Tham khảo tài liệu 'plastics engineered product design part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 420 Plastics Engineered Product Design metalworking permits tolerances as low as one tenth of those on the drawing assuming the latter being reasonable for plastics. Under difficult circumstances it has been proved to be a good practice if critical dimensions are kept smaller in the mold first and then being revised after a test run under production conditions thus permitting machining the cavity if required. In any case close tolerances should be applied on such dimensions only as directly related to invariable mold dimensions. Any other dimension which is related to a mold dimension in two different mold parts should allow a generous tolerance. Economical production requires that tolerances not be specified tighter than necessary. However after a production target is met one should mold tighter if possible for greater profit by using less material. Many plastics change dimensions after molding principally because their molecular orientations or molecules are not relaxed. To ease or eliminate the problem one can change the processing cycle so that the plastic is stress relieved even though that may extend the cycle time. Also used is heat-treat the molded part based on experience or according to the resin supplier s suggestions. Theoretical efforts to forecast linear shrinkage have been limited because of the number of existing variables. One way to solve this problem is to simplify the mathematical relationship leading to an estimated but still acceptable assessment. This means however that the number of necessary processing changes will also be reduced. The parameters of the injection process must be provided. They can either be estimated or to be more exact taken from the thermal and rheological layout. The position of a length with respect to flow direction is in practice an important influence. This is so primarily for glass-filled material but also for unfilled thermoplastics. The difference between a length parallel to 0 and perpendicular to 90 the flow .
