Abstract: Injection molding process of plastic drainage pump filter screen is simulated using Moldflow. Based on structural analysis, a reasonable gate location and cooling water path are determined. A three-factor, three-level orthogonal experiment was used to study effects of melt temperature (A), filling pressure (B), and mold opening time (C) on warpage deformation of plastic part. Optimal combination of process parameters was found to be A1B3C3, namely, melt temperature 220 ℃, filling pressure 80%, and mold opening time 6 s. Under this condition, maximum warpage deformation of plastic part was 0.6328 mm, which was reduced by 10.37% compared with previous value. Finally, based on analysis results of Moldflow, structure of injection mold was designed using UG software. According to structural characteristics of plastic part's snap-fit mechanism, cavity adopts an integral structure, and core is designed as a combined structure (main core + slider). Three sets of "slanted guide pillars + sliders" type side core-pulling mechanisms are used to solve demolding problem. To achieve sequential demolding, a fixed-distance parting mechanism with an internal "small tie rod + tie rod sleeve" structure was designed to increase mold opening resistance between fixed and moving mold plates using nylon plugs. Considering shape of plastic part and mold structure and cost, a round push rod demolding mechanism arranged along contour of plastic part's sidewall was designed. To avoid formation of defects such as air marks and air pockets, an venting groove structure was designed at cavity. This mold structure design is relatively reasonable and can provide useful reference and guidance for mold design of other plastic parts with similar structures.
Injection molding occupies an important position in modern mechanical industry due to its high production efficiency, good product dimensional consistency, automated operation, ability to mold complex plastic parts. However, many factors affect injection molding process, such as mold structure, material properties, and process parameters. Only by comprehensively considering influence of these factors can defects such as warpage, weld lines, and shrinkage marks in plastic parts be avoided. Traditional mold design requires repeated trial molding and modification processes to create a mold that meets requirements, leading to increased design and manufacturing costs and longer cycles. This fails to meet manufacturing industry's demands for high-quality injection molds and short design cycles. With introduction and application of Moldflow software, injection molding process can be simulated and analyzed before mold processing to predict potential defects, thereby optimizing molding process parameters. This allows for mold improvement and refinement during design phase, increasing success rate of first-time trials, shortening development cycle, and reducing production costs.
This paper takes a fully automatic drainage pump filter screen as research object and uses Moldflow software to simulate and analyze its injection molding process. It determines a reasonable gate location and studies influence of melt temperature, filling pressure, and mold opening time on maximum warpage deformation of plastic part. A three-factor, three-level orthogonal experiment combined with variance analysis is used to optimize injection molding process parameters to formulate optimal process scheme. Based on this, mold structure design is completed using UG 3D modeling software.

Figure 1 shows 3D model of drainage pump filter screen, with dimensions of 134 mm * 80 mm * 53 mm, classifying it as a small to medium-sized plastic part. Sidewalls consist of arc and elliptical surfaces, and sidewalls have three protruding structures formed by buckles higher than main body, making molding difficult. Different lateral core-pulling mechanisms in different directions are required to achieve molding and demolding of plastic part. Wall thickness of plastic part is relatively uniform, with an average thickness of 3 mm. Surface of this plastic part must be smooth, without obvious warping or deformation, and it must also possess certain strength, wear resistance, good toughness, and corrosion resistance, with a precision of MT5 grade or higher.

 

Fig. 1 3D diagram of structure of filter
(a) Interior (b) Exterior

Drainage pump filter screen is mainly used to filter impurities in water during drainage and must be usable in both fresh and seawater. Acrylonitrile-butadiene-styrene (ABS) plastic possesses advantages such as high strength, good wear resistance, acid, alkali, and salt resistance, water resistance, low-temperature resistance (can be used at -40 ℃), and good surface gloss of molded parts. It is widely used in automobiles, electrical instruments, mechanical parts, and can meet performance requirements of drainage pump filter screens. Therefore, ABS plastic of grade TFX-210 was selected as molding material. Recommended process parameters for this material are shown in Table 1.

