Abstract A mold with a special structure is designed for injection molding of oil pipe accessory cover of engine twist-bending machine. Structure of mold is a 6-plate type without ejector mechanism, using a latent gate, mold is opened after 3 times of parting. Three kinds of mechanisms are set in mold to realize automatic production of plastic parts, namely: a simple circular arc slider core pulling mechanism to realize core pulling of inner wall of elbow; hydraulic cylinder drives threaded rotation core pulling mechanism to realize demolding of external thread characteristics of plastic part; push plate pushes out plastic part to achieve complete demoulding.

Structure of accessory cover of engine torsion machine oil pipe is shown in Figure 1, total production is 2 million. Plastic part includes 3 main features: ①cap; ②liquid elbow; ③center tube with external thread which is screwed together with another bottle cap. There are two design difficulties for mold of molded plastic parts: ①Core pulling and demolding of elbow of engine oil pipe accessory cover; ②Demolding of outer thread of center tube of disc. Integrating use requirements and material characteristics of plastic parts, plastic parts are made of modified materials ABS+PC, with a shrinkage rate of 0.5% to 0.6%.

 

Figure 1 Engine oil pipe accessory cover

Single cavity structure in mold layout is set up as shown in Figure 2. Single cavity uses a single parting surface for parting, cavity is divided into a cavity plate insert and a core insert. In view of demoulding requirements of external thread of central pipe, inner wall core, external thread core, cylindrical core are divided on core insert. Internal core and external thread core are tubular cores, column hole core is a cylindrical core, outer thread core is sleeved outside column hole core, and also inside inner wall core. External thread core can be rotated independently to perform function of rotating external thread during demolding.

 

Figure 2 Mould single cavity structure setting
Elbow of engine oil pipe accessory cover is a liquid outlet elbow, and inner wall of elbow needs to be pulled out by elbow core. Part of formed elbow is elbow core shown in Figure 2.
Mold uses a single latent point gate for gating, latent gate uses a cylindrical runner to supply material, runner waste is pulled out of cavity plate insert using a draw rod, and at the same time draw rod pushes plastic part out of core insert to achieve demolding.

Taking into account comprehensive factors such as molding accuracy of plastic parts, layout of mechanism, injection capacity of existing injection molding machine in factory, and cost of mold manufacturing, it is planned to use a 8-cavity layout for injection [1-4]. Mold multi-cavity layout is shown in Figure 3. . When designing, set 8 cavities on same insert for easy processing. Gating system uses an "H"-shaped balanced layout, which requires use of a 3-level runner and a 1-level main runner. The overall cooling of cavity plate inserts uses a water channel for cooling, using a φ10mm straight-through pipeline.

 

Figure 3 Mold multi-cavity layout design

In order to realize automatic injection production of plastic parts, demoulding mechanism shown in Figure 4 is arranged on the side of movable mold to realize automatic demoulding of plastic parts [5-8]. For sub-type setting of plastic part, correspondingly set up core-pulling mechanism of 8 elbow cores, core-pulling mechanism of 8 threaded cores and push plate ejection mechanism required for complete demolding of plastic part from mold.

 

Figure 4 Dynamic model

Mold base uses a non-standard mold base with no ejector multi-plate mold, as shown in Figure 5, composed of plates 1 to 8. Fixed mold base plate 1 and fixed mold plate 2 constitute fixed mold part of mold base, push plate 3 is used for plastic molding. Final push out of part is demolding bearing plate, movable mold plate 4 is used to install core insert, plates 5-8 are fastened together by screws, are used to install threaded core-pulling mechanism and elbow core-pulling mechanism. Thread core pulling and demoulding mechanism is shown in Figure 6.

 

Figure 5 Mould structure

1. Fixed mold base plate 2. Fixed mold plate 3. Push plate 4. Movable mold plate 5. Upper gear plate 6. Diaphragm 7. Lower gear plate 8. Movable mold base plate 9. Upper end cover 10. Upper bearing 11. Transmission gear Shaft 12. Lower bearing 13. Lower end cover 14. Intermediate transmission gear 15. Central core 16. Threaded sleeve 17. Threaded core 18. Compression block 19. Inner wall core insert 20. Guide groove block 21. Arc sliding Block 22. Pin 23. Locking block 24. Drive block

 

