Abstract: A two-color injection mold was designed for molding a two-color plastic part of a medical electronic detector stand. Based on production requirements of different materials and colors for plastic part, first injection base part was molded from polycarbonate (PC), and second injection insert was molded from PC/acrylonitrile-butadiene-styrene (ABS). Melt temperature of second injection material was 35 ℃ lower than that of first injection material to ensure smooth molding of second injection. Based on formulation of molding processes for first and second injections, a two-color mold for molding plastic part was designed. Two-color mold adopts a rotary structure and consists of two sub-molds, both of which are two-plate molds. Gating system for both sub-molds adopts a cold runner + hot nozzle configuration. To address issue of flash formation during second injection molding of two-color molds with slider mechanisms, a first-injection core-pulling slider mechanism with a triangular cross-section inclined guide post was designed. This mechanism improves upon commonly used cylindrical inclined guide posts by replacing them with triangular cross-section inclined guide posts, adds spring pins, and modifies inclined guide post holes into U-shaped grooves. These measures eliminate need for side core pulling during first injection molding, ensuring tight sealing of mold cavity in second injection mold and increasing part quality pass rate to over 99%.
For modern medical electronic testing devices or instruments, based on product appearance design or testing needs, it is often necessary to design structural plastic parts of some testing devices or instruments as two-color plastic parts, achieving mass production through two-injection molding. In injection molding, two-shot molding is a process that uses two sets of molds and two barrels of a two-shot molding machine to form plastic parts containing two different materials in two separate molding processes. Two-shot molding is an important molding process in injection molding, and its molded products have following advantages: firstly, high product precision and stable quality; secondly, good structural strength and durability; thirdly, small fit clearance and good appearance; fourthly, low production cost, while also increasing added value as composite plastic parts. Two-shot molding has evolved into various molding methods according to practical needs of plastic part design. Three most common are: ① two-shot molding with different materials (mixed material molding); ② bounded two-color two-shot molding with different/same materials and different colors (two-color molding); and ③ two-shot molding with same material but different colors (mixed color molding).
Different two-shot molding methods have different applications. Mixed material molding mainly solves problems related to assembly of plastic parts and feel of grip, while two-color and mixed color molding mainly solve problems related to color and appearance of plastic parts. In two-shot molding, bonding between materials occurs in two ways: one is through mechanical bonding via features such as undercuts, i.e., pseudo-bonding; the other is through fusion of materials.
In two-color molding, second material is typically injected after first material has cooled and solidified, resulting in a clear bonding boundary between two materials. Molded products are often parts combining soft and hard plastics. First injection is of hard plastic, and second is of soft plastic. This method of mold making is called a forward mold, and it is currently the most common mold-making method in two-color molding production. However, there are also a few reverse molds, where first injection is of soft plastic and second is of hard plastic. This is generally due to product structure limitations and is only used when a forward mold is not possible. Difficulty of mold making and molding is very high in this case. Reverse mold technology is relatively rare and is still under further exploration.
This paper addresses requirement of using two different colored materials for molding a medical electronic detector bracket. A two-color molding process and a rotary two-color molding die were designed for this plastic part. An improved triangular cross-section inclined guide post slider mechanism was designed within die, which is highly suitable for development of two-color molding dies with side core pulling. This design provides valuable reference for research and development of similar dies.

Structural composition of two-color plastic part for a medical electronic detector bracket is shown in Figure 1. Part consists of a base part C1 and an insert body C5, and is molded using a two-color molding method. First injection molding of two-color molding base component, C1, has a basic shape of a 118 mm * 17 mm * 7 mm frame. Its basic structure consists of two sides connected by five central crossbeams. Each side has five side holes C3, totaling ten. Four bosses C2, each 9 mm high, are located on four central crossbeams. Bottom surface of base component also features multiple reinforcing ribs and small local bosses. Second injection molding of two-color molding process forms four independent insert bodies C5 located on four bosses C2. During two-color molding process, molding sequence is as follows: based on first injection molding of base component C1, four insert bodies C5 are then formed in second injection molding, thus forming two-color plastic part C4 for medical electronic detector bracket.