Table 1 Recommended process parameters of ABS materials

3D solid model of drainage pump filter screen constructed in UG software was imported into Moldflow software, model was meshed and optimized. Because wall thickness of drainage pump filter screen is relatively uniform, a double-layer mesh type was selected for meshing plastic part. Meshed model is shown in Figure 2. Grid statistics column shows that model has 1 connected region, 51,156 triangular grid cells, a surface area of 444.117 cm2, a maximum and minimum aspect ratio of 8.17 and 1.16 respectively, an average aspect ratio of 1.60, matching and mutual percentages of 94.4% and 92.7% respectively. Grid has no defects such as overlapping, intersecting cells, free edges, indicating that it is suitable to select a two-layer grid for analysis.

 

Fig. 2 Mesh model of drainage pump filter

Gate is melt channel connecting runner and cavity in injection mold gating system. Whether its position is designed properly is directly related to molding quality of plastic part. A suitable gate position can reduce deformation and avoid generation of weld lines. Figure 3 shows optimal gate position of drainage pump filter obtained by Moldflow analysis. As shown in Figure 3, blue area has a high matching degree and is suitable as a gate location, while red area has a poor matching degree and greater flow resistance of melt in cavity. This area should be avoided when selecting a gate point.

 

Figure 3: Analysis results of gate location
Generally, there should be no obvious gate marks on outer surface of plastic part to avoid affecting product's appearance. Therefore, gate is usually placed inside plastic part. However, for this plastic part, this will inevitably increase difficulty of demolding and mold structure design. Considering appearance requirements and structural form of plastic part, a single-point gate injection method is adopted. In this experimental gating system, inlet diameter of main runner is 4 mm, gate sleeve is inserted into panel with a length of 47 mm and a taper of 5°. Cross-section of branch runner is circular with a diameter of 7 mm. Bottom diameter of vertical main runner is 4 mm with a taper of 5°. Starting and ending diameters of gate are 4 mm and 1.2 mm, respectively, and length is 3.5 mm. Figure 4 shows gating system of plastic part.

 

Figure 4 Gating system

After plastic part is filled and pressurized, it needs to be cooled to a certain temperature before it can be ejected from mold. Cooling time accounts for more than half of the entire production cycle. In order to achieve rapid cooling and thus improve production efficiency, a reasonable and effective cooling system needs to be designed. In addition, cooling system has a significant impact on molding quality of plastic part. Uneven cooling will cause residual stress in plastic part, which will lead to formation of warping defects. Generally speaking, for molds with shallow cavities, a cooling water pipe system can meet cooling requirements, while for plastic parts with deep cavities, cooling water wells and heat transfer rods are required for assistance. Cavity depth of this plastic part mold is 32 mm, which is a deep cavity structure. For cooling of fixed mold, a double-layer cooling water pipe is designed with a diameter of 6 mm. For cooling of moving mold, in order to meet cooling requirements of protruding parts, a combination of cooling water pipes and cooling water wells is used, with diameters of 6 mm and 8 mm, respectively. Pure water with a Reynolds number of 10,000 was selected as cooling medium, with an inlet temperature of 25 ℃. Cooling water channel layout is shown in Figure 5.

 

Figure 5 Cooling water channel

Molding process of drain pump filter screen plastic part was simulated using a "cooling + filling + holding pressure + warpage" analysis sequence. Causes of warpage were analyzed to obtain degree of influence of different factors on warpage deformation of plastic part. Injection molding process parameters used in preliminary analysis were: melt temperature 240 ℃, mold opening time 4 s, filling pressure 60%, and remaining process parameters were set to default. Figure 6 shows mold flow analysis results of warpage deformation of plastic part. As shown in Figure 6, maximum warpage deformation of plastic part is 0.7060 mm, slightly higher than maximum allowable warpage deformation requirement (0.7 mm). Further analysis revealed that warpage deformation caused by uneven cooling was 0.1588 mm, indicating that designed cooling system could achieve a good uniform cooling effect; warpage deformation caused by orientation effect was 0.0715 mm, which could be ignored; warpage deformation caused by uneven shrinkage was 0.5725 mm, indicating that uneven shrinkage was main cause of warpage deformation of plastic part. Adverse effects of uneven shrinkage can be controlled by optimizing process parameters.

