Molding and demolding of inner and outer wall annular grooves in cylindrical thin shells is a difficult point in design of molds for such plastic parts, especially design of inner annular groove molding part and demolding mechanism, which is a key factor affecting success of mold structure design. Design of demolding mechanism for inner wall annular groove of a cylindrical tube is limited by three characteristic conditions of plastic part: first, inner diameter of cylindrical tube; second, whether top of annular groove is closed; and third, whether there are additional features such as internal threads at lower end of cylindrical tube opening of annular groove. Design of demolding mechanism for inner wall annular groove varies depending on characteristic conditions of plastic part. This paper, based on injection molding of a synchronous sealing shell plastic part in a machine tool oil pump assembly, and drawing on design of inner wall annular groove demolding mechanism described in literature, innovatively designs a composite mechanism of an inward sliding block + arc-shaped tile-shaped lifter for demolding inner wall annular groove of plastic part, taking into account characteristics of plastic part itself. In mold of plastic part, a spring-driven half-slider mechanism for outer wall annular groove is also designed for demolding outer wall annular groove. Combining structural characteristics of inner and outer wall annular groove demolding mechanisms, a multi-plate two-plate mold is designed for automated injection molding of plastic part. Structural design method of this mold has good reference value for design of injection molds for similar plastic parts.

Structure and shape of synchronous sealing shell plastic part are shown in Figure 1. Plastic part consists of two stepped columnar thin shells, namely upper columnar shell ZS and lower columnar shell ZX. Upper columnar shell ZS is a simple smooth-walled columnar shell, while lower columnar shell ZX has several additional features. Lower cylindrical shell ZX features two grooves C1 and two directional markings L1 in the middle of its outer wall, along with two semi-circular grooves YC1, YC2, and YC3. Inner wall has a full-circle circular groove YC4, with each YC1, YC2, YC3, and YC4 having a semi-circular radius of 1.2mm. Top, located on extension wall of upper cylindrical shell ZS, has 12 internal teeth NC and 12 external teeth WC. Boundary between upper cylindrical shell ZS and lower cylindrical shell ZX is a circular platform TCj. Upper cylindrical shell ZS has an outer diameter of 58.4mm and a height of 30mm, while lower cylindrical shell ZX has an outer diameter of 71.4mm and a height of 30mm. Average wall thickness of plastic part is 2.1m, with a maximum thickness of 4.4m and a minimum of 1.5m. Plastic part is made of 15% glass fiber reinforced polyamide, with a shrinkage rate of 0.65%~0.73%, and a total production volume of 600,000 pieces. Molding accuracy MT4~MT5.

 

Z_S - Upper cylindrical shell; Z_X - Lower cylindrical shell; C₁ - Groove; L₁ - Directional marking: Y_C1, Y_C2, Y_C3 - Half-ring circular groove; Y_C4 - Full-circle circular groove; N_C - Internal tooth; W_C - External tooth; T_j - Circular platform.
Figure 1 Synchronous sealing shell plastic part

Based on molding analysis of structural features of plastic part shown in Figure 1, it was found that main difficulties in designing plastic part molding die are as follows: The biggest difficulty lies in demolding of circular groove YC4 on inner wall of lower column shell ZX, because design of demolding mechanism of YC4 is affected by two aspects. First, it is affected by internal gear NC and external gear WC; second, it is limited by inner diameter of lower column shell ZX. Therefore, demolding mechanism design of YC4 cannot directly adopt differential slider mechanism to implement core-pulling demolding in the form of internal core shrinkage as described in literature, nor can it achieve demolding by using lifter assembly method. Secondly, demolding of semi-annular grooves YC1, YC1, YC1 on outer wall of lower column shell ZX, as well as demolding of groove C1 and parting mark L1, must be performed. Demolding direction cannot be set to mold opening direction axially with plastic part; instead, a side core-pulling demolding method perpendicular to mold opening direction is required.

