Figure 1 shows a modified ambulance filter cartridge cover. Its main body is a pentagonal cover B0. Bottom edge uses a lip structure and is reinforced with reinforcing ribs j1 to prevent shrinkage deformation. Bottom edge has five corners, C1~C5, where three positioning rectangular posts d1, d2, and d3 are set at C1, C3, and C4. A side-hole type positioning rectangular post d4 is set on one side of C5. An air inlet pipe G0 is obliquely upward on cover B0. Inner wall of G0 is a hollow tube h1, and end of h1 has a rounded corner feature b1. Outer wall of G0 pipe is equipped with a front clamp r0, two intermediate intermittent clamps r1 and r2, an indicator arrow mark r4. Rear outer wall also features two screw posts z1, z2 and a slanted side hole h2 (coaxial with z1 and z2). A back-cut area Dk is formed between lower end of G0 and corner C5, with j1 and j2 serving as reinforcing ribs. According to ISO-20457 dimensional tolerance standards for plastic parts, critical dimensional tolerances must be controlled within ±0.1 mm, and non-critical dimensional tolerances must be controlled within ±0.3 mm.

 

Figure 1: Ambulance-modified filter box cover
Plastic material is glass fiber + talc modified polypropylene (PP) plastic, grade V-7000. Material performance parameters are shown in Table 1. It exhibits significant anisotropy, with coefficient of thermal expansion in flow direction lower than that in vertical direction (assembly tolerance design must be considered).

Table 1 Material Performance Parameters
Melt flow index: 1~2 g/min; Molding shrinkage: 0.6%~1.0% in flow direction, 1.2%~1.8% in perpendicular direction (talc reduces anisotropy, but mold compensation design is still required). Injection barrel temperature should be set at 210~250℃ to avoid glass fiber degradation due to high temperature; mold temperature should be set at 40~60℃ to balance crystallinity and cooling rate. Material has good chemical resistance, tolerating machine oil, weak acids, weak alkalis, and most solvents, but prolonged contact with strong oxidants (such as concentrated sulfuric acid) or halogenated hydrocarbons should be avoided. Adding UV stabilizers allows for long-term outdoor use and prevents aging and embrittlement. Gate location in mold design should avoid uneven strength caused by glass fiber orientation. Glass fiber easily causes wear on mold parts; draft angle should be ≥1° to prevent scratching surface of plastic part. Post-treatment generally involves annealing (80~100℃×2h) to reduce internal stress and improve dimensional stability of mold parts.

Molding of plastic parts mainly presents following four difficulties:
(1) Difficulty in matching material properties with process, primarily manifested in control of material shrinkage and warpage. In PP+10%GF+10%TD composite materials, addition of glass fiber (GF) and talc (TD) reduces shrinkage rate of material, but uneven distribution of both may lead to anisotropic shrinkage. Lip structure (bottom edge) and reinforcing rib j1 area are prone to warpage due to uneven shrinkage, requiring optimization by controlling mold temperature and holding time to balance cooling rate. Orientation of glass fiber in flow direction can lead to local strength differences, especially at roots of reinforcing rib j1 and positioning pillars d1~d4, which may form weak points. Solution: Adjust injection speed (medium to low speed) and gate position to reduce influence of glass fiber orientation on mechanical properties.
(2) Difficulty in filling small features. Difficulties in ensuring integrity of small features: This is mainly reflected in fact that microstructures such as clamps (r0, r1, r2), indicator arrows (r4), and screw posts (z1, z2) are prone to material shortages due to insufficient material flow. Main body wall thickness is 1.5~2.5mm, and there are multi-directional reinforcing ribs, which easily lead to insufficient filling at the end of melt flow. Solution: Optimize injection process, increase barrel temperature to 250~280℃ to improve melt flow, and increase local injection pressure to 100~120MPa to ensure sufficient filling of thin-walled areas.
(3) Difficulty in demolding complex structures 5-28. Complex structure of plastic part mainly affects demolding and fullness of filling of local features. Difficulties in demolding are mainly reflected in following: ① G0 tube hole h0 is a hollow tube, and its inner wall is obliquely pulled for demolding; ② Demolding of tail end b1 of h1; ③ Demolding of outer wall of G0 tube; ④ Demolding of upper side hole h2 and screw posts z1, z2 of outer wall of G0 tube: 5G. Demolding of undercut area D0 below outer wall of pipe; demolding of 6 positioning rectangular column dk. Demolding directions of these local features are numerous and concentrated in small areas, requiring use of oblique push or slider mechanisms for demolding, but this may lead to demolding interference.
(4) Defect prevention and quality control are difficult. Following issues exist in prevention and quality control of plastic part defects: ① Molding of rounded corner b1 at the end of hollow tube h1 needs to avoid microcracks caused by stress concentration, and surface roughness of core needs to be high (Ra≤0.8μm). Solution: Use mirror steel S136 to reduce inner wall shrinkage and collapse; ② Stability control of critical dimensions (±0.1mm), tolerance requirements of positioning posts (d1~d4) and clamp mating surfaces are strict, and gate layout needs to be optimized through mold flow analysis to reduce dimensional fluctuations caused by shrinkage differences; ③ Deformation suppression in non-critical areas, tolerance of non-critical areas of pentagonal cover body (B0) needs to be controlled within +0.3mm, the overall warping needs to be reduced through symmetrical design of reinforcing rib j1 and uniform cooling of conformal water channel; ④ Weld lines and porosity risks, air inlet pipe G0. Weld lines easily form at junctions with main body and in multiple clamping structures, affecting appearance and strength. Countermeasures include: optimizing gate location, prioritizing fan-shaped gates, increasing melt front temperature, and adding flow promoters if necessary. Regarding surface quality and exposed glass fiber, exposed glass fiber can lead to surface roughness, especially in decorative areas such as indicator arrow R4. Corresponding measures include: adjusting process and appropriately reducing glass fiber content to below 8%.
In summary, overcoming molding difficulties of plastic parts requires following: ① In mold design, adopt a multi-slider + inclined push composite demolding mechanism, conformal cooling water cooling, and optimize gating system design; ② In terms of refining injection process parameters, injection pressure and speed need to be controlled in stages, combined with holding pressure curve adjustments to ensure dimensional stability; ③ In terms of sampling and debugging, use mold flow analysis to predict defect locations, focus on monitoring undercut demolding, glass fiber distribution, and compliance rate of key dimensions during trial molding stage. Through these measures, molding challenges caused by material shrinkage, structural complexity, and precision requirements can be systematically solved, achieving high-quality mass production.

