Design of Injection Mold of Printer Skeleton Based on CAD/CAE

Time:2023-05-26 11:17:01 / Popularity: / Source:

1 Process analysis of plastic parts

Skeleton of printer is shown in Figure 1. There are undercuts, inclined holes, threads and multiple deep ribs, which require stable dimensions and high strength, cannot change with ambient temperature. Material selection is PA6+GF30%, and shrinkage rate is 0.7%. It has characteristics of high heat distortion temperature and strength, good fluidity, good filling and molding. Maximum external dimension of plastic part is 348 mm*73 mm*41 mm, main wall thickness is about 1.7 mm, and wall thickness is relatively thin. During actual injection, 2-point feeding is required to prevent insufficient filling of cavity.
Design of Injection Mold 
Figure 1 Printer skeleton

2 Mold structure design

Injection mold of printer skeleton has a structure of 2 cavities, a hot runner gating system is used to reduce waste and improve injection molding efficiency. Two of undercuts and inclined holes of molded plastic part can be core-pulled and demolded by using a combination of inclined guide pillars and inclined sliders, the other thread needs to use a hydraulic cylinder and inclined slider mechanism for inclined core-pulling and demoulding. Due to presence of deep ribs in many parts of plastic part, parting surface is relatively complicated. Multiple inserts are assembled in movable and fixed molds to facilitate processing and exhaust. At the same time, multiple flat push rods are used to push out to ensure smooth release of molded plastic parts. Cooling systems are installed on movable mold and fixed mold side to ensure sufficient cooling of plastic parts.

2.1 Parting surface design

There are undercuts, inclined holes and threads on the side wall of printer frame, and a side core-pulling mechanism is required for demoulding. Since demoulding direction of reverse buckle is oblique, parting surface is designed on the surface of inclined slider (see Figure 2). Parting surface design must take into account processing feasibility, assembly convenience and structural rationality to ensure stable and reliable mold structure. Shape around parting surface can be designed according to shape of plastic part, and inclined surface should be extended as much as possible to ensure a smooth parting surface, avoid steep insertion surfaces, prevent mold parts from being damaged by collisions and unable to produce normally.
Design of Injection Mold 
Figure 2 parting surface design

2.2 Hot runner system design

Size of printer frame is large and wall thickness is thin. It adopts 2-point latent gate. Gating system uses a hot runner to reduce waste and improve molding quality and strength of plastic part. Position of gate is analyzed and adjusted by Moldflow software. Hot runner gating system is shown in Figure 3. A comparative analysis of common point gate hot runner (Scheme 1) and needle valve sequential hot runner (Scheme 2) was carried out. Option 1 is to open gates G1 and G2 at the same time, and Option 2 is to open gate G1 first, and gate G2 to be opened after a delay to fill cavity. Since requirements for plastic parts focus on strength and molding quality, there are no strict requirements on appearance. It is only necessary to compare and analyze filling time and weld line position of two schemes, and determine the best scheme according to analysis results.
Design of Injection Mold 
Figure 3 gating system
2.2.1 Comparative analysis of conversion point pressure
Pressure at V/P conversion point is pressure when speed control is switched to pressure control. When filling volume reaches 99%, it can be switched, and then pressure control is carried out. After filling volume reaches 100%, pressure is maintained and fed to ensure that cavity is completely filled, so that molded plastic parts do not appear obvious shrinkage marks, lack of material and other appearance defects. It can be seen from Figure 4 and Figure 5 that pressures at transition points of Scheme 1 and Scheme 2 are 63.75 and 63.66 MPa respectively, pressure flow path and distribution of V/P transition points are displayed in different colors, pressures of V/P transition points of Scheme 1 and Scheme 2 are close to each other, pressure is low, which is convenient for injection molding, can ensure stable and reliable molding quality of plastic parts, and is suitable for mass production.
Mold structure design 
Figure 4 Scheme 1 V/P conversion point pressure
Mold structure design 
Figure 5 Scheme 2 V/P conversion point pressure
2.2.2 Comparative analysis of weld lines
Since skeleton is a structural part, impact on strength of plastic part is first considered when determining hot runner system solution. When two streams of material flow meet, weld marks will be produced. If there are long weld marks on the surface of plastic part, it will affect structural strength of plastic part, especially when temperature difference is large, deformation and stress cracking will easily occur. It can be seen from Figure 6 that when the first solution is adopted, a long weld line is formed in the middle of plastic part, which affects molding quality of plastic part. It can be seen from Figure 7 that the second scheme adopts needle valve sequential feeding, and there is no intersection of two material flows in the middle of plastic part, so there will be no stress concentration that will affect quality and performance of plastic part.
Mold structure design 
Figure 6 Scheme 1 weld line
Mold structure design 
Figure 7 Scheme 2 weld line
Since V/P conversion point pressures of Scheme 1 and Scheme 2 are similar, and weld line of Scheme 2 is obviously better than Scheme 1, it is determined to select Scheme 2 for mold structure design, and final needle valve hot runner system is shown in Figure 8. By using junction box, cylinder, air valve and timing controller to control feeding time of each hot nozzle, timing control valve needle advances and retreats, accurately controls filling sequence of molten plastic in mold, avoids generation of weld lines, and reduces stress concentration, improve strength and dimensional accuracy of molded plastic parts, and prolong service life of plastic parts.
Mold structure design 
Figure 8 Needle valve hot runner system