 

Figure 1. Dual-color plastic part of medical electronic detector bracket
C1—Basic plastic part; C2—Convex platform; C3—Side hole; C4—Dual-color plastic part; C5—Plugging sheet body; I—Basic plastic part; II—Dual-color plastic part with plugging sheet body; III—Bottom surface of plastic part

Based on molding requirements of plastic part, base part and insert body are molded using materials of different colors. Base part material is polycarbonate (PC) plastic of grade CALIBRE IM 401-11 (Cathay Plastics, Inc., USA), with a shrinkage rate of 0.51%~0.69%. Insert body is made of Multilon T-3011 TG5667, a PC/acrylonitrile-butadiene-styrene (ABS) modified material (Teijin Corporation, Japan), with a shrinkage rate of 0.52%~0.65%. This material is an ABS-modified PC alloy. Base part is off-white, and insert body is black; both materials require color modification. For two-color molding, melting point of first injection molding material must be higher than that of second injection molding material to prevent second injection molding material from melting first injection molding material at boundary between two materials, which would prevent second injection molding from completing properly. Therefore, in material selection design for this two-color plastic part, CALIBRE IM 401-11 has a melt temperature range of minimum 280 ℃, maximum 320 ℃, and a rated melt temperature of 300 ℃; Multilon T-3011 TG5667 has a melt temperature range of minimum 230 ℃, maximum 300 ℃, and a rated melt temperature of 265 ℃, which is 35 ℃ lower than melt temperature of CALIBRE IM 401-11, thus meeting basic requirements for two-color molding materials of this plastic part.

In two-color molding production, injection control systems for first and second injections of two-shot injection molding machine are separate, forming two completely independent operating systems, allowing for free adjustment of parameters of first and second injections. However, in actual two-shot molding, molding parameters of first shot may affect quality of second shot. This is because second shot uses plastic part from first shot as a core, and base plastic part from first shot can be considered a molding component of mold cavity for second shot. This molding component has a very important influence on molding quality of second shot. Therefore, when adjusting molding process parameters of first shot, it is necessary to consider whether it will have an adverse effect on second shot. To meet requirements of two-color molding for this plastic part, molding processes for first and second shots are set separately.

Molding process parameters for first injection of basic component are as follows: Material drying: 90℃/2h (infrared oven); Nozzle temperature: 285~290℃; Mold temperature: 80~85℃; Injection time: 1~2s, injection pressure: 90~110 MPa, holding time: 4~8s, holding pressure: 40~60 MPa, three-stage holding pressure, back pressure: 3~5 MPa; Cooling time: 12~15s.

Molding process parameters for second injection of insert body are: ① Drying material at 80℃/2 h (infrared oven); ② Nozzle temperature 255~260℃; ③ Mold temperature 60~80℃; ④ Injection time 1~2 s, injection pressure 60~80 MPa, holding time 2~4 s, holding pressure 30~50 MPa, two-stage holding pressure, back pressure 2~3 MPa; ⑤ Cooling time 5~8 s.

When designing a rotary two-color molding mold using forward mold design method, basic structure of two-color molding mold consists of two sets of secondary sub-molds. First sub-mold is used for molding base plastic part of first injection, and second sub-mold is used for molding insert body of second injection. Two sub-molds are arranged symmetrically at 180° on turntable of injection molding machine's moving mold platen. Moving mold structure is same for both, but fixed mold structure is different. Main difference in fixed mold structure lies in different fixed mold cavity insert structures within mold cavity composition. When designing fixed mold cavity insert for first injection, injection molding area in first sub-mold needs to be sealed, while this second injection molding area is left open in second injection, second sub-mold. Based on this design principle, two-color mold design for two-color plastic part of medical electronic detector bracket is as follows.

In this two-color molding mold design, mold cavities of first injection, first sub-mold, and second injection, second sub-mold need to be designed separately, as follows.
3.1.1 Design of the First Injection Base Part C1 Molded Part
Design of base part molded part is shown in Figure 2. Since shape of base part is relatively simple, a planar parting surface can be used to part mold cavity to obtain main molded part cavity insert 1 and core insert 2. Based on this, combined with plastic part structure shown in Figure 1, 10 side cores are designed for side core pulling and demolding due to 10 side holes on both sides of base part. These include side cores 5, 6, 7, and 8. Two side sliders are used to drive these 10 side cores: a left slider 3 and a right slider 4. Five side cores are integrated on left slider 3, and five side cores are integrated on right slider 4. Thus, only two side sliders are needed to achieve molding and core pulling demolding of five side holes on each side of base part. In addition to driving 10 side cores for core pulling, left slider 3 and right slider 4 also participate in molding of most of outer surface of base part. This is done to facilitate design of second injection insert body molding cavity. In this design, four insert pieces 11 are placed in first injection molding cavity to block molding positions of insert body in second injection molding cavity. These four insert pieces 11 are then removed from second injection molding cavity to obtain molding positions of insert body. Molded part in second injection molding cavity only requires machining of four modified insert pieces 11'; the other molded parts can have same structure as molded parts in first injection molding cavity. Alternatively, modified insert pieces 11' can be omitted, and only cavity insert 1' of second injection molding cavity needs to be machined. This simplifies machining and saves on mold manufacturing costs. In gating system setup for first injection molding cavity of base part, two sets of gating systems are used to gating cavity. One set uses a hot runner H1 to supply two submarine gates G1 for gating cavity, and the other set has same design. In mold cavity cooling design, one Ø10 mm water channel is used for cooling both cavity insert 1 and core insert 2, one Ø8 mm water channel is used for cooling both left slider 3 and right slider 4. Final molded two-color plastic part is ejected from mold by multiple flat ejector pins 12 after second injection molding of insert body. Runner waste from first injection molding of base body and runner waste from second injection molding of insert body are ejected from mold by their respective pull rods 9 after second injection molding.