Considering above molding difficulties, mold structure design is as follows:

Addressing filling difficulties in plastic part molding process, considering that upper surface of B0 has appearance requirements and cannot have a gate, mold flow software Moldflow was used for analysis. After optimizing gate position (restricted gate), CAE model is shown in Figures 2(a) and (b). In model, g2 is cavity side gate, R1 and R4 are runners of side gate g2. To ensure that cavity is located in the center of mold and to prevent injection pressure eccentricity, point gate runners R0, R1, and R2 are used to supply material to side gate runner R3 through point gate g1. In cavity cooling, it is optimized to have 6 cooling pipes W1~W6, of which W1, W2, W3, and W5 are water channels with contoured cooling water wells. W1~W3 are located on fixed mold side, and W4~W6 are located on moving mold side. CAE optimization analysis results are shown in Figures 2(c) to (h).

 

Figure 2 CAE Analysis Model and Results
Through above optimization analysis, the overall molding effect is controllable, but attention should be paid to porosity, cooling efficiency, filling end quality to further improve yield rate and dimensional accuracy of plastic parts.

Mold structure design is as follows.
(1) Cavity parting and molding part design are shown in Figures 3(a) and (b). Main molding parts of cavity are obtained by parting with planar parting surface PL0 and stepped parting surfaces PL1 and PL2: cavity plate insert 1 and core insert 10. Among them, in order to facilitate molding of reinforcing rib j1, multiple local inserts 2 to 9 are separated from cavity plate insert 1. Since multiple rib molding grooves need to be set in molding part of reinforcing rib j2, inner wall insert 11 needs to be separated from core insert 10 again for easy processing. For molding of inner wall of hollow tube h1, since b1 is an undercut, inclined push block 13 is used for molding and side-pull demolding. Remaining part of inner wall of hollow tube h1 is molded and demolded by cylindrical core insert 12. For demolding of d4, after adjusting its demolding direction to be same as core-pulling direction of cylindrical core insert 12, d4 insert 14 is set, and together with cylindrical core insert 12, it is driven by inclined slider 15 to perform inclined core-pulling demolding (F1 direction). Inclined slider 15 uses parting surface PL2 for parting. Another function of inclined slider 15 is to perform molding and core-pulling demolding of most of Dk area, as shown in Figure 3(c). For demolding of z1, z2, and h2, as shown in Figure 3(d), they are integrated into inclined core-pulling demolding according to F2. Therefore, inclined slider 16 (PL3 parting) is set to perform molding and core-pulling demolding of these three features. Inclined slider 16 also performs molding and side core-pulling demolding of remaining part of Dk area.

