Abstract: Considering industrial design characteristics of printer roller guide plate plastic part and production requirements of molding modified alloy polycarbonate / acrylonitrile-butadiene-styrene plastic, a three-plate mold with a multi-point gate was designed for injection molding of this plastic part. Mold cavity layout is one cavity per mold to control molding accuracy of plastic part at MT3-4 level. Unlike conventional three-plate molds, this mold employs following design measures to ensure molding requirements of plastic parts: First, use of main guide pillars, secondary guide pillars, and mold core positioning cones ensures molding accuracy of basic molding dimensions of plastic parts. Second, by adding insert venting measures and optimizing injection molding process parameters, deformation of plastic parts after molding is effectively reduced and controlled, ensuring molding shape accuracy of plastic parts. Added molding inserts facilitate mold cavity venting, with insert gaps less than 0.02 mm. Insert design requires a 1 mm extension at hole edge to reduce difficulty of insert fitting and avoid flash due to insert gaps. Third, to improve cooling efficiency of runner waste and plastic parts, two Ø10 mm cooling water channels are provided in stripper plate, and ten Ø10 mm cooling water channels are provided in molded part of mold cavity. Mold also includes two inclined guide pillar slider side core-pulling mechanisms for demolding two end faces of plastic parts. Final demolding of plastic parts is achieved by multiple ejector pins evenly ejecting parts.
In industrial structural design of small and medium-sized printers, structural design of plastic components within printing function parts is a highly complex task. These plastic components generally have complex structures, requiring consideration of reliability of their function, molding method, and stability of molded part's shape to prevent functional failure due to deformation after a period of printer operation. Printer roller assemblies and their peripheral accessories are typical examples of such parts. Based on complex structural characteristics and dynamic service life requirements of a new printer roller guide plate plastic component, this paper selects optimal molding material and molding process for its injection molding, designs a three-plate injection mold with three gates for its injection molding production. Mold design, selection of demolding methods, demolding mechanism design, overall mold structure layout all provide valuable reference for mold design, can offer useful insights for design of molding dies for similar plastic parts. Specific design process of this plastic part mold is described below.

Figure 1 shows printer roller guide plate plastic part. Front structure of part includes 6 rectangular slots, 1 triangular hook, 1 straight hole, 1 rectangular hole, 1 material mark, 8 rectangular slots and 2 open slots formed by reinforcing ribs, 3 rectangular straight holes, 1 corner hole, and 1 irregular hole. Two hollow pin-hole posts are located on the left end face. Right end face features a multi-hole end support structure, consisting of multiple side circular holes, 1 bracket, 1 polygonal through hole, 1 concave surface, 1 arc-shaped hole, 3 rectangular slots, and 1 arc-shaped edge protector. Back of part has 4 rectangular slots and multiple short ribs. Both front and back of part have numerous reinforcing ribs, forming 6 rectangular slots and 2 open slots on the front and 12 short straight ribs on the back. Average wall thickness of plastic part is 1.5 mm, with a maximum wall thickness of 2.6 mm and a minimum of 1 mm. External packaging dimensions of plastic part are 320 mm * 100 mm * 80 mm. Production volume is 500,000 pieces, and molding precision is MT3-4.

 

Fig. 1 Plastic part of printer drum guide plate
q1～q8—Rectangular groove; j1, j2—Joint; s1—Material identification; h1, h2, h9—Rectangular straight hole; h3, h4—Lateral circular hole; h5—Polygonal hole; h6, h6′—Circular through hole; h7—Arc-shaped hole; h8, h10—Special-shaped hole; c1—Corner hole; w1—Edge protection; g1—Three legged hook; z1—Bracket; L—Left side; R—Right side; Rp—Right end face; p1—Camber concave; a—Front 3D view; b—Reverse 3D view; c—3D view of the right end face

