Abstract: Addressing challenges in mold design for smoke alarm plastic parts, such as difficulties in opening gate, demolding difficulties for local features, and high requirements for molding precision, this paper proposes an automated injection molding production method. Firstly, based on technical route of plastic part design and manufacturing analysis, a feasibility analysis of mold structure layout is conducted for molding of plastic part. Then, CAE analysis is used to optimize gating and cooling system design of single-cavity mold. In optimized single-cavity gating system, a single-point horn gate is used for gating. With a gate diameter of 0.8mm, injection time is approximately 0.5584s, filling pressure is approximately 30MPa, flow front temperature difference is 3.8℃, gate freezing time is 11.57s, warpage is less than 0.25mm, injection cycle can be controlled within 36.65 s. Mold designed based on CAE analysis features a two-cavity layout, a two-platen cold runner structure, single mold opening and single ejection. Side core-pulling sliders are used for molding and demolding straight side walls of plastic part to ensure integrity of wall's appearance. For situations where there are both side holes and undercuts in confined spaces, a centralized demolding design method is used, employing a multi-feature lifter for side core-pulling demolding. To facilitate mold manufacturing, temporary fixing screws are added to slider, which facilitates fitting of slider parting surface during mold assembly. Additionally, tie rods are added to ejector plate to facilitate installation of numerous ejector components.
In injection molding of plastic parts, design of gating system is a key factor. The most important factors in gating system design are gate location and number of gates. In most practical cases, optimal gate location and number may not meet actual needs of mold structure design. A compromise in setting gate location and number of gates must be reached after comprehensively considering arrangement of demolding mechanisms such as sliders, lifters, ejector tubes, ejector pins, inserts, and cooling channels. This optimization process requires, on the one hand, referring to layout design based on production experience in Design and Manufacturing Analysis (DFM) report, and on the other hand, comprehensively utilizing CAE technology to obtain a more accurate gate location and number. Such mold design avoids rework, repair, and even mold scrapping, while also significantly improving mold manufacturing efficiency. Smoke detectors are a type of fire-fighting electronic product widely used in fire-fighting facilities, with a large quantity and many styles. Encapsulation shells of internal electronic components of smoke detectors are generally mass-produced using injection molding. For higher-grade products, encapsulation shell material is often modified acrylonitrile-butadiene-styrene plastic (ABS) for injection molding, type of modification varies depending on working conditions. Encapsulation shell plastic parts generally have many features such as fastening studs, self-adaptive assembly clips, component positioning ribs, side holes, and appearance requirements are high. Mold structure design of such plastic parts is quite difficult. This paper, considering requirements of injection molding housing of a medical smoke alarm using ABS/polycarbonate (PC) material, uses CAE to optimize molding scheme of housing plastic part, solving demolding problem on inner and outer walls of plastic part. A two-plate mold was designed for injection molding production of this plastic part. Design methods used in mold design can provide a useful reference for mold design of similar plastic parts.

Structure and shape of smoke alarm housing plastic part are shown in Figure 1. Plastic part is a semi-cylindrical shell with an average wall thickness of 2.0 mm, a maximum wall thickness of 3.1 mm, a minimum wall thickness of 0.6 mm. Main body of plastic part is a semi-cylindrical shell, with following features: T1 and T1’ are two screw posts; T2, T2', T3, and T3' are four retaining ribs; T4 and T4' are two hooks; T5 and T5' are two snap-hole ribs, with snap-holes designated k1; T6 and T6' are reinforcing ribs on inner wall of disc T7, with a thickness of 0.8 mm; T7 is side wall of disc, with an inner wall having a draft angle of 0.5°, an outer wall that is straight, an average disc thickness of 2 mm; T8 is a pillar hole; T9 is material identification characters; T10 is a small locating post; T11 is two round holes; and T12 is a square hole. Outer radius of cylindrical shell of plastic part is 28.7 mm, its length is 64.1 mm. Draft angle of cavity surface of plastic part is set at 0.5°, and that of core surface is set at 0.3°. The total production volume of plastic parts is 1 million units, with a molding precision of MT4~MT5. Depending on customer grade requirements, 50,000 injection-molded parts require surface electroplating treatment, while the rest do not.

