According to structural characteristics and technical requirements of plastic parts, a pair of 1 three-plate injection mold with 1-cavity was designed. It uses point gate feeding, through T-shaped block, oblique guide post + slider, slider built-in oblique push block and other composite core pulling mechanisms, which solves problem of inverted mold release in multiple directions. Production practice shows that mold structure design is reasonable, core-pulling mechanism is stable, work is reliable, installation is convenient, space is small, mold parts are easy to replace, molded quality of produced plastic parts is good, and good economic benefits have been achieved.

Figure 1 shows housing of a certain vacuum cleaner, which is an assembly and requires high appearance and dimensional accuracy. When designing and manufacturing mold, ensure appearance and size of molded plastic part. Plastic part material is ABS+PC. ABS is an acrylonitrile-butadiene-styrene copolymer, which is a kind of thermoplastic polymer material with high strength, good toughness, easy processing and molding; PC is a polycarbonate plastic with high strength, wide temperature range, high transparency, free dyeability, and high fatigue resistance; ABS+PC combines excellent characteristics of two types of plastics and is widely used in household appliances, automotive interior parts, communication equipment and lighting equipment. Molding temperature of ABS+PC is 230~300 ℃, and shrinkage rate is 0.5%.

 

Figure 1 Plastic part structure
Structural characteristics and technical requirements of plastic parts are as follows: ①Maximum external dimension is 180 mm*120 mm*65 mm; ②Appearance is high, defects such as weld marks, shrinkage dents and spots are not allowed; ③Plastic parts are assembly parts with high dimensional accuracy; ④Plastic parts have many undercuts, which increases difficulty of mold design and manufacturing, and it is necessary to install an oblique push block in slider to achieve demolding.

According to plastic parts structure and output requirements, combined with difficulty of mold processing, mold adopts a 1-cavity layout. Inner surface of plastic part has reinforcing ribs and screw posts, which are formed by movable mold. After mold is opened, plastic part remains in movable mold; outer surface is formed by fixed mold, fixed mold has no inserts and other assembled parts, which can ensure surface quality of plastic part.

Core and cavity plate adopt a combined structure, each component is processed separately, which saves manufacturing time. Core uses die steel 718, which has superior mechanical properties and machinability, simple processing technology, can extend service life of die parts. Parting surface of molded part should avoid sharp corners and sharp edges to ensure reliable sealing. In order to facilitate processing and maintenance, movable mold is designed with multiple inserts, and inserts are fixed by step positioning.

Gate is core of gating system, its form and position directly determine molding quality of plastic part. Mold has a one-mold, one-cavity structure, adopts two-point feeding at one point gate to shorten filling time, ensure complete filling and small temperature differences in filling process. Advantage of point gate is that it does not affect appearance of plastic part. Condensate of pouring system can be automatically separated from plastic part when mold is opened. Disadvantage is that it is not easy to process. Point gate is shown in Figure 2.

 

Figure 2 Gate

Plastic parts have multiple inverted buckles in different directions. Both movable mold and fixed mold need to be designed with core pulling mechanisms. Fixed mold adopts "T-shaped block pressing block + T-shaped block + insert" for core pulling, movable mold adopts "inclined guide post". +slider" for core pulling, inverted buckle in sliding direction of slider adopts “oblique push block” for core pulling in slider, angle between inclined guide post and mold opening direction is 18°, diameter is φ16 mm. Inclined guide column and pressing block are fixed on fixed mold plate. When mold is opened, inclined guide column drives slider to realize core pulling, and at the same time, inclined push block is driven out of the reverse buckle. When mold is closed,  pressing block acts on sliding block to reset and push inclined push block to reset at the same time. Mold has multiple lateral core-pulling mechanisms, reasonable structure, safe, stable and reliable core-pulling action. Core-pulling mechanism is shown in Figure 3.

 

Figure 3 Side core pulling structure
1. Inclined guide post 2. Slider insert 3. Slider 4. Press block 5. Slider insert 6. T-shaped block 7. T-shaped block press block 8. Oblique push block 9. Slider 10. Press block

In order to improve cooling efficiency, mold adopts a straight-through waterway temperature control system, in which diameter of waterway is φ6 mm, distance between cooling waterways is controlled at about 36 mm. Among them, 3 sets of circulating water channels are designed on the side of fixed mold,1 set of circulating water channels are designed on the side of movable mold. Due to majority of area of molded plastic part of slider, cooling water circuit should also be designed in slider. Cooling circuit of mold is crisscrossed, total cooling area reaches about 60% of total area of plastic part. Temperature of all parts of mold is balanced, cooling is fast and sufficient, which improves production efficiency of mold.

Overall size of mold is 450 mm * 450 mm * 516 mm, and structure is shown in Figure 4. Mold is opened after injection, pressure holding, cooling, and molding. When mold is opened, sliding block of injection molding machine drives movable mold. Mold is first opened from parting surface PL1. At this time, fixed mold plate and stripper plate are separated, gate aggregate and plastic part are automatically separated, T-shaped block 14 drives T-shape block 13 to break away from plastic part. Mold opening distance of parting surface PL1 is 10 mm, which is controlled by limit nail; then movable mold is opened from parting surface PL2, fixed mold seat plate and stripper plate are separated, gating system aggregates are separated from nozzles and drawing rod, casting system aggregates fall off; finally mold is opened from parting surface PL3, movable and fixed molds are separated. Inclined guide column 3 drives slider 5 to achieve core pulling, and at the same time oblique push block 16 is driven away from undercut. After completing opening stroke of 30 mm, ejector rod of the injection molding machine pushes push rod fixing plate 9 and push plate 10, finally plastic part is pushed out by push rod to complete one injection molding. When closing mold, injection molding machine slide block drives movable mold to close mold, pressure block pushes slide block to reset oblique push block, T-shaped block pressure block pushes T-shaped block to reset, reset rod pushes push rod fixed plate and push plate to reset, all parts are reset. Then start next injection molding.

 

Figure 4 Mould structure
1. Fixed mold base plate 2. Stripping plate 3. Inclined guide column 4. Fixed template 5. Slider 6. Press block 7. Movable template 8. Cushion block 9. Push rod fixed plate 10. Push plate 11. Movable mold Seat plate 12. Slider insert 13. T-shaped block 14. T-shaped block press block 15. Core 16. Oblique push block 17. Cavity plate

Set T-shaped block + pressing block + insert core pulling mechanism in fixed mold, which solves demolding problem of plastic part's outer surface undercut; set inclined guide post + slider in movable mold, and set inclined push block core pulling mechanism in slider, which solves problem of demolding of plastic part with multi-directional reverse buckle. By controlling mold opening sequence, core-pulling mechanism can operate reasonably, work stably, and quality of molded plastic parts is good.