| A new plastic molding technique, rotational molding is capable of producing hollow plastic parts. In this regard, the mold is subjected to a biaxial rotation under heating, resulting in that the rotating plastic powder in the mold contacts the inner surface of the mold, changes into the melt and shapes hollow plastic parts. Rotational molding has many advantages over other molding process, including①The mold does not endure very high pressure and does not need very high strength, so the equipment and the mold are simple, conveniently made and have a low cost.②Rotational molding is particularly suitable to produce the complicated large hollow product.③Better performance multiplayer products, such as double wall product with high impact strength out-layer and good chemical stability inner-layer can be produced by the combination of different materials.④The product has equal thickness, without corner scrap and welding line, so the process saves raw materials.⑤There is not inner stress in the product because of the zero press molding, and there is not distortion and crack.The main study work is summarized as follows:1. Study on the choice of truck fender's material Choosing principle of the resin material was put forward. Three factors should be considered when choose a kind of resin material. They are fluidity of the resin melt, shape and size of the resin powder. According to the choosing principle of resin material, the material was chosen for truck fender. Impact capability, MFI and thermal stability were tested. The results indicate that the impact capability raises stably along with the heating time. The impact capability would decline when it reached to the peak value if the material was still heated. The MFI is stable in a long heating time. The analysis of infrared spectrum shows that the material has good thermal stability. At last we learn that the best truck fender can be made in the 290 centigrade heated for fifteen minutes using DOW2440LLDPE..2. Study on heating time algorithm for truck fenderTheory of thermal exchange was studied and a mould about heat transfer was put forward. From the mould we learn that the heat first comes to the surface of the mold and the transfer to the inner of the mold, the heat reaches the resin material, and ultimately the resin material is melt. There are two stages through the mold. They are the heating time of mold and the heating time of the resin material. According to the theory of thermotransport and interfacial movement, the heating time algorithm was brought forward. The heating time algorithm for mold was analyzed. We confirmed that the material for mold was aluminum and the thickness of the mold was five millimeter. By using the heating time algorithm, the theoretic heating time was 1.25 minutes, compared with the practical time 1.45 minutes, the correspond tolerance was 13.8 percent(less than 20 percent). It is in the permit scale of engineer. The heating time algorithm for the product was studied. Through the experiment, we ascertained the thickness of the product. By using the heating time algorithm, we calculated the theoretic heating time was 10.85 compared with the practical heating time 11.6 minutes, the correspond tolerance was 6.5 percent(less than 20 percent). It is in the permit scale of engineer. The reason for that is the product has complicated surface. Dead center exists. By using blocks with different heating transfer rate, we can decline it.3. Study on the design of truck fender and the moldThe outline for design was analyzed first. In the design of truck fender, the thickness was checked according to loading analysis. Near wall, fillet radius, enhancement rib, removal angle and tolerance were optimized. According to the shape of truck fender and the molding request, mold meeting face, quad, arris, angle and venthole were optimized. They were used in manufacture, and the truck fender with excellent performance was produced.
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