Plastic part is a shell with waterproof sealing function, as shown in Figure 1, with an outer dimension of 56.8 mm * 30.5 mm * 69.5 mm, and an oblate cylindrical shell. Outside is wrapped with ABS hard plastic, side wall and bottom of one side of inside contain TPE soft plastic sealing structure. Upper surface of shell is open to install internal parts, and there is a runway-shaped through hole in the middle of bottom. Edge of through hole is TPE soft plastic, which has a sealing and waterproof function. There are two circular holes on outer large surface of shell extending to internal TPE soft plastic, and there is a larger circular hole connected to surface of internal soft plastic, as shown in Figure 2.

 

Figure 1 Three-dimensional structure of plastic part
(a) Front (b) Bottom

 

Figure 2 Sectional structure of plastic part

According to analysis of plastic part structure, outer large surface needs to have a slider side core pulling structure to form circular hole. Two circular holes on the side of plastic part are penetrated by soft plastic. Slider structure of two circular holes requires two different slider inserts during two-color molding. Difficulty of mold structure of molded plastic part lies in how to determine its demolding direction and demolding method. Following three demolding schemes are now considered.
Scheme 1: According to conventional two-color injection mold design, ABS is used as first color and TPE soft plastic is used as second color. Large surface of opening bottom of shell is used as parting surface, opening faces downward, and inside of shell is used as moving mold. However, it will cause second color soft plastic to be inside first color hard plastic, that is, on moving mold side. It is impossible to use conventional two-color injection mold to use different fixed mold inserts to mold second color plastic, and it is impossible to provide space for second color plastic. Conventional two-color injection mold demolding scheme is not feasible.
Scheme 2: Demolding method is same as Scheme 1. TPE is selected as first color to be injected first, and hard plastic of shell is injected as second color later. In this way, hard plastic structure of shell can be molded in fixed mold block of second color, and molding structure is feasible. However, melting temperature of ABS is higher than that of TPE. If TPE with a low melting temperature is injected and molded first, then ABS with a high melting temperature contacts TPE, already molded TPE will melt again and deform. Scheme 2 is not feasible in injection molding sequence.
Scheme 3: Opposite to demoulding direction of previous two schemes, internal structure of shell is used as fixed mold of mold. In this way, ABS hard plastic of shell is used as movable mold for first color injection during injection, and TPE soft plastic is molded in fixed mold of second color. This scheme is feasible for both molding and injection sequence, but in actual injection process, when first color mold is opened after injection, semi-finished product of first color will cool, shrink and stick to fixed mold, and gates of both colors need to be designed inside shell. Internal space of shell cannot be designed with a hot nozzle, so Scheme 3 is not the best solution.
After research, it was decided to make improvements based on Scheme 1, with ABS as first color and TPE as second color. Large surface of bottom opening of shell is used as parting surface, opening faces downward, and inside of shell is used as movable mold. Second color TPE plastic is concentrated in the middle part of plastic part. Mold parts of this part can be separately inlaid. Inlaid parts are replaced when second color is injected, and serve as fixed mold of second color in conventional two-color injection mold. How to convert these two inserts on movable mold side has become a design difficulty. It is decided to install middle two-color conversion insert on next layer of movable mold (pallet). Design a cylindrical rotating rod in the center of pallet and through pallet, and fix top of rotating rod on movable mold. Use rotating rod to lift movable mold, then use hydraulic cylinder to rotate movable mold. Mechanism on movable mold is rotated 180° to achieve purpose of two-color replacement. When rotating rod lifts movable mold for conversion, semi-finished product of first color needs to be fixed on core, which can be fixed by side slider structure of shell. However, there are two circular holes on the side of molded plastic part that need to cooperate with two-color conversion, so it is necessary to design a different small slider structure inside large slider structure on both sides to achieve two-color conversion.

According to above analysis, there are two key points in plastic part molding: ① After first color is injected, small slider with two circular holes on molding side needs to be driven, while large slider that wraps the entire plastic part to be molded remains stationary; ② After second color is injected, large slider needs to be driven, and after large slider returns to first color position, it needs to be reset by fixed mold of first color. Slider is driven by inclined wedge of fixed mold, and different fixed mold inclined wedges are designed according to different driving opportunities. Driving requirements of small slider are relatively simple. It only needs to be driven to open after first color is injected, and second color is also kept open, then driven to close on first color side. That is, opening and closing of small slider are completed on first color side, so only inclined wedge needs to be installed on fixed mold side of first color. Small slider remains stationary when second color is injected, injection pressure is supported by one-word lock and large slider structure. Structure of small slider is shown in Figure 3.

