Structure of two-color mold is closely related to two-color injection molding machine. There are seven categories of two-color machines, but two categories are commonly used. Correspondingly, there are two types of two-color injection molds, namely Asian two-color molds and European two-color molds. Two-color injection molding machines in Taiwan and Japan have two parallel injection systems. In European two-color injection molding machine, one barrel is horizontal and one barrel is vertical, and there is only one two-color mold. But principle is basically similar. Shape of upper and lower rows of front mold is different, rear mold rotates 180 degrees to form  front mold once and twice.
Asian two-color molds usually have two molds, front mold is mounted on fixed plate of machine, and rear mold is mounted on rotary plate. Back molds of two molds are exactly same. First front mold is different from second front mold. When plastic injection of first color is completed, front and rear molds are opened, back mold is rotated 180 degrees with product of first color to reach injection position of second color, and second injection is performed to obtain a two-color part. In this cycle, there will be a mold primary product and a secondary product in each molding cycle.
Two Asian two-color mold bases can also be simplified into one mold base depending on mold structure. For Asian two-color mold base, when ordering mold base, you need to pay attention to position of reference angle guide post and guide sleeve not to shift, otherwise it will be difficult to close mold after rotating. Two Asian two-color mold bases, when ordering mold bases, you need to pay attention to offset of reference angle guide post and guide sleeve, which will not affect mold clamping.

1. Before designing, be sure to understand parameters of injection molding machine, such as type of injection molding machine, size of screw hole of code, specification of turntable, size and ejection position of ejector, size and specification of shaft; when product is layout, make sure that maximum dimension of mold is smaller than rotating diameter of turntable.
2. First injection product should be placed on non-operating side, and it is difficult for operator to grasp first-time material. When first injection is designed to feed glue at point gate of three-plate mold, first-time injection runner needs to be designed with a ballistic runner device to facilitate fully automatic production. After first injection, product should be rotated 180 degrees for second injection, and it is just turned to operating side for easy access to product. Special attention should be paid to whether demoulding action of soft glue nozzle is reliable.
3. Gate must be automatically cut when glue is fed once, so as to avoid residual protruding secondary glue position and formation of defective products. Secondary gate should cover primary gate as much as possible; secondary gate should be designed in a concealed part as much as possible, adopting latent gate, horn gate, point gate or bottom overlap to inject glue. Be careful not to impulse primary plastic with material flow of secondary injection molding, leaving marks on product surface.
4. Pay attention to repetitive positioning accuracy of moving and fixed molds; slope drop of all penetrating surfaces should be as large as possible, which should be more than 0.1mm. When sealing, try to use surface sealant instead of rubbing surface sealant.
5. Basic principles of two-color mold design:
(1) 1 time for hard rubber and 2 times for soft rubber;
(2) 1 time for transparent materials and 2 times for non-transparent materials;
(3) One time for plastics with high molding temperature and 2 times for plastics with low molding temperature;
6. Shapes of two fixed molds are different, and two materials are formed separately. Shapes of two movable molds are exactly same. Movable and fixed molds must be matched after rotating 180º at the center. Rotation inspection must be done during mold design. When mold is processed, it is necessary to improve processing accuracy to ensure that difference between dimensional accuracy of two movable molds is not more than 0.01mm.
7. When designing cavity for second injection, in order to avoid inserting (or scratching) cavity to damage glue position of product that has been formed for first time, primary hard glue can be partially avoided. However, strength of each sealing position must be carefully considered, that is, during injection molding, will plastic be deformed under high injection pressure, which may lead to possibility of cracking in second injection?
8. When injection molding, size of first injection molding product can be slightly larger, so that it can be pressed tighter with another cavity during second molding, in order to achieve role of sealing glue. Note that during second injection, will flow of plastic impulse product that has been molded first time, causing it to deform glue position? If this is possible, we must find ways to improve it.

Figure 1 shows product diagram of phone's two-color buttons. Numbers and letters on the top of keys are molded by two-color injection molding. Material used for two injections is ABS, buttons with two-color injection molding can avoid defects that handwriting on printed buttons is easy to wear off.
Mold design is shown in Figure 2. It adopts a two-color mold structure with a middle plate ejection rotating. Middle plate ejection rotating two-color mold is an integral two-color mold, one mold base, one ejection system, and it is only installed on the side of secondary injection. As name implies, movable mold does not need to rotate whole set of molds, movable mold only rotates after middle plate is ejected and then closes mold. Top numbers and letters of phone's two-color buttons are molded during first injection, glue position fills up entire numbers and letters during second injection, forming a button with a smooth top. In the first injection, a point gate was used to inject glue at the top of button avoiding numbers and letters. Second glue is injected into top surface of connecting bridge between each button, and point gates are also used. There are some capital letters "O", "P", "Q", "A", "B", "R", "D", etc., as well as numbers "0", "4", "6", "8 ", "9" and so on have a closed structure. If secondary material cannot be filled into closed structure of primary material, "seesaw" mold structure or middle plate ejection rotating mechanism must be adopted.

 

Figure 1 Product picture of two-color telephone buttons

 

 

Figure 2 Die diagram of two-color buttons for telephone
1. Fixed mold seat plate 2. Runner push plate 3. Locating pin 4. A plate 5. Nylon buckle 6. Middle plate 7. Movable mold backing plate 8. Spring 9. Push rod fixed plate 10. Push plate 11. Move Mold backing plate 12. Soft plastic sprue sleeve 13. Soft plastic fixed mold cavity 14. Core 15. Hard plastic fixed mold cavity 16. Core 17. Hard plastic sprue sleeve 18. Hard plastic movable mold insert 19. Inlay Fixing plate 20. Guide bushing 21. Support sleeve 22. Spline shaft 23. Soft rubber movable mold insert 24. Push rod 25. Push rod
This set of molds uses middle plate to eject and rotate, which can replace "seesaw" mold structure. Note that primary movable mold insert 18 and secondary movable mold insert 23 adopt different top structures. Top structures of two movable mold inserts are different, and they are fixed and non-rotating. Hard rubber movable mold insert 18 is made of several raised steel materials at non-number and letter strokes to collide with fixed mold. After mold is opened, spline shaft 22 pushes middle plate 6 and core 14 out of movable mold, semi-finished product of primary glue position is separated from movable mold insert 18, after rotating to secondary injection station, middle plate is retracted to close mold. Semi-finished product of primary glue position is combined with secondary movable mold insert 23, and vacant position of piercing groove is convenient for filling of secondary glue. After injection is completed, mold is opened, nozzle material is taken out, and plastic part is ejected on secondary injection side. Whole set of molds are layout 2X2, with two holes each for one-shot injection and two-shot injection.