 

1. Cavity plate insert 2. Insert 4. Insert 5. Insert 6. Insert 7. Insert 8. Insert 9. Insert 10. Core insert 11. Internal insert 12. Cylindrical core insert 13. Angled push block 14. d insert 15. Angled slider 16. Angled slider
Figure 3. Parting Design and Molded Part Design
(2) Gating System. Gating system is designed according to Figure 2(a). Gate g2 is designed as a fan-shaped side gate with a thickness of 1.2 mm, an unfolding angle of 60°, and an inlet cross-sectional dimension of 5 mm × 1.2 mm.
(3) Cooling System. Cooling system is designed as a conformal cooling channel according to Figures 2(a) and (b). Among cooling pipes W1~W3 and W5 with water wells, distance between water well cooling part and cavity surface is (12±0.5) mm. Core/cavity temperature difference is controlled within ±3℃.
(4) Exhaust System. A three-dimensional exhaust network is used for exhaust. Exhaust groove depth is 0.03 mm. Inserts 2-9 on cavity plate insert also have an exhaust function. Auxiliary venting inserts (8 φ0.8 mm micro-holes) are provided in weld line area. Gap between cylindrical core insert 12 and inclined push block 13 is 0.01~0.02 mm, which also serves as a venting function. Multiple push rods 17 are provided at the bottom of reinforcing rib j₂ forming groove (see Figure 4), with a single-sided gap of 0.02 mm to facilitate venting within groove.

 

1. Cavity plate insert 12. Core insert 13. Cylindrical core insert 14. Angled slider 15. Angled slider 16. Push rod 17. Pull rod 18. Pressure bar 19. Slider seat 20. Hydraulic cylinder 21. Pressure bar 22. Slider seat 23. Hydraulic cylinder 24. Sub-guide post 25. Nylon buckle 26. Reset rod 27. Push rod fixing plate 28. Push plate 29. Push plate 30. Moving mold base plate 31. Spacer block 32. Moving mold plate 33. Precision positioning block 34. Fixed mold plate 35. Stripper plate 36. Fixed mold base plate 37. Main guide post 38. Fixed distance tie rod 39. Guide sleeve
Figure 4 Three-dimensional structure of mold
(5) Ejection system design is shown in Figure 4(a). Final demolding of plastic part is achieved by 12 φ6 mm push rods 17 and inclined push blocks 13. Solidified material in Rₐ runner is pulled out by a pull rod 18. Inclined push block 13 is mainly for forming of b₁ and side demolding.
(6) Slider side core pulling mechanism. Slider mechanisms M1 and M2 are provided for inclined core pulling drive of inclined sliders 15 and 16, respectively. Both mechanisms have similar structures, employing a hydraulic cylinder to directly drive inclined core-pulling slider mechanism. Taking mechanism M1 as an example, inclined slider 15 is mounted on slider seat 20 via a pressure strip 19, and core is pulled using a hydraulic cylinder 21. After inclined slider 15 completes core pulling, it is positioned by a limiting glass bead at its bottom.
Mold is designed as a three-plate mold with a one-cavity layout. Mold base guide assembly consists of two types of guide pillars: main guide pillars 37 (4 sets) and secondary guide pillars 25 (4 sets). Three sequential openings of three-plate mold are controlled by control components such as nylon buckles 26 and spacer rods 38. To ensure accurate mold closure and positioning, precision positioning blocks 33 are installed on opposite sides of slider mechanisms M1 and M2. Mechanisms M1 and M2 are mainly installed on moving mold side.
Two-dimensional structure of mold is shown in Figure 5, with three opening surfaces KM₁, KM₂, and KM₃, with opening distances of 80, 15, and 145 mm, respectively. Mold operation process: After mold closes and injection molding machine completes filling, holding pressure, and cooling processes, it prepares to open mold. Moving mold below KM₁ surface descends in mold opening direction. Due to opening control spring 48 and control of nylon buckle 26, mold first opens at KM₁ surface, separating solidified material from R₂ and R₃ runners. Moving mold continues to descend, and due to pull of fixed-distance tie rod 38, stripper plate 35 follows, opening KM₂ surface and pushing out waste material from R₀ and R₁ runners. Moving mold continues to descend, opening KM₃ surface, and plastic part is removed from cavity plate insert 1, remaining on one side of moving mold. Subsequently, hydraulic cylinders 21 and 24 drive inclined sliders 15 and 16 respectively to complete inclined core pulling. As moving mold continues to descend, injection molding machine's ejector pin pushes ejector plate 29, which in turn pushes lifter block 13, ejector pin 17, and pull rod 18 to push plastic part, forcing it to eject from core insert 1 and inner wall insert 11, thus completing demolding. Simultaneously, lifter block 13 completes side core-pulling action. Plastic part finally remains on lifter block 13, and is then completely demolded by a robotic arm.

 

10. Core insert 11. Inner wall insert 12. Cylindrical core insert 13. Angled push block 15. Angled slider 16. Angled slider 17. Push rod 18. Pull rod 20. Slider seat 21. Hydraulic cylinder 23. Slider seat 24. Hydraulic cylinder 25. Secondary guide pillar 27. Reset rod 28. Push rod fixing plate 29. Push plate 35. Stripper plate 36. Fixed mold base plate 37. Main guide pillar 38. Fixed distance tie rod 40. Spring 41. Support pillar 42. Short tie rod 43. Upper pull rod 44. Positioning ring 45. Sprue sleeve 46. Push plate guide pillar 47. Limit block 48. Spring
Figure 5 Two-dimensional structure of mold