Main difficulty in molding plastic parts lies in complexity of part's structure, making basic filling relatively difficult. Complex design of plastic part structure serves two purposes: firstly, to ensure functionality of printer roller assembly, and secondly, to prevent potential quality defects during injection molding, primarily warping. To address this issue, plastic part structure is designed with 8 transverse ribs and 2 longitudinal ribs on the front side and 12 short transverse ribs on the back side. Rib arrangement primarily affects processing difficulty of plastic molded part, location of mold gate, and filling of mold cavity. Molding of plastic parts presents several challenges: First, asymmetrical structure of left and right ends makes even filling difficult, potentially leading to defects such as over-filling or underfilling in certain areas. Second, complex structure of part makes it prone to insufficient filling due to poor venting in some areas during injection molding. Third, high precision required for molding necessitates robust mold design to ensure accuracy. Fourth, molded part needs good rigidity to guarantee stable performance and a long service life after assembly in printer. Fifth, demolding is challenging, particularly for left and right ends. Two hollow cylindrical joints on left are deep, using separate molded parts results in insufficient rigidity and strength, making them prone to breakage. Sixth, complex structure and high clamping force of right end face make it difficult to design molded parts, requiring specialized demolding mechanisms for this area. To address aforementioned issues, basic solutions are as follows: First, modified polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) is selected as plastic part material to reduce potential deformation issues from perspective of material fundamental properties, while also ensuring good flowability of filling material within mold cavity. Second, multi-point gating is used in mold cavity to ensure even filling at each filling end, reducing warping deformation caused by uneven filling. Third, repositioning methods are employed on main molding components of mold cavity, such as setting mold core jaws or parting surface ramps for positioning. Design incorporates several key elements: 1) improving parting line closure and positioning accuracy to ensure molding precision of plastic part; 2) establishing venting channels such as venting grooves to enhance venting within mold cavity and prevent insufficient filling in localized areas; 3) installing inserts in specific locations to reduce machining difficulty and enhance venting; 4) designing a slider-side core-pulling mechanism for demolding on the left and right sides; 5) evenly distributing multiple ejector pins to ensure balanced ejection and prevent deformation of plastic part during ejection; 6) optimizing injection molding process parameters to ensure uniformity and consistency of molding in all localized areas of plastic part.

Preferred molding material is high-heat-resistant PC/ABS (Chi Mei PC-365 from Taiwan), which features high heat resistance and high impact resistance. Its heat distortion temperature is 95℃, meeting requirement of maintaining dimensional stability even under conditions of high-volume paper printing. Material has a melt flow rate of 13 g/10 min, which is better than that of polymethyl methacrylate (PMMA) and PC. Its density is 1.1 g/cm³, water absorption is 0.4%, molding shrinkage is 0.5%, elongation at break is 85%, flexural strength is 73 MPa, and flexural modulus is 2255 MPa, meeting rigidity requirements of plastic parts. It also exhibits high impact strength at low temperatures. Material has stable chemical properties, particularly good indoor UV stability and electrical properties. It also has excellent flame retardancy (UL 94 5VB) and a wide color range, making it suitable for various office environments.

(1) Mold Cavity Partition Design. In molds, molded parts of single-cavity molds are generally made of alloy steel, which is more expensive than mold base material, to meet molding needs of different plastic materials for different shaped plastic parts. Therefore, considering manufacturing costs, molded parts of mold cavity are generally made of expensive alloy steel, assembled with inserts to form mold cavity. Each molded insert is then installed in fixed mold plate, moving mold plate, or other mechanical components of mold frame, forming mold cavity when mold is closed. Design of this plastic part's mold cavity also follows this design method. Figure 2 shows mold cavity parting design. As shown in Figure 2a, in 3D design software UG, based on a molding shrinkage rate of 0.5%, plastic part is magnified by 1.005 times to obtain mold cavity model. First, mold opening direction is determined to be F0 direction. Then, main parting line is obtained at maximum outer contour of mold cavity model. Finally, main parting line is extended to obtain main parting surface shown in Figure 2a, which consists of multiple different surfaces. In mold opening direction F0 upwards, main parting surface is used to part mold cavity to obtain cavity insert and core insert respectively. Based on determined main parting surface, right and left parting surfaces are set according to side core pulling requirements to obtain side core pulling molded part, as shown in Figure 2b.

