Design of Injection Mould for External Thread Water Spray Gun Base

Time:2021-01-08 21:28:00 / Popularity: / Source:

molded plastic parts 
Figure 1 Water spray gun holder
Figure 1 shows a water spray gun holder with external threads, made of ABS. There are external threads on plastic part, which are used to connect water gun and water pipe. External dimension is ϕ62.22mm*24.8mm, which is cylindrical. There are 4 through holes in the middle of plastic part. Each through hole has a different diameter to adjust water flow. Size of through hole is small. 3 buckles on the back are evenly distributed around plastic part, there are 4 convex and concave positions on outer surface.

1 Analysis of molding process

(1) Conventional forming method is to form external thread of slider, external thread of plastic part can be demolded through movement of slider.
molded plastic parts 
Figure 2 Gate location
(2) Based on past experience of developing similar molds, mold uses point gates for gating, and gates are set on ribs of plastic parts to be molded. Positions of two gates are shown in Figure 2.
(3) Size of 4 through holes on plastic part are ϕ4, ϕ6.8, ϕ6.5, ϕ8.39mm, depth of small hole is 16.2mm. During injection, position of small hole is prone to flash, and molding is small. To facilitate maintenance of mold, insert core with 4 through holes into panel. There should be no flash on parting surface of hole, otherwise it will affect water flow and cause mold development to fail.
(4) Three buckles on the back of plastic part must be demoulded with an oblique push structure. Due to small volume of plastic part, in order to prevent interference when three oblique push structures are pushed out, push-out distance of oblique push structure must be limited. At the same time, in order to ensure strength of moving model core and avoid opening cooling water circuit, size of diagonal push structure should not be too large.
(5) There are 4 convex and concave positions on outer surface of plastic part, which cannot all be molded with sliders, nor can all be molded on a movable mold. Parting surface can be made into a special-shaped surface, of which two convex and concave positions are formed by sliders, two convex and concave positions are formed by a movable mold, as shown in Figure 2.

2 Modeling design and mold flow analysis

molded plastic parts 
Figure 3 Bubbles in mold flow analysis
CREO is a 3D software with modeling design, mold flow analysis and mold design functions. Use functions of rotation, stretching and shell extraction under CREO modeling module to shape plastic parts; then use CREO modeling flow analysis module for mold flow analysis. Set melt filling time to 1s, injection temperature to 235℃, mold temperature to 60℃, maximum injection pressure to 155MPa, which can produce good moldability of plastic part, temperature of melt front is normal. Although there are several weld marks on molded plastic part, they are not obvious, there are several bubbles, such as black spots shown in Figure 3. Defects such as weld marks and bubbles on molded plastic parts can be solved by making cores into inserts.

3 Mould structure design

molded plastic parts 
Figure 4 Mould structure
1. Movable mold seat plate 2. Push plate 3. Push rod fixed plate 4. Spacer 5. Oblique push rod 6. Guide column 7. Movable template 8. Guide sleeve 9. Movable mold insert 10. Core insert 11 . Slider 12. Fixed template 13. Fixed mold insert 14. Stripping plate 15. Fixed mold seat plate 16. Positioning ring 17. Pull rod 18. Pull rod 19. Fixed distance pull rod 20. Limit nail 21. Slider limit Position post 22. Inclined guide post 23. Screw 24. Nozzle
Structure of molded plastic parts adopts a simple three-plate mold base, size of mold base is 320mm*300mm*331mm, ejection stroke is 50mm, and injection pressure is 1600kN. In order to save manufacturing cost and processing difficulty of injection mold parts, both fixed mold and movable mold adopt the insert type, and mold structure is shown in Figure 4.

01 Mold work process

Mold working process: After injection is completed, under action of push rod of injection molding machine, stripper is first separated from fixed mold plate, distance between stripper and fixed mold plate is 100mm under action of fixed distance rod. Then fixed mold base plate and stripper plate are separated, under action of stripper plate pull rod, distance between fixed mold base plate and stripper plate is 8mm, finally fixed mold plate and movable mold plate are separated. Under action of packing force, plastic part is glued to movable mold core. Under action of inclined guide column, double-sided slider moves away from center of mold. A limit nail is set on chute. When sliding 20mm, limit nail prevents slider from moving. When movable mold moves to extreme position, under action of push rod of injection molding machine, all inclined push structures in mold are driven together to start action. Under guidance of T-shaped groove, oblique push block is separated from buckle of molded plastic part. When limit nail in mold touches movable mold plate, ejection mechanism stops moving. At this time, six oblique push structures have separated from molded plastic part, and oblique push rod pushes molded plastic part out of movable mold core. Then ejection mechanism starts to reset under action of push rod of injection molding machine. When 6 inclined push structures in movable mold are completely reset, next injection molding starts.

02 Movable and fixed mold insert design

Mould structure design 
(A) Movable mold inserts
Mould structure design 
(B) Fixed mold insert
Figure 5 Movable and fixed mold inserts
Due to small volume of plastic part, there are two sliders in mold, which is suitable for a mold with two cavities. There is a convex and concave position in the middle of molded plastic part, which can be used as a component molding surface. Parting surface of movable mold insert is a "convex" type, as shown in Figure 5(a), material in the middle of core can enhance strength of core. In order to strengthen strength of fixed mold insert, two independent inserts are made, as shown in Figure 5(b).

