For previous reading, please refer to Mold Design Guide (Ventilation System Design and Mold Temperature and Cooling System Design).
Two-color molds utilize a two-color/two-material injection molding machine to produce products using two types of plastic materials (primarily hard plastic with a supplement of soft plastic) and different colors. They are particularly suitable for molding various buttons with permanent markings. Advantages are: simultaneous molding shortens production cycle and improves production efficiency.
Injection Molding Machine
I. Three Types of Two-Color Injection Molding Machines
Two-color injection molding machines are like plastic sculptors who can perform magic tricks, mainly achieving two-color effects through three methods:
Rotary type: Mold can rotate 180°, first injecting A-color part, then combining it with B-color part after rotation.

 

Transverse type: Mold slides horizontally, completing two injections at different stations

 

Co-injection type: Two injection systems work synchronously, achieving synchronous two-color molding through a special runner

 

Molding process for two-color/two-material products is as follows (taking two materials, material 1 and material 2, as an example):
(1) Inject material 1 first; →(2) Eject runner after mold opening; →(3) Rotate rotary table 180° to the other side; →(4) Close and lock the mold; →(5) Inject material 2 again, and simultaneously inject material 1 into other mold cavity; →(6) Hold pressure, material 1 and material 2 cool simultaneously; →(7) Open mold, eject finished product and sprue, and eject only sprue from other mold cavity; → (8) Repeat process (3) (rotate turntable 180° to the other side).
Molding types:
1. Two-color molding 2. 3. 4. 5 Core pulling type

 

Parallel injection gun two-color injection molding machine

 

Two-color dual-mold co-injection molding principle

 

Rotary plate size
Rotary plate is an auxiliary plate, installed on motor plate. Structural dimensions of rotary plate are shown in Figure 12.1.8. Rotation is driven by hydraulic pressure of core pulling mechanism and can rotate +/-180°. It is only suitable for two-color/two-material injection. Dimensions of rotary plate and mold are shown in Figure 12.1.9. Enlarged view of central cooling water channel is shown in Figure 12.1.10.

 

When molding sandwich or camouflage plastic parts, auxiliary plate (mixing nozzle plate) is installed on fixed machine plate of equipment. Two nozzles pass through mixing nozzle on auxiliary plate, as shown in Figure 12.1.13. Injection system uses a single nozzle in mold, suitable for molds with typical structures; mixing nozzle has an independent heating coil.

 

Sandwich injection principle is shown in Figure 12.1.4, involving injection in three stages to fill entire cavity. The first stage, outer shell forming part, has the largest filling volume, with molten material filling most of cavity. The second stage continues filling cavity based on the first stage, ensuring sandwich material is not exposed. The third stage has a relatively fixed filling volume, only needing to seal gate area. Currently, sandwich injection for plastic parts still has some unresolved defects, mainly including frequent color mixing on the surface of plastic part, inability of light-colored surface layer to completely cover dark-colored sandwich material, uneven filling of sandwich material, and a low sandwich ratio of approximately 20% or less.

 

12.2 Mold Structure
ARBURG 520C injection molding machine is used for two-color/two-material injection molding. Mold structure is described below.
12.2.1 General Structure
Structure of two-color/two-material mold is shown in Figure 12.2.1. Compared with ordinary (single-color) molds, mold structure has following characteristics:
Mold has two independent ejection mechanisms; Vertical end injection, entering through mold parting surface or between sprue ejector plate and front mold plate (for three-plate molds), as shown in Figure 12.2.6 for three-plate molds; Cooling for rear mold is achieved through center of rotating plate, then introduced into rear mold via mold base plate; Mold is fixed on rotating plate and requires locating pins for positioning, ensuring accurate alignment of ejector pins; Locating pins are required between mold base plate, square iron, and backing plate for positioning; To ensure good fit between front and rear molds after rear mold rotates 180°, dimensions of mold side pins and sprue edge (for three-plate molds) must be symmetrical and consistent.

 

Key points for design and manufacturing of dual-color/dual-material molds:
(1) To ensure mold can rotate on rotary plate, maximum height and width dimensions of mold should be within range of diameter of inscribed circle of Green prism & 750mm; when mold is fixed to rotary plate with a pressure plate, maximum width of mold is 450mm and maximum height (length) is 590mm; in addition, to meet requirements of mold positioning and ejector hole position dimensions, minimum width of mold is 300mm and minimum height (length) is 400mm, as shown in Figure 12.2.2.

 

 

(2) Due to horizontal and vertical injection nozzle end faces of equipment For a planar structure, mold nozzle must meet planar contact requirements, as shown in Figure 12.2.3.
(3) Ensure center position dimension of mold positioning and ejection is 12060.02, as shown in Figure 12.2.1.
(4) For two-material injection molds, if shrinkage rates of two materials are different, scaling of mold cavity will also be inconsistent. When performing second injection, part formed in the first molding has already shrunk. Therefore, sealing surface of mold in second molding should be actual size of part. Alternatively, it can be reduced (on one side) by 0.03mm to control sealing, as shown in Figure 12.2.4.

 

(5) For front mold cavity in secondary forming process, ensure non-sealing mating surfaces are clear to avoid pinching or scratching surface of first-injection molded part. See Figure 12.2.4 for pinching prevention and Figure 12.2.5 for scratch prevention.

 

(6) Avoid sharp-angle joints between two plastic materials. Sharp-angle joints result in significant heat loss, hindering fusion and causing corners to easily separate, as shown in Figure 12.2.6.

 

(7) When selecting two plastic materials for part, pay attention to their bonding effect. Commonly used plastic material combinations are shown in table below.

 

Note: (1) "+" indicates a good combination; "-" indicates a slightly worse combination; "θ" indicates a poor combination.
(2) Remaining blanks indicate bad combinations.
12.2.3 Cooling Method of Rear Mold
For cooling of two-color/two-material molds, front mold is same as that of ordinary (single-color material) molds; while rear mold is installed on a rotary plate, and cooling water is introduced from middle of rotary plate. There are usually two ways for cooling water to enter rear mold:

 

As shown in Figure 12.2.7, cooling water enters mold base plate and exits from side, then flows through a hose into pad plate, and finally enters rear mold cavity for cooling.

 

12.3 Mold Example
(1) As shown in Figure 12.3.1, molded plastic part is made of two-color material. Front mold adopts a half-frame structure as shown in Figure 12.3.2, consisting of two sets (four pairs per set) of half-frame blocks. By changing two sets of half-frame cavities, forming of inner core (hard plastic) and outer plastic material (soft plastic) after rotation is achieved. Rear mold utilizes lifter mechanism at both ends of plastic part as shown in Figure 12.3.3, so that after first forming, plastic part is fixed on lifter and rotated into another set of half-frame cavities. After second forming, plastic part is ejected.

 

(2) As shown in Figure 12.3.4, mold is a push-plate mold, and molded part is a two-color "button". Rear mold of two sets of cavities inside mold is same, while front mold is different. After first molding of part, it remains on the rear mold and ejects sprue; rear mold rotates 180° and closes into another set of front molds, completing second molding of part, ejecting part, and ejecting sprue.

 

(3) As shown in Figure 12.3.5, formed plastic part "disc shell" has a ring of a different color. Rear mold cavity is same, but front mold is different. After first forming, sprue is ejected through gate plate; after rotating 1808 for secondary forming, plastic part is ejected by ejector pins, and sprue gate plate is ejected.