 

Figure 1. Plastic parts of smoke alarm shell
T1, T1'—Screw post; T2, T2', T3, T3'—Positioning ribs; T4, T4'—Hook; T5, T5'—Buckle hole reinforcement ribs; k1—Buckle hole; T6, T6'—Reinforcing ribs; T7—Disc side wall; T8—Column hole; T9—Material identification characters; T10—Small positioning column; T11—Circular hole; T12—Square hole
Plastic parts are made of ABS/PC alloy JH960-HT08 manufactured by Kingfa Science & Technology Co., Ltd. This alloy is a high-performance engineering plastic that improves stress cracking resistance and processability of PC, while also possessing excellent heat resistance, impact resistance, chemical resistance, and dimensional stability. This alloy material is a halogen-free flame-retardant material, achieving a flame retardant rating of 3.0 mm 5VA, and it does not use halogen flame retardants, making it more environmentally friendly. Material shrinkage rate is 0.46%~0.52%.

In mold design, feasibility analysis for injection molding of plastic parts (DFM report stage) must start by improving manufacturability of parts. First, it considers factors such as manufacturing difficulty, cost, efficiency of parts and various processes. In scheme design stage, mold supplier arranges engineers to design 2D and 3D drawings of mold parts. Next is machining stage, where mold is finely machined. Then comes assembly stage, where multiple parts of mold are assembled as a whole. Following this is trial molding stage, which verifies mold performance and product quality. Finally, acceptance stage verifies that dimensions of injection molded sample are consistent with design drawings.
Based on above manufacturing procedures, plastic part DFM analysis is conducted from following aspects:
Firstly, feasibility of mass production of molding. As can be seen from structure of this plastic part, wall thickness varies from 0.6 mm to 3.1 mm. This wall thickness range is considered a thin-shell plastic part in structural design, making injection molding entirely feasible. Considering the total production volume, injection molding with two or more cavities is necessary to meet production efficiency requirements. However, too many cavities will reduce molding accuracy of plastic part. After calculation, a two-cavity mold layout is more suitable for this plastic part. In terms of automated molding production, key lies in automatic demolding of plastic part and runner waste. Demolding of plastic part is divided into demolding of outer wall and inner wall. Inner wall, as shown in bottom view in Figure 2, is difficult to demold due to studs T1/T1', requiring use of an ejector pin (ES) for molding and ejection. Difficult-to-demold features also include T4, T5, T4', and T5'. T4 and T4' can be automatically demolded using a lifter mechanism. T5 and T5', due to hole k1, also require lifters for side core-pulling demolding. However, due to space constraints, T4 and T5 on same side can only use one shared lifter AL for side core-pulling demolding, T4' and T5' on the other side can also only use one shared lifter AL for side core-pulling demolding. Another issue to consider regarding demolding inner wall of plastic part is ensuring balanced and deformation-free ejection from core insert. This requires consideration of balanced arrangement and number of ejector pins. Based on distribution characteristics of clamping force, setting multiple ejector pins Ej in Figure 2 can ensure balanced and deformation-free ejection of plastic part. Demolding of outer wall of plastic part mainly considers demolding of T7 outer wall. T7 outer wall is a straight wall without a draft angle; therefore, demolding of T7 outer wall can only be carried out using a side-pulling method to ensure its straight wall characteristic. Furthermore, from perspective of machining molded parts, CNC milling is difficult for features T2, T3, T2', T3', and T8. A better approach is to use a split-insert method, separating four local inserts Ib and IZ from moving mold core insert. This allows for easier acquisition of these local feature molded parts using wire EDM and CNC milling.

 

Figure 2. Analysis of demoulding of plastic part and layout of mechanism components.
T7, T11, T12—Same as Fig. 1; ES—Ejector sleeve; Ej—Ejector pin; Ib—Insert block; AL—Lifter; IZ—Cylindrical insert
Secondly, controllability of key factors. Key factors for successful injection molding of plastic parts lie in design of gating system and effectiveness of temperature control system. These two factors need to be judged in conjunction with CAE mold flow analysis. Based on structural characteristics of plastic part and aforementioned demolding mechanism components, without considering requirements of inserts, lifters, ejector pins, and subsequent surface electroplating, optimal position of a single gate in mold cavity is set at optimal gate position Yz recommended by CAE analysis as shown in Figure 3a. However, Yz is difficult to meet molding requirements of plastic part. Main reason is that if it is set at Yz, gate can only be a point gate or a direct gate among cold gates, or a direct hot gate. These will leave gate marks on the surface of plastic part, affecting surface electroplating. Therefore, gating system setting after gate position optimization is shown in Figure 3b. Optimized gate position is set on inner wall of one side end of plastic part. Gate g0 adopts form of a bull's horn gate, which can effectively avoid aforementioned surface quality problems, facilitate opening of demolding mechanism components, and does not affect opening of mold cavity cooling water channel. Based on a thorough consideration of molded part arrangement in demolding mechanism, single-cavity mold uses four water channels for cooling temperature regulation. Water channel WL1 is used for cavity cooling, WL2 and WL3 for core cooling, and WL4 for cooling T7 sidewall outer wall forming slider.

