Injection mold – Two-color injection molding

For two-color injection molds (also called 2C or 2K molds), as the name suggests, two different types of plastic are injected into the mold sequentially. Due to the increasing market demand for these plastic parts, more and more injection molding suppliers are adopting this technology.

1. Material Combination
For two-color injection molding, there are various material combinations for plastic parts. For example, the first injection uses a hard plastic, and the second injection uses a soft plastic. For this combination, the selection of hard and soft plastics, and the structural design of the plastic part, must have sufficient bonding strength to ensure the two components are firmly bonded together. Another common two-color injection molding combination is using the same material but different colors. For example, multi-colored car taillights. For car lights, 3C and 4C molds are no longer uncommon.

2. Mold Structure

2.1 Semi-finished Product Relocation
For molds, the simplest method is relocation. For this process, a 2C mold needs two cavities, while the number of cavities in a multi-component mold should be even.

The first material is injected into half of the cavity, and the second material is injected into the other half. The arrangement of mold cavities in an injection mold should allow a displacement system or robot to move parts from the front half to the back half of the cavity.

Therefore, the parts should be either linearly or circularly distributed.

For example, consider a coaster molded in a mold using a hard/white – soft/blue material combination. In the first mold cavity, the coaster blank is injection molded with the hard material. In the first injection, the area to be filled with the soft material in the second injection is left as a recess. All recessed areas are connected, so that one feed point is sufficient for the soft material. After the mold opens, the semi-finished part from the first half of the cavity is moved to the second half, where the coaster is completed.

For each cycle, one half of the cavity contains semi-finished parts, and the other half contains finished parts. Therefore, the number of parts emerging from the machine is always half the number of cavities. That is, two cavities correspond to one finished part, four cavities correspond to two finished parts, and so on. Two injection molding units are needed to simultaneously injection mold all the mold cavities.

The image below shows a coaster, depicting both the semi-finished product (after the first injection) and the finished product emerging from the mold.

The first type of hard material enters the mold from the right. Simultaneously, a soft material is injected into the center of the mold. The mold opens, and the finished part is ejected from the left. The semi-finished part on the right is transferred to the cavity on the left by a moving system.

Two-color product

Two-color mold moving half

Fixed mold half
Moving mold half
Moving mold half
Moving mold half and fixed mold half

2.2 Rotation

Another common form of 2C mold is to use an injection molding machine with a rotating table or a rotating mechanism designed into the mold. The semi-finished part does not need to be moved but remains in the mold. Instead, the moving mold half rotates 90° driven by the injection molding machine.

The rotation angle can be large or small depending on the size, complexity, and shape of the plastic part. For larger 2C parts, the moving mold half can rotate 180°. This type of injection mold has two cavities, one for soft materials and one for hard materials.

For smaller 2C parts, such as injection molds, there might be eight cavities: four for soft materials and four for hard materials. The moving mold half can rotate 45° between injecting the hard and soft materials.

The example below is a cap with a hard material interior and a soft material seal on the exterior.

The image below shows the front and back of the cap, with both materials injected through a hot runner with a needle valve.

In the moving mold half, four molding inserts are identical. The stationary mold half has two different cavity inserts. These are used for molding the hard plastic semi-finished part and the sealing area for the soft plastic part, respectively. The stationary mold half molds the soft plastic cavity, and the seal’s outline is machined in the semi-finished part cavity. The seal’s outline intersects with the rear mold half, thus molding the required hard and soft plastic areas.

The image below shows the difference between the two stationary mold halves. For this mold, the material is molded through different stationary mold halves. They intersect on the semi-finished part and between the semi-finished part and the seal.

The moving mold half requires a rotatable template, while the fixed template remains stationary with the machine. The moving mold half rotates upwards by 90° via a gear and then back. Both halves of the mold must fit precisely at both the 0° and 90° positions, so a guiding and positioning device must be designed on the mold. As shown in the diagram, a guiding device is installed on the side of the mold. The moving mold half has eight positioning slots, and the fixed mold half has four positioning guide rails.

The moving mold half has four identical cores.

The diagram below shows the two halves of the mold, and you can also clearly see that the fixed mold part is used for two different cavities for hard and soft plastic.