2.3 Design of oblique core-pulling structure

Stripping direction of side wall of plastic part is oblique, and it is necessary to design an inclined slider (see Figure 2) for side core-pulling demoulding. Inclination angle of undercut at the 1st and 2nd places of oblique slider is 26.5º, core-pulling demoulding is directly performed by using inclined guide column and inclined slider, while inclination angle of undercut at thread is 78.5º,    mechanism combining inclined slider 3, horizontal wedge block and hydraulic cylinder must be used for side core-pulling demoulding to ensure smooth demoulding of plastic part.
2.3.1 Design of core-pulling structure of inclined guide column
Outer hole and undercut of plastic part are formed by inclined guide pillars and inclined slider side core-pulling mechanism. Outer core-pulling mechanism is composed of oblique slider 1, oblique slider 2, oblique guide column, bead, back plate, wear-resistant block and other parts. Its structure is shown in Figure 9. Inclined slider is driven by inclined guide column to realize side core-pulling and demoulding of plastic part. Since undercut surface is inclined, movement direction of slider should be consistent with direction of undercut slope, otherwise plastic part will be damaged.
Mold structure design 
Figure 9 Core-pulling structure of inclined guide pillar
Core-pulling direction of oblique sliders 1 and 2 is downward along slope, as shown in Figure 10, oblique sliders are smaller, and diameter D of oblique guide column is φ16 mm. Extraction distance of inclined guide post (that is, movement distance of slider along inclined plane) S1=S+4 mm, S is distance between forming position of side forming part and limit that does not hinder axial extrusion of plastic part, and S is 4 mm, that is, extraction distance of inclined guide post is S1=8 mm. Angle between inclined slider and inclined guide post is α=b+d (b is inclination angle of slider, 26.5°; d is inclination angle of inclined guide post, which is 13°), that is, α=26.5°+13°=39.5 °. Locking angle of back plate of inclined slider is c=d+(2°~3°), and c=15°. Matching length of inclined guide post is L1≥1.5D (D is diameter of inclined guide post), that is, L1≥24 mm, and L1 is taken as 30 mm. Length of inclined guide post L2=(S1cosb/tgd) / cosd+D/2+L1, that is, L2=31.01/0.974+8+30=69.83 mm, and L2 is taken as 70 mm.
Mold structure design 
Figure 10 Structural dimensions of core-pulling structure of inclined guide column
2.3.2 Design of inclined core-pulling mechanism driven by hydraulic cylinder
Undercut angle of outer thread of plastic part is 78.5º, and it is impossible to use inclined guide post to drive inclined slider to demould, so hydraulic cylinder core pulling mechanism is used to realize side core pulling. Core-pulling mechanism is composed of oblique slider 3, hydraulic cylinder, bead, wedge block and other parts. Structure is shown in Figure 11. Wedge block is driven by piston rod of hydraulic cylinder so that T-shaped groove drives inclined slider 3 to realize side core-pulling and demoulding of plastic part. Because thread undercut surface of plastic part is oblique and has a large angle, direction of motion of oblique slider 3 should be consistent with direction of thread undercut slope, otherwise thread will be damaged.
molded plastic parts 
Figure 11 Hydraulic cylinder core-pulling structure
Core-pulling direction of inclined slider 3 is downward along slope, as shown in FIG. 12 . Thread undercut of plastic part is 7 mm. Core-pulling stroke K of inclined slider 3 is obtained by adding 3 mm to the amount of undercut, that is, 10 mm. Piston rod of hydraulic cylinder drives horizontal movement distance of wedge block K1=Kcosg/sine, g=f-(90°-e), K is core-pulling stroke of inclined slider, which is taken as 10 mm; e is angle between T-shaped groove and horizontal direction, which is 26.5°; f is inclination angle of oblique slider 3, which is 78.5°. After calculation K1=(Kcosg)/sine=(10cos15°)/sin26.5°=21.65 mm, core-pulling stroke of hydraulic cylinder is 22 mm.
molded plastic parts 
Figure 12 Hydraulic cylinder core-pulling structure size

