L=[L21+k)-(3/4)△° Wherein Nominal dimension of the external shape of plastic parts k Average shrinkage of plastic; plastic of molds taking 1/3 1/6 of the dimension olerance of corresponding plastic par Formula for depth dimension of cavity H=[Hn01+k)-(2/3)△]° (2-2) Wherein: Hp-Nominal dimension in the altitude-direction of plastic parts 2)Computation of Core Working Dimension to contained dimension which gradually decreases due to the abr orking Cores are used to mold the internal shape of plastic parts. Its working dimension also belongs 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 upper limit as the contained dimension and take the lower deviation as dimension tolerance. The specific formula is as follows Formula for radial dimension of core =[(+k)+(3/4)△ (2-3) Wherein: p-Radial nominal dimension of the internal shape of plastic parts Formula for altitude dimension of core h=[bn(1+k)+(2/3)4] Wherein: hp--Nominal dimension in the depth-direction of plastic parts 3)Computation of the Position Dimension of Molds(such as dimension of center-to-center distance of holes) The formula is C=C.(1+k)±δ/2 Wherein: Cp--Position dimension of plastic parts 3. Computation for the Dimension of Threaded Ring Cavity and Threaded Core 1)Computation for the Dimension of Threaded Ring Cavity Dn=[Dm(1+k)-△]° D=[D(+k)-4 [Dn(1+k)-△]° Wherein: Dm-Dimension of pitch diameter of threaded ring cavity D-Dimension of major diameter of threaded ring cavity D Dimension of minor diameter of threaded ring cavity Dpm--Nominal dimension of pitch diameter of plastic parts' external thread; D Nominal dimension of major diameter of plastic parts external thread D ominal dimension of minor diameter of plastic parts' external thread+δ L = [L（1+ k）− (3/ 4)Δ] p （2-1） Wherein: Lp —— Nominal dimension of the external shape of plastic parts; k —— Average shrinkage of plastic; Δ —— Dimension tolerance of plastic parts; δ —— Manufacturing tolerance of molds, taking 1/3 ~ 1/6 of the dimension tolerance of corresponding plastic parts. Formula for depth dimension of cavity: +δ H = [H（1+ k）− (2 / 3)Δ] p （2-2） Wherein: Hp —— Nominal dimension in the altitude-direction of plastic parts. 2) Computation of Core Working Dimension Cores are used to mold the internal shape of plastic parts. Its working dimension also belongs to contained dimension which gradually decreases due to the abrasion of core 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 upper limit as the contained dimension and take the lower deviation as dimension tolerance. The specific formula is as follows: Formula for radial dimension of core: = + + Δ −δ l [l（1 k） (3/ 4) ] p （2-3） Wherein: lp —— Radial nominal dimension of the internal shape of plastic parts. Formula for altitude dimension of core: = + + Δ −δ h [h（1 k） (2 / 3) ] p （2-4） Wherein: hp —— Nominal dimension in the depth-direction of plastic parts. 3) Computation of the Position Dimension of Molds (such as dimension of center-to-center distance of holes) The formula is: C = C（p 1+ k）± δ / 2 （2-5） Wherein: Cp —— Position dimension of plastic parts. 3. Computation for the Dimension of Threaded Ring Cavity and Threaded Core 1）Computation for the Dimension of Threaded Ring Cavity δ δ δ + + + = + − Δ = + − Δ = + − Δ [ 1 ] [ 1 ] [ 1 ] （ ） （ ） （ ） D D k D D k D D k s ps l pl m pm （2-6） Wherein: Dm —— Dimension of pitch diameter of threaded ring cavity; Dl —— Dimension of major diameter of threaded ring cavity; Ds —— Dimension of minor diameter of threaded ring cavity; Dpm —— Nominal dimension of pitch diameter of plastic parts’ external thread; Dpl —— Nominal dimension of major diameter of plastic parts’ external thread; Dps —— Nominal dimension of minor diameter of plastic parts’ external thread;