 

Fig. 6 Analysis results of warpage deformation
Orthogonal experiments were used to optimize injection molding process parameters and study effects of melt temperature (A), filling pressure (B), and mold opening time (C) on warpage deformation of plastic part. Three levels were selected for each within recommended material parameter range, and an L9 (33) orthogonal array was designed. Design of orthogonal experimental factor levels is shown in Table 2. Orthogonal experiments determined optimal process parameter combination to be A1B3C3, meaning that melt temperature, filling pressure, and mold opening time were 220 ℃, 80%, and 6 s, respectively, resulting in minimum warpage deformation of plastic part. Figure 7 shows warpage deformation mold flow analysis results of pump filter screen plastic part under optimal process parameters. As can be seen from Figure 7, maximum warpage deformation under all effects was 0.6328 mm, with deformations caused by uneven cooling, uneven shrinkage, orientation effects being 0.1417, 0.5079, and 0.0654 mm, respectively. Compared with initial parameters, warpage deformation caused by all effects, uneven cooling, uneven shrinkage, and orientation effects decreased by 10.37%, 10.77%, 11.28%, and 8.53%, respectively. Results indicate that optimization of process parameters reduced maximum warpage deformation of plastic part, meeting precision requirements of part.

 

Table 2. Orthogonal Experiment Factor and Level Settings

 

Fig. 7. Analysis Result of Optimized Warpage Deformation

Molding part is a component with high requirements in all aspects of the entire injection mold. It must not only be able to produce plastic parts that meet specifications, but also satisfy requirements of reliability, efficiency, ease of maintenance, and processing. Cavity structures generally have two types: integral and modular. Considering that this plastic part has three snap-fit structures on its sidewall, a side-pull mechanism is needed to assist in demolding. An integral cavity is not suitable. Therefore, molding part is made into an inner mold insert and fixed to moving and fixed mold plates of mold frame with screws. Three snap-fits are formed by sliding block assembly. Under above conditions, moving and fixed mold plates that fix molding part can be made of general materials, without need for high-quality materials and special heat treatment like molding part, thus saving material costs, facilitating later disassembly and maintenance. Figure 8 shows structural design of moving and fixed mold inserts. Material selected is 3Cr2Mo, with a heat treatment hardness of 48~50 HRC. Parts of cavity used for molding are mirror-polished, while parts of core used for molding are ordinary-polished. Fit with mold plate is a transition fit with a tolerance of H7/m6.

 

Fig. 8 3D structure diagram of molding parts

Plastic part has three snap-fit structures, requiring a side core-pulling mechanism. There are various forms of side core-pulling mechanisms. For internal pulling structures, a lifter type side core-pulling mechanism is generally used; while for external pulling structures, a slanted guide post or hydraulic cylinder type side core-pulling mechanism is typically used. Based on structural characteristics of plastic part, a "slanted guide post + slider" type side core-pulling mechanism is adopted, as shown in Figure 9. Because three snap-fits on plastic part are relatively small, slider is designed as a single piece to ensure its strength. Protruding mounting platforms are designed on both sides of slider, secured with square pressure strips. Square pressure strips and moving mold fixing plate form a T-shaped guide groove for guiding slider. Inclination angle of inclined guide post is generally 15°~25°. If angle is too large, inclined guide post will bear a large bending force, which is detrimental to its service life. If angle is too small, it will inevitably increase length of inclined guide post, which is not conducive to a compact mold structure and makes inclined guide post prone to wear. This mold uses an 18° angle. Plastic part clip protrudes 2 mm, and safety distance is 4 mm. Calculated core-pulling distance is 6 mm. Inclined guide post is fixed to slider clamping block. Slider clamping block thickness is 28 mm, and calculated length of inclined guide post is 47 mm. Number and diameter of inclined guide posts are generally determined by size of slider. We take one inclined guide post per slider, with a diameter of 10 mm. Wedging angle of wedge block needs to be 1°~2° larger than inclination angle of inclined guide post, so we use 20°. To make structure compact, inclined guide post is fixed to wedge block, clamped by fixed mold plate. Since three sliders are all small sliders, ball bearing positioning is sufficient. To prevent sliders from falling out of mold during sliding, screws are used to limit sliders.

