the forming design of mold, of which the following is a commonly-used one L=L×k erein L-Working dimension of molds forming parts Lp--Nominal dimension of plastic parts external shape, k Average shrinkage of plastic Dimension tolerance of plastic parts Manufacturing tolerance of molds usually taking 50% When the upper and lower tolerance are either positive or negative value, it is a seldom-used tolerance, which can easily result in faulty calculation; thus, it needs to be modified during design before calculating the working dimension and tolerance, for example Dimension of plastic parts 1002, re-defined as the medium value.3-01, dimension and tolerance of its molds: 10.35 Dimension of plastic parts 10_04, re-defined as the medium value 9.7_0,1, dimension and tolerance of its molds: 9.75-0 05 This principle is very important, since when modifying the product chart of plastic parts, it must be modified into the medium dimension in accordance with the requirements of dimension and tolerance of drawings. Several instances of such dimension and tolerance see Table 2-2 Table 2-2: several instances of dimension and tolerance Dimension and Plastic Shrinkag Working Manufacturing Dimension and Tolerance of plastic Dimension Tolerance Tolerance of mold 10±0.1 HIPS 0.5% 1005 ±0.05 10.05±0.05 1005 HIPS 0.5% 1005 0025 10.050 10 HIPS 0.5% 10.35 9.75 2.2 Side Core-pulling Mechanism The flanks of plastic parts are usually provided with holes or flutes, as indicated in Fig. 2-9 Under such cases, side-direction forming cores must be employed to form plastic parts. However, Ich forming cores must be fabricated into active parts so that they can be pulled out prior to the stripping of plastic parts. The mechanism for pulling out and restoring such active forming cores called core-pulling mechanismthe forming design of mold, of which the following is a commonly-used one: L = L × k p (2-9) δ = Δ× p Wherein: L—— Working dimension of molds’ forming parts; Lp —— Nominal dimension of plastic parts’ external shape; k —— Average shrinkage of plastic; Δ —— Dimension tolerance of plastic parts; δ —— Manufacturing tolerance of molds; p —— Proportion, usually taking 50%. When the upper and lower tolerance are either positive or negative value, it is a seldom-used tolerance, which can easily result in faulty calculation; thus, it needs to be modified during design before calculating the working dimension and tolerance, for example: Dimension of plastic parts 0.4 0.2 10+ + , re-defined as the medium value 0.1 0.1 10.3+ − , dimension and tolerance of its molds: 0.05 0.05 10.35+ − ; Dimension of plastic parts 0.2 0.4 10− − , re-defined as the medium value 0.1 0.1 9.7+ − , dimension and tolerance of its molds: 0.05 75 0.05 9. + − . This principle is very important, since when modifying the product chart of plastic parts, it must be modified into the medium dimension in accordance with the requirements of dimension and tolerance of drawings. Several instances of such dimension and tolerance see Table 2-2. Table 2-2: several instances of dimension and tolerance Dimension and Tolerance of Plastic Parts Plastic Shrinkage Working Dimension Manufacturing Tolerance Dimension and Tolerance of Molds 10 ± 0.1 HIPS 0.5% 10.05 ± 0.05 10.05 ± 0.05 0.05 100 + HIPS 0.5% 10.05 0.025 0 + 0.025 050 10. + 0 10−0.05 HIPS 0.5% 10.05 0 −0.025 0 05 0.025 10. − 0.4 10 0.2 + + HIPS 0.5% 10.35 0.05 0.05 + − 0.05 35 0.05 10. + − 0.2 10 0.4 − − HIPS 0.5% 9.75 0.05 0.05 + − 0.05 75 0.05 9. + − 2.2 Side Core-pulling Mechanism The flanks of plastic parts are usually provided with holes or flutes, as indicated in Fig.2-9. Under such cases, side-direction forming cores must be employed to form plastic parts. However, such forming cores must be fabricated into active parts so that they can be pulled out prior to the stripping of plastic parts. The mechanism for pulling out and restoring such active forming cores is called core-pulling mechanism