With rapid development of plastics industry, plastic products are becoming increasingly popular. Due to their low cost and high plasticity, plastic products are widely used in daily life and are now widely applied in various industries, such as household appliances, automotive industry, and everyday hardware. Molding process of plastic products is very complex, and injection molding is one of the most important molding methods for plastic processing. Currently, nearly 60% of plastic products are made using injection molding.
In current rapid development of plastics industry, product quality requirements are becoming increasingly stringent, while production cycles are becoming increasingly shorter. Traditional injection molding processes rely on long-term experience accumulated by designers for mold design, followed by repeated mold trials and modifications under actual working conditions to gradually improve design scheme. The entire process is time-consuming and costly, and it is still difficult to obtain optimal design scheme, which greatly limits development of injection molding technology. Computer-aided engineering (CAE) technology can greatly shorten mold design time, reduce number of mold trials and modifications, thereby significantly shortening R&D cycle and improving production efficiency. Moldex3D mold flow analysis software can effectively simulate potential quality problems in plastic parts based on design scheme, assisting designers in optimizing design scheme and providing better injection molding process parameters, thus saving costs and improving design quality and process reliability.
This paper takes shell of a household socket as research object. Pro/E software was used to model and analyze socket shell, designing three injection schemes. Injection molding process was simulated using Moldex3D software, and schemes were compared to find optimal injection scheme. Subsequently, four factors—melt temperature, cooling time, holding pressure time, and mold temperature—were selected to design an orthogonal experiment. Warpage deformation was used as result to explore optimal process parameters, optimal process parameters were simulated and verified. Finally, it was concluded that optimizing process parameters can effectively control warpage deformation of socket shell and improve product quality.

Socket shell was modeled using Pro/E software. Socket shell consists of an upper cover and a lower cover, and it is planned to be produced in a one-to-two manner. Three-dimensional models of upper and lower covers are shown in Figure 1. Maximum dimensions of socket's top cover are 86mm * 258.8mm * 46mm, with an average wall thickness of 2.9mm; maximum dimensions of bottom shell are 86mm * 258.8mm * 32mm, with an average wall thickness of 2.6mm. Product has a uniform and moderate thickness. However, dimensions of top and bottom covers differ, requiring separate gate designs. Furthermore, to meet assembly requirements of top and bottom covers, product deformation must be minimized.

 

Image 1: 3D model of socket
Mesh is foundation of mold flow analysis, and accuracy of mesh generation directly affects realism of mold flow analysis results. Model is imported into Moldex3D software in STP format. Before meshing, model needs to be repaired. After repair, mesh size is set to 2, and boundary layer mesh (BLM) method is used to generate mesh, as shown in Figure 2(a). Typically, BLM provides at least 5 element layers in thickness direction for the entire plastic part to ensure accuracy of analysis results. By examining model details, we can see that number of solid mesh elements is 634,259, including 617,069 plastic parts, 17,190 cold runner elements, and 139,356 surface mesh elements. After meshing, poor meshes were repaired to ensure that product mesh quality met analysis requirements. Repaired mesh is shown in Figure 2(b).

 

Figure 2 Mesh Generation
Choice of materials determines service life and performance of product. For household appliances, electrical insulation is the most important. Sockets contain metal springs, contact switches, and fuses. During use, safety hazards such as overload and poor contact should be avoided. In this paper, acrylonitrile-butadiene-styrene plastic (ABS, XT-18T) was selected as plastic material. ABS has excellent heat resistance, flame retardancy, and impact resistance. It is not easily deformed or discolored, and also has excellent electrical insulation properties, making it one of the best materials for producing sockets. Figure 3(a) shows viscosity-shear rate curves of ABS at different temperatures. As temperature increases, viscosity of ABS decreases, a trend more pronounced at low shear rates. Figure 3(b) shows specific volume-temperature (PVT) curves of ABS under different pressures. As pressure increases, specific volume of ABS decreases, while temperature increases lead to an increase in specific volume.

 