 

Figure 3 Small slider structure
Drive of outer large slider remains stationary after first color is injected. Injection of second color molten plastic is completed in closed state. Then large slider is driven to open by wedge on fixed mold side of second color. After molded plastic part is ejected and demolded, large slider is reset and closed by wedge on fixed mold side of first color. Wedge on fixed mold side of large slider of first color is responsible for reset and closing action, and wedge on fixed mold side of second color is responsible for opening action. Two color wedges of large slider are designed differently, and wedge on fixed mold side of first color cannot drive slider to open when mold is opened. Wedge on fixed mold side of second color needs to enter closed wedge hole when mold is closed.
In order to meet above driving requirements, a new type of wedge mechanism is designed. 1/4 of circular cross section is machined on cylindrical wedge to form a small plane. At the same time, a sliding insert controlled by a spring is designed on the side of wedge hole of large slider. Plane orientation on wedge and sliding insert are used to control drive of slider by wedge, as shown in Figure 4. Plane of inclined wedge on first color fixed mold side faces obliquely downward. When mold is closed, downward plane of inclined wedge can be pressed on plane of sliding insert, driving large slider to reset and close. When mold is opened, inclined wedge on the first color fixed mold side moves upward, inclined wedge does not act on large slider, and large slider remains closed. Plane of inclined wedge on second color fixed mold side faces upward. When mold is closed, inclined wedge on second color fixed mold side will contact sliding insert with a circular arc surface. Under action of circular arc surface of inclined wedge and guide angle of sliding insert, sliding insert slides and compresses spring until inclined wedge completely passes over sliding insert. Sliding insert is reset under action of spring and is stuck on upper plane of inclined wedge. When mold is closed, inclined wedge on second color fixed mold side enters closed slider inclined wedge hole. After second color plastic is injected, mold is opened, and inclined wedge on second color fixed mold side uses upward plane to drive sliding insert to drive large slider to open, completing drive of large slider in second color.

 

Figure 4 Internal structure of two-color wedge

According to above design, demoulding problem of molded plastic parts can be solved. According to demoulding structure, it can be determined that moving inserts need to be designed around and at the bottom of plastic parts to be molded, they are all convertible structures. Pouring system gate can be designed at the top of plastic part to be molded. Top of plastic part is the first color ABS hard plastic, and top is not appearance surface. Second color TPE soft plastic is molded at the bottom of hard plastic. A small hole can be designed at hard plastic as gate position of second color, so that second color TPE soft plastic can enter bottom of ABS hard plastic through hole for pouring. In order to save raw material costs and ensure quality of molded plastic parts, both types of plastics are cast by hot runner. Hot runner is divided into two parts. Part located in the center of mold is used as the first color hot runner system, and main nozzle of injection molding machine provides molten material; part located at the top side of mold is used as second color hot runner system, and auxiliary nozzle newly added at the top of injection molding machine provides molten material. Pouring system is shown in Figure 5.

 

Figure 5 Casting system

According to above design, main structure of two-color conversion shown in Figure 6 is obtained. Top side is injection position of second color, and the other side is injection position of first color. First mold opening: After mold is injected into first color plastic, mold is opened, and movable mold moves backward as a whole. Inclined wedge on fixed mold side of first color does not act on large slider, large slider mechanism does not open and remains closed. Small slider inside large slider drives small slider to open under action of small inclined wedge (see Figure 3), and small slider drives two internal inserts to move backward to form two circular holes of first color semi-finished product, providing space for second color plastic to be filled.

 

Figure 6 Main structure of two-color mold conversion
Two-color mechanism conversion: After mold is fully opened, machine top rod pushes rotating rod at the center of bottom of mold, top of rotating rod is connected to movable mold plate, and rotating rod lifts movable mold plate. Pallet under movable mold plate remains stationary, and center insert of movable mold is installed on pallet, because large slider of first color is not opened when mold is opened, most of semi-finished product is on slider, and large slider is wrapped on the bottom surface of semi-finished product. When movable platen is lifted, center insert of first color movable mold is separated from semi-finished product, semi-finished product is wrapped by large slider mechanism and lifted as a whole, as shown in Figure 7. After movable platen is lifted, hydraulic cylinder slider drives movable platen to rotate 180°, then machine top rod is reset, movable platen is refitted with pallet, and slider mechanism wrapping semi-finished product molded in first color is converted to position of second color, and at the same time, interior of semi-finished product is also fitted with second color movable mold insert, realizing two-color conversion of movable mold insert.

 

Figure 7 Conversion structure
Mold closing: After two-color mechanism conversion is completed, mold starts to close. For small slider, second color fixed mold side is not installed with wedge mechanism, and small slider remains open to ensure that second color plastic is filled into 2 circular holes. For large slider, second color fixed mold wedge uses downward arc surface to press sliding insert inside slider. Sliding insert retracts and gives way under action of wedge. When mold is closed, upward plane of second color fixed mold side wedge passes over sliding insert, enters sliding insert and large slider. At the same time, sliding insert is reset under action of spring and stuck on wedge plane to prepare for driving slider when opening mold (see Figure 4).
Second mold opening: Mold is closed to inject second color plastic. After injection is completed, mold is opened again. Large slider is opened under drive of second color fixed mold side wedge, and injection molded plastic parts stay on moving mold insert.
Two-color mechanism conversion reset: After second mold opening, machine top rod drives rotating rod, and rotating rod pushes moving mold plate. Plastic part is pushed up together with parts on moving mold plate. moving mold insert of two-color molding is separated from plastic part. At the same time, push plate is used to push plastic part out of moving mold. Robot takes molded plastic part out of mold to complete demolding action. Hydraulic cylinder slider drives rotating rod to rotate movable platen 180° to reset, and large slider in open state returns to position on first color side. Machine top rod is reset, and movable platen is reset to be close to pallet again.
Second mold closing: Mold is closed for second time. When closing mold, inclined wedge on first color fixed mold side uses downward plane to push sliding insert on large slider to drive large slider to close and reset, preparing for next injection cycle.