Based on above analysis, single-cavity molding scheme designed to overcome difficulties in molding plastic parts is as follows: As shown in Figure 2, in parting setting of plastic part mold cavity, when mold opening direction is set to FZ⁃, outer wall parting is set as follows: Outer wall is divided into upper and lower molding parts using P1 parting surface, first obtaining upper outer wall insert I2; then lower outer wall molding part is separated from inner wall molding part of plastic part using P12 parting surface, resulting outer wall molding part on P1 and P2 surfaces is further divided using P3 parting surface to obtain two outer wall half-molding sliders S1 and S2. Half-molding sliders S1 and S2 realize upper semi-annular circular grooves YC1 and YC1 on outer wall of lower column shell ZX of plastic part through side core pulling. Side core pulling demolding of YC1, groove C1 and parting mark L1; in parting process of inner wall forming part, in order to facilitate demolding of inner tooth NC and outer tooth WC, inner parting surface P4 is used to part upper and lower inner wall forming parts to obtain upper cylindrical insert I1 and lower cylindrical insert I3' (lower cylindrical insert I3' is initial forming part of lower cylindrical shell ZX); again, in order to facilitate design of demolding mechanism of YC4, lower cylindrical insert I3' is further parted using inner parting surface P5 to obtain toothed cylindrical core insert I4 and lower inner wall cylindrical insert I3, then lower inner wall cylindrical insert I3 is divided according to parting segmentation shown in FZ-direction view in Figure 2, lifters X1 and X2 are obtained by using parting surfaces P6 and P7 respectively, inward sliding blocks S4 and S3 are obtained by using parting surfaces P8 and P9 respectively. Lifters X1 and X2, and inward sliding blocks S4 and S3, respectively complete molding and demolding of a portion of circular groove YC4.

 

I₂ - Outer wall insert; I₁ - Upper cylindrical insert; I₃ - Lower inner wall cylindrical insert; I₄ - Toothed cylindrical core insert; T_z - Core support platform; y₁, y₂ - Slanted dovetail guide rails; Y_c - Circular groove; S₁~S₄ - Half slider numbers; X₁, X₂ - Lifters; P₁~P₉ - Parting surfaces; F_Y-, F_Y+, F_X-, F_X+, F_Z-, F_Z+ - Core pulling direction along mold coordinate axis
Figure 2 Single-cavity parting design.
This design prevents ejection jamming issues that can occur with all lifters and avoids lack of inward movement space when using all inward sliding blocks. Compared to a differential sliding block mechanism, strength of each molded part is effectively guaranteed (design of differential sliding block mechanism is limited by inner diameter of inner wall of annular groove, which can easily lead to poor structural strength of molded parts). Lifters X1 and X2 are tile-shaped, with angled dovetail guides y1 and y2 respectively, which slide and engage with angled dovetail groove on core support platform Tz, providing guidance for angled sliding ejection of X1 and X2. Demolding of YC4 relies on movement directions of lifters X1 and X2, inward sliding blocks S4 and S3, respectively: FY-, FY+, FX-, and FX+. Assembly form of single-cavity molded part obtained after single-cavity parting is shown in 3D diagram of single-cavity parting in Figure 2.
Under this parting arrangement, and in conjunction with Figures 2 and 3, demolding of plastic part can be achieved as follows: As shown in Figure 3(a), firstly, when surfaces P1 and P4 open simultaneously, outer and inner walls of upper cylindrical shell ZS of plastic part first detach from inserts I1 and I2; secondly, when surface P3 opens (as shown in Figure 2), half-slider S1 and S2 move in FX+ and FX- directions respectively, outer wall of lower cylindrical shell ZX of plastic part separates from half-slider S1 and S2 to achieve demolding; thirdly, inward sliding blocks S3 and S4 move towards central axis of plastic part to complete partial separation from inner wall of lower cylindrical shell ZX (also achieve demolding of a part of YC4), inward core-pulling distance of inward sliding blocks S3 and S4 is 5mm; as shown in Figure 3(b). As shown, in fourth step, insert I3 is pushed by push plate via reset rod RP, two lifters X1/X2 are pushed by push plate via ejector rods Ej1/Ej2, and insert I4 is pushed by push plate, together ejecting plastic part in FZ+ direction. During this process, two lifters simultaneously move in FY+ and FY- directions to separate from plastic part (achieving partial demolding of YC4). Angle of lifters X1/X2 is 7°, and core-pulling distance on centripetal side is 3.4mm. Plastic part remains on insert I4. In fifth step, insert I3 continues to be ejected in FZ+ direction, pushing plastic part off insert I4 to achieve complete demolding (in this step, internal teeth Nc and external teeth Wc are mainly demolded from insert I4; plastic part will automatically fall off when it finally remains on insert I3).