 

Fig. 2 Mould cavity parting design
Cav—Cavity insert; Core—Core insert; LP—Left parting surface; RP—Right parting surface; PL—Main parting surface; F0—Mould opening direction; Product—Plastic parts; I1～I6—Sub-inserts; I7—Partial insert; I8, I9—Joint inserts; SL—Left slider; SR—Right slider; D1—Truncated cone; W1—Cavity insert water channel; W2—Core insert water channel
(2) Insert design. In molded cavity parts, for groove features, hole features, etc. of plastic parts, local small inserts need to be set to reduce difficulty of mold processing and ensure smooth air venting of mold cavity. Setting of small inserts is shown in Figure 3a. Among inserts installed on one side of cavity insert, text inserts I11 must be installed to facilitate engraving of text on molded part by CNC engraving machine. Since CNC engraving machine is generally small and cannot accommodate large molded parts, holes must be machined from large molded parts such as cavity insert using wire cutting before text insert is installed. Small inserts for other small-sized hole features are also designed for ease of machining. These local features often have deep ribs and narrow widths, depth of some areas may be very deep, making it difficult for general CNC machine tools to access them. Electrical discharge machining (EDM) would be time-consuming and expensive, so small inserts are also used. When designing small inserts I1 to I6 and I10, to facilitate assembly of molded part and reduce insert gaps, a mounting platform is generally installed on one side of rectangular bottom of insert to facilitate installation on cavity insert. Platform has a single-side width of 5 mm. To prevent molten plastic from flowing into gap between small insert and cavity insert during injection molding, causing flash defects in plastic part, small insert is designed in two stepped shapes. Lower half of step has same size as hole size in plastic part (i.e., hole edge size shown in Figure 3), while upper half of step is larger than lower half by a full circumference of hole edge, with hole edge extension set to 1 mm. Main reason for hole edge extension is that, in actual production, it is difficult to ensure that size of small insert matches size of reserved hole in cavity insert to accommodate it. This can cause molten material to enter these gaps during injection molding when mold cavity is closed, resulting in flash in plastic part. Hole edge extension design effectively prevents this problem. Design for right end face molding insert is shown in Figure 3b, and design for left end face joint molding insert is shown in Figure 3c. Shallow ribs on core insert can be formed using electrical discharge machining (EDM) because these areas require less machining time. Therefore, core insert can be a single-piece insert to ensure sufficient strength. Installation gap between small inserts is less than 0.02 mm to ensure effective venting while preventing flash from forming on plastic part. To ensure accuracy of mold cavity closure, a core positioning cone is installed in single-cavity mold for closing positioning control of cavity insert and core insert. Core insert, cavity insert, and all small inserts involved in molding process are made of S136 steel. S136 steel has excellent hardness, toughness, and corrosion resistance, making it suitable for producing high-quality, long-life molds.

 

Figure 3. Design of sub-inserts for cavity mold
I1～I5, I10—Sub-inserts; I11—Text insert; I12, I15—Hole inserts; I13—Right end groove insert; I14—Tripod hook insert; I15, I16—Cylindrical insert; I17—Outer cylindrical insert; I18—Inner cylindrical insert; j2—Joint; d1—Hole edge expansion distance; v1—Hanging platform; hb—Hole edge

After single-cavity mold design is completed, a mold base and auxiliary mechanisms need to be added externally to ensure that mold cavity can realize its molding function on injection molding machine. Therefore, in mold design, layout design shown in Figure 4a is required to optimize mold structure design and save mold manufacturing costs. Selection of mold base is based on standard of being able to accommodate molded parts of mold cavity, install demolding mechanism and other mechanisms. Since mold cavity will use a three-point gate for casting, a three-plate gate mold base is selected. To ensure that mold base can smoothly realize three-plate mold function, under basic premise that single-sided dimension of mold base is 50 mm larger than molded part size of mold cavity, in order to save mold base material, structural parts used in mold need to be pre-arranged to rationally select mold base. Mold cavity layout adopts a one-cavity design. Required structural components include: four reset rods at position P1, right slider at position P2, four ejector guide pillars at position P3, four secondary guide pillars at position P4, two precision positioning blocks at position P5, cooling water channels at position P6, screws at position P7, and left slider at position P8. Final mold base selected is LKM FCI 4050 standard mold base from LK. A top view of mold structure layout is shown in Figure 4b. Because guide mechanism components wear after a period of mold operation, to prevent a decrease in accuracy during cavity reset and closure, four sets of main guide pillars/sleeves, four sets of secondary guide pillars/sleeves, two precision positioning blocks are arranged in mold to ensure reset and positioning accuracy of mold cavity during closure.

