With improvement of people's living standards, requirements for various plastic products are also constantly increasing. In order to enhance competitiveness of plastic products, two-color molds have been used more and more widely in the market in recent years. In terms of two-color mold design, many scholars have conducted relevant research. Zhang Weihe took two-color plastic handle as research object and designed a two-color injection mold for handle. He focused on introducing problems and solutions that occurred during trial molding of mold when injecting soft TPE. Liu Qingdong took interior parts of Bentley cars as research object and designed a two-color injection mold for interior parts of cars. In view of difficulty of injection of plastic parts with flat rectangular frame structure, he adopted multi-point injection gate gating system design method. Lu Jianjun et al., using a car door outer edge strip plastic part as an example, introduced a new type of guillotine blade-type two-color injection mold design. Guillotine blade uses an inlaid structure, by controlling lifting and lowering movement of guillotine blade, hard and soft plastics are injection molded in a combined cavity. Based on research of aforementioned experts and scholars, this paper takes molding characteristics of a special island structure in a two-color button plastic part of a certain product as an example, illustrating its special injection mold structure design compared to traditional two-color molds, solutions to problems encountered during molding process. Conventional two-color plastic parts rarely exhibit island formation; even when islands appear, their number is relatively small, their area is relatively large. Conventional designs allow each island to be injected individually. In this case, due to limitations of plastic part's structural characteristics, a total of 8 small islands (H1 to H8) appeared. In mold design, since direct injection into these small, isolated islands is not feasible, a rear mold floating plate structure was specially designed. This floating plate lowers float, creating a gap between float and first-injection plastic part, achieving secondary injection. This injection method and optimized design for addressing molding process issues yielded excellent results after trial molding, providing valuable reference for similar two-color mold designs.

Figure 1 shows part structure diagram of button. The entire part is composed of two materials: green represents soft plastic TPE surface, and white represents hard plastic PC surface, with only character area being outer surface. Product has a very simple circular shape, with a medium to small size, measuring ϕ107.5 mm * 16.5 mm. There are three main difficulties in molding plastic parts: (1) Design of gating system: Main body of PC material for first injection is 1.3 mm thick. Elastic ribs connected to main body are located at five touch points (center, top, bottom, left, and right). These ribs are 1.0 mm wide and 0.7 mm thick. Rear mold also has many small ribs, especially outer ring ribs, which are 1.0 mm thick and 5.5 mm deep, making filling very difficult. (2) Since secondary injection cannot be directly injected into surface, problem of leaving marks on the surface of injection point needs to be solved. In particular, injection problem of letters R/e/o/a and inner ring H1~H8 (8 islands) in the middle ring closed by plastic part is key and difficult point of this mold design; (3) In view of characteristic of TPE material in secondary injection being prone to air entrapment, it is necessary to solve problems of flow marks, joint lines and air marks on the surface of plastic part, so that plastic part can obtain excellent appearance quality.

 

Figure 1 Button Structure Diagram

Mold adopts a simplified fine-gate mold base. Maximum external dimensions of mold are 330 mm * 490 mm * 326 mm. It consists of two sets of molds for primary and secondary injection, with a single set weighing 400 kg, classifying it as a small mold. A characteristic of two-color molds is that two rear molds must be identical. By rotating identical rear molds and using two different front molds at different nozzle positions, same product can be produced with two different materials and colors. Compared to traditional two-color molds, this two-color mold adds a floating plate structure to bottom of rear mold B plate. Its function is to control float's movement to achieve island-like injection during secondary injection. The overall mold structure diagram is shown in Figure 2.

 

1-Faceplate; 2-Sprue Plate; 3-A Plate; 4-B Plate; 5-Square Iron; 6-Ejector Pin Faceplate; 7-Ejector Pin Base Plate; 8-Base Plate; 9-Locking Module; 10-Flange; 11-Primary Pump Nozzle; 12-Water Channel; 13-Sprue Hook; 14-Primary Front Mold Insert; 15-Primary Front Mold Core; 16-Return Pin Limiting Post A; 17-Pressure Plate; 18-Rear Mold Core; 19-Return Pin; 20-Ejector Rod; 21-Modible Ejector Block; 22-Modible Ejector Block Seat; 23-Pin; 24-Ejector Sleeve; 25-Lower Float Plate; 26-Upper Float Plate; 27-Float; 28-Limiting Screw; 29-Shovel Base; 30-Wear-Resistant Block; 31-Slider; 32-Angled Guide Post; 33-Spring; 34-Secondary Pump Nozzle; 35-Secondary Front Mold Core; 36-Return Pin Limiting Post B; 37-Ejector Pin; 38-Ejector Mesh 39 - Limit switch; 40 - Parting surface tube positioning block; 41 - Guide post; 42 - Pull rod screw
Figure 2 Mold Assembly Diagram

The overall structure of button plastic part is a semi-circular shape. Front mold surface of the entire part is external appearance surface, composed of two parts. PC material body of first injection molded part is 1.3 mm thick, which is relatively thin. The only visible areas are white characters and a white ring in the middle. Challenge lies in elastic ribs connected to body body at five touch points in the middle, top, bottom, left, and right. These ribs are 1.0 mm wide and 0.7 mm thick. There is also a ring-shaped rib around rear mold with a thickness of 1.0 mm and a depth of 5.5 mm, making filling very difficult. To address this, mold structure was designed with an 11-point fine gate injection method. Optimized injection point positions must ensure that there are no defects such as weld lines on the surface, while also ensuring even injection and reducing injection pressure. One injection point is placed on or near elastic thin-walled ribs at five touch points in the middle, top, bottom, left, right to ensure injection needs of thin-walled ribs. In addition, six injection points are distributed along rib above ring-shaped thin-walled rib in the rear mold to ensure injection needs of the entire molded part and ring-shaped thin-walled rib, as shown in Figure 3(a). Second injection is made of TPE soft rubber. Since all mold surfaces are exterior surfaces, and front mold surface cannot be used for injection points, mold was designed with a three-point fine gate that connects to rear mold's large gate, allowing injection from bottom parting surface. A special vertical flipping design was also implemented at injection points. Three rear mold slider core-pulling structures were added to complete runner ejection. This design slows down injection speed under same injection pressure and speed. Combined with optimized venting pins, venting grooves, and venting inserts, it solves problem of air trapping in TPE material during secondary injection and avoids surface gate marks, resulting in excellent appearance quality for plastic part, as shown in Figures 3(b) and (c).