 

Figure 2 Design of basic plastic part forming parts
1—Cavity insert; 2—Core insert; 3—Left slider; 4—Right slider; 5, 6, 7, 8—Side cores; 9—Sprue puller; 10—Small insert; 11—Plugging Sheet inserts; 12—Blade ejector; H1—Hot nozzle; R1—First injection gating cold runner; G1—Submarine gates; PL—Parting surface; C1—Basic plastic part; C2—Convex platform;
Gating system for first injection basic part forming cavity is shown in Figure 3a. Two gating systems are set up for gating this cavity. Runners used are R1 and R1'. Runners R1 and R1' each have two submarine gates G1 for feeding material. Therefore, basic part forming cavity uses four submarine gates G1 for injection. Gating system for second injection gating cavity is shown in Figure 3b. When second injection gating cavity is being molded, four independent gating cavities need to be gated separately. Therefore, runners R2 and R2' are provided on parting surfaces of left slider 3 and right slider 4 to supply material to four side gates G2. Four side gates G2 are used for gating four gating cavities respectively. Installation configuration of four inserts 11 in first injection cavity on cavity insert 1 is shown in Figure 3b.

 

Fig. 3 Design of first and second injection gating systems
1, 2, 3, 4, 8, 11 are same as Fig. 2; R1, R1'—First injection gating cold runners; R2, R2'—Second injection gating cold runners; G2-Side gates
3.1.2 Cavity Design of Insert Body C5
Layout of two-color molding die is shown in Figure 4a. Structure of second injection molding cavity of insert body C5 is basically same as that of first injection molding cavity. That is, components of first injection molding cavity of base part C1 are simply rotated 180° around axis O-O' of injection molding motor platen turntable to obtain second injection molding cavity assembly of insert body. However, cavity insert 1' of second injection molding cavity is different from cavity insert 1 of first injection molding cavity. That is, four insert inserts 11 are removed from cavity insert 1'. Space formed after removing four insert inserts 11 is cavity space of four insert bodies, which is used for second injection molding of four insert bodies. In this way, second injection molding can be carried out after first injection molding cavity, and two-color plastic part of medical electronic detector bracket can be formed. Shape of second injection molding cavity is shown in Figure 4b. In components of second injection molding cavity, base component of first injection molding actually serves as a molding part of second injection molding cavity.

 

Fig. 4 Layout of dual-color mold cavity and design of second injection plugging sheet body C5 mold cavity
1-12 are same as Fig. 3; 1’—Second injection cavity insert; 2'—Second injection core insert; 3'—Second injection left slider; 4'—Second injection right slider; 5', 6'—Second injection side cores; 12'—Second injection blade ejector; O-O'—Injection molding machine moving plate turntable axis; V2—Second injection mold cavity; C1—Basic plastic part

Two-color molding mold for medical electronic detector bracket consists of two sub-molds, final design of which is shown in Figure 5a. In two sub-molds, moving mold M1 of first injection mold and moving mold M2 of second injection mold have same structure, are arranged symmetrically along a 180° axis when installed on injection molding machine's moving platen turntable. Fixed molds differ in three ways: First, molding surfaces of cavity insert 1 in first injection mold D1 and cavity insert 1' in second injection mold D2 are different, and cavity insert 1 has four insert pieces 11, while cavity insert 1' has these four insert pieces 11 removed. Second, first injection mold D1 only has two wide locking blocks 15 to lock left slider 3 and right slider 4, while second injection mold D2 has two triangular section inclined guide pillars 14 and 14', two narrow locking blocks 16 and 16'. Left slider 3' and right slider 4' are driven by side core pulling. Locking of left slider 3' and right slider 4' is achieved by an inclined locking surface in fixed mold plate 19. Third difference lies in gating system. As shown in Figure 5b, in gating system of first injection mold, two hot nozzles H1 supply material to four submarine gates G1 through cold runners R1 and R1' (Figure 3a). In gating system of second injection mold, two hot nozzles H2 supply material to four side gates G2 through cold runners R2 and R2' (Figure 3b). Both first and second injection molds are two-plate hot runner molds, single-opening, with opening surface being PL surface. Ejection mechanisms of both sub-molds are identical, both using a conventional ejector plate ejection mechanism with ejector plate 23.

