Fig 2-7: inlaying modular core 2.1.2 Dimension Design The working dimension of molding parts refers to the dimension in direct contact with plastic parts in the cavity and core. Its precision directly influences the precision of plastic parts 1. Factors Relating with Working Dimension 1)The Shrinkage of Plastic Parts Due to the nature of plastic(to expand when hot and to shrink when cold), the dimension of plastic parts after molding and cooling is smaller than that of the cavity 2)Manufacturing Tolerance The manufacturing tolerance directly influences the dimension tolerance of plastic parts 3-1/6 of the tolerance of plastic parts is usually taken as the manufacturing tolerance of cavity and core and the surface roughness ra is0.8 um.4 um 3)Abrasion loss during Use The abrasion and restoration during production can lessen the dimension of cores and enlarge the dimension of cavities Therefore, shrinkage plays a more important part in the dimension of plastic parts when molding large parts, whereas when molding small parts, the influence of manufacturing tolerance ly gre between several percent and parts per thousand. For specific shrinkage of plastic please refer to relevant manuals or instructions to plastic products Usually, the working dimension of cavities and cores is determined according to such three factors as the shrinkage of plastic, the manufacturing tolerance of cavity and core parts as well ) Computation of Cavity Working DimensioN Cavities are mold parts for forming the external shape of plastic parts. Its working dimension is a kind of containment dimension which can gradually get larger due to the abrasion of cavity in use.Hence, to leave some space for mold-repair after abrasion and for the convenience of fitting and assembly, when designing mold, it might as well take the lower limit as the containment dimension and take the upper deviation as dimension tolerance. The specific formula is as followsFig.2-7: inlaying modular core 2.1.2 Dimension Design The working dimension of molding parts refers to the dimension in direct contact with plastic parts in the cavity and core. Its precision directly influences the precision of plastic parts. 1. Factors Relating with Working Dimension 1) The Shrinkage of Plastic Parts Due to the nature of plastic (to expand when hot and to shrink when cold), the dimension of plastic parts after molding and cooling is smaller than that of the cavity. 2) Manufacturing Tolerance The manufacturing tolerance directly influences the dimension tolerance of plastic parts. 1/3~1/6 of the tolerance of plastic parts is usually taken as the manufacturing tolerance of cavity and core, and the surface roughness Ra is 0.8 um ~0.4 um . 3) Abrasion Loss during Use The abrasion and restoration during production can lessen the dimension of cores and enlarge the dimension of cavities. Therefore, shrinkage plays a more important part in the dimension of plastic parts when molding large parts, whereas when molding small parts, the influence of manufacturing tolerance and abrasion loss is relatively greater. Commonly-used shrinkage of plastic parts is usually between several percent and parts per thousand. For specific shrinkage of plastic please refer to relevant manuals or instructions to plastic products. 2. Computation of Working Dimension Usually, the working dimension of cavities and cores is determined according to such three factors as the shrinkage of plastic, the manufacturing tolerance of cavity and core parts as well as the abrasion loss. 1) Computation of Cavity Working Dimension Cavities are mold parts for forming the external shape of plastic parts. Its working dimension is a kind of containment dimension which can gradually get larger due to the abrasion of cavity in use. Hence, to leave some space for mold-repair after abrasion and for the convenience of fitting and assembly, when designing mold, it might as well take the lower limit as the containment dimension and take the upper deviation as dimension tolerance. The specific formula is as follows: Formula for radial dimension of cavity: