Length of plastic part is 248mm, width is 203mm, wall thickness is 2mm, and material is ABS. ABS is a thermoplastic engineering plastic with a wide range of uses. It has excellent comprehensive physical and mechanical properties, good low-temperature impact resistance, stable size, good molding performance, and small shrinkage. Plastic parts A, B, and C have undercuts, which are difficult to remove from mold. A complex core-pulling mechanism needs to be designed. Wall thickness of undercut at B is 1.1mm, which is relatively thin. There are many and long ribs on the surface, and molded plastic parts are easy to lack materials. Therefore, injection process requires a relatively high injection pressure and a long dwell time. Quality of outer surface of plastic part is high. Since bottom maximum contour line has a slanted line and a stepped turning line, it is necessary to control quality of flash at parting part and the overall deformation tolerance of molded plastic part. There are many and deep ribs on inner and outer surfaces of plastic parts, which makes it difficult to exhaust during molding. Therefore, exhaust of insert should be designed at molding ribs, and position of push rod should be set as close to ribs as possible.

 

Distribution of mold cavity is related to layout of gating system. Gating system includes: main channel, runner and gate. Considering that there should be no gate marks on outer surface of plastic part, gate is designed as a latent type, gating from inner surface of plastic part to be formed, mold has a structure of 2 cavities, and gate is set on the side without core-pulling structure, as shown in Figure 2. This design can make gate positions of two cavities same, process is same and relatively short. There is a pulling rod at the end of main runner, and head of pulling rod is Z-shaped, which can pull gating system congealed material to moving mold side during parting of moving and fixed molds, can also accommodate condensate at the front end of melt when mold is closed and poured, and it also has exhaust effect. Latent gate is ground on the top of push rod and connected to oblique gate at the end of runner, so that gate can be designed on inner surface of plastic part to be molded, without affecting appearance quality of molded plastic part, and it is convenient for molding release of piece. When demolding, under action of push rod and pull rod, plastic part and condensate of gating system are ejected, and gate is automatically cut off.

 

Parting surface is designed at the largest contour of plastic part, and it is necessary to facilitate processing of mold parts and exhaust of cavity, as shown in Figure 3. Since parting surface of plastic part is stepped and has an inclined surface, lateral force suffered during injection is relatively large. Therefore, while guide post and guide sleeve are guiding, a concave-convex cone surface is added around parting surface for secondary positioning. Top of concave-convex cone surface should be evacuated, top should be made into a plane, and surrounding corners can be avoided for better positioning of cone surface. Sealant position of parting surface should extend a certain distance along original curved surface to ensure that parting surface fits tightly and prevent overflow. A plane parting surface should be added at the end of parting surface, plane can be used as a benchmark for processing and maintenance.

 

Mold structure is shown in Figure 4. There is a 1.5mm undercut at plastic part A (see Figure 1). This undercut core-pulling must be completed before moving mold plate 40 and fixed mold plate 37 are parted. Inclined slider 33 can slide in guide chute formed by pressing block 43 and fixed mold core 38. When mold is opened, under action of multiple parting mechanism and spring 30, fixed mold plate 37 and fixed mold seat plate 36 are separated first. That is, it is separated at PL1, then inclined guide column 32 fixed in fixed mold seat plate 36 drives inclined slider 33 to complete core pulling in fixed mold, spring 31 and positioning screw 34 play a positioning role to prevent inclined slider 33 from loosening after core is pulled. After mold closing, inclined wedge 35 presses oblique slider 33, oblique angle of oblique wedge 35 is 2° greater than oblique angle of oblique guide post 32, and core pulling distance of oblique slider 33 is preferably 5mm.

 

1. Distance screw 2, spring 3, core fixing plate 4, hydraulic cylinder pull rod 5, guide slider 6, hydraulic cylinder fixing plate 7, hydraulic cylinder 8, pull rod fixed 9, core 10, core fixing block 11 , oblique pressure block 12, core 13, fixed mold core 14, sprue sleeve 15, positioning ring 16, guide sleeve 17, guide post 18, spring 19, oblique core 20, wedge 21, movable mold seat plate 22, Support block 23, movable mold cushion block 24, pull hook 25, pin seat 26, pin 27, compression spring 28, pressure rod 29, distance screw 30, spring 31, spring 32, inclined guide column 33, inclined slide block 34, positioning Screw 35, wedge 36, fixed mold base plate 37, fixed template 38, fixed mold core 39, movable mold core 40, movable template 41, push rod fixed plate 42, push plate 43, briquetting block 44, push rod 45, pull rod
Figure 4 Die structure

Since there is a long rectangular hollow boss at part B (see Figure 1), and inner hole of boss forms a 23mm undercut, it is necessary to design a lateral core-pulling mechanism for demoulding. Considering that core-pulling distance here is longer, and side hole wall of boss is thinner, only 1.1mm, and outer surface of boss can not have flash, it is necessary to design a hydraulic secondary core-pulling, as shown in enlargement of F in Figure 4, that is, first core pulling is performed to loosen side hole on inner surface of rectangular boss, then second core pulling is performed to complete outer surface core pulling, which can prevent damage to molded plastic parts during core pulling, and core-pulling distance on outer surface should be greater than undercut 23mm. One end of hydraulic cylinder pull rod 4 is fixed in pull rod fixing block 8, core 9 is fixed in core fixing block 10, pull rod fixing block 8 and core fixing block 10 are connected by screws. Core 9 can slide in core 12 holes, core fixed plate 3 is connected with core 12 by screws, and core fixed plate 3 is locked when oblique pressing block 11 is matched. Working principle: After mold is opened, oblique pressing block 11 is separated from core fixing plate 3, and hydraulic cylinder rod 4 starts to move. Rod fixing block 8, core fixing block 10, and hydraulic cylinder rod 4 drive core 9 to slide a distance of H1 in core 12, inner hole of boss is pulled out for the first time. Then core 9 touches core fixing plate 3 again, which drives core 12 to move, carries out core pulling on outer surface of boss, and completes hydraulic secondary core pulling. Hydraulic secondary core pulling mechanism is guided by guide slider 5. Heart-pulling distance must satisfy the formula: H3>H2+H1, where H3 is maximum core-pulling stroke of hydraulic cylinder; H1 is the first core-pulling distance of core 9, which is 25mm; H2 is core-pulling distance of core 12, H2= boss undercut length 23 + margin, H2 can be 26mm, and H3 can be 55mm.

Since there is an oblique column at inner surface C of plastic part (see Figure 1) to form an undercut, it is necessary to design a core-pulling structure in movable mold, wedge 20 is fixed in movable mold pad 23, and oblique core 19 forms an oblique column. When mold is opened, under action of spring 2 and secondary parting mechanism, movable mold pad 23 is separated from movable mold plate 40, that is, separated at PL2, then wedge 20 leaves inclined core 19, oblique core 19 moves obliquely under action of spring 18 to complete oblique core pulling. Distance screw 1 plays role of distance, and when mold is closed, inclined wedge 20 presses inclined core 19 to reset.

Due to complex structure of mold, it is necessary to complete core pulling three times in sequence, so a sequential parting mechanism of hook and pressing rod is designed. When opening mold, because draw hook 24 has pulled moving mold pad 23, under effect of spring 30, fixed mold plate 37 and fixed mold seat plate 36 are separated from PL1. When fixed mold core pulling is completed after being separated by a certain distance, pressure rod 28 fixed on fixed mold seat plate 36 touches tail of inclined plane of pull hook 24, pull hook 24 rotates around pin 26, and pull hook 24 is decoupled from movable mold cushion block 23. Under action of spring 2 and distance screw 29, movable mold cushion block 23 and movable mold plate 40 are separated at PL2, and after a certain distance, oblique core pulling of movable mold spring is completed. Afterwards, due to fixed distance of distance screw 1, movable mold plate 40 and fixed mold plate 37 are separated from PL3, and three-time sequence is realized.

Molding parts are parts that determine shape and size of plastic parts. Molding parts of plastic parts mainly include: fixed mold core 38 on the outer surface of molded plastic part, dynamic mold core 39 on the inner surface of molded plastic part, and various undercuts of molded plastic part core pulling mechanism. Fixed model core 38 is embedded in fixed mold plate 37, and movable model core 39 is embedded in movable mold plate 40. Since plastic parts have many and deep ribs, in order to facilitate exhaust, processing and maintenance, inserts are used to form ribs. Small inserts are fixed with shoulders at the end, pressure plate is pressed against steps, and large inserts are fixed with screws. Molded parts are all provided with a demoulding slope, material of movable mold core 39 and fixed mold core 38 can be 738H mold steel, material of various core-pulling mechanism sliders can be H13 mold steel. 738H is a pre-hardened mold steel with good machinability, good polishing and electro-corrosion performance. H13 has high wear resistance, dynamic mold core is shown in Figure 5.

 

To shorten molding cycle of mold, design of cooling system is more important. Due to large shape of plastic part, in order to ensure sufficient cooling, cooling water enters movable mold core 39 from movable mold plate 40, and enters fixed mold core 38 from fixed mold plate 37. There are cooling water channels around plastic part, and diameter of water hole is φ8mm. To prevent water leakage, cooling water hole cannot touch push rod hole and small inserts. Leak-proof rubber rings should be provided at upper and lower joints of movable mold plate 40 and movable mold core 39, fixed mold plate 37 and fixed mold core 38, cooling water circuit of movable mold is shown in FIG. 6 .

 

Plastic melt enters closed cavity through mold gating system, mold is opened after being pressurized and cooled. Under action of multiple parting mechanisms, mold is first separated at PL1, under action of core-pulling mechanism in fixed mold slider, core-pulling at A on the upper surface of molded plastic part is completed first, after separating for a certain distance, hook 24 of multiple parting mechanism is separated from movable mold pad 23. Under action of spring, mold is divided at PL2, then under action of movable mold spring oblique core-pulling mechanism, core-pulling at C of molded plastic part is completed. Finally, under action of fixed-distance screw, mold is divided at PL3, movable and fixed molds are separated. Under action of hydraulic secondary core-pulling mechanism, core-pulling of hollow boss at the side B of molded plastic part is completed, then push plate 42 drives pulling rod 45 and push rod to push out molded plastic part and gating system condensate.