 

R_P - Reset rod; E_j1-E_j2 ejector pins; V₁, V₂ - Mold cavity; w₁~w₄ - Water channel; G₁, G₂ - Submerged gate; R₀ - Main runner; R₁ - Runner.
Figure 3 Separation of Single-Cavity Molded Part and Plastic Part and Design of Two-Cavity Arrangement

Based on above single-cavity mold parting and demolding design, to meet mass production requirements of plastic part, mold cavity layout adopts a 2-cavity layout, as shown in Figure 3(b). Two cavities are V1 and V2. Both cavities use submarine gates (G1 and G2), sharing a main runner R0 and a branch runner R1. For cavity cooling, 10mm diameter cooling channels are opened in two half-sliders S1 and S2, insert I3, and insert I2, with corresponding channel numbers w3, w4, w2, and w1.

As shown in Figure 4, with 2-cavity layout, mold is designed as a two-plate multi-plate mold based on a cold runner system, featuring three mold openings and two ejection functions. In mold base, an additional mold opening mechanism (K1 surface) is added between fixed mold base plate 1 and fixed mold plate 2 to allow insert rod 13 to be pulled out from inward sliding block 22 and half sliding block 18 in advance, thus pre-engaging locking mechanism and ensuring that core is pulled out before mold cavity opening surface K2 opens. There are four insert rods 13, corresponding to control two half sliding blocks S1 (part 18) and S2, four inward sliding blocks S13 (part 22), S3', S4, and S4'. An upper push plate 6 is added for final ejection and demolding of plastic part, and a backing plate 8 is added to facilitate installation of TZ insert 24. During mold assembly, fixed mold plate 2 and push plate 6 are floating plates, moving mold plate 7 and backing plate 8 are fastened together with screws. Mold has three opening surfaces: K1, K2, and K3. A fixed-distance long pull rod 31 controls opening distance of K1 surface, a nylon buckle 37 is used for friction drive between push plate 6 and fixed mold plate 2.

 

1. Fixed mold base plate; 2. Fixed mold plate; 3. Guide pillar; 4 & 5. Guide sleeve; 6. Upper push plate; 7. Moving mold plate; 8. Pad plate; 9. Mold foot; 10. Secondary push plate; 11. Secondary push plate; 12. Moving mold base plate; 13. Insert rod; 14. Upper strip; 15. Upper stop; 16 & 20. Spring; 17 & 19. Spring core rod; 18. Half slider; 21. Lower baffle; 22. Inward sliding block; 23. Lower pressure strip; 24. T_z insert; 25. I₂ cavity insert; 26. I₄ toothed cylindrical core insert; 27. I₃ core insert; 28. T_z core support platform; 29. RP reset rod; 30. Fixed-distance short pull rod; 31. Fixed-distance long pull rod; 32. Sprue sleeve; 33. Positioning ring; 34. X₁ lifter; 35. X₂ lifter; 36. E_j1/E_j2 ejector rod. 37 - Nylon buckle; 38 - Push plate guide post; 39 - Support post; 40 - Return spring; 41 - Nylon buckle; 42 - I₁ insert; K1, K2, K3 - Mold opening surface; E₁, E₂ - Ejector surface; I₁, I₂, I₃, I₄, T_z, y₁, y₂, S₁~S₄, X₁, X₂, P₁~P₅, W₁~W₂, E_j1, E_j2, V₁, V₂, G₂, RP - same as Figure 3; S₃', S₄' - Inward sliding block; F_y-, F_y+, F_x-, F_x+, F_z-, F_z+ - same as Figure 2
Figure 4 Mold Structure
To achieve fourth and fifth steps of demolding of plastic part, ejection mechanism in mold uses a secondary ejection mechanism. Primary ejector plate 11 and secondary ejector plate 10 are used for this function, positions of two ejection surfaces are E1 and E1. In ejection mechanism, ejector plate 6 is connected to secondary ejector plate 10 through screws, reset rod RP (part 29), and two move synchronously. Ejector rods Ej1 (part 36)/Ej2 and toothed cylindrical core insert I4 (part 26) that push lifter are installed on primary ejector plate 11, move synchronously with primary ejector plate 11. Fixed-distance short tie rod 30 is used to control position of E1 surface. Nylon buckle 41 is used to drive primary ejector plate 11 to be ejected by friction of secondary ejector plate 10.

