Abstract: A three-plate injection mold with an automatic unthreading mechanism consisting of a gear drive + core-pulling hydraulic cylinder and a slanted guide post side core-pulling mechanism was designed for a water pipe connector with an internal thread at one end and a smooth hole at the other. First, considering structural characteristics of plastic part, point gate injection, gate location, and DC circulating cooling circuit were determined. Injection molding process was simulated and analyzed using mold flow simulation software Moldflow. Results showed that filling time, shrinkage index, cavitation distribution, and warpage of plastic melt were all within reasonable ranges, verifying rationality of gating and cooling system designs. Second, mold structure was designed, with main parting surface located on axial surface of plastic part. The other two parting surfaces were sequentially parted to achieve automatic shedding of solidified material from gating system. Since plastic part is a tubular part with internal threads, to address difficulty of demolding internal threads, two sets of motion mechanisms were designed: a gear transmission mechanism driving rotation of threaded core and a core-pulling hydraulic cylinder for linear movement, to achieve automatic thread removal. Molding and core-pulling of smooth hole portion of plastic part were achieved using a slanted guide post side core-pulling mechanism. Through mold flow analysis, rationality of mold structure was verified, mold design and development cycle were shortened, and molding quality of plastic part was effectively improved.
Water pipe connectors are widely used parts in bathroom products. They are tubular parts with internal threads, and their molding process is injection molding. In process of injection mold design, rationality of gating system and cooling system design will seriously affect quality of product. In the past, traditional mold development model could only rely on experience of designers to select gate position and cooling method. Later, repeated mold repair and trial molding were required to meet usage requirements of plastic parts, which increased mold development cost. At present, CAE simulation technology is becoming increasingly mature and widely used. Gating system and cooling system are determined by using mold flow simulation analysis software, and injection molding process parameters are set to simulate injection molding process, thereby verifying rationality of determined mold flow analysis model. We used mold flow analysis software to simulate and analyze important molding process parameters of plastic parts, determined gating system and cooling system of mold, shortened mold structure design cycle, and ensured molding quality of plastic parts.
Currently, regarding difficulty of demolding internal threads, references generally employ a threaded core rotation + push plate ejection mechanism. Threaded core rotates under drive of transmission mechanism, and internal thread is demolded under action of push plate. Ejection force of push plate is provided by injection molding machine's ejection system, and its magnitude is determined by clamping force of plastic part. During ejection process, ejection force remains constant, which may damage internal thread. Furthermore, if a push plate ejection mechanism is used, plastic part can only be placed vertically, inevitably increasing mold height and making it difficult to guarantee mold processing accuracy, thus increasing mold manufacturing costs. To address this problem, we designed a gear transmission mechanism + core-pulling hydraulic cylinder automatic demolding mechanism for internal threads, realizing that threaded core rotates while moving linearly with core-pulling hydraulic cylinder, effectively avoiding damage and deformation of internal thread of part.

Water pipe joints operate in a humid environment and need to withstand high-pressure water flow. Therefore, warpage deformation of molded plastic part should be small to meet assembly accuracy requirements, prevent water leakage during use. According to working environment and molding requirements of water pipe connector, nylon 66 (PA66) + 50% glass fiber (GF) was selected, that is, PA66 reinforced with GF with an addition ratio of 50%. This is a commonly used modified engineering plastic with excellent mechanical properties and impact resistance, good wear resistance, dimensional stability and good chemical properties. Plastic part is mass-produced. Volume of plastic part is 2.1 cm3, draft angle is 1°, average wall thickness of plastic part is 2.9 mm, surface of plastic part is required to be free of floating fibers, deformation is required to be within 0.08 mm, and precision of plastic part is MT4. Figure 1 is a schematic diagram of water pipe connector. As can be seen from Figure 1, plastic part is a stepped rotating body with a length of 34 mm, a maximum arc diameter of 14 mm, and a minimum diameter of 8.3 mm. One end of plastic part is an internal thread, and the other end is a hollow tube. Nominal diameter of internal thread is 8 mm and pitch is 1.2 mm. Therefore, an automatic unscrewing mechanism needs to be designed to solve problem of difficult demolding of internal thread.

