Effective measures to solve the problem of oil loss after oil spraying on low-hardness soft plastic parts
Low-hardness soft plastic parts (such as those made from TPU, EVA, and PVC) often experience oil shedding after spray coating. This occurs when the coating is not firmly bonded to the surface and easily falls off, affecting not only the product’s appearance but also its performance. This problem is related to a variety of factors, including the plastic material properties, surface treatment process, spray material selection, and curing process. Targeted measures are needed to improve the coating’s adhesion.
Optimizing the surface pretreatment process for plastic parts is fundamental to resolving the oil shedding problem. The surfaces of low-hardness, soft plastic parts often harbor contaminants such as release agent residue, oil, and dust. These contaminants can hinder the adhesion of the coating to the part surface, leading to oil shedding. Therefore, the surface of the plastic part must be thoroughly cleaned before oil spraying. Solvent cleaning can be used, using volatile solvents such as alcohol and isopropyl alcohol to wipe the surface to remove oil and release agent. For plastic parts with rougher surfaces, ultrasonic cleaning can be used, using the vibrations of ultrasound to remove fine surface impurities. Furthermore, for materials with lower polarity (such as TPU), plasma surface treatment can be used. Plasma bombardment of the plastic surface introduces polar groups, increasing surface energy and enhancing the adhesion of the coating to the part.
Selecting the right spray material is crucial to preventing oil loss. Low-hardness soft plastic parts possess a certain degree of elasticity and flexibility. The spray material must match the part’s shrinkage and hardness. Otherwise, when the part is deformed under stress, the coating will easily peel due to stress concentration. For elastic materials such as TPU, elastic polyurethane coatings should be selected. These coatings offer excellent flexibility and adhesion, expanding and contracting with the deformation of the part. For PVC parts, vinyl chloride-vinyl acetate copolymer coatings can be used. They are highly compatible with PVC and form a strong bond. Furthermore, the type and ratio of the curing agent in the spray material must be appropriately selected. For example, in polyurethane coatings, appropriately increasing the proportion of isocyanate curing agent can increase the coating’s crosslinking density and enhance adhesion.
Adjusting the spraying process parameters can effectively improve the coating’s bonding strength. Excessively thick spraying can increase internal stress in the coating, making it prone to cracking and peeling. Therefore, the spraying thickness must be controlled. Generally, a single spraying thickness of no more than 30μm is recommended. For a thicker coating, multiple thin sprayings can be used. Spraying pressure and distance must also be optimized. Excessive pressure or close distance can lead to uneven coating distribution, resulting in bubbles and pinholes. Excessive pressure or long distance can result in a thin coating and incomplete coverage. Typically, spraying pressure should be controlled between 0.2-0.4MPa and a spraying distance of 150-250mm. Furthermore, the temperature and humidity of the spraying environment must be controlled. Excessive temperature can cause the coating’s solvent to evaporate too quickly, prematurely curing the coating surface, preventing internal solvent removal and forming bubbles. Excessive humidity can impair the coating’s curing performance. It is recommended to maintain a temperature between 20-30°C and a relative humidity between 50% and 70%.
Improving the curing process is crucial for ensuring coating adhesion. Low-hardness soft plastic parts have poor heat resistance, and excessively high curing temperatures can cause deformation. Therefore, a low-temperature curing process is required, typically controlled between 60-80°C. Curing time varies depending on the coating type, typically 30-60 minutes. For applications requiring improved curing efficiency, ultraviolet (UV) curing can be employed. UV-curable coatings cure rapidly under ultraviolet light, minimizing the time the part spends exposed to high temperatures. Furthermore, the heating rate during the curing process must be controlled. A slow ramp prevents stress between the coating and the part surface due to different thermal expansion coefficients, improving bond strength. After curing, sufficient cooling should be performed to ensure a complete bond between the coating and the part surface, preventing oil loss due to uneven cooling.
Strengthening the compatibility testing and adjustment of plastic material can reduce the oil loss problem at the source. Different batches of low-hardness soft plastic materials may have different compositions, resulting in different compatibility with the spray material. Therefore, compatibility testing is required before production, and the plastic parts after spraying are subjected to adhesion tests (such as cross-cut tests and bend tests) to ensure that the coating adhesion meets the standards. If compatibility issues are found, you can communicate with the material supplier to adjust the formula of the plastic material. For example, the content of polar groups in the TPU material can be appropriately increased to improve its compatibility with the coating. In addition, for plastic parts that need to be used for a long time or in harsh environments, a layer of primer can be applied to the surface of the plastic part before spraying. The primer should have good compatibility with both the plastic part and the topcoat, play a transitional role, and enhance overall adhesion.
