Electric meter end cap, as shown in Figure 1, features a thin-walled structure with dense air holes on one side to facilitate heat dissipation from underlying components and also serve as a decorative feature. Plastic part has dimensions of 110 mm * 115 mm * 55 mm, with an average wall thickness of 2.2 mm and a wall thickness of 3.2 mm at air holes. End cap is made of PC + 10% glass fiber, with a density of approximately 1.19 g/cm³ and a shrinkage of 0.376%. Material processing parameters are shown in Table 1. This material is a reinforced plastic with a heat deflection temperature exceeding 120℃. It exhibits high mechanical properties and excellent UV resistance, making it suitable for outdoor device exteriors. Based on 3D model, part's volume was measured to be approximately 43.4 cm³ and its weight to be approximately 51.7 g.

 

Figure 1 End Cap Structure

Table 1 Material Molding Process Parameters
According to plastic part requirements, part deformation should be less than part's isotropic length multiplied by material shrinkage rate, surface should be free of visible defects such as weld marks and shrinkage marks. Because end cap is secured to main body via snap fasteners, a certain degree of fit precision is required, placing high demands on the end cap's strength and warpage. To ensure smooth demolding, end cap is designed with a 2° draft angle.

A mold flow simulation model for end cap was established and a solid mesh was generated, as shown in Figure 2. A total of 1.07 million tetrahedral elements were generated. Mesh defects can negatively impact analysis results and even render calculations impossible. Therefore, a mesh model diagnostic is necessary. Statistical results indicate that the overall mesh 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 contains no mismatched, correlated, or overlapping elements, making it suitable for mold flow analysis.

 

Figure 2 Mesh Model

A reasonable and reliable gating system ensures that plastic melt enters mold cavity in an optimal flow state, improving part quality and molding speed. Design of gate location, form, and number significantly impacts the overall molding quality of part. Improper gate placement can easily lead to underfilling or jetting, as well as defects such as shrinkage holes, warping, and surface weld marks that affect appearance quality. Therefore, it is important to rationally design gate form, number, and location.
Moldflow analyzed gate location of part. Results, shown in Figure 3, indicate that recommended optimal gate location is at position 1 in mesh area in the middle of end cap. Considering flow resistance of part's pores, placing gate here would increase injection pressure, potentially causing underfill in certain areas and hindering complete filling of molded part.

 

Figure 3: Gate Position Simulation Results
Based on actual situation, gate was placed at position 2, using a single-point feed gate. This ensures complete filling of molded part and eliminates noticeable defects in exterior. Placing gate on the side facilitates trimming, reduces post-production trimming work, and ensures a high-quality exterior.

Meter end cap is thin. Uneven mold cooling can easily cause defects such as warping and uneven stress in the part, impacting molding quality. Furthermore, temperature fluctuations in injection mold affect production efficiency. To minimize temperature differences between different parts of part, cooling system utilizes a hybrid cooling system using traditional water channels and baffles. Cooling channels are divided into upper and lower sections, surrounding part. Due to concave structure at the bottom of end cap, baffles are used for forced heat dissipation. Cooling water pipe has a diameter of 10 mm. Cooling system layout is shown in Figure 4.

 

Figure 4 Cooling Channel Layout

Based on aforementioned gating system and cooling scheme, end cap molding process was simulated to study molding process parameters of plastic part, including filling time, pressure, weld marks, shrinkage marks, and warpage. During analysis, melt temperature was set at 300℃, mold temperature at 95℃, and holding time at 10 seconds. Mold flow analysis results for end cap are shown in Figure 5.

 

Figure 5 Mold Flow Analysis Results for End Cap
Figure 5 (a) shows that it takes approximately 1.94 seconds for plastic melt to fill cavity. During filling process, cavity does not experience underfilling. However, due to unilateral placement of gate, balanced filling is not achieved. Time difference between melt reaching cavity is large, resulting in different cooling times and a tendency to cause warpage. Figure 5 (b) shows that during speed-pressure switching, maximum filling pressure reaches 180 MPa, which is relatively high. When melt fills the other side, it must flow through pore area. The greater flow resistance creates a pressure drop on melt, increasing required pressure. During filling process, when two streams converge, weld marks are likely to form. Weld mark distribution results for end cap shown in Figure 5(c) show that weld marks are primarily concentrated in pore area, affecting surface quality. Figure 5(d) shows that melt flow front temperature ranges from 290℃ to 332℃. Both high and low temperature zones are located on the side away from gate. Temperature difference exceeds generally allowable 20℃, which can easily cause melt retention. Mold temperature in Figure 5(e) shows a significant temperature difference of nearly 15℃ on same side of part. Mold temperature in inner corners around end cap reaches 55℃, requiring increased cooling capacity in cooling system. Figure 5(f) shows that the total deformation of end cap due to all effects is approximately 1.065 mm. This deformation is located on the side of end cap and tends to shrink inward. Deformation is significant, requiring subsequent warpage optimization. Analysis results indicate that initial design had certain issues that affected quality of part. These issues primarily focused on: ① Unbalanced melt flow prevented simultaneous flow to the ends of cavity, necessitating a redesign of gating system; ② Shrinkage and deformation along sides of end cap necessitated structural adjustments and addition of ribs to enhance rigidity of end cap; and ③ Significant temperature differences in mold necessitated a redesign of cooling system, refinement of cooling channels, and addition of lateral cooling channels to improve cooling.