 

Fig. 3 CAE optimization analysis
WL1~WL4—Water line; g0—Bull horn gate; R0, R1—Runner; Yz—Best location of gate
With gate g0 dimension optimized to Ø0.8 mm, CAE analysis results are shown in Figs. 3c~3h. As shown in Figure 3c, cavity filling time is 0.5584 s, with balanced filling time on both sides of filling path. As shown in Figure 3d, pressure difference from gate to filling end is approximately 30 MPa, which does not place high demands on injection molding machine. As shown in Figure 3e, temperature difference at flow front is approximately 3.8 ℃, eliminating problem of flow stagnation caused by excessive cooling at flow front. As shown in Figure 3f, among four cooling channels, temperature difference between inlet and outlet of cooling water channels does not exceed 1 ℃. Further analysis revealed that cooling time of plastic part can be accelerated to 24.53 s, which is 10.22 s shorter than cooling time without cooling conditions, indicating effective pipe cooling. As shown in Figure 3g, in cooling settings, gate freezing time is 11.57 s, ensuring sufficient holding time in mold cavity, and molding cycle can be controlled within 36.65 minutes. As shown in Figure 3h, maximum warpage deformation is 0.245 5 mm. mm, and after verification, this deformation amount did not exceed range of molding tolerance grade specified by MT4.
Overall, using gate g0 for filling mold cavity of plastic part is more reasonable, can meet molding requirements and production efficiency control requirements of plastic part.

Single-cavity molded part designed based on DFM analysis is shown in Figure 4a. Single cavity is parted using parting surface PS to obtain cavity insert 1 and core insert 5. Based on this, a second insert split is performed, arranged as shown in Figure 2 to obtain T7 slider 2, T8 cylindrical core, 2 lifters 4 (AL), 2 T2 inserts 6 (Ib), 2 T3 inserts 7 (Ib), and 2 ejector tubes 8 (ES). In design of lifter 4, a centralized demolding method is used to integrate T5 retaining rib and T6 reinforcing rib side hole into one lifter 4 for side core pulling demolding. Similarly, T5' and T6' are integrated into another lifter 4' for side core pulling demolding, as shown in Figure 4b.

 

Figure 4 Single cavity parting and forming part setting
T5, T6, k1—The same as Fig. 1; PS—Main parting surface; DZ—Mouth opening direction; 1—Cavity insert; 2—T7 slider; 3—T8 cylindrical core; 4, 4'—lifter (AL); 5—Core insert; 6—T2 inserts (Ib); 7—T3 inserts (Ib); 8—T1 Ejector Sleeve (ES); 9—g0 gate insert; Pa—Plastic part; g0—Bull horn gate; DZ—Mouth opening direction

Mold core structure after cavities are arranged in a two-cavity configuration is shown in Figure 5. Single-cavity insert 1 is rotated and copied to obtain two-cavity insert 1'. Similarly, single-cavity core insert 5 is rotated and copied to obtain two-cavity core insert 5'. Gates g0, parts 2, 4, 6, 7, and 8 are same as in single-cavity configuration. To ensure accurate repositioning when two cavities are closed, tapered positioning grooves Cg and frustums Co are provided at four corners of cavity insert 1' and core insert 5'.