 

Fig. 5 Dual-color mold structure
1, 1'—Cavity insert; 3, 3'—Left slider; 4, 4'—Right slider; 11—Plugging sheet inserts; 13—Plug in box; 14, 14'—Triangular section angle pin; 15, 15'—Wide locking block; 16, 16'—Narrow locking block; 17—Fixed mouldbase plate; 18—Hot runner partition plate; 19—Fixed mold plate; 20—Moving mold plate; 21—Mould foot; 22—Ejector retainer plate; 23—Ejector plate; 24—Mouldbase moving bottom plate; 25—Ejector sleeve; 26—Hot runner plate; 27—Locating ring; H1, H2—Hot nozzles; PL—Parting surface; O-O'—Injection molding machine moving plate turntable axis; M1—Moving mold of first injection Mold; M2—Moving mold of second injection mold; D1—Fixed mold of first injection mold; D2—Fixed mold of second injection mold; Z1, Z2—First and second injection mold

In this two-color molding mold, a first injection core-pulling slider mechanism driven by a triangular cross-section inclined guide post was designed. In commonly used inclined guide post driven slider mechanism designs, cylindrical inclined guide post drives slider to perform side core pulling through driven inclined guide post hole. However, when this mechanism is applied to this two-color molding mold, it will cause problems with molding quality of plastic parts. Specifically, as shown in Figure 5a, when left slider 3 and right slider 4 of first injection mold are switched to position of second injection mold, they will become left slider 3' and right slider 4'. During this process, when mold is first opened, left slider 3 and right slider 4 are driven open by cylindrical inclined guide pillars on their respective first injection molds. When they are switched to position of second injection mold, become left slider 3' and right slider 4', they are driven closed by cylindrical inclined guide pillars on second injection mold. Since base part after first injection molding still has a certain temperature before demolding, base part has a certain dimensional change after losing tightness of left slider 3 and right slider 4. This makes it difficult for left slider 3' and right slider 4' to completely wrap base part when they close in second injection mold. As a result, left slider 3' and right slider 4' cannot be completely closed. In this state, molding cavity of second injection mold is prone to not being tightly closed during injection molding, resulting in flash and burrs, which reduces molding quality of two-color plastic part of medical electronic detector bracket. Solution is to keep left slider 3 and right slider 4 closed after first injection molding, opening only after second injection molding. Based on this requirement, a first-injection core-pulling slider mechanism driven by a triangular cross-section inclined guide post 14, as shown in Figure 6, was designed. Taking modification of left slider 3' as an example, inclined guide post hole of commonly used cylindrical inclined guide post slider core-pulling mechanism was first modified into an open "U-shaped groove" as shown in Figure 6. Then, two sets of spring-driven spring pin 29 assemblies were installed on both sides of groove.

 

Figure 6. Triangular section angle pin slider mechanism
T1, T2—Unlock plane; T3—Drive plane; pin; F0—mould opening direction; F1—Core pulling direction; 3'—Left slider; 14—Triangular section angle; 28, 28'—Spring; 29, 29'—Spring pin; U1—U-shaped groove
In addition, commonly used cylindrical angled guide post is modified into a triangular section angled guide post 14 as shown in Figure 6. Triangular section angled guide post 14 has three planes T1, T2, and T3. Among them, T3 plane is used to drive left slider 3' in second ejector mold to pull core laterally. That is, when moving mold M2 moves downward according to F0 and drives left slider 3' to move downward, triangular section angled guide post fixed on one side of fixed mold D2 of second ejector mold will drive left slider 3' to pull core according to F1 due to obstruction of spring pins 29, 29'. When second ejector mold closes, surfaces T1 and T2 will compress spring pins 29 and 29' respectively, allowing triangular section inclined guide post 14 to enter "U-shaped groove". This achieves following effects: when second ejector mold closes, triangular section inclined guide post 14 can enter "U-shaped groove", and when mold opens, triangular section inclined guide post 14 can drive left slider 3' to perform side core pulling in F1 direction. Thus, in the entire two-color molding mold, in first ejector mold, there is no need to set inclined guide posts to drive left slider 3 and right slider 4 to open; while in second ejector mold, triangular section inclined guide posts 14 and 14' can be easily inserted into left slider 3' and right slider 4' (since right slider 4' and left slider 3' have similar structures, right slider 4' is driven by inclined guide post 14'), and when second ejector mold opens, it can drive left slider 3' and right slider 4' to perform side core pulling, ensuring automated and smooth demolding of two-color plastic parts for medical electronic detector bracket. By modifying cylindrical inclined guide post mechanism into a triangular cross-section inclined guide post slider mechanism, problem of flash during molding of plastic parts was solved, and yield rate of molded parts increased to over 99%.