Original design had gate offset to one side, resulting in unbalanced filling during part molding process. Melt could not simultaneously flow to the ends of cavity, leading to different pressures and cooling rates on both sides of end cap.
To ensure balanced cavity filling, gate remained in the center of end cap. Based on gate position simulation results, optimized gate position is shown in Figure 6: a overlapping fan gate with a sprue A diameter of φ12 mm and gate B dimensions of 12 mm * 1.2 mm.

 

Figure 6 End Cap Gate Arrangement
After completing meshing and connectivity diagnosis, an end cap filling analysis was performed. Results are shown in Figure 7. Optimized model has a filling time of approximately 2.4 s, and pressure at filling speed/pressure switch is reduced from 180 MPa before optimization to 86.77 MPa. Optimization effect is significant, allowing use of an injection molding machine with a lower holding pressure. Number of weld marks in heat dissipation hole area is significantly reduced after optimization, ensuring surface molding quality of plastic part. Low-temperature area at melt front is reduced, and surface temperature is consistently above 300℃, resulting in better melt flow properties than before optimization.

 

Figure 7 Optimized Filling Simulation Results

In original design, cooling water channels were only distributed in a portion of end cap, resulting in uneven cooling inside and on the sides of end cap. Optimized water channels were added on both sides, number of baffles was increased to effectively cool perimeter and interior of end cap, as shown in Figure 8. By optimizing cooling water circuit, temperature difference between different parts of end cap was reduced, and temperature difference on same side of plastic part was controlled within 5℃, a significant improvement compared to original solution. Maximum temperature of circuit coolant reached 25.75℃, and minimum temperature was 25℃, with a temperature rise of no more than 3℃, meeting requirements, as shown in Figure 9.

 

Figure 8 Optimized cooling water circuit

 

Figure 9 Optimized plastic part cooling results

Initial warpage analysis shows that end cap deforms inward on both sides. This is because end cap is a thin-walled plastic part. When local strength is insufficient, shrinkage stress will cause warping on the side of end cap. To prevent significant deformation and increase end cap's strength, reinforcing ribs were added internally, as shown in Figure 10.

 

Figure 10 Optimized end cap model
After warpage optimization, deformation analysis caused by all effects is shown in Figure 11. Maximum deformation is reduced from 1.065 mm in original solution to 0.645 mm. This shows that modifying plastic part structure can achieve some results in optimizing warpage.

 

Figure 11 Overall deformation caused by all effects

A reasonable parting surface can simplify mold structure, ensure smooth demolding of plastic part, and ensure good molding quality. Taking into account design principles and structural characteristics of end cap, parting surface is selected on outer surface of end cap to ensure surface quality. Because plastic part has a shrinking force, part is retained on movable mold side during mold opening.

Molded part consists of two components: cavity plate insert and core. While ensuring part quality, design of molded part should facilitate subsequent processing, assembly, use, and maintenance. Due to small size of end cap, cavity plate uses a modular insert structure to save material, control costs, facilitate processing and subsequent replacement, reducing impact of thermal deformation on subsequent molding accuracy. Core also adopts a modular structure. Cavity plate insert and core are shown in Figure 12.

 

Figure 12 Cavity Plate and Core Structure Design

Side of end cap contains a blind-hole recess with a snap-fit design. Its parting direction is inconsistent with mold opening direction, hindering part ejection during mold opening and preventing direct demolding. Therefore, a side parting and core pulling mechanism were required during design process. Because recesses on both sides of end cap are shallow and core pulling distance is short, an inclined guide pin core pulling mechanism can be used. As shown in Figure 13, inclined guide pins are fixed to fixed mold, and side core sliders are mounted on movable mold. When movable and fixed molds separate, side core sliders and inclined guide pins move relative to each other. Inclined guide pins move side core sliders up and down, separating them from part and facilitating ejection mechanism to eject part from core.

 

Figure 13 Side Core Pulling Structure

Based on structural characteristics of plastic part, the overall structure of injection mold for end cap is shown in Figure 14. It utilizes a combination of non-standard components and standard parts. Mold dimensions are 560 mm * 400 mm * 450 mm.

 

Figure 14 Mold Structure
1. Movable Mold Base Plate 2. Ejector Plate 3. Ejector Fixing Plate 4. Core Fixing Plate 5. Core 6. Side Core Sliders 7. Plastic Part 8. Cavity Insert 9. Cavity Insert Fixing Plate 10. Fixed Plate 11. Fixed Mold Base Plate 12. Locating Ring 13. Angled Ejector 14. Foot Pad

During production of end cap, Kingfa JH720-R0G10 was used. It contains 10% glass fiber, which increases surface strength of part and provides good electrical insulation. Surface of part was treated with a leather grain finish. Injection molding machine process parameters were set as shown in Table 2. Production verification confirmed that end cap mold design was rational, capable of stable and reliable mold opening and closing. Molded part had excellent surface quality and consistent dimensions with designed dimensions. As shown in Figure 15, surface was free of defects such as floating fibers, shrinkage marks, weld marks, and deformation, meeting appearance requirements.

Table 2 End Cap Molding Process Parameters

 

Figure 15 Actual Electric Meter End Cap