3 Mold working process

Due to large output of plastic parts, mold is designed as a structure with 2 cavities. Side wall undercuts, inclined holes and threads of plastic parts use inclined sliders for side core-pulling demoulding. Each plastic part uses 2 latent gates to feed material, so that gate aggregate can be automatically cut off when demoulding, reducing labor intensity and improving productivity. Final design of injection mold structure is shown in Figure 13. Maximum dimension of mold is 600 mm*500 mm*590 mm. Standard mold base is used. Material of guide parts is GCr15, and spatial position accuracy is ±0.01 mm. Mold plate material is S55C, side of frame must be vertical, tolerance from side to reference is 0~0.01 mm, and thickness tolerance of all mold plates is 0~0.02 mm. Since there are 6 inclined sliders on the side of movable mold, in order to ensure strength of mold, thickness of bottom of movable mold set plate must be not less than 1.5 times depth of frame opening, so as to prolong service life of mold. Support column must be reasonably arranged in the middle of mold, its height tolerance is 0.1~0.15 mm to ensure stable production of mold, and molded plastic parts do not produce flash, so as to improve molding quality of plastic parts. After completing design of molded parts, reasonably arrange push rods and cooling water channels to ensure that mold is pushed out in balance and cooling effect of plastic parts is excellent. HTF-320t injection molding machine is used for mass production according to parameters of injection melt volume, tie rod distance, maximum template thickness and clamping force.
molded plastic parts 
Figure 13 Die structure
1. Moving mold seat plate 2. Positioning pin 3. Cushion block 4. Hydraulic cylinder 5. Wedge block 6. Inclined slider 7. Moving mold cover plate 8. Moving template 9. Balance block 10. Straight positioning block 11. Press Tight block 12. Fixed mold cover plate 13. Pressure bearing block 14. Fixed mold core 15. Hot runner fixed plate 16. Fixed mold seat plate 17. Hot runner plate 18. Pressing plate 19. Cylinder 20. Positioning ring 21. Heat Nozzle 22. Blocking air column 23. Sealing ring 24. Fixed template 25. Insulation plate 26. Air plug 27. Inclined slider 28. Cooling water nozzle 29. Inclined guide column 30. Slider pressure plate 31. Slider seat 32 .Slider wear-resistant block 33. Support column 34. Push rod fixed plate 35. Push plate 36. Reset rod 37. Support column 38. Limit column 39. Push rod 40. Push rod 41. Moving model core 42. Guide column 43. Water plug 44. Guide sleeve 45. Water block 46. Air valve 47. Junction box 48. Compression block 49. Positioning column 50. Reset lever 51. Spring 52. Push plate guide column 53. Push plate guide sleeve 54. Rubber block
Ejection of plastic part and reset of mold are completed by ejector rod of injection molding machine. After moving and fixed molds are closed, molten plastic is pressed into hot runner system of mold under action of screw of injection molding machine. Air valve 46 controls valve needle of cylinder 19 to work. Valve gate G1 is opened, melt enters cavity formed by movable platen 8 and fixed platen 24 through hot runner plate 17 and hot nozzle 21 for filling and molding. After a delay of 1.6s, valve gate G2 is opened to continue filling, pressure is maintained, then cooled and shaped. After injection is completed, mold is opened, movable mold set plate 7 is separated from fixed mold set plate 12, inclined guide column 29 drives inclined slide block 27 to perform side core pulling and demoulding. When set mold opening stroke of 450 mm is reached, piston rod of hydraulic cylinder 4 drives wedge block 5 and inclined slider 6 to perform thread side core-pulling and demoulding, then ejector rod of injection molding machine drives reset rod 36 to drive push plate 35 and push rod fixing plate 34 to move, push rods 39 and 40 push plastic part out of cavity, gate condensate and plastic part are automatically cut off and separated to complete an injection cycle. When clamping mold, piston rod of hydraulic cylinder 4 first drives inclined slider 6 to reset before movable and fixed molds can be closed. Then push plate 35 and push rod fixed plate 34 are pulled back by injection molding machine slider, finally pushed back by reset rod 50 and reset spring 51, demoulding mechanism and side core pulling mechanism reset and enter next injection cycle.

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