Abstract: Based on analysis of process performance of housing of a mining wireless data acquisition instrument, a three-plate two-color mold with a split structure and a two-cavity layout was designed, featuring a fixed mold and a sliding block core-pulling mechanism. To achieve two-stage injection molding of this product, two sets of cavities with different structures and two sets of cores with same structure were designed. Addressing design challenge of having part of product's outer surface located in moving mold cavity, a fixed-mold core-pulling method was used to convert lower half of cone-shaped outer surface of front shell into a fixed mold cavity for molding. A fixed-mold "bent pin + slider" core-pulling structure was designed. For front shell core, which is relatively high and has a narrow slit at its root, this core was designed as an insert, saving expensive materials and simplifying processing. Both injection molding processes used a single-point gate gating system. Moldflow2021 software was used to analyze and determine appropriate gate location, verifying rationality of gating system design and reducing number of mold repairs. To address design challenge of internal threaded undercut, threaded core was designed as a movable insert. Specifically, a hexagonal through-hole with protrusions was machined inside fixed mold slider and threaded core. A hexagonal shaft with grooves was fixed to slider with a pin. Threaded core can be inserted into or removed from hexagonal shaft through hexagonal hole, thus simplifying mold structure. Mold has been put into production, its operation is smooth and stable, producing high-quality two-color products.
In recent years, with increasing requirements for appearance and function of plastic products, two-color injection molding has been applied more and more widely. Two-color injection molding is currently a relatively advanced injection molding process. It uses a two-color mold and is equipped with a two-color injection molding machine to directly produce two-color products composed of two different colors of materials. Intrinsically safe wireless data acquisition instruments are widely used in monitoring and testing of parameters such as coal mine gas, safety devices, and equipment operation. Data acquisition instrument shell is assembled from a two-color front shell and a two-color back cover, with circuit boards and electronic components installed inside. Shell is positioned by convex and concave stop, is fastened with screws, which causes soft rubber of front shell and back cover to be squeezed to achieve a seal. In addition, seal at the button of front shell is made of soft rubber injection molding, with a sealing level of IP67. Based on our design experience, we analyzed injection molding process performance of a certain mine wireless acquisition instrument as an example. we designed a three-plate two-color mold with a fixed mold and a slider core pulling mechanism using UG NX2312.

Outer shell of mine wireless acquisition instrument is assembled from a two-color front shell and a two-color back cover, with an annual output of 50,000 pieces. First color product inside front shell and back cover is made of hard rubber, while second color product on the outside is made of soft rubber. In addition to providing protection, insulation, and dustproofing, it also adds waterproof, shockproof, and anti-collision functions, meeting requirements of harsh working environment of coal mines. This type of two-color product has a thin wall thickness, high precision requirements, many column holes, bone positions, and other structures, many undercuts, and internal thread undercuts. Its mold structure is complex and typical. Two-color front shell is shown in Figure 1. It is a deep shell structure product with an average wall thickness of 3.4 mm and an external size of 99 mm * 47 mm * 41.4 mm. Purple-red part P1 is first transparent product, made of polycarbonate (PC) with a volume of 36.4 cm3. Orange part P2 is second color product, made of thermoplastic elastomer (TPE) with a volume of 19.1 cm3. Two-color back cover is shown in Figure 2. It is a plate-like product with an external size of 76.9 mm * 47 mm * 11.2 mm. Cyan part N1 is first color product, made of PC with a volume of 7.6 cm3. Orange part N2 is the second color product, made of TPE with a volume of 5.2 cm3. Two materials mentioned above have good compatibility and meet requirements of two-color injection molding. Shrinkage rate of first color product material PC is low, generally 0.5%~0.7%, and is taken as 0.6%, with poor fluidity. Second-color product is made of TPE, a soft rubber that provides waterproofing, shock absorption, and drop protection. Its shrinkage rate is same as first-color product during design.