03 Slider design

Mould structure design 
Figure 6 Slider structure
Thread on plastic part can be formed with two sliders. As shown in Figure 6, demolding distance of two sliders is 20mm. Running distance of sliders is aligned. It is suitable to use inclined guide post to drive slider. In actual work, when fixed and movable mold plates are separated, two sliders are separated under action of inclined guide post, slide 20mm, stop after contacting limit nails, sliders are completely separated from molded plastic part. Movement of two sliders is synchronized with movement of moving and fixed mold plate, which will not extend mold opening time. Considering influence of slider quality on sliding, during injection production, two sliders should be distributed on left and right sides of mold, sliders should be supported by chute. Slider material adopts GS738, which shortens forming part for carbonitriding treatment. Treatment layer has effect of corrosion resistance and can prevent high temperature melt from corroding slider. A wear-resistant sheet is installed on inclined surface of slider. Material of wear-resistant sheet is 40Cr, heat treatment hardness is 48~52HRC, which has function of pressure resistance, wear resistance and lubrication.

04 Gating system design

Mould structure design 
Figure 7 Gating system
Mold adopts a three-plate mold structure, feeding method is a point gate, and gating system is shown in Figure 7. Gate is located at stress position of plastic part to be molded. Mold flow analysis results show that there are several bubbles on molded plastic part and actual production proves that this gating method can avoid flow marks, shrink marks, weld marks and other undesirable phenomena on plastic parts, meet injection requirements. It is also convenient to repair flash on molded plastic part.

05 Inclined pusher

Mould structure design 
Figure 8 Inclined pusher assembly
1. Screw 2. T-shaped block 3. Oblique push rod 4. Plastic parts
Molded plastic part is pushed out by 3 oblique push components, as shown in Figure 8, without push rods. Because there are three oblique push components arranged in mold, if pushing distance is large, three oblique push components will interfere. Demolding distance of upper buckle position of plastic part is less than 2.1mm, and slope of oblique push assembly is 84°. When ejection distance is 50mm, lateral movement distance of oblique push assembly is 50mm * cot84°= 5.25mm, which is greater than demolding distance, molded plastic part can be demolded normally.
Oblique pusher component is made of FDAC and heat-treated. Oblique pusher is composed of oblique push rods and T-shaped blocks. T-shaped block is connected with push plate by screws and is installed on push rod fixing plate. Oblique push rod is easy to wear, strain or break. In order to prevent oblique push rod from being worn or broken when moving up and down, moving guide groove of oblique push rod is installed on movable mold plate. Material of guide groove is 40Cr, heat treatment hardness is 48~52HRC, which plays a role of lubrication. T-shaped block and guide groove are made of same material, which can protect sliding position of inclined thrust assembly from being worn. When ejector rod of injection molding machine is ejected, oblique pusher component pushes out and moves horizontally in T-shaped groove under action of guide groove. Combined movement of two constitutes an oblique demolding movement. When movement reaches end point, slider separates from plastic part. In order to facilitate maintenance of oblique pusher assembly, through holes are provided in bottom plate and push plate. When repairing, only need to loosen screws from through holes of movable mold base plate and take out slider.

06 Movable mold insert

Mould structure design 
Figure 9 Core insert
There are four apex cylinders on plastic part. These cylinders are very important to performance of plastic part. Core of these cylinders may be worn or broken during injection production, molded plastic part is prone to flash. In order to facilitate maintenance of mold, core of shaped cylinder is made into an insert, as shown in Figure 9, material is GS2311 and it is carbonitrided.

07 Cooling system

Function of cooling system is to control mold temperature within a certain range, so that molten polymer material is uniformly cooled in mold cavity, so that molded plastic part is shaped, separated from mold under action of ejection mechanism. If mold temperature is too high, molded plastic part cannot be quickly cooled and shaped, plastic part is easily deformed under action of ejection mechanism during demolding; if mold temperature is too low, molten polymer material has poor fluidity in cavity and cannot fill entire cavity, molded plastic part will be undershot. Arrangement of cooling system in mold should follow principle that temperature of each part can be cooled uniformly to ensure molded plastic parts are fully and evenly cooled. At the same time, cooling water channel cannot communicate with inclined push hole, push rod hole, etc. to avoid water leakage. Molding temperature of ABS material is around 230℃, and demolding temperature is generally around 60℃.

08 Mold parts materials

Mold base is made of LMK mold base. Material of main parts is: 45# steel is used for movable mold plate, fixed mold plate, stripper plate, push rod fixed plate, push plate, etc. Small insert material in fixed and movable molds is GS2311; Material of large insert is 718; material of slider is 738; material of oblique push component is FDAC, and it is treated by carbonitriding.
According to structural performance and characteristics of plastic part, mold adopts a three-plate mold structure, gating system is designed as a point gate, side core pulling mechanism is used in two directions of mold, and buckle position of molded plastic part is demoulded by an oblique diagonal push component. Production practice has proved that mold structure is reasonable, production efficiency is high, and molded plastic parts are demolded smoothly, which can provide a reference for molding of similar plastic parts.

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