 

Fig. 5 Design of dual mold cavity forming parts
Ve—Vent; Ge1, Ge2—Air groove; hv—Air hole; 1'—Dual cavity insert; 5'—Dual core insert; 9—Bull horn gate inserts; PS, g0, 2, 4, 6, 7, 8—The same as Fig. 4; 2'—The same slider as 2; R0—Main Runner; R1, R2—Runner; LP—Runner Platform; Cg—Cone groove; Co—Cone platform; H—Hanging hole
To ensure that two gates g0 are not blocked, main runner R0 is offset, and a cold slug well is opened at its lower end. Two ends of runner R1 are extended and cold slug wells are set. Molten material enters two gates g0 through two ends of R2. To facilitate machining of bull horn gate g0, two gate inserts 9 are used to combine to obtain shape of runner R2 and two gates g0. To facilitate processing and handling of molded parts, two lifting holes H are evenly machined on parts 1' and 5' respectively. Cavity insert 1' is made of alloy steel 2738, while core insert 5' and parts 2, 6, 7, 8, 9 are made of alloy steel H13.

The overall mold structure 3D diagram is shown in Figure 6. Mold is a two-plate mold structure with two cavities, V1 and V2. Structural composition of each cavity and molding parts are designed as shown in Figure 5. Single mold opening surface is PS surface. Mold ejection mechanism is a single ejection mechanism. Push plate 17 pushes four lifters 4, four push tubes 8, hook-shaped pull rods 30, and several ejector pins to perform a single ejection, simultaneously ejecting two plastic parts in cavities V1 and V2 to achieve complete demolding. Slider 2 and 2' adopt a conventional inclined guide post slider mechanism structure. For example, in structure of slider 2', its components include part 2', part 20, and parts 23-27. To facilitate parting surface modification during slider 2' assembly, a temporary fixing screw 23 is added to fix slider 2' in corresponding slider groove within fixed mold plate 11 during modification. After modification, screw 23 is removed. In mold component design, parts weighing more than 5 kg must have lifting holes H for easy installation.

 

Fig. 6 Mold structure
1~9, 1', 2', 5', Ps—Same as Fig.4 and Fig.5; 10—Upper clamping plate; 11—Fixed mold plate; 12—Moving mold plate; 13—Safety bar; 14—Mould foot; 15—Dust shield; 16—Ejector retainer plate; 17—Ejector plate; 18—Lower clamping plate; 19—Supporter block; 20—Angular pin; 21—Locating ring; 22—Sprue bushing; 23—Fixing screw; 24—Guide bar; 25—Wear plate; 26—Stop block; 27—Guide rails; 28—Return pin; 29—Return spring; 30—Z-shaped sprue puller; 31—Ejector; 32—Copper blockage; 33—pull rods; 34—Wear resistant circular Spacer; V1, V2—Mould cavity numbers
A wear-resistant round pad 34 is installed on fixed mold plate 11 at the top of reset rod 28 to extend service life of reset rod 28. Two pull rods 33 are temporarily installed on push plate 17. Through these pull rods 33, after ejector elements on push plate 7 are installed, when push plate assembly is installed into bottom of moving mold plate 12, tightening pull rods 33 compresses reset spring 29, forcing ejector elements of push plate assembly into moving mold plate 12, facilitating installation of ejector assembly.

Installation of structural parts in mold is shown in Figure 7. Double-cavity insert 1' and core insert 5' are fastened with screws into corresponding slots of fixed mold plate 11 and moving mold plate 12, respectively. Installation fit between two in slots is a clearance fit, that is, length and width of each side are 0.02mm smaller than length and width of corresponding inner slot of mold plate. Molding parts of sliders 2 and 2' are also clearance-fitted with mold cavity, with a clearance of 0.01 mm on each side. Clearances of lifter 4, ejector pin 31, and push tube 8 are treated similarly. Screw 23 and tie rod 33 are removed after mold is installed. Mold uses four sets of guide pillars 35/shoulder guide bushings 36 for mold opening and closing movements. Lifter seat of lifter 4 uses an articulated type for easy installation. Six evenly distributed support pillars 38 are added to bottom of moving platen 12 to enhance its strength. Ejector pin 31 and push tube 8 are made of alloy steel SKD61 to ensure a service life of over 1 million cycles. Lower end of reset rod 28 uses a delay design with a delay distance of 5 mm to facilitate mold closing on PS surface.

 

Figure 7. Mold structure installation
g0'—Bull horn gate; 35—Guide pillar; 36—Shoulder guide bush; 37—Distance stopper; 38—Support pillar; 39—Stop pin; 40—Ejector guide pin; Other numbers are same as Fig. 4-Fig. 6
To ensure smooth movement of mechanism components, taking lifter 4 as an example, movement clearances of mechanism components are arranged as follows: clearance between lifter and mold core is -0.015~0.025 mm; clearance between lifter seat and lifter cylinder is -0.03~0.05 mm; clearance between bronze guide block and lifter is 0.30 mm for two straight surfaces and 0.05~0.10 mm for two inclined surfaces.