 

Fig. 1 Two-color front shell
P1—First-color part; P2—Second-color part

 

Fig. 2 Two-color back cover
N1—First-color part; N2—Second-color part
Front and back covers have a fit precision of MT3. After assembly, they are exposed on outer surface, requiring a high level of aesthetics. Other than these, there are no special requirements. Key points and difficulties of this mold design are as follows: Firstly, as shown in Figure 1, if maximum outline of front shell in Figure 1a is taken as parting line (red), cavity for forming conical outer surface of front shell (see ellipse in Figure 1a) is located in moving mold cavity, which does not conform to design principle of a two-color mold (i.e., moving mold product has same shape, while fixed mold product can have different shapes). Therefore, this must be converted into a fixed mold cavity. This results in outer surface and internal threads of conical shape being undercut, right side wall of front shell being a straight wall with multiple undercuts (see ellipse in Figure 1b), hindering mold opening movement. Therefore, a suitable fixed mold core-pulling mechanism must be designed. Secondly, due to presence of closed narrow slots, button holes, numerous pillar holes, and reinforcing ribs in this product, a rational design of moving mold core is required. Except for right side wall of front shell, which has a 0º slope (this outer surface is formed by a slider), all other sides have reasonable slopes to facilitate product demolding. As shown in Figure 2, two-color back cover has a simple structure with no undercuts, and cavities of second-color product are all located in fixed mold.

As mentioned earlier, elliptical portion of front shell in Figure 1a needs to be converted into a fixed mold cavity. This is achieved using a fixed mold core-pulling method. Based on this, according to parting surface selection principle, parting surface design for second-color product of front shell is shown in Figure 3a, at its maximum outline. According to two-color mold design principle, moving mold cavity and parting surface of first and second-color products must be identical. Therefore, glue area of second-color product must be used as parting surface sealing of first-color product. Therefore, parting surface design for first-color product of front shell is shown in Figures 3b and 3c (red lines).

 

Figure 3 Design of parting surface for front shell
Design of back cover parting surface is shown in Figure 4 (red line), located at maximum outline of product. Its shape is planar, which has advantage of good sealing effect, facilitates processing and mold fitting. Second color of back cover can be molded in fixed mold cavity, which conforms to design principle of two-color mold.

 

Fig. 4 Design of parting surface for back cover
Considering batch size, volume, appearance requirements of product, as well as mold cost, injection position and method, mold of this plastic part is designed as a split mold. Its structural feature is that fixed mold structure of two sub-molds is same, and secondary injection position of moving mold structure is different. Each sub-mold adopts a one-mold two-cavity layout. Each cavity is a single-point gate for material injection. Cavity layout is shown in Fig. 5. Distance from center of two products to center of mold is not equal in length direction. Reason is that volume of two products is different. We strive to balance filling and stress of two cavities.

 

Fig. 5 Cavity layout

Whether gating system design is reasonable will directly affect whether mold can inject qualified plastic products. Selection of gate location is a key aspect of gating system design. Software Moldflow2021 can be used to simulate optimal gate location on plastic part. Figure 6 shows analysis results of simulated gate location. Blue area represents the best gate matching and is suitable for setting gate, while red area represents the worst gate matching and is best not to set gate in this area. As can be seen from Figure 6, gate can be set on the top of product. Considering appearance, structure and two-color injection molding process of product, single gate location of first color product of front shell is selected to left of top button, and single gate location of second color product is selected in rectangular groove on the right side of top; single gate location of first color product of rear cover is selected to left of top center, single gate location of second color product is selected to right of top center and located in groove.

 

Fig. 6 Analysis results of gate position
Based on above, two-color mold gating system designed based on experience and its mesh division are shown in Figure 7. Because front shell and rear cover have significantly different volumes, to ensure balanced flow during filling of both cavities, simultaneous filling, product quality, and reduced trial molding, cross-section and length dimensions of gate were modified, optimized multiple times. Final simulated filling time is shown in Figure 8. It can be seen that gating system of each sub-mold fills evenly and flows smoothly, filling both cavities almost simultaneously, verifying rationality of designed gating system.

 

Fig. 7 Design of gating system
1—Gating system for first-color part; 2—First-color part; 3—Gating system for second-color part; 4—Second-color part

 

Fig. 8 Filling time

Forming parts mainly consist of fixed mold core, moving mold core, forming slider, etc., and are key components of injection molding. They have high performance requirements and should be made of high-quality materials. Two-color mold consists of two sets of molding parts: fixed mold core, moving mold core, and inserts are all made of 1.2344 material, requiring quenching followed by tempering to a hardness of 48-50 HRC. Fixed mold slider is also made of 1.2344 material, requiring quenching followed by tempering to a hardness of 50-52 HRC.
Design of fixed mold core for first color product is shown in Figure 9a. It is a modular structure with dimensions of 150 mm * 180 mm * 65 mm. Cavity of front shell is formed by cavity on fixed mold core and two sliders, while cavity of rear cover is formed by cavity on fixed mold core and inserts. To ensure alignment accuracy of moving and fixed mold cores, three recessed openings are designed, as shown in elliptical areas in Figures 9a and 9b. Venting grooves are provided around two cavities on parting surface. Depths of primary and secondary venting grooves are 0.02 mm and 0.3 mm, respectively, both with a width of 5 mm and lengths of 4 mm and 6 mm, respectively, to facilitate venting.

