, considering the aging attenuation of the label adhesive (about 20%), the 180° peel strength will be increased to 0.5 N/mm as the inspection standard.

Test Sample Preparation
1. Selection of Test Panels

Select one each of the uncoated, shot-blasted, bare color passivated, blue film passivated, and powder-coated appearance parts provided by Supplier A as the peel strength test panels to analyze the differences in surface energy and peel strength of different process-treated surface appearance parts from the same supplier. In addition, one powder-coated appearance part each is provided by Supplier A, Supplier B, and Supplier C to analyze the differences in surface energy and peel strength of the same process-treated surface appearance parts from different suppliers.

2. Selection of Test Label Materials

The test selects PET labels from Brand A and Brand B as the label test comparison materials.

3. Selection of Test Items

According to the national standard GB/T 25978-2018 "Road Vehicles - Plates and Labels," three test items, namely 180° peel strength, thermal cycle performance, and thermal aging performance, are chosen as the test items for this study. It should be noted that the full-size inspection and color difference inspection of the labels, as two routine inspection items, are not reflected in this study.

Test Verification Results and Analysis
1. Surface Energy of Different Surface-Treated Materials

Test method: Clean the surface of the object to be labeled with isopropyl alcohol, draw a short line with a dyne pen, and observe whether the short line will deform (contract). If the dyne liquid spreads out (does not contract or deform), then the surface energy of the solid is higher than the dyne value of the dyne pen; if the dyne liquid contracts into a mass, then the surface energy of the solid is lower than the dyne value of the dyne pen. The higher the value, the easier it is to wet/adhere; the lower the value, the more difficult it is to wet/adhere. According to the above test method, the listed test results are obtained. From the numerical results, the surface energy of the appearance parts decreases in the following order: uncoated > shot-blasted > bare color passivated > blue film passivated > powder-coated. This indicates that labels applied to uncoated appearance parts are the least likely to fall off, while those applied to powder-coated appearance parts are the most likely to fall off, consistent with the market feedback on label detachment problems. In addition, the surface energy values measured for the three suppliers providing the powder coating process are close, so it can be concluded that the differences in surface treatment processes among different suppliers do not have a significant impact.

2. Initial Adhesion and Holding Adhesion of the Same Brand of Labels
Initial adhesion refers to the ability of the adhesive to quickly adhere to the surface of the object to be bonded under slight pressure in an extremely short period of time. It determines whether the label can quickly stick to the surface without curling edges, shifting, or falling off. Holding adhesion refers to the ability of the adhesive to resist long-term displacement or detachment under continuous stress (usually shear force) parallel to the bonding surface. It measures the durability of the label's adhesion over a long period of time. According to relevant research, the strong correlation between the time after labeling and the adhesive surface can also be proven from the morphology of the adhesive surface. The longer the time, the better the leveling of the adhesive. After 24 hours, the leveling reaches its peak and maintains the maximum adhesive value.