Figure 3 Material property curves
Gating system design is a crucial aspect of injection mold design, directly impacting injection molding and product quality. To improve production efficiency, reduce production costs, shorten molding cycle, and minimize post-processing, this paper adopts a two-cavity molding method (one cavity for top cover and one for bottom cover). Based on structure of plastic part, three initial gating schemes were designed. Gate locations were chosen in the side areas of top cover and bottom shell, avoiding main appearance surfaces, using both side gates and submersible gates. Side gates have a simple cross-sectional shape, are easy to process, and facilitate design of venting holes to easily expel air from cavity. However, stress concentration is prone to occur at gate, and gate is difficult to separate from part automatically, sometimes requiring manual trimming, which may affect appearance of plastic part. Submarine gates have a concealed injection position, small gate marks, can automatically discharge material, meeting automation needs. They can also effectively reduce stress concentration and improve mechanical properties of part. However, due to concealed injection port, venting design is more difficult, there is a risk of air bubbles and burns.
Injecting material at the middle of side allows for a relatively symmetrical melt flow path, resulting in a more uniform filling effect and pressure distribution, which helps reduce stress concentration and deformation. However, injection from middle can easily lead to warping concentrated at both ends. Therefore, design of comparative scheme changes gate position to 1/3 of plastic part. By adjusting gate position, filling process is altered to improve product's deformation. Submarine gates have a small cross-section, and a single gate may lead to excessively high injection pressure, large residual stress, and uneven filling. Therefore, two gates are set in the top cover and bottom shell respectively. Due to differences in size and wall thickness between two parts, separate gate sizes are required. Optimal injection method was determined through comparison using Moldex3D mold flow software.
Three injection schemes are shown in Figure 4: Scheme 1 uses a side-gated gate, located at one-third of product. Bottom shell gate size is 1.5mm * 3mm, and top shell gate size is 2mm * 4mm, as shown in Figure 4(a); Figure 4(b) shows Scheme 2, with same gate as Scheme 1, but injection position is in the middle of plastic part; Scheme 3 uses a submarine gate, with a small-end diameter of 1.5mm for bottom shell gate and 2mm for top shell gate.

 

Figure 4 Gating System Design
During injection molding, cooling time accounts for approximately 2/3 of the entire production cycle. Inadequate cooling can lead to warping or surface defects (shrinkage marks, dents, etc.) in product, also affect dimensional stability, appearance quality of product. Cooling water system was constructed using Moldex3D software. Cooling pipes were arranged in a straight line, with 10 pipes on each of moving and stationary mold sides. Pipe diameter was 8mm, pipe spacing was 24mm. Cooling system is shown in Figure 5. This design was used in all three schemes.

 

Figure 5 Cooling System Design

Initial process parameters were set as follows: filling time 2.5s, holding time 10s, holding pressure set to 50% (referencing maximum injection pressure), melt temperature 225℃, mold temperature 60℃, cooling time 15s. Analysis sequence was set to instantaneous analysis: cooling + filling + holding pressure + cooling + warpage. Mold flow analysis results are shown below.
Injection filling stage is a critical stage in injection molding process. Moldex3D can record melt flow wavefront position at different times during filling process. Flow wavefront results can be used to check for short-shot problems, predict possible locations of seam lines and encapsulation. Simulation results show that filling of all three injection schemes is relatively uniform, with no short-shot phenomenon (no unfilled areas). Figure 6 compares maximum filling time required for three schemes: Scheme 1 requires 2.646s to complete filling; Scheme 2 requires 2.563s; and Scheme 3 requires 2.484s. Scheme 3 requires the shortest filling time, reducing it by 6.1% and 3.1% compared to the first two schemes, respectively.

 

Figure 6 Comparison of Filling Time
Flow shear stress is one of sources of internal stress in molded plastic parts. Excessive shear stress may lead to residual stress problems, resulting in unstable dimensions and uneven wall thickness of product. Shear stress should be less than maximum value allowed by material database. Maximum shear stress of three schemes is shown in Figure 7, which are 1.547, 1.500, and 1.350 MPa, respectively. Difference is small, and all three schemes are within allowable stress range, with Scheme 3 performing the best.

 

Figure 7 Comparison of Shear Stress
Entrapment refers to phenomenon where gas in cavity cannot be discharged from injection mold in a timely manner during filling process, leading to injection molding defects such as material shortage and scorching. The best solution to entrapment is to improve mold's venting structure. Figure 9 shows simulation results of entrapment location in plastic parts. As can be seen from figure, all three schemes exhibit varying degrees of air trapping, requiring additional venting. Air trapping locations in all three schemes are mostly concentrated on inner wall of top cover, while bottom shell shows less entrapment, mostly at edges or corners of plastic part, having a smaller impact on product. Scheme 3 has a higher probability of air trapping than the other two schemes. This is because Scheme 3 uses multi-gate injection molding, where multiple molten material flows converge in mold cavity during filling process, forming weld lines. Gas is easily trapped at weld lines, increasing likelihood of air trapping.

 

Figure 9 Comparison of Encapsulation Location
Warpage is one of the most common defects in plastic parts, occurring in any injection molded product, and is also one of the most difficult defects to resolve. Severe warpage can affect product assembly, even its performance and reliability.
Figure 10 shows a comparison of warpage deformation for three schemes. Comparison reveals that all three schemes exhibit larger warpage deformation in upper cover, mainly distributed at both ends; lower shell shows slightly smaller warpage deformation. Maximum warpage displacement for Scheme 1 is 1.518 mm, for Scheme 2 it is 1.525 mm, and for Scheme 3 it is 1.437 mm. Based on warpage results, Scheme 3 performs best.

