End cover of electric meter is shown in Figure 1. It has a thin-walled structure as a whole, with dense pores on one side to facilitate heat dissipation of device below and serve as a decoration. Appearance size of plastic part is 110 mm * 115 mm * 55 mm, average wall thickness is 2.2 mm, and wall thickness at pores is 3.2 mm. End cap material is PC + 10% glass fiber, with a density of about 1.19 g/cm3 and a shrinkage rate of 0.376%. Material processing parameters are shown in Table 1. This material is a reinforced plastic with a heat deformation temperature of over 120℃. It has high mechanical properties and excellent UV resistance, and is suitable for outer surface of outdoor devices. According to three-dimensional model, volume of plastic part was measured to be approximately 43.4 cm3, and weight was approximately 51.7 g.

 

Figure 1 End cover structure

Table 1 Material molding process parameters
According to requirements of plastic parts, deformation of plastic part should be less than length of plastic part in all directions x material shrinkage rate, and there should be no obvious welding marks, shrinkage marks and other appearance defects on the surface. Since end cover and main body of electric meter are fixed by buckles, a certain degree of matching accuracy is required, there are high requirements for strength and warpage deformation of end cover. In order to ensure smooth demoulding of plastic parts, end cover is designed with a draft angle of 2°.

An end cover mold flow simulation model was established and solid mesh was divided, as shown in Figure 2, with a total of 1.07 million tetrahedral elements. When there are defects in mesh, it will have a negative impact on analysis results and even make calculation impossible, so mesh model needs to be diagnosed first. According to the statistical results, it can be seen that the overall meshing quality is good, with a maximum aspect ratio of 89.2, a minimum aspect ratio of 1.08, and an average aspect ratio of 8.61. Mesh has no incorrectly matched units, no relevant units, and no overlapping units, and mold flow analysis can be performed.

 

Figure 2 Grid model

A reasonable and reliable pouring system can ensure that plastic melt enters mold cavity in an optimal flow state, improving quality and molding speed of plastic parts. Design of gate location, form and quantity has a great influence on the overall molding quality of plastic parts. Unreasonable gate settings may lead to insufficient filling or spraying, and may also produce defects such as shrinkage holes, warping, and surface weld marks that affect appearance quality. Therefore, gate form, quantity, and location need to be reasonably designed.
Moldflow was used to analyze gate position of plastic part. Results are shown in Figure 3. Recommended optimal gate position is located at position 1 of mesh area in the middle of end cover. Considering influence of flow resistance of pores in plastic part, if gate is set here, injection pressure will increase, and insufficient filling will easily occur in local areas, which is not conducive to complete filling of plastic part to be formed.

 

Figure 3 Gate position simulation results
According to actual situation, gate is arranged at position 2 and a single-point feeding gate is selected to ensure that plastic part to be formed is completely filled and there are no obvious defects in appearance area; setting gate on the side makes it easier to trim, reduces later trimming marks on plastic parts, and ensures appearance quality.

Thickness of end cover of electric meter is small. If mold is cooled unevenly, defects such as warping deformation and uneven stress of plastic parts may easily occur, which will affect molding quality of plastic parts. At the same time, temperature changes in injection mold affect production efficiency of plastic parts. In order to reduce temperature difference between various parts of plastic part, cooling system adopts traditional water channel + water barrier mixed cooling method. Cooling water channel is divided into upper and lower parts, arranged around plastic part. Due to concave structure at the bottom of end cover, a water barrier is used for forced heat dissipation. Diameter of cooling water pipe is φ10 mm. Cooling system layout is shown in Figure 4.

 

Figure 4 Cooling duct layout

Based on above-mentioned pouring system and cooling scheme, end cap molding process is simulated and molding process parameters of plastic parts are studied, including filling time, pressure, weld marks, shrinkage marks, warpage deformation, etc. During analysis process, melt temperature was set to 300 ℃, mold temperature was 95 ℃, and holding time was 10 s. End cap mold flow analysis results are shown in Figure 5.

 

Figure 5 End cover mold flow analysis results
As can be seen from Figure 5(a), time required for plastic melt to fill cavity is about 1.94 s. During filling process, cavity was not underfilled, but because gate was skewed to one side, filling balance could not be achieved. Time difference between melt reaching cavity was large, resulting in different cooling times and prone to warping deformation. Figure 5(b) shows that during speed-pressure switching, maximum filling pressure reaches 180 MPa, and pressure value is too high. When melt fills the other side, it needs to flow through pore area. The larger flow resistance produces a pressure drop on melt, resulting in an increase in required pressure. During filling process, when two material flows come together, weld marks are easily produced. From end cap weld mark distribution results shown in Figure 5(c), it can be seen that weld marks are mainly concentrated in pore area, affecting surface quality. As shown in Figure 5(d), melt flow front temperature is between 290 and 332℃. High temperature area and low temperature area are both located on the side away from gate. Temperature difference exceeds generally allowed value of 20℃, which can easily cause melt retention. Mold temperature in Figure 5(e) shows that temperature difference on same side of plastic part is close to 15℃, which is a large temperature difference. Mold temperature in inner corners around end cover reaches 55℃. Cooling capacity of cooling system needs to be strengthened. Figure 5(f) shows that all effects of end cover cause an overall deformation of about 1.065 mm, which is located on the side of end cover, showing a tendency to shrink inward, with a large amount of deformation, and it needs to be optimized for warping in later stage.
According to analysis results, it can be seen that there are certain problems in initial plan that affect molding quality of plastic parts, mainly focusing on: ① Melt flow is unbalanced and cannot reach end of cavity at the same time, so gating system needs to be redesigned; ② Side of end cover shrinks and deforms, structure needs to be adjusted and ribs added to improve rigidity of end cover; ③ Mold temperature difference is large. In order to improve cooling effect, cooling system is redesigned, cooling pipes are refined and improved, and lateral cooling water channels are added.

