[Abstract] Structure of injection mold for camera lens cover is introduced. It uses point gates for feeding, ejection mechanism uses a fixed side ejection mechanism to ensure a good appearance and smooth ejection of plastic part. Fixed mold cavity and movable mold core adopt a cyclic cooling method, and mold can be fully cooled after injection molding, which not only shortens molding cycle, but also improves quality of plastic parts.

For plastic parts such as camera lens caps, customers have high requirements for appearance, and it is difficult to control appearance quality during production. In particular, defects such as parting surface marks, water lines, scratches, and black spots are particularly outstanding. In order to obtain higher appearance quality of plastic parts in injection molding process, various influencing factors in injection molding process must be considered reasonably during mold design, such as fluidity of plastic materials, selection of gate positions, mold release form of plastic part, and temperature control of mold.

Figure 1 shows camera lens cover, material used is ABS, and shrinkage rate is 0.5%. Plastic part material has good gloss, certain abrasion resistance, stable forming size, can ensure a good matching relationship between plastic part and plastic part. Final molded plastic part needs to be tightly and compactly matched with buckle of upper cover, which is convenient for customers to disassemble and install. Where surface of plastic part is appearance surface (see Figure 1), sanding treatment is required in the later stage, so these surfaces must be guaranteed to be free of flash, no traces of parting surface, etc., otherwise it will be more obvious after sanding treatment, surface of the plastic part should be bright and feel better.

 

Figure 1 Camera lens cover
Maximum diameter of plastic part is ϕ55.6mm, height is 7.7mm, and wall thickness is 2mm. Diameter of plastic part is 53.5mm, which is used to connect with camera lens body. Connection method is that cover rotates and snaps into camera body, so this position (position A) is internal shape, which requires high dimensional accuracy and appearance accuracy. Requirements are not high, and parting surfaces are allowed here, which can be used as main parting surfaces of fixed side and movable side. Outer surface of plastic part is appearance surface, surface must be smooth, and there should be no undesirable phenomena such as flashing, so there can be no traces of gate here, and point gate must be placed on inner side. At the same time, movable side is appearance surface and ejector rod cannot be set, there are many shapes left on fixed side at this time. It needs to be designed such that after mold is opened, plastic part is left on fixed side and ejected on fixed side.

 

Figure 2 Parting diagram of plastic parts

Mold is a three-plate mold with a thin nozzle, and plastic part is a camera lens cover. Since parting structure of camera lens cover is relatively simple, and cavity core adopts mosaic structure, here is no more cumbersome, this article only focuses on mold structure of camera lens cover.

 

Figure 3 Dimensions and appearance requirements of plastic parts

(1) Point gate location. Mold flow analysis of plastic part shows that the best pouring position of plastic part is in the middle of camera lens cover. Figure 4 shows mold flow analysis diagram. In order to avoid feeding marks on the surface of plastic parts and ensure uniform feeding, mold uses point gates for feeding, and gate location is selected in the middle of inner side of camera lens cover.

 

Figure 4 Mold flow analysis diagram
(2) Runner design. Runner block is designed to have plastic parts left on fixed side and eject plastic parts, so a runner extension sleeve is required. Specific structure is shown in Figure 5, which is composed of part 17, part 18, part 19, and part 21.
(3) Runner demoulding. This sub-mold can realize automatic flow channel. As shown in Figure 5, when mold is opened, due to action of spring 33, nozzle push plate 2 and backing plate 3 are separated, point gate on plastic part is pulled off at this time. After mold is completely opened, tie rod assembly 32 pulls off interface between main runner and injection molding machine nozzle.

In order to ensure appearance of plastic part, parting surface of plastic part is located at F as shown in Figure 6, with one mold and one cavity arranged.
Purpose of fixed side inserts is to facilitate processing technology and reduce processing cost.
Purpose of parting movable side insert: Since outer surface of plastic part needs sanding treatment, and as can be seen from Figure 3 of plastic part, there are two types of sanding on outer surface, so when manufacturing insert, it needs to be sanded separately. .

As shown in Figure 5, plastic part remains on fixed side, and plastic part is ejected on fixed side.
Function of mold lockers 11 and 31: when mold is opened, 7 plates are first pulled out. When mold is pulled to a certain distance, mold locker opens, 6 plates and 5 plates are separated.