 

Figure 10 Warpage Deformation
In production of high-precision products, flatness of specific planes is required. For two assemblies of socket shell, flatness of assembly surfaces needs to be evaluated to avoid large deformations that could prevent a tight fit. Flatness of assembly surfaces was measured in warpage deformation results. Figure 11 shows a comparison of flatness of three schemes. Figure shows that flatness ranges for three schemes are -0.463~0.388; -0.463~0.380; and -0.434~0.352, respectively. Scheme 3 performs best.

 

Figure 11 Flatness
Table 1 summarizes above results. It can be seen that Scheme 3 performs better in terms of filling time, shear stress, gate pressure, warpage. Scheme 2 shows better encapsulation performance. Based on comprehensive comparison, Scheme 3 is selected as final injection scheme.

 

Table 1 Comparison of Scheme Results
Maximum warpage of Scheme 3 is 1.437mm, which is relatively large and has a significant impact on assembly of plastic part. Many factors affect deformation, including holding time, holding pressure, and filling time. Optimizing warpage by adjusting process parameters is a quick and convenient method.
To find key factors affecting warpage and explore optimal process parameters, this paper uses orthogonal experimental design. Four factors were selected: melt temperature, mold temperature, holding time, and holding pressure. Four levels were selected within parameter range recommended by Modex3d software. Table 2 shows orthogonal factor level table (where holding pressure is based on maximum injection pressure).

 

Table 2 Orthogonal Factor Level Table
Using the total warpage deformation as measurement index, an L16 (45) orthogonal experimental table (Table 3) was designed. Simulation experiments were conducted based on orthogonal experimental table to obtain final warpage results, as shown in Table 3 below.

 

Table 3 Orthogonal Experiment Results
Simulated data was processed and summarized to obtain Table 4, range analysis table. Range value reflects magnitude of influence of each factor on experimental results. The larger range value, the greater influence of factor on experimental results; conversely, the smaller range value, the smaller influence. Indicator for this experiment is warpage deformation value; the smaller indicator, the better. From Table 4, it can be seen that warpage deformation gradually decreases with increasing holding time; with increasing holding pressure, warpage value first decreases, then increases; with increasing melt temperature and mold temperature, warpage deformation continuously increases. Influence of four process parameters on warpage deformation, ranked from largest to smallest, is: holding time > melt temperature > mold temperature > holding pressure.

 

Note: R - Range; K - Sum of all levels; k - Average of all levels.
Table 4 Range Analysis
To improve accuracy of experiment, an analysis of variance (ANOVA) was performed, and results are shown in Table 5. Melt temperature and holding time were significant influencing factors on warpage value, while mold temperature and holding time had no significant effect on warpage deformation. From the F-values in the table, it can be seen that order of importance of factors affecting warpage is C>A>B>D, i.e., holding time>melt temperature>mold temperature>holding pressure, consistent with range analysis results. Combining range and ANOVA results, optimal process parameter combination is A1B1C4D2.

 

Table 5 ANOVA
Optimal process parameters obtained through orthogonal experiments are: melt temperature 200℃, mold temperature 45℃, holding time 25s, and holding pressure 55%. Using this process parameter combination, a Modex3d simulation was performed. Figure 12(a) shows warpage value under optimal process parameters. Maximum warpage of socket casing was 0.863 mm, an improvement of approximately 40% compared to 1.437 mm in initial analysis (Scheme 3 above); flatness range decreased from -0.434 to 0.352 to -0.170 to 0.101, demonstrating a significant optimization effect [Figure 12(b)]. Warpage results show that optimized process parameters significantly improve warpage deformation of parts compared to initial process parameters, effectively reducing warpage deformation and improving product quality.

 

Figure 12 Warpage deformation and flatness after optimization

(1) This paper takes upper and lower covers of a socket as research object, uses Pro/E 3D modeling software to model socket and analyze model, designs three different glue injection schemes, and compares filling, pressure holding, and warpage aspects of three schemes using Moldex3D simulation software. Finally, third scheme is determined to be the best glue injection scheme. To optimize warpage deformation of Scheme 3, orthogonal experiments were conducted using four factors: melt temperature, mold temperature, holding time, and holding pressure. Range and variance analyses were performed on experimental results. Results showed that factors influencing warpage deformation of socket shell, from largest to smallest, were: holding time > melt temperature > mold temperature > holding pressure. Optimal combination of process parameters was determined to be: melt temperature 200℃, holding time 25s, holding pressure 55%, and mold temperature 45℃.
(3) Mold flow simulation was performed on optimal process parameters. Maximum warpage deformation after optimization was 0.863mm, a reduction of 0.574mm compared to before optimization, representing an optimization of nearly 40%, effectively improving molding quality of socket shell.