According to current structure and quality requirements of two-color automotive parts, characteristics of two-color molds are introduced, and two-color engine baffle plate of an automobile is taken as an example. With help of UGNX software, key parts of two-color mold are designed respectively: gating system, molded parts and ejection system, etc. Automobile two-color mold designed according to this has been successfully tested, design results can be promoted and applied to design and manufacture of two-color molds for automobile exterior parts with same structural characteristics.

Two-color injection mold is to inject two materials of different colors (different materials) into same mold through a specially designed mold and a special injection molding machine, so as to achieve purpose of two materials based on one plastic part [1~2]. Two materials can be different colors or different materials. In automobile industry, since automobile exterior parts need to be installed and coordinated with various assembly parts, sealing performance is often considered, precision of plastic parts is relatively high. Traditional method is to assemble hard rubber plastic parts with other parts and then seal them with soft rubber, which usually causes relatively large assembly errors and increases labor costs. At present, with emergence of polymer thermoplastic elastomers (TPE), two-color injection molding can perfectly combine thermoplastic elastomers and plastics to produce two-color plastic parts based on soft and hard materials, thereby meeting performance requirements of automotive industry's exterior parts. Two-color plastic parts have many advantages such as stable quality, short molding cycle and high production efficiency.

Engine deflector structure is composed of a main body and a plate pad, as shown in Figure 1.

 

Figure 1 Two-color deflector structure
1. Hard plastic (PP) 2. Soft plastic (TPE)
Main body is made of polypropylene (PP), and board pad is made of TPE (thermoplastic elastomer). Performance of two parts of material is quite different, plastic parts require soft and hard rubber to be closely combined. Therefore, two-color injection molding technology must be used to solve problem. Appearance of plastic parts requires no weld lines, no sink marks, no discoloration and other injection defects, and surface roughness value of plastic parts is Ra1.6μm. Main body size of deflector is 277*127*67mm, average wall thickness is 2.3mm, and material is polypropylene, which not only has good flow properties, but also has excellent flexibility and impact resistance. Pad material is TPE, size is 312*123*45mm, average wall thickness is 2.3mm, material is TPE, structure is composed of different resin segments and rubber segments. Therefore, TPE has excellent properties such as rubber elasticity and aging resistance, and also has thermoplasticity. It can be produced by processing methods such as injection and extrusion, and can be produced continuously, recycled material can be recycled. Emergence of this new type of material also contributes to large-scale promotion and application of two-color plastic parts in automotive industry.

Traditional two-color mold is also called an overmold. Two materials required by this molding method are molded in two parts: after hard rubber is molded out of the first mold, it is placed in another mold for second time Injection molding. This molding method usually requires two molds, and does not require a special two-color injection molding machine for production. Structure of mold is basically same as that of ordinary injection mold. Specific ones are simple structure, low mold cost, low production efficiency, and low precision of plastic parts.
At present, more advanced two-color molds basically have two materials concentrated in one mold to complete injection molding, and a special two-color injection molding machine is required for production. Structure of this mold is: mold is composed of two fixed molds and two movable molds. Movable mold has same structure and can be rotated. After hard rubber is formed, plastic part and movable mold rotate 180° together. Soft rubber fixed mold forming area is closed to form soft rubber. This kind of mold can quickly form two different materials into a single two-color plastic part, which reduces a lot of labor costs, improves added value of plastic part, has high quality stability. Manufacturing cost of plastic part can be reduced by 20%~30% compared with overmolding. This article adopts second form.

Overall design of mold is shown in Figure 2. Fixed mold of mold is composed of two cavities to form PP and TPE respectively, and movable mold is also composed of two cores, but geometry is
same. It can be seen from Figure 2 that two positioning rings are designed on fixed mold seat plate, which are matched with two nozzles of two-color injection molding machine. Since two movable molds must be ejected at different times, a set of ejection mechanism is designed for each movable mold. In addition, due to use of a double movable mold, injection pressure of entire mold during injection molding will increase accordingly. In order to increase support strength, three support blocks are designed under movable template.

 

Figure 2 Overall view of two-color mold of deflector
1. Positioning ring 2. Fixed mold base 3. Fixed template 4. Moving template
5. Ejector plate 6. Movable mold seat plate 7. Support block

Since movable mold is continuously rotating in two-color injection molding and is in dynamic coordination with fixed mold, if a cold runner structure is adopted, cold material in main runner will seriously hinder cooperation between molds. At the same time, in order to ensure low consumption of injection pressure and ensure quality of plastic parts, hot runner system is adopted for gating system, as shown in Figure 3.

 

Figure 3 Hot runner gating system
1. Soft rubber (TPE) 2. Hot nozzle 3. Hot runner plate 4. Hot runner socket
5. Horn type latent gate 6. Hard plastic (PP) 7. Fan gate
Main body of baffle is made of hard rubber, which does not need to be ejected after first injection molding. Therefore, there is no need to consider separation of plastic part and gate. At the same time, in order to ensure flow quality of hard rubber molding, eliminate spray pattern and internal stress, a fan-shaped side gate is designed for PP plastic parts, as shown in Figure 3. However, when movable mold of PP plastic part is rotated by 180° and second fixed mold cavity is used to form TPE, separation of gate material and plastic part during ejection should be considered. In order to realize automatic production, movable mold of TPE plastic part A horn-shaped latent gate is designed. As shown in Figure 4, runner only starts on the side of movable mold, cross-section adopts a trapezoid shape. Considering processing technology, horn-shaped gate is designed into two inserts and CNC machining is used. In order to ensure matching accuracy of two inserts, a T-shaped positioning is designed on mating surface of insert.

