白④ Fig 1-9: improvement design for too small holes' pitch and space to wall The holes used for fastening on the plastic products as well as other holes under stress should be reinforced with a convex edge designed thereon, as indicated in Fig. 1-18. The fixing hole can be designed in the form of a screw hole with sinking head as illustrated in Fig. 1-19a), whereas that in Fig 1-19b)is generally not recommended. The form as shown in Fig. 1-19c)can be used instead so that the core can be better set m山 Fig1-10: reinforcement of holes 国國 Fig. 1-11; types of fixing holes 1.2.7 Screw Screws on a plastic part can be formed during the injection process. They may be created by machining after the injection process. For a plastic part which is subject to frequent assembly and For design of screws on a plastic part, the following principles are followed 1. The diameter of injected external screws should not be smaller than 4 mm. Internal screws sl not be smaller than 2 mm. Length of fit should be reduced to less than 1.5-2.0 times of the diameter. in order to minimize the accumulative error on the screw pitchFig.1-9: improvement design for too small holes’ pitch and space to wall The holes used for fastening on the plastic products as well as other holes under stress should be reinforced with a convex edge designed thereon, as indicated in Fig. 1-18. The fixing hole can be designed in the form of a screw hole with sinking head as illustrated in Fig. 1-19a), whereas that in Fig. 1-19b) is generally not recommended. The form as shown in Fig. 1-19c) can be used instead so that the core can be better set. Fig.1-10: reinforcement of holes Fig.1-11: types of fixing holes 1.2.7 Screw Screws on a plastic part can be formed during the injection process. They may be created by machining after the injection process. For a plastic part which is subject to frequent assembly and disassembly, the part may be inserted with a metal screw. Fig.1-12: screw design on plastic parts For design of screws on a plastic part, the following principles are followed: 1. The diameter of injected external screws should not be smaller than 4 mm. Internal screws should not be smaller than 2 mm. Length of fit should be reduced to less than 1.5~2.0 times of the screw diameter, in order to minimize the accumulative error on the screw pitch