 

Fig. 1 Water pipe joint

Three-dimensional model was imported into moldflow software, specified as a two-layer mesh type, global mesh edge length was set to 0.35 mm. Mesh was completed and repaired by mesh processing tool to ensure feasibility of mold flow analysis and reliability of analysis results. Mesh matching rate after repair was 85.6%, maximum aspect ratio of mesh was 10, connectivity was good. It can be seen that all information of mesh quality met conditions of mold flow analysis.

Basic function of gate is to accelerate melt from runner to quickly fill cavity. In most cases, gate (except for direct gate) is part with the smallest cross section in the entire gating system. Reasonable selection of gate location is an important link to ensure quality of plastic parts. Optimal gate location of plastic part was obtained through analysis of moldflow software, as shown in Figure 2. As shown in Figure 2, dark blue area represents optimal gate location, situated near raised annular band. To ensure smooth filling of mold cavity by molten plastic, gate is positioned on raised annular band. A small platform on this band prevents noticeable gate marks from appearing on the outer surface, thus placing gate on this platform.

 

Figure 2 Gate position simulation results

Mold flow analysis using Moldflow software requires establishment of a mold flow analysis model, which mainly includes gating system and cooling system. Based on production volume requirements and product structure characteristics, a two-cavity mold with a point gate is adopted. If a common side gate is used for casting, plastic melt will climb, resulting in poor melt flow and insufficient filling. Since flow of glass fiber reinforced PA66 is poor, flow of PA66 and glass fiber are inconsistent, in order to ensure that glass fiber is evenly dispersed and does not accumulate or delaminate to form floating fibers, melt must quickly fill cavity. In addition, if a side gate is used for casting, secondary removal is required, leaving obvious marks on the surface of plastic part. Cooling system adopts a common DC circulation cooling circuit with symmetrical arrangement of upper and lower molds. In order to improve uniformity of mold cooling, a water baffle is added to cooling circuit. Mold flow analysis model is shown in Figure 3.

 

Fig. 3 Moldflow analysis model

Based on determined gating system and cooling system model, analysis sequence of "filling + holding pressure + cooling + warpage" is selected. Injection molding process parameters set during mold flow analysis are shown in Table 1. To meet requirements for quality of molded plastic parts, we conducted a simulation analysis of four molding process parameters: filling time, shrinkage index, cavitation location, and warpage deformation, in order to verify whether determined analysis model is reasonable and to provide a reliable basis for subsequent mold structure design. Analysis results are shown in Figure 4.

Table 1 Injection molding process parameters

 