As shown in Figures 6 and 7, after mold is installed on injection molding machine, its injection function is realized according to following steps.
(1) First, mold closes at PS surface. Injection molding machine's barrel nozzle fills and holds pressure in cavities V1 and V2 through sprue bushing 22. Then, injection molding machine's cooling system is activated to cool plastic part within mold cavity, awaiting mold opening.
(2) Mold opening surface Ps opens. During mold opening, injection molding machine's moving platen drives moving mold mounted on it to move downwards (DZ-), mold opens at PS surface. During opening process, two inclined guide pillars 20 drive their corresponding sliders 2 and 2' to complete side core pulling. Simultaneously, upper surface of plastic part is also pulled out from cavity insert 1', remaining on core insert 5, following moving mold as it moves downwards (DZ-).
(3) Ejection. Moving mold travels a certain distance, reaching PS surface opening width sufficient to ensure plastic part can be removed from mold. Then, injection molding machine's ejector cylinder activates, pushing ejector plate 17 to eject in reverse direction of DZ-. This pushes ejector pins 31, lifters 4, ejector tubes 8, and other ejector elements on ejector plate 7, ejecting plastic part from core insert 5' and leaving it on these elements.
(4) Part Removal. Plastic part is removed from ejector elements by injection molding machine's robot arm.
(5) Mold Reset and Closure. When mold resets and closes, reset spring 29 pushes ejector plate 17 to reset first. Then, mold moves in reverse direction of DZ-, completely closing at PS surface before starting next injection cycle.

Physical mold is shown in Figures 8a and 8b, and physical plastic product is shown in Figures 8c and 8d. Production inspection revealed that mold structure is reasonable, easy to process and maintain, can meet shape and dimensional accuracy control requirements of plastic parts. High precision of fit between components ensures stability and reliability of mold during long-term use. Looking at actual plastic parts, mold cavity and core, after fine machining, have a low surface roughness value (Ra0.8) and high gloss, guaranteeing surface finish and dimensional accuracy of plastic parts.

 

Fig. 8 Mold and plastic part physical objects
2', 2, 20, 35, V1, V2, PS—Same as Fig.4-Fig.7; T1~T5—Same as Fig.1
As can be seen from actual plastic parts, parts have a smooth appearance, high surface finish, no obvious shrinkage, flash, bubbles, or other defects, and color is uniform. After quality inspection, basic dimensions of plastic parts met design requirements, tolerance range of key dimensions was controlled within specified MT4~MT5 grade range. Mechanical properties of plastic parts were good, strength, hardness, and toughness met customer's requirements.
After batch testing, mold's production efficiency and cost controllability were good, meeting requirements for large-scale, high-efficiency production of plastic parts. Furthermore, mold was easy to maintain, with minimal downtime and high production reliability.

(1) For injection molding of smoke alarm housing plastic parts, a single-cavity gating system was optimized using CAE, employing a single horn gate with a gate size of Ø0.8 mm. Under single-gate gating, optimized injection molding results of single-cavity gating system were: filling pressure approximately 30 MPa, filling time 0.5584 s, flow front temperature difference less than 3.8 ℃, holding time approximately 11 s, warpage less than 0.25 mm, injection cycle less than 36.65s.
(2) Mold structure is a two-plate mold with a two-cavity layout. For molding of straight wall of outer end face of plastic part, a side slider mechanism is used for molding and core pulling demolding, resulting in a smooth and flat surface. To address difficulty in demolding concentrated undercuts and side holes in certain areas, a merging method is used to integrate two side core pulling features into a single lifter for side core pulling demolding.
(3) In mold structure optimization design, three optimization designs were implemented to facilitate assembly: first, a temporary fixing screw is added to slider, which is beneficial for mold fitting at slider parting surface; second, a temporary limit adjustment rod is added to ejector plate, which is beneficial for installation of ejector plate mechanisms with many ejector elements, effectively ensuring operational safety during mold manufacturing; third, lifting holes are opened on parts weighing over 5 kg to facilitate lifting and prevent work-related accidents.