 

Figure 9. Structure diagram of side core-pulling mechanism

Plastic part is molded using a three-plate mold, requiring three mold openings, as shown in Figure 10a. To ensure that fixed mold plate A and moving mold plate B open last, mold opening resistance needs to be added between them, typically using nylon plugs or clips. This mold is a small to medium-sized mold, suitable for medium-batch production, so nylon plugs are sufficient. For controlling opening sequence and distance between fixed mold plate A, runner plate, and panel, a built-in fixed-distance parting mechanism is used due to ample internal space in mold base. This mechanism consists of a "small tie rod + tie rod sleeve" combination, as shown in Figure 10b. Based on mold base width, diameter of small tie rods is determined to be 20 mm, and there are four rods. Opening distance between fixed mold plate A and runner plate is controlled by stroke of small tie rods, which is set to 125 mm. Opening distance between panel and runner plate is controlled by stroke of tie rod sleeve, which is set to 10 mm.

 

Fig. 10 Diagram of mold opening sequence and apart mechanism of fix distance

Demolding is last step in molding process of plastic parts, and design of demolding mechanism directly determines quality of ejection of plastic parts. Common demolding mechanism forms include push rods, push tubes, push plates, etc., and application occasion is determined by multiple factors such as shape of plastic parts, mold structure, and allowable stress points of plastic parts. Demolding force F required to eject plastic parts can be obtained by formula (1).

 

Where: is unit normal pressure generated by plastic parts on core, which is taken as 8~12 MPa; is side area of plastic parts that wrap around core; is coefficient of friction, which is generally taken as 0.15~1.0; is draft angle. =988 7 mm2, taken =10 MPa, =0.5, and substituted into formula (1) to calculate required demolding force as 4770 N. It can be seen that demolding force is not large. Considering shape of plastic part and cost of mold, demolding mechanism adopts a round pusher with a diameter of 6 mm. In order to ensure smooth ejection and uniform force, there are 10 round pushers, and positions are arranged along contour of side wall of plastic part. Specific layout is shown in Figure 11.

 

Fig. 11 Position layout of round putters

During injection process, presence of gas in cavity will not only reduce filling speed of melt, but also cause defects such as flow marks, air marks, and air cavities on the surface of plastic part. Therefore, gas needs to be discharged in time. In production practice, venting system of injection mold needs to be continuously improved after trial molding until qualified products can be produced before mold venting is considered sufficient. Generally, for small to medium-sized molds with shallow cavities and simple structures, mold's own clearance is sufficient for venting, such as parting surface, ejector pins, runners, mating surfaces of other moving parts. However, for mold cavities with deeper and more complex structures, a separate venting system needs to be designed. Common venting methods include adding venting rods, adding cold slug wells at air traps, using permeable steel, and creating venting slots. This mold has a deep cavity, which easily leads to air pockets on sidewalls of plastic part. In addition to utilizing mold's own clearance for venting, a venting device needs to be designed. Considering all factors, creating venting slots is the most suitable approach. Venting slots are generally located on parting surface, preferably on one side of cavity, as shown in Figure 12.

 

Figure 12 Exhaust slot structural layout

Figure 13 shows cross-sectional structure of injection mold for drain pump filter screen. According to GB/T 11335-2008, select mold base with model number FCI-3335-A90-B90-C120-L330-EGP. Working process of injection mold is as follows:

 