Demolding mechanism design includes three types of mechanism designs, as follows:
(1) Spring-driven fixed mold half-slider mechanism. As mentioned above, both half-slider mechanisms S1 and S2 are spring-driven half-slider mechanisms installed on lower surface of fixed mold plate 2. They are symmetrically structured. Taking mechanism S1 as an example, its main components include parts 13 to 18. Half-slider S1 (part 18) is installed on fixed mold plate 2 by upper pressure bar 14 and is driven by spring 16 at its tail end to pull core in FX direction. When mold is closed, spring 16 is in a compressed state, and part 18 is locked by insertion rod 13. When mold is opened, insertion rod 13 releases locking limit on part 18, and spring 16, due to release of tension, drives part 18 to perform side core pulling.
(2) Spring-driven moving mold inward sliding block mechanism. Four retractable sliders S4 (part 22), S4', S3, and S3' are all spring-driven retractable slider mechanisms located on one side of moving mold. Four mechanisms have same structure. Taking mechanism S4 as an example, its components include parts 13 and 19 to 23. Retractable slider 22 of S4 is installed on moving mold plate 7 using lower pressure bar 23. It is driven to perform inner core pulling action by tension of spring 20 after insertion rod 13 releases locking of part 22. Inner core pulling distance is 5mm (Figure 5).

 

1~42, I₁,I₂,S₁,S₂, S₃, S₃'-same as Figure 4,
Figure 5 Structure and installation of mechanisms S1 and S3
(3) Tile lifter + secondary ejection mechanism. Main function of mechanism is to realize fourth and fifth steps of demolding of plastic part. Main components of mechanism include parts 6, 10, 11, 24, 26, 27, 28, 29, 30, 34, 35, 36, 38, 40, and 41. Short pull rod 30 is installed on moving mold base plate to limit ejection distance of primary push plate 11. Nylon buckle 41 is installed on primary push plate 11 to pull primary push plate 11 out synchronously. Under a certain resistance, two can separate. Lower end of RP reset rod 29 is installed on secondary push plate 10, upper end is fastened to upper push plate 6 with screws to ensure that upper push plate 6 and secondary push plate 10 are ejected synchronously. I3 core insert 27 is installed on upper push plate 6, which participates in molding of plastic part, is used for final ejection and demolding of plastic part. Lower end of Ej1 top rod 36, etc., is mounted on primary push plate 11. Upper end is set as a T-shaped guide rail, which slides in cooperation with T-shaped groove set at lower end of tile lifter X1 lifter 34, etc., to push top X1 lifter 34 and X2 lifter 35 out. TZ core support platform 28 is used to guide ejection of X1 lifter 34 and X1 lifter 35. Inclined dovetail groove is set inside, which slides in cooperation with corresponding dovetail groove guide rails y1 and y2. Lower end of I4 insert 26 is mounted on primary push plate 11. During ejection, driven by ejector pins of injection molding machine, primary ejector plate 11, secondary ejector plate 10 simultaneously complete their first ejection in FZ+ direction, reaching ejection position surface E1. At this point, upper ejector plate 6, X1 lifter 34, X2 lifter 35, I4 insert 26, and Ej1 ejector pin 36 will also eject simultaneously. After primary ejector plate 11 reaches surface E1, it is held back by short pull rod 33 and cannot continue upwards. Plastic part remains on I3 core insert 27 and I4 insert 26 on upper ejector plate 6. Secondary ejector plate 10 overcomes friction of nylon buckle 41 and continues to eject upwards, pushing I3 core insert on upper ejector plate 6 to push plastic part off I4 insert 26, thus achieving complete demolding. Ej1 ejector pin 36 ejects 28mm along FZ+ direction from TZ core support platform 28 (Figure 6).