 

Figure 9 Design of fixed and moving mold inserts for first-color product
1—Core pin of fixed mold insert; 2—Cover cavity; 3—Air vent; ; 4—Shell cavity; 5—Slider one; 6—Slider two; ; 7—Core pin; 8—Cover core of moving mold insert; 9—Shell core; ; 10—Button core; 11—Ejector pin
Moving mold insert design for first-color product, as shown in Figure 9b, is a modular structure with dimensions of 150 mm * 180 mm * 40 mm. This design facilitates machining, mold fitting, and maintenance. As shown in Figure 9c, front shell core protrudes too much from parting surface (32 mm), there is a narrow groove 0.6 mm wide and 1.8 mm deep along core on parting surface, making core machining very difficult. Furthermore, using a monolithic design wastes high-quality steel and machining costs. Therefore, core is divided into inserts (gray). To facilitate processing and demolding, four column holes at four corners of front shell need to be separated into ejector pins. Additionally, rear cover core has four insert pins at its four corners, with three protruding slots corresponding to fixed mold core.
Fixed mold core design for second-color product is shown in Figure 10. It is a modular structure, differing from fixed mold core of first-color product in that two cavities of fixed mold core are replaced with those of second-color product, as shown in dotted area in Figure 10. Everything else remains unchanged. Moving mold core structure for second-color product is identical to that of first-color product.

 

Fig. 10 Design of fixed mold insert for second-color product
Two-color injection molding machine used in this two-color mold is a horizontal parallel nozzle with a fixed nozzle distance. Considering factors such as product output, point gate feeding, and mold cost, a simplified fine-gate mold base type was selected.

As mentioned above, a proper fixed mold core pulling mechanism must be designed for front shell product to ensure smooth mold opening. Fixed mold slider core-pulling mechanism has a complex structure, is one of key and difficult aspects of mold design. Core-pulling distance is a crucial parameter in design of this mechanism, and it is generally determined by adding a safety margin of 2-5 mm to undercut distance T of plastic part. Using UG NX2312 software, undercut distances on the left and right sides of first-color front shell are measured to be 22 mm and 5 mm respectively, undercut distances for second-color product are 22 mm and 3.8 mm respectively. Therefore, left slider core-pulling distance for both first and second-color front shell products is taken as 27 mm, and right slider core-pulling distance for both is taken as 10 mm.
As mentioned above, mold base type for both sub-molds is a simplified fine-gate mold base, which provides conditions for fixed mold slider core-pulling mechanism. Considering relatively large left-side core-pulling distance (27 mm), large demolding force for first and second-color products, a "slider + bent pin" structure is adopted. Taking into account factors such as product output, mold complexity, and cost, it was ultimately decided that core-pulling mechanism for left internal thread of first-color product would be a movable insert. This means threaded core is movably connected to the rest of slider, as shown in Figure 11. A hexagonal through-hole with protrusions is created inside slider A14 and threaded core A13. A hexagonal shaft with a groove, A19, is fixed to slider using pin A18. Threaded core of slider is inserted into or removed from hexagonal shaft through hexagonal hole. After first injection molding, threaded core remains in the front shell of first-color product. After second injection molding, threaded core is ejected along with two-color front shell. Apart from this, the rest of slider adopts an integral structure, with its molding part (at point N in Figure 11) used to mold lower half of conical outer surface of first-color product. Left slider of second-color front shell product adopts an integral structure. Forming part of slider is used to form lower half of conical outer surface of second-color product. It is important to note that threaded core has a hexagonal through hole in axial direction. Its left end face must be sealed with glue to mate with slider, and its right end face must be sealed with glue to mate with core insert to prevent second-color melt from entering this hole.

