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Tham khảo tài liệu 'modern plastics handbook 2011 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 8.100 Chapter Eight mold can be textured and the part will reproduce the textured surface. Raised numbers and letters can also be molded-in and labels can be molded into the surface. Referring again to Fig. 8.32 the nominal wall W can range from a bare minimum of 0.060 to over 1.500 in there really is no technical limit with a variation of 0.020 in. Holes A D are not economically feasible to mold-in and are usually cut in as a postmolding operation but large openings can be molded-in. Parts cannot be trimmed in the mold but cores can be placed in the mold and removed from the part by hand after molding. Metal inserts H and bosses F can also be molded into the part but only from the outside surface. External ribs and internal wood or metal stiffeners can also be molded-in. When needed internal ribs bosses and other structures are usually glued in place or corrugated structures are used to increase strength and stiffness. The use of reinforcement mats strategically located in the wall may eliminate the need for structural details. Outside undercuts G are possible using slides or a split mold. Although it is possible to mold some parts with no draft at least a 1 to 2 draft is desirable for economical molding. 8.3.10 Machining Plastics can be machined just like other materials and often with less scrap because the stock is more readily formed to a size closer to the finished shape. Machining is the process which can provide the tightest tolerances because of the shrinkage inherent in the other processes. The use of CNC machining centers makes reproducibility very high. However one-off prototypes are most economically made with traditional machine tools because the setup cost is lower. The greatest limitation to the use of machining is the lack of a wide selection of materials available. Stock must be extruded or cast from the basic resin and this is only done for a limited number of resins and rarely for more than one grade of each resin. A limited number of sizes are