In original plan, gate was biased to one side, resulting in unbalanced filling during molding process of plastic part. Melt could not flow to the end of cavity at the same time, resulting in different pressures and cooling rates on both sides of end cap.
In order to make cavity filling as balanced as possible, it is still necessary to set gate in the middle of end cover. Combined with gate position simulation results, gate position in optimization plan is shown in Figure 6. It is designed as an overlapped fan-shaped gate. Diameter of main channel A is φ12 mm, and size of gate B is 12 mm * 1.2 mm.

 

Figure 6 End cap gate layout
After completing meshing and connectivity diagnosis, end cap filling analysis was performed. Results are shown in Figure 7. Optimized mold filling time is about 2.4 s, pressure during filling speed/pressure switching is reduced from 180 MPa before optimization to 86.77 MPa. Optimization effect is obvious, and an injection molding machine with a smaller holding pressure can be selected; number of optimized weld marks is significantly reduced in heat dissipation hole area, which can ensure surface molding quality of plastic part; area of low-temperature areas at melt front is reduced, and surface temperatures are all above 300℃, which has better melt flow performance than before optimization.

 

Figure 7 Optimized filling simulation results

In original plan, cooling water channels were only distributed in part of end cover, resulting in uneven cooling inside and on the sides of end cover. Optimized waterway adds cooling waterways on both sides, increases number of water baffles to effectively cool down surroundings and inside end cover, as shown in Figure 8. By optimizing cooling water path and reducing temperature difference between various parts of end cover, temperature difference on same side of plastic part is controlled within 5℃, which is a significant improvement over original plan. Maximum temperature of loop coolant is 25.75 ℃, and minimum temperature is 25 ℃. Temperature rise does not exceed 3 ℃, which meets usage requirements, as shown in Figure 9.

 

Figure 8 Optimized cooling water path

 

Figure 9 Optimized cooling results of plastic parts

From initial warpage analysis, it can be seen that deformation of end cap is inward shrinkage on both sides. Reason is that end cap is a thin-walled plastic part. When local strength is insufficient, shrinkage internal stress will cause warping deformation on the sides of end cap. In order to prevent large deformation of end cover and increase strength of end cover, reinforcement ribs are added inside end cover, as shown in Figure 10.

 

Figure 10 Optimized end cover model
After warpage optimization, deformation analysis caused by all effects is shown in Figure 11. Maximum deformation amount was reduced from 1.065 mm in original plan to 0.645 mm. It can be seen that optimizing warpage deformation by modifying structure of plastic parts has a certain effect.

 

Figure 11 Overall deformation due to all effects

A reasonable parting surface can simplify mold structure, enable smooth demoulding of plastic parts, and ensure good molding quality of plastic parts. Based on design principles and structural characteristics of end cover, parting surface is selected on outer surface of end cover to ensure surface quality of plastic parts. Since plastic parts have shrinkage and tightness, plastic parts should be left on the side of movable mold when opening mold.

Molded parts include cavity plate inserts and cores. On premise of ensuring quality of plastic parts, design of molded parts should facilitate later processing, assembly, use and maintenance. Due to small size of end cover, cavity plate adopts a combined insert structure, which can save materials, control costs, facilitate processing and later replacement, and reduce impact of thermal deformation on later molding accuracy. Core also adopts a combined structure, cavity plate insert and core are shown in Figure 12.

 

Figure 12 Cavity plate and core structural design

There is a snap-on blind hole groove on the side of end cover, and its parting is inconsistent with direction of mold opening. It will hinder push-out of plastic part when opening mold, making it impossible to demould directly. Therefore, it is necessary to add lateral parting and core-pulling mechanisms during design process. Since grooves on both sides of end cover are shallow and core-pulling distance is short, an inclined guide pillar core-pulling mechanism can be used. As shown in Figure 13, inclined guide pillar is fixed on fixed mold, and side core slider is installed on movable mold. When movable mold and fixed mold are separated, relative displacement occurs between side core slider and inclined guide pillar. Side core slider moves up and down under action of inclined guide pillar, and is separated from plastic part, which facilitates push-out mechanism to push plastic part out of core.

 

Figure 13 Side core pulling structure

According to structural characteristics of plastic part, the overall structure of injection mold of end cover is shown in Figure 14, using design of non-standard parts and selection of standard parts. The overall dimensions of mold are 560 mm * 400 mm * 450 mm.

 

Figure 14 Mold structure
1. Moving mold base plate 2. Push plate 3. Push rod fixed plate 4. Core fixed plate 5. Core 6. Side core slider 7. Plastic part 8. Cavity insert 9. Cavity insert fixed Plate 10. Fixed mold plate 11. Fixed mold base plate 12. Positioning ring 13. Inclined push rod 14. Foot pad

During production process of end cap, material is Kingfa JH720-R0G10, which contains 10% glass fiber inside, which can increase surface strength of plastic part and has good electrical insulation. Surface of plastic part is treated with texture. Set injection molding machine process parameters as shown in Table 2. After production verification, end cover mold design is reasonable, can open and close mold stably and reliably. Surface quality of molded plastic parts is excellent, and production dimensions are consistent with design dimensions. As shown in Figure 15, there are no defects such as floating fibers, shrinkage marks, weld marks, deformation, etc. on the surface, which meets appearance requirements.

Table 2 End cap forming process parameters

 

Figure 15 Actual meter end cover