 

Figure 5 Mould assembly drawing

1. Fixed mold seat plate 2. Nozzle push plate 3. Backing plate 4. Square iron 5. Fixed plate 6. Fixed side push plate 7. Movable plate 8. Movable mold seat plate 9. Mandrel pad 10. Mandrel Fixing plate 11, 31. Clamping device 12. Bracket 13. Magnet 14. Guide column guide sleeve kit 15. Ejector 16. Support column 17. Gate extension sleeve 18. Runner insert plate ① 19. Runner insert Parts plate Ⅱ 20. Guide column guide sleeve kit 21. Gate sleeve 22. Limit rod 23. Reset rod 24, 33. Spring 25. Pressure plate 26. Fixed side core Ⅱ 27. Fixed side core Ⅰ 28. Plastic parts 29 Movable side cavity Ⅰ 30. Movable side cavity Ⅱ 32. Limit rod kit 34. Push plate guide post and guide sleeve kit 35. Limit screw and spring kit 36, 39. Screw 37. Precision positioning component 38. Limit screw 40. Pin

 

Figure 6 Parting diagram of mold insert

Cooling system of mold adopts a circulating cooling water channel to ensure that temperature of water inlet and outlet is between 2℃ and 3℃, so as to ensure that plastic parts are fully and uniformly cooled before mold is opened. Layout of waterways is shown in Figure 7. Due to high requirements for appearance of plastic parts, mold plate and cavity must be connected to a waterway, and a sealing ring needs to be added at the joint.

 

Figure 7 Cooling water circuit

If plastic parts are not exhausted smoothly, it will cause following problems: difficulty in filling, pores, voids, loose tissue, local carbonization and scorching, obvious flow marks and fusion marks, etc. Due to high requirements for appearance of plastic part, in order to avoid above problems, it is necessary to exhaust air for each insert. For example, movable side cavity, as shown in Figure 8.

 

Figure 8 Schematic diagram of exhaust

When mold is closed, injection molding machine's mold opening and closing system drives movable mold to move to fixed mold direction. Since spring 35 is compressed and needs to be restored to its original state, movable plate 7 and movable mold seat plate 8 are pushed to close mold first, and at the same time, return rod spring works to make ejector rod retreat. Then guide post enters guide sleeve, parting surface is aligned with fixed mold, accuracy of alignment is guaranteed by mold clamping guide mechanism. After movable mold and fixed mold are aligned, cavity on fixed mold plate and core fixed on movable mold plate are combined to form a closed cavity with same shape and size as plastic part. Mold cavity is locked by clamping force provided by clamping system of injection machine during injection molding process to prevent it from being expanded by pressure generated when plastic melt fills cavity. Plastic melt injected by injection molding machine from nozzle enters mold through main runner opened on fixed mold, then enters cavity from runner and gate. After melt fills cavity and undergoes pressure holding, feeding and cooling, mold is opened.
When mold is opened, injection molding machine pulls movable mold back. Due to action of spring 33, part 2 and part 3 are separated first, gate position of break point is pulled at the same time. One end of tie rod assembly 32 controls mold opening distance of part 2 and part 3 (must be greater than length of runner). The other end of tie rod assembly 32 controls mold opening distance of fixed mold base plate 1 and part 2, pulls off connection between main flow channel and nozzle. Movable mold continues to retreat. Main parting surfaces of part 6 and part 7 are opened. When they are retracted to a certain distance, mold clamp 31 opens. When distance between piece 6 and piece 7 reaches distance specified by brace 12, surface between piece 6 and piece 5 opens, piece 6 starts to move away from piece 5 until distance specified by limit screw 38 is reached. Movable mold continues to retreat, magnet 13 opens, surface between part 8 and part 7 opens. At the same time, part 8 drives pull rod 22, and then pulls ejector plate to move forward to eject plastic part.

Core and cavity of mold can be processed by cutting blank first, then rough machining, CNC machining, heat treatment. At the same time, design sanding jigs for sanding treatment.

Structure of this mold is relatively complicated. Both moving and fixed molds adopt inlay structure and point gate feeding. Due to appearance requirements of plastic part, it is necessary to design plastic part to remain in fixed mold and eject plastic part from fixed side. Use Pro/e, MoldFlow and other software to design a complete mold. Use UG to generate processing program, process dynamic and fixed model cavity and electrode on CNC machine tool to ensure processing accuracy.