With rapid economic development, cosmetics and skin care products have gradually become daily consumer goods in life. In competitive cosmetics consumer market, novel cosmetic packaging is one of main reasons for attracting consumers to buy, and it is also an important form of cosmetics value. There are a lot of advertising and its packaging is also convincing sales. The first thing that attracts consumers' attention when buying cosmetics is outer packaging, so packaging design is very important. Cream bottles are often used in packaging of skin care products. There are two types of plastic and glass. Structure of cream bottle is shown in Figure 1.

Surface requirements of cream bottle: outer surface is beautiful, inner and outer wall surfaces are smooth after forming, surface roughness value is Ra0.1~0.2μm, no visible black spots, gate marks, sink marks, surface flow marks, weld marks, deformation, cracks and insufficient filling are allowed. Size requirements: outer diameter is (φ71.5±0.25) mm, inner hole is (φ60±0.2) mm. Functional requirements: Plastic parts and lid rotate smoothly, which can protect contents of container from contamination after assembly. Cream bottle material is PETG. Its transparency allows consumers to visualize contents of container. It has excellent impact resistance and toughness to ensure that it is not easily damaged during use. It also has strong acid and alkali resistance, but it has poor fluidity and high viscosity. Mold requires that surface of molded plastic parts is bright, runner is smooth, and exhaust design is reasonable. Plastic part is a transparent body, with a mass of 80g, a capacity of 115mL, a minimum wall thickness of 1.55mm, and a maximum wall thickness of 5.9mm. All connections are rounded to avoid stress concentration and cracks.

 

Plastic parts are mass-produced and service life of mold is 1 million times. In order to reduce production cost, mold is designed with a 4-cavity structure and a balanced layout. Flow balance analysis is shown in Figure 2.

 

1. Oblique guide column 2. Pressure block 3. Fixed mold cavity plate 4. Hot runner system 5. Pull rod 6. Return spring 7. Slider 8. Fixed mold insert 9. Limit nail 10. Moving model core 11. Moving Mould waterway inserts 12. Fixed mould seat plate 13. Gate sleeve 14. Positioning ring 15. Runner plate 16. Fixed mould fixed plate 17. Fixed mould plate 18. Moving mould plate 19. Moving mould fixed plate 20. Moving mould seat plate
It can be seen from Figure 1 that mouth of plastic part has threads, and design needs to be divided into slider. Shrinkage rate of PETG is 1.005 and fluidity is poor. In order to facilitate melt molding, needle valve type HUSKY hot runner is used, without manual trimming, fixed mold is designed as an insert structure, and insert gap is used to strengthen exhaust. On appearance surface of fixed mold molded plastic part, draft angle of mold is designed to be 1.5°, and draft angle of movable mold is designed to be 1° under premise of not sticking to mold to damage plastic part. Plastic parts have a large packing force, if push plate is used to push out, it may cause mouth flanging or internal deformation of plastic parts. In summary, mold is pushed out in two stages. In the first stage, slider and push rod are pushed out at a certain distance; in second stage, plastic part is pushed out by push rod after being separated from core. Mold structure is shown in Figure 3.

 

Threaded slider is divided in movable mold, and plastic part is formed in fixed mold. At the same time, insert is split at the bottom of fixed mold to enhance exhaust and cooling, as shown in Figure 4.

 

 

Design core components are slider, push block, movable model core, and waterway insert. Slider and inclined surface of push rod are coordinated and positioned. Spiral waterway insert can fully cool plastic part, as shown in Figure 5. Cavity part includes: fixed mold cavity plate, fixed mold insert and hot runner system. Design of water path on molding insert can enhance heat exchange between hot runner and bottom of the plastic part, and prevent crystallization at gate from whitening, as shown in Figure 6.

 

 

 

During injection molding, mold temperature directly affects melt filling and molding quality of plastic parts. A good cooling system will shorten molding cycle and improve production efficiency. Due to thicker wall of molded plastic part, a hot runner system is used to adjust mold temperature, cavity insert water path and fixed mold water path are designed in parallel, as shown in Figure 7. According to mold arrangement structure, 2 cavities are provided with 1 set of water channels, there are 4 sets of water channels for fixed mold insert and fixed mold. As shown in Figure 8, temperature in cavity can be fine-tuned separately. Movable mold cavity uses 4 separate sets of water channels. Movable mold core water path is shown in Figure 9. Cooling water enters from top surface and flows out of threaded mouth to enhance cooling effect.