Fig. 13 Structure diagram of injection mold for drainage pump filter
1—Mounting plate; 2, 12, 15, 16, 20, 23, 31, 38—Hexagon socket cap screw; 3—Push rod backing plate; 4—Ejector retainer plate; 5—Square iron pad; 6—Moving die fixed plate; 7—Core; 8—Cavity; 9—Fixed die fixed plate; 10—Runner plate; 11—Panel; 13—Locating ring; 14—Sprue bush; 17—Wedge block; 18—Slider angle pin; 19—Sliding block; 21—Push rod; 22—Banking column; 24—Stop pin; 25—Push rod plate guide sleeve; 26—Push rod plate guide post; 27—Spring; 28—Release link; 29—Nylon Plug; 30—Sprue puller; 32—Stopper bolt; 33—Support column; 34—Guide pillar straight; 35—Headed guide bush; 36—Small pull rod; 37—Straight guide bush; 39—Small pull rod sleeve; 40—Sliding batten; 41—Positioning wave bead
(1) After mold and injection molding machine are installed together, ABS plastic granules are heated to a molten state. Molten plastic is injected from injection molding machine nozzle through gating system into cavity 8 of injection mold for pressure holding, cooling, and solidification processes.
(2) Injection molding machine drives moving mold part of mold to move. Due to increased mold opening resistance between moving mold fixing plate 6 and fixed mold fixing plate 9 caused by nylon plug 29, fixed mold fixing plate 9 and runner plate 10 are opened first. At this time, solidified material in gating system is pulled off plastic part by pull rod 30, and fixed mold fixing plate 9 is pulled out, completing the first mold opening.
(3) Driven by small tie rod 36, small tie rod sleeve 39, and hexagonal head screw 38, runner plate 10 and panel 11 are opened sequentially, allowing solidified material to fall from sprue sleeve 14, completing second mold opening.
(4) Moving mold continues to move. At this time, because pulling force of injection molding machine is much greater than mold opening resistance between moving mold fixing plate 6 and fixed mold fixing plate 9, moving mold fixing plate 6 and fixed mold fixing plate 9 are opened, molded plastic part detaches from cavity 8 and wraps around core 7, completing third mold opening.
(5) During third mold opening, slider 19, driven by inclined guide post 18, synchronously detaches from plastic part and moves to limit screw 32, completing lateral core pulling.
(6) Injection molding machine ejector operates, ejector pad 3 and ejector fixing plate 4 drive ejector 21 to move and eject plastic part, completing the demolding.
(7) Injection molding machine ejector retracts, under action of spring 27, ejector pad 3, ejector fixing plate 4, and ejector 21 complete their reset.
(8) Driven by injection molding machine, moving mold part moves in opposite direction, performing process opposite to mold opening, and mold closes.
(9) Repeat above steps to enter next injection cycle.

Taking drain pump filter screen plastic part as an example, optimal gate position of plastic part was analyzed using Moldflow software, mold gating system and cooling system were designed. Based on this, warpage deformation of plastic part was investigated using "cooling + filling + holding pressure + warpage" analysis sequence. Using orthogonal experiments combined with analysis of variance, and with maximum warpage deformation of plastic part as evaluation index, effects of melt temperature (A), filling pressure (B), and mold opening time (C) on warpage deformation of plastic part were studied. Results show that melt temperature has the greatest impact on warpage of plastic part, mold opening time has least impact, and filling pressure has an intermediate impact. Optimal process parameter combination obtained is A1B3C3. Simulation analysis of molding process based on optimal process parameters reveals that maximum warpage after optimization is 10.37% lower than maximum warpage under initial state, indicating that molding quality of injection mold can be improved through process parameter optimization.
Using UG software, molding parts, gating system, cooling system, side core-pulling mechanism, fixed-distance parting mechanism, demolding mechanism, and venting system of a three-plate injection mold for plastic part were designed. To ensure uniform cooling and shorten cooling cycle, a double-layer cooling water pipe was designed for fixed mold, while moving mold uses a combination of "water pipe + water well" cooling. Venting grooves were created on fixed mold inserts to avoid affecting surface quality of plastic part due to cavitation. To achieve smooth ejection of plastic part with uniform force, round ejector pins were designed, with their positions evenly distributed along sidewall contour of plastic part. Throughout design process, using a combination of UG and Moldflow for injection mold design can improve molding quality of plastic parts, shorten mold development cycle, and reduce mold manufacturing cost.