Chapter 2 Mold design 2.1 Molding parts Molding parts refer to those in direct contact with plastics to form the shape of plastic parts, wherein those constituting the contour of the plastic parts are called cavities and those constituting the internal shape of the plastic parts are called cores. Since the cavity and core directly contact the plastics of high temperature and pressure and rub with the plastic parts when protruding, it is thus required that they are provided with sufficient intensity, rigidity, hardness, abrasion resistance corrosion resistance as well as low enough surface roughness 2.1.1 Structural design 1. Cavity Structural Design 1)Integral Cavity Directly cut cavity in the die set plate as indicated in Fig. 2-1. The advantage thereof is that the processing cost is relatively low. Yet the molding board material for making the die set is usually common medium carbon steel which is short in service life if used as cavity parts, whereas selecting materials with high performance shall result in high production cost. Usually, for the mold and precision of plastic parts which are less than 10000 times of molding, relatively low requirements are made; therefore, for molds of simple shape, integral structure can be adopted ig.2-1: integral ca 2) Integral Embedded Cavity Use high quality materials(high-carbon steel or alloy tool steel)which are slightly larger than the external shape of the plastic parts(wall thickness of sufficient intensity must be ensured)to make the cavity parts and embed them into the molding plate thereafter, as indicated in Fig. 2-2 The advantage is that the service life of the cavity parts can be ensured and meanwhile the material cost is reduced. Furthermore, it is easy and convenient to repair and replace the cavity parts if they are damaged
Chapter 2 Mold Design 2.1 Molding Parts Molding parts refer to those in direct contact with plastics to form the shape of plastic parts, wherein those constituting the contour of the plastic parts are called cavities and those constituting the internal shape of the plastic parts are called cores. Since the cavity and core directly contact the plastics of high temperature and pressure and rub with the plastic parts when protruding, it is thus required that they are provided with sufficient intensity, rigidity, hardness, abrasion resistance, corrosion resistance as well as low enough surface roughness. 2.1.1 Structural Design 1. Cavity Structural Design 1) Integral Cavity Directly cut cavity in the die set plate as indicated in Fig. 2-1. The advantage thereof is that the processing cost is relatively low. Yet the molding board material for making the die set is usually common medium carbon steel which is short in service life if used as cavity parts, whereas selecting materials with high performance shall result in high production cost. Usually, for the mold and precision of plastic parts which are less than 10000 times of molding, relatively low requirements are made; therefore, for molds of simple shape, integral structure can be adopted. Fig.2-1: integral cavity 2) Integral Embedded Cavity Use high quality materials (high-carbon steel or alloy tool steel) which are slightly larger than the external shape of the plastic parts (wall thickness of sufficient intensity must be ensured) to make the cavity parts and embed them into the molding plate thereafter, as indicated in Fig. 2-2. The advantage is that the service life of the cavity parts can be ensured and meanwhile the material cost is reduced. Furthermore, it is easy and convenient to repair and replace the cavity parts if they are damaged
H7/m6 7/m6 egral e 3)Insertion and Splice Cavity For cavities which are of complicated shapes or are damageable in certain parts, design the parts hard to be processed or easily damaged into insert form and embed them into the basal body he cavity, as indicated in 2-3 Fig 2-3: local insertion and splice cavity For large and complicated cavity mold, the four walls of the cavity can be separately processed and inlaid into the mold sleeve and finally fitted with the soleplate, as indicated in 1g. 2-4: split-type cavit
Fig.2-2: integral embedded cavity 3) Insertion and Splice Cavity For cavities which are of complicated shapes or are damageable in certain parts, design the parts hard to be processed or easily damaged into insert form and embed them into the basal body of the cavity, as indicated in 2-3. Fig.2-3: local insertion and splice cavity For large and complicated cavity mold, the four walls of the cavity can be separately processed and inlaid into the mold sleeve and finally fitted with the soleplate, as indicated in Fig.2-4. Fig.2-4: split-type cavity
4)Threaded Ring Cavity Threaded ring cavity is a kind of active insert used to mold the outer thread of the plastic parts, which shall be protruded together with the parts after molding and dismounted outside the mold. Fig 2-5 shows an integral threaded ring cavity whose length of fit is 5mm-8mm. To make it easy for assembly, the remaining parts are made into 3.5obliquity, and a four-sided plane is modified at the lower extreme so that it will be convenient to screw it from the plastic parts with To sum up, cavity structures in more common application are integral embedded cavity and insertion and splice cavity 8/8 2. Core Structural Desi Integral punch costs too many materials and the working load for cutting and processing amounts too high. Therefore, hardly any such structure exists in modern mold structures which is instead preoccupied with integral embedded punch and inlaying modular punch, as indicated in Fig 2-6 and 2-7 多
4) Threaded Ring Cavity Threaded ring cavity is a kind of active insert used to mold the outer thread of the plastic parts, which shall be protruded together with the parts after molding and dismounted outside the mold. Fig.2-5 shows an integral threaded ring cavity whose length of fit is 5mm~8mm. To make it easy for assembly, the remaining parts are made into 3°~ 5° obliquity, and a four-sided plane is modified at the lower extreme so that it will be convenient to screw it from the plastic parts with tools. To sum up, cavity structures in more common application are integral embedded cavity and insertion and splice cavity. Fig.2-5: threaded ring cavity 2. Core Structural Design Integral punch costs too many materials and the working load for cutting and processing amounts too high. Therefore, hardly any such structure exists in modern mold structures which is instead preoccupied with integral embedded punch and inlaying modular punch, as indicated in Fig.2-6 and 2-7. Fig.2-6: core structure
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 follows
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. 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:
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;