 

1. Fixed mold base plate 2. Runner plate 3. Fixed mold fixed plate 4. Fixed mold plate 5. Movable mold plate 6. Dynamic mold fixed plate 7. Movable mold base plate 8. Push block 9. Slider 10. Inclined guide column 11.Tie rod
Launch mechanism is shown in Figure 10. Movable mold fixing plate 6 and movable mold base plate 7 are fixed by screws, movable mold plate 5 is connected to movable mold fixing plate 6 through a limit nail and a spring, movable mold base plate 7 and fixed mold base plate 1 are fixed on injection molding machine. When mold is opened, ejector rod of injection molding machine drives movable mold seat plate 7 to move backwards, friction between movable mold plate 5 and movable mold fixed plate 6 is greater than friction between fixed mold plate 4 and movable mold plate due to return spring and thread tightening force. Fixed mold plate 4 and movable mold plate 5 are opened at PL1 first, and plastic part is separated from fixed mold first.
Movable mold plate continues to move backwards. When distance between fixed mold plate 4 and movable mold plate 5 reaches distance set by pull rod, manipulator moves down and sucks plastic part, movable mold seat plate 7 continues to move backward under action of ejector rod of injection molding machine. Return spring is forcedly compressed, movable mold plate 5 and movable mold fixed plate 6 begin to separate at PL2, plastic part is separated from movable mold core. Because inclined guide post is installed on movable mold fixed plate 6, sliding block is driven to move outward, and threaded sliding block is installed on sliding block. Threaded slider moves with slider, but when movable mold plate 5 and movable mold fixed plate 6 just start to separate, slider, push plate and push rod push out plastic part together, so push out area is large, force is uniform, and mouth of plastic part will not be deformed. After mold is opened for a certain distance, opening distance of threaded slider becomes larger, plastic part has been loosened, and pushing force becomes smaller. After plastic part is separated from movable mold core and threaded slider, manipulator moves upward and grabs plastic part onto conveyor belt. At this time, mold clamping starts, movable mold fixed plate 6 and movable mold seat plate 7 move forward, and movable mold plate 5 remains stationary under action of return spring. Movable mold plate 5 and movable mold fixed plate 6 are first closed. After movable mold plate 5 and movable mold fixed plate 6 are closed, movable mold plate 5, movable mold fixed plate 6, and movable mold seat plate 7 move forward together. Fixed mold plate 4 and movable mold plate 5 begin to mold until mold clamping is completed. High-pressure mold clamping completes an injection cycle.

 

During molding process, air in mold must be discharged. PETG material has poor fluidity and high shearing force in screw. Reasonable exhaust design can avoid flow marks and weld marks of molded plastic parts caused by trapped air, reduce injection pressure, shorten injection time, pressure holding time and molding cycle, eliminate internal stress of plastic part, prevents deformation and warping. Exhaust grooves are set on parting surface of cavity, fixed mold insert and movable mold core, as shown in Figure 11 and Figure 12.

According to size of mold and weight of plastic parts, FNX280Ⅲ injection molding machine is selected, and clamping force is 2.8*103kN. According to material characteristics of PETG, pre-dry it at 65°C for 3~4 hours, and set melting temperature at 220~240°C; in order to ensure sufficient cooling of mold core and plastic parts, cooling water at 20°C is used to avoid shrinkage on the surface of plastic parts. Because outer surface of plastic part is a high-gloss mirror, cavity temperature is set to 50°C to ensure that melt flows in cavity.

Set clamping force to 70%, install and debug mold, use high-pressure and low-speed injection, inject 10%, 30%, 50%, 70% of material in the early stage, check whether melt filling balance of each section meets requirements.

When proportion of injected material is 90%, molded plastic part is almost full. Check exhaust effect of cavity, increase holding pressure appropriately, observe surface shrinkage of plastic part and change of parting surface of plastic part, until molding is released. After molding, surface of plastic part is full and does not shrink. At the same time, observe whether there is flash on parting surface of plastic part.

Proper holding pressure and stable cooling time can ensure good molding quality of plastic parts. Equipment is required to fully automatic and stable molding, continuous injection several times, observe whether mold opening and closing, guide column in and out, slider movement, pull rod opening and closing, and reset are smooth.

The first mold trial found that there are weld marks on the surface of plastic part, because bottom of plastic part has a large projection area and inner wall is thick, which affects balanced flow of melt. Solution is to set an exhaust groove, reduce injection speed, increase diameter of hot runner and injection hole of the injection molding machine nozzle, and then weld mark disappears after mold is tried again.
When designing cream bottle mold, according to structure and material characteristics of cream bottle, mold classification, exhaust system, ejection mechanism, cooling system were designed and planned. Through production practice verification, mold action is stable and reliable, solved phenomenon of welding marks and silver lines on the surface of PETG plastic parts, achieved expected production effect, and provided a reference for similar mold design.