Working principle of designed two-color molding mold is shown in Figure 7. Its working process is as follows:

 

Fig. 7 Working principle of dual-color mold
M1—Moving mold for first injection mold; M2—Moving mold for second injection mold; D1—Fixed mold for first injection mold; D2—Fixed mold for second injection mold; 23, 23'—Ejector plate; F0—Mold opening direction; O-O'—Injection molding machine turntable axis; PL—Parting surface; B1—Fixed mold plate for injection molding machine; B2—Moving mold plate for injection molding machine; B3—Turntable; J1, J2—First and second injection barrels; Z1, Z2—First and second injection mold
(1) Moving mold platen of injection molding machine drives first and second injection molds to close at mold opening surface PL. Closed state is: first injection mold D1+M1, second injection mold D2+M2.
(2) First injection barrel opens first, injecting first injection of plastic (first color PC) into forming cavity of first injection mold of base component. Second injection barrel closes first, and waits for forming cavity of first injection mold to complete filling, pressure holding, and cooling processes before opening mold.
(3) Moving platen of injection molding machine pulls moving molds M1 and M2 of first and second injection molds backward by pressing F0, and two sub-molds open synchronously at PL surface. As shown in Figure 5, when opening, left slider 3 and right slider 4 of first injection mold do not pull core and remain in a closed state. Left slider 3' and right slider 4' of second injection mold are driven by triangular cross-section inclined guide pillars 14 and 14' to pull core without contact. Ejection mechanisms (push plates 23 and 23') of both sub-molds do not operate.
(4) 180° rotation for exchange. After moving mold PL surface opens, turntable on injection molding machine's moving platen rotates 180° counterclockwise around O-O' axis, exchanging moving molds of two sub-molds. Then, two sub-molds close again at PL surface. At original position of first injection mold, combined state is D1+M2, forming first injection mold; at original position of second injection mold, combined state is D2+M1, forming second injection mold.
(5) Simultaneous Double-Injection Molding. Mold closes a second time at PL surface, first and second barrels open simultaneously for injection molding. First injection material (first color PC) is injected into forming cavity of D1+M2 sub-mold, and second injection material (second color PC/ABS) is injected into D2+M1 sub-mold. After both sub-molds have completed injection molding, two sub-molds open at PL surface.
(6) Mold Opening and Ejection. After injection molding machine's moving platen moves backward to a certain position by pressing F0, ejector cylinder, mounted on injection molding machine's moving platen and located at the bottom of second injection mold, actuates, ejecting push plate 23 of moving mold M1 of first injection mold in D2+M1 sub-mold. This ejects molded medical electronic detector bracket two-color plastic part, and after ejection, push plate 23 quickly resets. During this process, moving mold M2 of second injection mold, located at position of first injection mold, does not eject.
(7) 180° Rotation Return Exchange. Rotating turntable of moving mold injection molding machine rotates 180° clockwise around O-O' axis and returns, exchanging moving mold again. Two sub-molds close at PL surface, and two-color mold returns to state of step (1), i.e., first injection mold D1+M1 and second injection mold D2+M2. Reciprocating injection cycle of steps (5) and (6) is then initiated. One two-color plastic part for a medical electronic detector bracket is formed in a single mold opening.

A two-color, two-shot injection molding process and a two-shot molding mold were designed to meet molding requirements of two-color plastic part for medical electronic detector bracket. In material selection, compatibility of two-color materials was considered. Melting temperature of second injection material must be lower than that of first injection material, and injection molding is performed at a relatively low pressure.
Designed two-color molding mold adopts a rotating two-sub-mold combination structure. Both sub-molds are two-plate molds, and gating system uses a cold runner + hot nozzle configuration. In mold, a first-shot core-pulling slider mechanism with triangular cross-section inclined guide post for two-color molds was designed. This design solves problem of incomplete sealing of second-shot mold cavity caused by conventional inclined guide post slider mechanism when there is a need for side core pulling in two-color molds, and improves qualified rate of molded plastic parts to more than 99%.