Figure 4 Simulation analysis of injection molding process parameters
Filling time refers to time from start of injection to filling of mold cavity by plastic melt. This molding process parameter plays an important role in molding cycle and quality of plastic parts. From filling time analysis results in Figure 4a, it can be seen that material flow filling time is 2.077 s, which can fill cavity in a short time. For glass fiber reinforced PA66 material, the shorter filling time, the less likely it is to have floating fiber phenomenon on the surface of plastic part, and there is no phenomenon of incomplete filling. No short shot phenomenon was found in analysis log, indicating that filling is complete and filling time is reasonable. Shrinkage marks refer to defects such as depressions and collapses on the surface of plastic parts due to excessive shrinkage during cooling and solidification process. Shrinkage mark index reflects relative probability of shrinkage marks appearing in a certain area on the surface of plastic part. The larger value, the higher probability of this phenomenon in this area. Shrinkage mark index of qualified plastic parts is required to be less than 5%. As can be seen from analysis results of Figure 4b, maximum shrinkage mark index of plastic part is 0.73%, and there is slight shrinkage on outer surface, which does not affect assembly of product. As can be seen from analysis results of Figure 4c, air cavitation is mainly distributed on two end faces of water pipe joint, and there is almost no air cavitation in other areas, indicating that there is air entrapment on two end faces. However, air cavitation is basically distributed on the edge near end face, close to parting surface of plastic part. In addition, as can be seen from size of plastic part in Figure 1, plastic part is a small part, so venting volume is not large. Therefore, gap of parting surface and gap between ejector pin and mold plate can be used for natural venting to eliminate a large number of air cavitation. Because water pipe joint operates in a high-pressure water environment, it is prone to leakage, so its deformation must be within 0.08 mm. Warpage deformation analysis results in Figure 4d show that maximum deformation of plastic part is approximately 0.05 mm, occurring on the surface near end face of water pipe joint. This is mainly due to uneven shape and structure of these areas, leading to inconsistent wall thickness and uneven shrinkage, resulting in warpage deformation. Based on structural dimensions of plastic part and proposed process requirements, warpage deformation is within a reasonable range, ensuring molding accuracy and process requirements of plastic part.
Through above process parameter analysis, design scheme of water pipe joint's gating system and cooling system is verified to be reasonable.

Based on structural characteristics of plastic part in Figure 1 and production batch requirements, a two-cavity mold structure is designed. Taking PL in sectional view of Figure 1 as main parting surface, it can be seen from mold flow analysis that mold adopts point gate injection, so a three-plate mold structure must be used. In this mold, a fixed-distance parting mechanism with spring parting and tie rod fixed distance is used to realize three-stage sequential parting of mold, as shown in Figure 5. To ensure mold manufacturing precision, the overall height of mold must be controlled. Therefore, placing plastic part horizontally, compared to vertical placement, effectively avoids thread damage and deformation. Internal thread core forms an M8*75 thread, and smooth hole core forms a 22 mm deep inner hole. Threaded core and smooth hole core intersect at surface A in Figure 1. Internal thread core-pulling mechanism is designed as a gear-driven hydraulic cylinder automatic unthreading mechanism, while smooth hole demolding mechanism is designed as a sliding block core-pulling mechanism with a slanted guide post. Mold's ejection mechanism is designed as an ejector pin ejection mechanism, with ejection point being outer surface of plastic part. To reduce ejection marks on outer surface of plastic part, ejector pin is designed as a forming ejector pin, and a spring-loaded return pin is used to achieve pre-reset of ejector pin.

 

Figure 5 Spring parting tie rod spacing mechanism

3.2.1 Positioning of side core: This plastic part has an internal thread at one end and a smooth hole at the other. Threaded end requires an automatic unthreading mechanism, while smooth hole at the other end is pulled using a common lateral core pulling mechanism with a slanted guide post and side slider. Threaded side core and smooth hole side core collide on surface A in Figure 1 during mold reset. To ensure coaxiality of inner hole of plastic part, both cores must be concentric. We uses a positioning method with a tapered hole to ensure coaxiality of two cores. Positioning inner hole and boss are designed on the end faces of threaded side core and smooth hole side core, respectively. Positioning structure and dimensions are shown in Figure 6.

 

Figure 6 Positioning of side core
3.2.2 Design of slanted guide post lateral parting and core pulling mechanism: Figure 7 shows lateral parting and core pulling mechanism. Inclined guide post side core-pulling mechanism mainly consists of a light hole side core 10, a side slider 11, an inclined guide post 12, and a wedge block. As shown in Figure 1, light hole depth is 22 mm. In order to facilitate demolding, core-pulling distance s is taken as 27.5 mm. Inclined guide post is installed on fixed mold plate, and slider is installed on moving mold plate. Installation angle α of inclined guide post is 18°, and working length L is 113 mm. According to core-pulling distance verification formula L=s/sinα, we get L=89 mm＜113 mm, so length of inclined guide post is sufficient to complete required core-pulling distance. In addition, slider is spring-reset and wedge block is locked. Wedge device of this mold is integrated with fixed mold plate. In order to quickly leave pressing surface of slider when mold is opened, avoid friction between wedge block and slider, inclined angle α' of wedge block is greater than installation angle α of inclined guide post, and α'=20°.