Final three-plate mold structure design is shown in Figure 5. Due to large size of plastic part, a multi-cavity layout would increase injection volume requirements of injection molding machine; simultaneously, mold structure size would also increase, raising requirements for mold capacity of injection molding machine; furthermore, considering molding accuracy requirements (MT3～4 grade), a single-cavity layout can better ensure that molded plastic part achieves this accuracy. Therefore, considering production quantity, molding accuracy of plastic part, and economic requirements of equipment, a single-cavity mold is more suitable. Three point gates, G1, G2, and G3, are used in gating system for mold cavity, as shown in the bottom view of fixed mold in Figure 5. Due to high heat generated by gating system, two cooling pipes (11In-11Out, 12In-12Out) are installed on both sides of gating sleeve 10 on stripper plate 12 for cooling waste material in main gating system. Inlet diameter of point gating gates G1, G2, and G3 is 1.3 mm, and horizontal runner uses a trapezoidal cross-section with dimensions of 6 mm * 5 mm * 10º. When pouring through three point gating gates, pressure difference at each filling end corresponding to point gating gate is controlled within 3 MPa, effectively preventing over-pressure issues in localized areas during filling.

 

Fig. 5 Mold structure
1—Cavity insert; 2—Core insert; 3—Left slider; 4—Right slider; 5, 6, 7—Small inserts; 8—Sprue puller; 9—Positioning ring; 10—Sprue bush; 11—Fixed clamp plate; 12—Runner stripper plate; 13—Fixed mold Plate; 14, 40—Angle pin; 15, 16, 28, 29, 31—Slider shaped sub-inserts; 17, 33—Spring; 18—Sliding rod; 19—Moving mold plate; 20—Ejector; 21—Ejector retainer plate; 22—Ejector plate; 23—Travel switch; 24—Ejector guide pillar; 25—Height limiting block; 26—Return pin; 27—Sealing ring; 30—Locking block; 32—Long pull rod; 34—Short Pull rod; 35—Guide pillar; 36—Locking mechanism; 37—Spacer parallel; 38—Auxiliary guide pillar; 39—Return spring; K1, K2, K3—Mould opening surface; G1, G2, G3—Pin-point gates; S1, S2—Slider mechanism; 1In～12In—Cooling water inlet; 1Out～12Out—Cooling water outlet
For mold cavity cooling, six pipes are installed on moving mold side for cooling molded parts on that side. Specifically, 1In-1Out, 2In-2Out, 3In-3Out, 5In-5Out are used for cooling core insert 2; 7In-7Out, 8In-8Out, 9In-9Out, 10In-10Out are used for cooling cavity insert 1; 4In-4Out is used for cooling left slider 3; and 6In-6Out is used for cooling right slider 4. Cooling pipe diameter is 10 mm, and inlet water temperature is ambient temperature (25 ℃). For demolding left and right side features of plastic part, two inclined guide pillar slider mechanisms S1 and S2 were designed for side core pulling demolding at both ends. Mechanism S1 is a commonly used inclined guide pillar slider mechanism, its components including left slider 3, parts 14 to 18, spring 17 for positioning left slider 3 after core pulling, providing auxiliary driving force during core pulling. Basic mechanism of mechanism S2 is same as mechanism S1, except that molded part embedded on the right slider 4 is different. Final demolding of plastic part uses multiple ejector pins 20. These ejector pins 20 are evenly distributed at reinforcing ribs and deep walls where clamping force of plastic part is relatively large, as shown in top view of moving mold in Figure 5. Because plastic part has side core-pulling mechanisms on both sides, to prevent interference between ejector pins below left slider 3, right slider 4 and molding head on slider during ejection, ejector pins 20 must be reset after ejection before left slider 3 and right slider 4 can be reset. Therefore, limit switches 23 are installed at both ends of push plate 22 of the ejector pin 20. Only after reset signal of limit switch 23 is issued can injection molding machine push moving mold to reset and close.
Mold has three opening surfaces, namely K1, K2, and K3. Opening sequence of these three surfaces is K1→K2→K3, and the closing sequence is K2→K3→K1. Mold opening and closing control mechanism is a commonly used fixed-distance tie rod assembly. A single assembly includes parts 32, 33, 34, and 36, for a total of four sets, arranged on fixed mold side.