Figure 6 Thread core pulling and demoulding mechanism
19. Inner wall core insert 25. Guide post 26. Fixed template guide sleeve 27. Movable mold push plate guide sleeve 28. Limit pull rod 29. Pull rod 30. Spring 31. Rack 32. Hydraulic cylinder seat 33. Connecting sleeve 34 .Hydraulic cylinder 35. Elastic ejector pin 36. Pull rod 37. Sprue sleeve 38. Positioning ring 39. Side pull plate 40. Cavity plate insert 41. Core insert

(1) Elbow core-pulling mechanism. Elbow core-pulling mechanism is composed of parts 20-24. After mold is opened on P1 side, locking block 23 unlocks arc slider 21, when P2 side is opened, driving block 24 with pin 22 drives arc slider 21 to slide along arc guide groove in guide groove block 20, and drives elbow core to pull core from inner wall of formed plastic elbow. Sequence of mold opening is: P1→P2→P3, sequence is reversed when closing mold.
(2) Thread core pulling mechanism. Thread core pulling mechanism consists of 3 parts: ①Thread core rotating core pulling mechanism composed of pieces 15-19 shown in Figure 5; ②Intermediate gear transmission mechanism, including pieces 9-14 shown in Figure 5; ③Hydraulic cylinder + rack drive mechanism that drives intermediate transmission gear mechanism to rotate, its components are parts 31 to 34 shown in FIG. 6.
Principle of operation of thread core pulling mechanism is: rack 31 is driven to slide by hydraulic cylinder 34, rack 31 then drives intermediate transmission gear 14 to rotate, intermediate transmission gear 14 drives gear on thread core 17 to rotate again, so as to drive thread core 17 under guidance of thread sleeve 16. While thread core 17 rotates, floating roof spring 30 lifts movable mold plate 4 upward, lifting distance is limited by pull rod 29 until thread core 17 is completely separated from plastic part.
(3) Mould push plate demoulding mechanism. Demolding of molded plastic part is carried out in two steps: ①When thread core is pulled and rotated, P2 surface is opened, push plate 3 and movable mold plate 4 are pushed up by spring 30, inner wall core insert 19 installed on movable mold plate 4 and push plate 3 jointly push plastic part out of central core 15; ②When P3 surface is opened, push plate 3 drives core insert 41 to push plastic part out of inner wall core insert 19 to realize complete release of plastic part. P3 surface is opened and fixed mold plate 2 is pulled by side pull plate 39, opening distance is limited by limit pull rod 28. Ejector pin 35 pushes out waste material of runner to make it demould.

Working principle of mold is divided into following 7 steps.
(1) Injection of melt. After injection molding machine completes melt filling, pressure holding, and cooling of cavity, mold stays in closed position and waits for injection molding machine to drive it to open.
(2) Thread demolding. Before mold is opened, thread core pulling mechanism must first complete rotating core pulling action, that is, under drive of driving mechanism,thread core 17 completes thread core pulling action, which corresponds to demolding of external thread of plastic part.
(3) Demoulding of outer wall. Mold is driven by injection molding machine, movable mold part retreats. Firstly, it is opened on P1 surface, most of outer wall of the plastic part is demolded. At the same time, locking block 23 in elbow core pulling mechanism unlocks arc slider 21 tight.
(4) Elbow hole is core-pulled and demoulded. When P2 surface is opened, driving block 24 drives arc slider 21 to force elbow core to rotate to complete core pulling and demolding action.
(5) Demoulding of center hole. When P2 surface is opened, center hole core 15 is drawn out to complete core pulling and demolding of hole.
(6) Plastic parts are completely demoulded. Movable mold continues to retreat, mold is opened at P3 surface, push plate 3 drives core insert 41 to push plastic part out of inner wall core insert 19 to achieve complete demolding of plastic part.
(7) Mold is closed. Closing process of mold is opposite to opening process.

A multi-plate no-ejection injection mold is designed for injection molding of plastic parts. Use of latent gates avoids problem of difficult gate opening and effectively ensures filling effect of cavity. Aiming at characteristics of slender elbow of plastic parts, a simple circular arc slider core pulling mechanism is designed to realize core pulling and demolding of inner wall of elbow. An 8-cavity threaded rotary core-pulling mechanism driven by a secondary drive by a hydraulic cylinder is designed to realize rotary core-pulling demoulding of external thread characteristics of plastic part. Push-out method is used to realize complete demolding of plastic part, which ensures that plastic part is not deformed during demolding.