 

Fig. 11 Movable threaded core design
A13—Threaded core; A14—Slider; A18—Dowel pin; A19—Hexagonal connector with groove; N—Molding part of slider
Considering that right-side core-pulling distance of first and second-color products is also relatively large (10 mm), for uniformity and ease of processing, a "slider + bent pin" core-pulling form is also adopted. Right slider of first-color product is used to form outer surface and undercut of right side wall of first-color product. Slider adopts a partially inlaid structure; only cylindrical core forming two blind holes uses an inlaid structure, while the rest is an integral structure. Right slider of second-color product is used to form outer surface of right side wall and undercut of second-color product. Slider is an integral structure.
Bent pin is a key component of fixed mold slider core-pulling system, serving both driving and locking functions. Its main parameters are designed as follows: Bevel angle of left bent pin in this two-color mold is 20°, and bevel angle of right bent pin is 18°. Effective working length of bent pin is equal to core-pulling distance divided by tangent of bevel angle, thus yielding working lengths of 82 mm and 36.4 mm for left and right bent pins respectively. Due to large forming area of slider in this mold, lateral force during injection is large. To prevent slider from retracting due to insufficient rigidity of bent pin, a 5° anti-locking structure is designed between tail of bent pin and groove of moving mold plate. All bent pins of two-color mold are fixed to fixed mold base plate with screws. Each slider is positioned using screw limits. Slider guide uses a rectangular pressure plate, which is fixed to fixed mold plate groove with screws. In summary, designed two-color mold fixed mold bent pin core-pulling structure is shown in Figure 12.

 

Figure 12 Fixed mold angular cam core pulling structure
A13—Threaded core; A14, A36—Slider; A15, A38—Plate; A16, A25, A33, A37—Screw; A17, A35—Stop screw; A27, A34—Angular cam; B13—Threaded core; B14, B34—Slider; B15, B36—Plate; B16, B23, B31, B35—Screw; B17, B33—Stop screw; B25, B32—Angular cam

Function of ejection system is to safely and without damage push plastic part away from moving mold core after mold opens. Ejection system of both sub-molds in this two-color mold is designed identically. It does not eject when first color is in position; instead, it ejects two-color product after mold opens at second color position. Designed ejection system is shown in Figure 13. Front shell uses a combination of ejector pins and ejector tubes for ejection. Specifically, four sets of ejector tubes, four ejector pins with diameters of 6 mm and 3.5 mm, and two ejector pins with a diameter of 3 mm are symmetrically arranged along bottom edge of inner wall of front shell. This is mainly because strength and rigidity near top edge of product are higher, and there are post holes at four corners, requiring a larger demolding force. Rear cover is a plate-shaped product with a smaller height, requiring less demolding force; therefore, a simple ejector pin method is used for ejection. To ensure smooth and reliable operation of ejection system, reset rods and ejection guide post guiding mechanisms are installed at four corners of top plate.

 

Figure 13 Ejector system
A5—Ejector plate; A6—Ejector retainer plate; A45—Ejector guide pin; A46—Ejector guide bushing; A47—Return pin; A48—Spring; A43, A49, A50, A59, A60, A63—Ejector pin; A52—First-color part of shell; A53—First-color part of cover; A61—Ejector sleeve

A reasonable cooling water channel design is crucial for improving molding quality and efficiency of plastic products. To enhance cooling effect of mold cavity, this two-color mold adopts a straight-through cooling water channel design, as shown in Figure 14. First color sub-mold has a total of 8 sets of water channels: 3 sets for moving mold, 3 sets for fixed mold, and 2 sets for stripper plate. Diameter of moving and fixed mold water channels is 8 mm, and diameter of stripper plate water channel is 10 mm. Cooling water circuit design for second color mold is same as that for first color mold. Production has proven that designed mold cooling system can ensure uniform cooling in all areas, short molding cycle, and high product quality.

 

Fig. 14 Layout of cooling water circuit

Based on analysis and design of above two-color mold structure, three-dimensional two-color mold structure designed using UG software is shown in Fig. 15. Engineering drawing of first color mold drawn from three-dimensional two-color mold drawing is shown in Fig. 16, and engineering drawing of second color mold is shown in Fig. 17. Combining Fig. 16 and Fig. 17, working process of this two-color mold is as follows.

 

Fig. 15 Three-dimensional two-color mold structure

 

Fig. 16 First sub-mold engineering drawing
A1, A4, A9, A10, A16, A22, A25, A29, A31, A33, A37, A39, A42, A62—Screw; A2—Support pillar; A3—Stop pin; A5—Ejector plate; A6—Ejector retainer plate; A7—Spacer block; A8, A43, A49, A50, A59, A60, A63—Ejector pin; A11—Moving mold plate; A12—Core insert; A13—Threaded core; A14, A36—Slider; A15, A38—Plate; A17, A35—Stop screw; A18—Dowel pin; A19—Hexagonal connector with groove; A20—Cavity insert; A21—Fixed mold plate; A23—Pulling rod; A24—Stripping board; A26—Fixed clamp plate; A27, A34—Angular cam; A28—Locating ring; slider; A41—Main core; A44—Moving clamp plate; A45—Ejector guide pin; A46—Ejector guide bushing; A47—Return pin; A48—Spring; A51—Cooling hole; A52—First-color part of shell; A53—First-color part of cover; A54—Core pin; A55—Small pull rod; A56— Mould spring; A57—Large pull rod; A58—Nylon lock；A61—Ejector sleeve