 

Figure 7 Lateral parting and core-pulling mechanism
1—Core-pulling hydraulic cylinder; 2—Electric motor; 3—Stop dog; 4—Wedge block; 5, 11—Sliding block; 6—Driven gear; 7—Driving gear; 8—Internal threaded side core; 9—Plastic part; 10—Unthread hole side core; 12—Inclined guide post
3.2.3 Design of internal thread demolding mechanism: Internal thread is formed by internal thread side core 8. We uses a gear transmission mechanism + core-pulling hydraulic cylinder for automatic demolding of internal thread. Driving wheel and two driven wheels are selected with standard gears and standard installation. Gear size parameters are shown in Table 2 . Core-pulling mechanism of internal thread consists of an electric motor 2, a driving gear 7, two driven gears 6 and a core-pulling hydraulic cylinder 1, as shown in Figure 7. Thread side core 8 is connected to driven gear 6 by a flat key.

 

Table 2 Gear dimension parameter
Driving gear 7 is driven to rotate by electric motor 2. Driving gear 7 drives two driven gears 6 to rotate in same direction to ensure that two internal threads have same direction of rotation. At the same time, core-pulling hydraulic cylinder 1 drives driven gear to slide axially on driving gear, realizing that thread core rotates while moving in straight line direction with core-pulling hydraulic cylinder. Two sets of motion mechanisms ensure requirements of internal thread forming accuracy of plastic part. Because plastic part is held tightly on core after molding, a large demolding force is required when plastic part just comes off threaded core. After that, core-pulling force provided by core-pulling hydraulic cylinder only needs to overcome friction of plastic part moving. Compared with gear transmission + push plate ejection unscrewing mechanism, it can better ensure that threads are not damaged or deformed. Table 2 shows gear size parameters. As shown in Figure 1, core-pulling distance of internal thread part is 12 mm. As shown in Table 2, thickness difference between driving gear and driven gear is 27 mm. When slider 5 contacts stop block 3, core-pulling hydraulic cylinder stops pulling core. Distance between slider and stop block is 25 mm (12 < 25 < 27). Therefore, required core-pulling distance is ensured, driven gear and driving gear are not separated axially. Change direction of oil in core-pulling hydraulic cylinder to reset internal thread side core and lock it by wedge block 4 for next pouring.

Water pipe connector mold is a three-plate mold structure with three parting surfaces, as shown in Figure 8 (P1, P2, P3). Specific working process is as follows:

 

