2. There should be an area larger than 0.5 mm without screw on its end, to facilitate manufacturing avoid burr, and effect guidanc ce at use. as shown in Fig. 1-12 3. For a plastic part which has two screws located along the same axis, the pitch and tightening irection should be the same. It simples mold structure design and the part production 1. 2. 8 Gear Fig 1-13: dimensions of a plastic gear The following principles are followed for plastic part design with gears The same material should be used for gears that match, to avoid problems caused by thrinkage 2. Follow the dimensional relations as below. H=D D1>1.5D t1=3m where H- the spoke thickness, mm D-the hub hole diameter, mm Di-the hub diameter, mm: tI-the spoke width, mm m-the gear modulus 3. Adopt the transition fit between the hole and shaft on a plastic gear. Avoid the key and key way design. Use the fastening method as shown in Fig. 1-13 1.2.9 Structure of Inserts Inserts are a metal part permanently inserted into a plastic part. They are used for various purposes. Some are used to enhance local strength, hardness, or wear of a plastic part. Some are used for electricity reasons, while others are used to improve the stability of its shape and dimensional precision. Though most inserts are typically made of metal, other materials may also be used Fig 1-14 shows some commonly used inserts. Fig 1-14(a) shows tubular inserts, with through holes blind holes, screw inserts, shaft inserts, and thin-walled tube inserts. Fig 1-14(b) shows cylindrical inserts such as screw bolts, pins, and connection pole and so on. Fig 1-14(c)shows non-cylindrical inserts such as conductive sheets and weld pieces. Fig 1-14(d)shows the small rod-shaped cross-cutting inserts of the car steering wheel. Fig 1-14(e) illustrates a PMMA housing inlaid with ABS plastic2. There should be an area larger than 0.5 mm without screw on its end, to facilitate manufacturing, avoid burr, and effect guidance at use, as shown in Fig.1-12. 3. For a plastic part which has two screws located along the same axis, the pitch and tightening direction should be the same. It simples mold structure design and the part production. 1.2.8 Gear Fig.1-13: dimensions of a plastic gear The following principles are followed for plastic part design with gears. 1. The same material should be used for gears that match, to avoid problems caused by thrinkage. 2. Follow the dimensional relations as below. H = D D1 >1.5D t1 = 3m where H – the spoke thickness，mm； D – the hub hole diameter，mm； D1 – the hub diameter，mm； t1 – the spoke width，mm； m – the gear modulus。 3. Adopt the transition fit between the hole and shaft on a plastic gear. Avoid the key and key way design. Use the fastening method as shown in Fig.1-13. 1.2.9 Structure of Inserts Inserts are a metal part permanently inserted into a plastic part. They are used for various purposes. Some are used to enhance local strength, hardness, or wear of a plastic part. Some are used for electricity reasons, while others are used to improve the stability of its shape and dimensional precision. Though most inserts are typically made of metal, other materials may also be used. Fig.1-14 shows some commonly used inserts. Fig.1-14(a) shows tubular inserts, with through holes, blind holes, screw inserts, shaft inserts, and thin-walled tube inserts. Fig.1-14(b) shows cylindrical inserts such as screw bolts, pins, and connection pole and so on. Fig.1-14(c) shows non-cylindrical inserts such as conductive sheets and weld pieces. Fig.1-14(d) shows the small rod-shaped cross-cutting inserts of the car steering wheel. Fig.1-14(e) illustrates a PMMA housing inlaid with ABS plastic