 

6~36 are same as Figure 4; V₁, E_j1, F_Z+- are same as Figure 4.
Figure 6 Installation of secondary ejection mechanism in mold

After mold is installed on injection molding machine, mold working principle (Figure 7) is as follows:

 

2-42, K₁, K₂, K₃, E₁, E₂, F_Z-, F_Z+- Same as Figure 4; E₀- Initial position surface.
Figure 7 Mold working principle
(1) Mold completes cavity injection. First, injection molding machine nozzle completes cavity filling, pressure holding, and cooling processes, then waits for mold to open.
(2) K1 surface opens. When injection molding machine moves moving mold to left of K2 surface of mold by pressing FZ-, mold first opens at K1 surface due to pushing action of return spring 40 and frictional attraction of nylon buckle 37. When it opens 15mm, insert rod 13 loses its limiting effect on inner sliding block 22. Under push of spring 20, four inner sliding blocks S3, S4, S3', and S4' in two mold cavities represented by inner sliding block 22 complete inner core pulling, thereby realizing partial core pulling and demolding of inner ring groove of inner wall YC4 of plastic part. As surface K1 continues to open, opening distance is 60mm. Insert rod 13 loses its limiting effect on half-slider 18. Driven by outward pull of spring 16, half-slider S1 and S12 complete side core-pulling demolding of semi-annular grooves YC1, YC1, YC1, groove C1, and directional marker L1 on outer wall of cylindrical shell ZX of plastic part. With K1 opening distance at 60mm, after surface K1 is fully opened, pointed tip of insert rod 13 remains in conical groove of half-slider 18, driving half-slider 18 to reset when mold is closed.
(3) Surface K2 opens. As moving mold continues to retract, when K1 surface opens by 60mm, fixed mold plate 2 is held back by fixed-distance long pull rod 31 and cannot continue to move to left. However, due to tension of spring 40, upper push plate 6 is held back by reset rod 29. Mold can only overcome frictional resistance of nylon buckle 37 to open mold opening surface K2. When K2 opens, plastic part is removed from I1 insert 42 and I2 cavity insert 25, remains on moving mold side.
(4) K3 surface opens / E1 surface ejects. As moving mold continues to move to left, once secondary ejector plate 10 is stopped by injection molding machine ejector pin 43, primary ejector plate 11 can no longer follow moving mold to left. Simultaneously, due to frictional attraction of nylon buckle 41, secondary ejector plate 10 also temporarily stops following moving mold to left. As moving mold continues to move, TZ insert 24 forces four lifters X1, X2, X1', and X2' of two cavities to complete lateral core pulling, thereby achieving lateral core pulling and demolding of remaining annular groove on inner wall of plastic part YC4. Required mold opening distance on K3 surface for this process is 28mm. Moving mold continues to move to left. Since idle stroke of short tie rod 30 has been used up, moving mold pulls primary push plate 11 via short tie rod 30, forcing it to overcome frictional attraction of nylon buckle 41 and separate from secondary push plate 10. Primary push plate 11 pulls four lifters X1, X2, X1', and X2' on it, and two I4 toothed cylindrical core inserts 26 of two cavities out of plastic part in FZ- direction, with a withdrawal distance of 20mm. Plastic part finally remains on I3 core insert 27 and automatically falls off, achieving complete demolding of plastic part.
(5) Reset. Mold is reset in FZ+ direction, and reset process is reverse of mold opening process.

An automated demolding sequence in plastic part molding mold was designed using a regional and step-by-step method. Based on this, parting parts and molding components of plastic part mold cavity were designed, resulting in a mold that is a two-cavity, two-plate multi-plate mold. Mold cavity gate employs a submerged gate on fixed mold side to achieve automatic demolding of runner waste. For features such as outer wall circular grooves, a fixed mold half-slider mechanism is designed for molding and demolding. For inner wall circular grooves, an inward-retracting slider + "tile"-shaped lifter is designed for molding and step-by-step demolding. Final demolding of plastic part is achieved through two ejections by upper ejector plate and ejection mechanism. Corresponding to demolding requirements of plastic part, mold has three opening operations and two ejection operations. Inward-retracting slider on moving mold side and half-slider on fixed mold side both use spring-driven core pulling, same insert rod locks half-slider and inward-retracting slider at same position on both fixed and moving molds. This reduces constraints on mold structure design, which helps to reduce mold manufacturing costs, simplify mold processing and assembly. Mold structure is ingeniously conceived, with good structural innovation design, and can provide useful reference for injection mold design of similar plastic parts.