 

Figure 4 Bullhorn-shaped latent gate
1. Horn-shaped latent gate 2. Gate insert Ⅰ 3. T-shaped positioning 4. Gate insert Ⅱ

In two-color molding process, two movable molds are rotated and ejected after each molding. Therefore, a set of independent ejection mechanisms with same structure but not synchronized should be designed for each movable mold. Ejector mechanism is composed of two ejector plates, ejector rods, runner ejector rods, reset rods, return springs, and support columns, as shown in Figure 5. Due to hot runner system, runners are all designed on the side of movable mold, ejector pin is designed under runner of horn-shaped gate. Therefore, when mold is opened, gating system naturally stays on the side of movable mold, then ejector pin of injection molding machine is pushed under ejector plate through ejector hole, plastic part and cold material of pouring system are pushed out through ejector rod and runner ejector under plastic part, automatic separation of plastic part and cold material of pouring system is smoothly realized.

 

Figure 5 Double ejector mechanism
1. Reset lever 2. Reset spring 3. Ejector 4. Support column
5.PP/TPE two-color plastic parts 6. Runner ejector rod
A reset spring is also designed in ejection mechanism. When ejector rod is ejected, ejector rod and reset rod are quickly reset due to action of reset spring, final reset accuracy needs to rely on contact between reset rod and parting surface of fixed mold plate. Since there is still space in ejector plate, in order to increase support of movable mold to plastic melt pressure, 8 support columns are designed inside ejector plate to enhance support strength of entire mold, thereby improving use of mold base life.

Molded parts are composed of a fixed mold and a movable mold. As shown in Figure 6, fixed mold is composed of two molding zones: A zone and B zone. Area A on the right side of fixed mold is used to form PP body part. In order to fit closely with parting surface of movable mold, reduce flash and burrs, all parts from edge of plastic part to parting surface 10mm beyond parting surface are all treated to avoid gap, so that clamping pressure is concentrated on parting surface of 10mm width. In addition, four side lock components are designed on fixed mold plate to achieve high-precision coordination between fixed and movable molds. Side boss part of parting surface is also treated to avoid air, and a wear plate is designed on the side of boss to enhance life of fixed mold. Necessary clearance treatments are designed for parts in contact with PP body in B area, which avoids unnecessary cooperation with PP body and also reduces resistance when final plastic part is ejected.

 

Figure 6 Fixed mold structure diagram
1. PP molding area A 2. Parting surface avoiding empty 3. Side lock 4. Wear plate
5. TPE molding area B 6. PP plastic part avoidance area 7. Exhaust groove
Since air is compressed when plastic melt flows inside mold cavity, in order to smoothly exhaust gas, a number of exhaust grooves are designed on fixed mold and melt of PP/TPE molding zone, so as to smoothly remove gas remaining in plastic part, eliminate defects of plastic part such as pores and burnt. As shown in Figure 7, movable mold is also composed of two parts, but molding structure is same, both of which are composed of a PP molding area and a TPE molding area. Since parting surface of fixed mold has been designed with an avoidance treatment, coordination of parting surface has been reduced. However, in order to reduce damage to entire mold caused by clamping force of injection molding machine, as many as 18 wear plates (material CrMoV, hardness 60HRC) are designed on free area of movable mold parting surface to share part of clamping force, thereby enhancing service life of mold.

 

Figure 7 Movable mold structure
1. TPE molding area 2. PP molding area 3. Wear plate 4. Side lock

Figure 8 is mold assembly diagram. It can be seen that fixed mold area is designed with two different molding areas, namely PP main body molding area and TPE molding area. Movable mold area is also composed of two molding areas, but geometry and structure are same.

 

Figure 8 Overall drawing of mold
a — —fixed mold area b — —movable mold area
When mold is closed for the first time, two molding areas of fixed mold of mold and movable mold form a fit to form PP main body of baffle plate and TPE plate pad. However, since there is no PP body in TPE molding area of fixed mold area, two-color plastic part is not formed in the first mold clamping, only PP body is formed. Starting from second mold clamping, due to rotation of movable mold area by 180°, PP body remaining in movable mold area (plastic part and fan gate are not separated) and TPE molding area of fixed mold area form a mold clamping fit. Second nozzle of injection molding machine injects TPE melt into mold cavity, thereby forming TPE gel around PP body. When cooling is over, ejection mechanism of movable mold area starts to operate, plastic part and horn-shaped latent gate are pushed out together through ejector rod, latent gate is also cut and separated from TPE plastic part. After mold is ejected, ejection mechanism is quickly reset due to action of return spring and return rod. Under action of rotating disk of injection molding machine, mold rotates 180° again, movable mold area that has ejected plastic part is again matched with fixed PP molding area to form next PP body. At this time, second molding area of movable mold area ejects already formed two-color baffle for the second time, thus starting next cycle .

(1) Two-color baffle mold uses a fan-shaped side gate for PP main body, and a horn-shaped latent gate for TPE part, which not only meets appearance requirements of plastic part, but also facilitates separation of plastic part from gate, improves production efficiency.
(2) Gating system adopts hot runner multi-point feeding method, which effectively reduces excessive reduction of melt front temperature and loss of injection pressure, helps to improve quality of plastic parts, reduces shrinkage and warpage.
(3) A variety of positioning structures are designed in two-color mold structure: T-shaped positioning is designed in horn-shaped gate assembly, 6 sets of T-shaped positioning components are designed between movable mold and fixed mold. So as to effectively ensure accurate fit between two-color mold structural parts.