Figure 8 Mold structure drawing
1—Moving mold seat plate; 2—Backing block; 3—Guide post; 4—Distance tie rod; 5—Moving plate; 6—Guide bush; 7—Core; 8—Cavity; 9, 45—Spring; 10, 12, 16, 31, 48—Screw; 11—Distance sleeve; 13—Locating ring; 14—Heel block; 15—Sprue puller; 17—Gate bush; 18—Fixed mold seat plate; 19—Stripper plate; 20, 43—Side slide; 21—Flat key; 22—Driven gear; 23—Release link; 24—Driving gear; 25—Resetting 26—Spring; 27—Push rod guide post; 28—Push rod guide sleeve; 29—Push rod fastening plate; 30—Garbage spike; 32—Supporting post; 33—Ejector pin; 34—Electric motor; 35—Core-pulling hydraulic cylinder; 36—Rolling bearing; 37—Threaded side core; 38—Rolling bearing; 39—Unthreaded hole side core; 40—Inclined guide post; 41—Fixed plate; 42—Side core fixing plate; 44—Pull rod; 46—Push stop; 47—Slider support plate
3.3.1 Injection Molding Process: After mold closes, injection molding machine pours material into two cavities through main runner, horizontal runner, vertical runner, and point gate, filling cavities completely.
3.3.2 Holding Pressure and Cooling Process: After molten plastic fills mold cavity, it is held under pressure and cooled according to set injection molding process parameters to obtain a complete plastic part.
3.3.3 Mold Opening Process: As injection molding machine opens mold, under preload of spring 9 in Figure 8, mold first separates at first parting surface P1. Under action of pull rod 15, vertical runner is pulled away from plastic part at point gate of mold cavity, and runner separates from plastic part. After P1 parting surface opens 120 mm, under pulling force of spacer rod 4, stripper plate 19 moves downward. At this time, P2 parting surface begins to open, stripper plate 19 pulls main runner solidified material out of sprue bushing 17 and scrapes runner solidified material off end of pull rod 15, completing automatic demolding of solidified material in gating system. After P2 parting surface opens 8 mm, stripper plate 19 stops moving under action of spacer sleeve 11. Under action of mold opening force, moving mold continues to move backward, forcing P3 parting surface to open. Interference of side cores keeps plastic part within moving mold.
3.3.4 Core Pulling: Inclined guide post 40 drives side core slider 43, causing it to move outward within guide groove of moving mold plate 5 until clear hole side core 39 is completely separated from plastic part, completing lateral core pulling of clear hole. Simultaneously, electric motor 34 drives drive gear 24, which in turn drives two driven gears 22, causing two internal thread side cores to rotate in same direction, separating them from plastic part. Simultaneously, under linear motion of core pulling hydraulic cylinder, lateral core pulling of plastic part's internal threads is completed.
3.3.5 Ejection: After reaching mold opening stroke, moving mold stops moving backward. Injection molding machine's central ejector pushes ejector plate 29, thereby driving ejector rod 33 fixed on ejector rod fixing plate to completely eject plastic part from mold.
3.3.6 Reset: Internal thread side core is reset by core-pulling hydraulic cylinder and locked by wedge block; smooth hole side core is reset by inclined guide post 40 and spring 45 and locked by fixed platen 41 (fixed platen and wedge device are made as an integral unit); ejector rod is reset by reset rod 23, and a pre-reset spring 25 is installed on reset rod to realize ejector rod reset first, preventing ejector rod and side core from colliding and interfering with each other during mold closing process. After reset is completed, injection molding process of next cycle continues.

Water pipe connector is a part of bathroom products. For a certain water pipe connector with internal threads, rationality of determined analysis model was verified by mold flow analysis software moldflow, and an automatic unscrewing injection mold structure was designed. Main work is as follows:
(1) First, combined with analysis results of optimal gate position and structural characteristics of plastic part, mold flow analysis model of plastic part was determined. This model includes gating system and cooling system. Then, using analysis model, injection molding process parameters were set for mold flow analysis. Analysis results showed that plastic melt filling time was 2.077 s, with no short shot phenomenon during filling and good consistency between two cavities; maximum shrinkage index of plastic part was 0.73%, with no obvious shrinkage or cavities; air pockets were mainly distributed on the edge of plastic part's end face, and natural venting was achieved using gap between parting surface and ejector pin, with good venting performance; maximum warpage deformation of plastic part was 0.051 mm, meeting requirements of molding process.
(2) Due to use of point gate for injection, injection mold was designed as a two-cavity, three-plate mold structure, using a spring-loaded tie rod structure to achieve three-stage parting of mold; since plastic part is a circular tubular part, parting surface was selected on axial plane of rotating body, and a slanted guide post side core-pulling mechanism was used to achieve core pulling of smooth hole part of plastic part, and two sets of motion mechanisms, a gear transmission mechanism and a core-pulling hydraulic cylinder, were used to achieve automatic demolding of internal thread. Actual production has proven that mechanism operates stably and reliably, effectively avoiding thread damage and deformation, reducing mold development costs, and providing valuable reference for design of injection molds for similar plastic parts.