Author: MULAN -Plastic Molding Manufacturer
Blow molding products are also called hollow blow molding products, a rapidly developing plastic processing method. The tubular plastic parison obtained by extrusion or injection molding of thermoplastic resin is placed in the split mold while it is hot (or heated to a softened state), and compressed air is injected into the parison immediately after the mold is closed to blow the plastic parison. It swells and clings to the inner wall of the mold, and after cooling and demoulding, various hollow products can be obtained. The manufacturing process of blown film is very similar in principle to blow molding of hollow products, but it does not use molds. From the perspective of plastic processing technology classification, the forming process of blown film is usually included in extrusion. We often find that blow-molded products will more or less fade after long-term storage, so why do blow-molded products fade? Let’s learn about the reasons for the fading of blow-molded products 1. Light fastness The light fastness of the colorant directly affects the fading of the product. For outdoor products exposed to strong light, the light fastness (light fastness) grade of the colorant used is an important indicator. If the light fastness is poor, the product will fade quickly during use. The light resistance grade of weather-resistant products should not be lower than grade 6, preferably grade 7 or 8, and indoor products can choose grade 4 or 5. The light resistance of the carrier resin also has a great influence on the color change, and the molecular structure of the resin caused by ultraviolet light will change and fade. Adding light stabilizers such as ultraviolet absorbers to the masterbatch can improve the light resistance of colorants and colored blow-molded products. 2. Acid and alkali resistance The fading of colored blow molded products is related to the chemical resistance (acid and alkali resistance, redox resistance) of the colorant. For example, molybdenum chrome red is resistant to dilute acid, but is sensitive to alkali, and cadmium yellow is not acid resistant. These two pigments and phenolic resin have a strong reducing effect on some colorants, which seriously affect the heat resistance and weather resistance of the colorant and cause fading. 3. Antioxidant Some organic pigments undergo macromolecular degradation or other changes after oxidation and gradually fade. The first process is high-temperature oxidation during processing, and the second is oxidation that occurs when encountering a strong oxidant (such as chromate in chrome yellow). After mixing lakes, azo pigments and chrome yellow, the red color will gradually fade. 4. Heat resistance The thermal stability of the pigment refers to the degree of thermal weight loss, discoloration and fading of the pigment at the processing temperature. The components of inorganic pigments are metal oxides and salts, which have good thermal stability and high heat resistance. The pigments of organic compounds will undergo changes in molecular structure and a small amount of decomposition at a certain temperature. Especially for PP, PA, and PET products, when selecting colorants, we should pay attention to the heat resistance of the pigment on the one hand, and consider the heat resistance time of the pigment on the other hand.