 

Figure 3: Injection Diagram

The biggest difference between this case and traditional two-color molds is addition of a floating plate mechanism at the bottom of rear mold B plate 4, enabling injection for eight islands (H1~H8). Float plate structure includes an upper float plate 26, a lower float plate 25, a float 27, and a return needle 19. Lower float plate 25 has movable top blocks 21 on its bottom left and right sides, connected to movable top block seats 22 via pins 23, then fixed to square iron 5. Movable top blocks 21 can rotate along pins 23. A plate of primary injection mold has a corresponding ejector rod 20 and a return needle limiting post A16. When mold is in primary injection, ejector rod 20 will press against movable top block 21, causing it to push float plate and lift float 27 until return needle limiting post A16 and return needle 19 coincide. Upward distance is limited to 0.7 mm, which is equivalent to a 0.7 mm reduction in plastic material at float 27 position in primary injection mold. Simultaneously, primary front mold core 15 adds a through-hole within float 27 area. A plate of secondary injection mold has a corresponding return needle limiting post B36, length of which is 0.7 mm greater than length of return needle limiting post A16. Mold eliminates ejector pin. During secondary injection, return pin limit post B36 pushes return pin 19, causing float plate to sink 0.7 mm, creating a 0.7 mm gap between float and primary injection part. This gap is then connected to main body of secondary injection part, eight isolated injection points (H1-H8) via pre-drilled holes on primary part, achieving purpose of injection. Rear mold float plate structure is the most innovative design feature of this mold. See Figure 4 for a schematic diagram of float state and Figure 5 for a diagram of float plate structure.

 

Figure 4: Schematic diagram of float state

 

3-A plate 4-B plate 5-Square iron 15-Primary front mold core 16-Return pin limit post A 18-Rear mold core 19-Return pin 20-Ejector rod 21-Modible ejector block 22-Modible ejector block seat 23-Pin 25-Lower float plate 26-Upper float plate 27-Float 35-Secondary front mold core 36-Return pin limit post B
Figure 5: Float plate structure

Venting system of mold has a significant impact on molding quality of plastic part, especially for products with high requirements for surface appearance. As shown in Figure 6(a), in addition to a ring-shaped venting groove on parting surface of front mold core for first injection, venting inserts were made on both sides and central circular position inside mold where air is easily trapped, totaling five inserts. Figure 6(b) shows that a ring-shaped venting groove was also made on parting surface of front mold core for second injection. Figure 6(c) shows that in addition to ejector sleeve and ejector pin holes for auxiliary venting, rear mold core features optimized design with blue venting inserts and red venting pins. In particular, four small venting inserts were added inside central float, achieving multi-angle, all-around venting. Combined with vertical flipping injection method of secondary injection runner, this solves problem of air trapping in TPE material during second injection, ensuring excellent product appearance quality. As shown in Figure 7 above, product molding effect clearly shows a very good product appearance.

 

Figure 6 Schematic diagram of exhaust system

 

Figure 7 Product molding effect diagram

(1) First mold closing: After primary injection mold completes injection, pressure holding, and cooling, first injection molding of part is completed; (2) First mold opening: Robot completes removal of sprue from first injection, two-color injection molding machine performs a 180° rotation and interchange of rear mold; (3) Second mold closing: After primary and secondary injection molds simultaneously inject, hold pressure, and cool, primary injection mold completes first injection molding of part, and secondary injection mold completes second injection molding of part; (4) Second mold opening: Robot simultaneously completes removal of sprue from primary and secondary injection molds, secondary injection mold ejects product → removes part → resets ejection system → injection molding machine performs a 180° rotation and interchange of rear mold, repeating a series of cyclical actions of mold closing, molding, mold opening, ejection, part removal, and rear mold interchange to carry out production task.

(1) The biggest design difficulty of this mold is to achieve H1~H8 through rear mold floating plate structure. For eight isolated injection points, since two-color mold must meet principle of consistency between two sets of rear molds, this floating plate structure is preferred solution to solve similar problems when an isolated structure cannot be injected individually;
(2) Gating system design is another highlight of this case. In primary injection gating design, injection of thin-walled part is achieved through a multi-point fine-water injection method, effectively solving problems of difficult filling and high injection pressure in thin-walled area. In secondary injection gating design, problem of sprue marks on product surface is solved by using a three-point fine-gate to large-gate injection method. Additionally, a special vertical flip design is used in the runner at injection point, and runner is ejected through a rear mold slider. Although this increases processing difficulty and cost of mold, this design slows down flow rate, effectively preventing air marks and flow lines, resulting in a better surface finish for plastic part.
(3) Mold venting system design addresses issue of poor venting and trapped air in injection molds for soft materials like TPE. To prevent this, mold is optimized with multi-angle, all-around venting methods, including venting grooves, venting pins, and venting inserts, based on product's structural characteristics in primary front mold core, secondary front mold core, and rear mold core. This successfully solves problem of poor appearance caused by trapped air in both primary and secondary injection molded parts.