 

Fig. 17 Second sub mold engineering drawing
B1, B4, B9, B10, B16, B20, B23, B27, B29, B31, B35, B38, B58—Screw; B2—Support pillar; B3—Stop pin; B5—Ejector plate; B6—Ejector retainer plate; B7—Spacer block; B8, B39, B45, B46, B55, B56, B59—Ejector pin; B11—Moving mold plate; B12—Core insert; B13—Threaded core; B14, B34—Slider; B15, B36—Plate; B17, B33—Stop screw; B18—Cavity insert; B19—Fixed mold plate; B21—Pulling rod; B22—Stripping board; B24—Fixed clamp plate; B25, B32—Angular cam; B26—Locating ring; B28—Sprue bushing fixing plate; B30—Sprue bushing; B37—Main core; B40—Moving clamp plate; B41—Ejector guide pin; B42—Ejector guide bushing; pin; B44—Spring; B47—Cooling hole; B48—Second-color part of cover; B49—Second-color part of shell; B50—Core pin; B51—Small pull rod; B52—Mould opening spring; B53—Large pull rod; B54—Nylon lock; B57—Ejector sleeve
(1) Mold closing. After threaded core A13 is manually installed on first-color sub-mold, two-color injection molding machine pushes moving molds of two sub-molds to close. Simultaneously, bent pins A27, A34, B25, and B32 drive fixed mold sliders A14, A36, B14, and B34 to accurately reset, ultimately closing and locking mold.
(2) Mold Injection Molding. Two nozzles of two-color injection molding machine inject PC molten material and TPE molten material into two sub-molds respectively. After filling, holding pressure, and cooling, material is molded, producing a first-color product, a complete two-color product.
(3) Mold Opening. When mold opens, under action of springs A56 and B52, nylon clips A58 and B54, it first opens from parting surface PL1. Gates of first and second die molds are simultaneously pulled off, and cast sprue separates from product. Then, due to action of large tie rods A57 and B53, nylon clips A58 and B54, mold opens again from parting surface PL2. Stripper plates A24 and B22 force cast sprue from gate sleeves A32 and B30, automatically detach them. At the same time as mold opens, sliders A14, A36, B14, and B34 complete lateral core pulling under action of bent pins A27, A34, B25, and B32, respectively. Finally, due to action of small tie rods A55 and B51, large tie rods A57 and B53, mold opens from parting surface PL3. After two moving molds have moved a certain distance, mold opening is completed.
(4) Mold demolding. After mold is fully opened, first-color product is not ejected. Ejector roller at second-color position pushes ejection system at that position to eject two-color product, completing demolding. After ejecting two-color product, injection molding machine rotates moving mold 180°, that is, moving mold at first-color position (including first-color product) rotates to second-color position, and moving mold at second-color position rotates to first-color position. This completes mold's current working cycle. It should be noted that in first working cycle, only first-color sub-mold is used for PC melt injection. After filling, holding pressure, and cooling, mold cavity is solidified to form first-color product, while second-color sub-mold is not used for injection.

(1) A fixed mold "bent pin + slider" core-pulling structure was designed, converting outer surface of front shell-like conical part into a fixed mold cavity for molding, solving design problem caused by outer surface of front shell-like conical part being located in moving mold cavity.
(2) To address design challenge of undercut internal thread at conical section of first-color front shell, a hexagonal through-hole with a protrusion was fabricated inside slider and threaded core. A hexagonal shaft with a groove was fixed inside slider with a pin. Threaded core of slider is inserted into or removed from hexagonal shaft through hexagonal hole. This design not only solved design difficulty but also simplified mold structure and reduced mold costs.
(3) Mold flow analysis can not only determine appropriate gate location but also verify rationality of gating system design, achieving balanced mold filling.
(4) A two-cavity, fixed-mold, slider, core-pulling, three-plate, two-color injection mold was designed. Mold has been put into production, its operation is smooth and stable. Two-color plastic products produced are of good quality, have achieved expected goals. It has certain reference value for mold design of similar two-color plastic products.