Working principle of mold is same as that of a common three-plate mold. Referring to Figure 5, specific process is as follows:
(1) Mold Closure. After injection mold closes and completes filling, pressure holding, and cooling processes on injection molding machine, it awaits mold opening.
(2) K1 surface opens. Moving mold platen of injection molding machine drives moving mold below K3 surface to retract. Under combined action of spring 33 and latch 36, mold first opens at K1 surface. Upon opening, three gate points break off, separating plastic part from runner waste.
(3) K2 surface opens. Moving mold continues to descend. When K1 surface opens 172 mm, due to pull of latch 36, mold opens at K2 surface. Stripper plate 12 pushes runner waste off pull rod 8 and automatically removes it. The K2 surface opens by 10 mm.
(4) K3 surface opens. Moving mold continues to descend, overcoming pull of latch 36, causing the mold to open at K3 surface. When K3 surface opens, inclined guide pillars of slider mechanisms S1 and S2 drive left slider 3 and right slider 4 to complete side core pulling.
(5) Ejection and Demolding. After moving mold descends a certain distance, injection molding machine's ejector pins hold push plate 22 stationary, thus keeping ejector pins 20 on push plate stationary on plastic part. As moving mold continues to descend, plastic part disengages from core insert 2 and automatically falls off ejector pins 20, achieving complete demolding.
(6) Reset. During reset, reset spring 39 pushes push plate 22 to reset first. Only after limit switch 23 sends a reset signal can moving mold continue to move upward to reset until it is fully closed. Mold then begins next injection cycle.

Injection molding process for plastic part is as follows: Pre-processing drying is essential to maintain a humidity level below 0.04%. Drying conditions are 100℃/3 h. Barrel temperature is 70℃ at feed section, melt temperature is 250℃, and constant barrel temperature is 200℃. Mold temperature is 80℃, injection pressure is 120 MPa, and holding pressure is 60 MPa to prevent product shrinkage. To minimize internal stress and prevent frictional heat, back pressure is set to 6 MPa. A medium injection speed is applied to optimize frictional heat, using multi-stage injection, progressing from slow to fast. After filling, a 2.2D (D is screw diameter) length is reserved at screw tip for holding pressure. After holding pressure, remaining buffer pad at screw tip is 5 mm. Because melt is sensitive to overheating, melt residence time in barrel does not exceed 5 minutes.

Based on injection molding requirements of printer roller guide plate plastic part, a three-plate mold was designed for its molding. Mold cavity layout is a single cavity, using three gates to ensure even filling of all areas within cavity. Pressure difference at the end of three gates is controlled within 3 MPa. For molded part, multiple small inserts are used in molding process, reducing difficulty of part processing and facilitating cavity venting. Insert gap is less than 0.02 mm. In insert design, hole edge extension distance must be designed, set to 1 mm, to reduce mold fitting difficulty and avoid flash due to insert gaps. PC/ABS melt has high heat, requiring increased cooling density in mold. Cooling channels must be provided in main runner to improve cooling rate of scrap and plastic parts, thereby increasing production efficiency. Mold opens in three stages, using a fixed-distance tie rod mechanism to control opening and closing sequence. Plastic part is finally demolded using ejector pins.