Two-color injection molding technology, also known as two-material injection molding, is a technique that uses a mixture of two plastic materials to injection mold plastic products. Its core principle is to use a mold assembly for shaping and welding within the mold. Therefore, the fundamental principle is to plasticize and mold two different plastic raw materials, and then weld them together using a mold to achieve the purpose of two-color injection molding.
1. Types of Two-Color Injection Molding Technology
1.1 Rotary Core Two-Color Injection Molding Technology
This technology, also known as rotary core two-color injection molding, works by first injecting the first type of plastic material into a small hole in the mold using an injection molding machine. Once formed, this becomes the first type of plastic. Then, the mold is rotated 180°, and the second type of plastic material is injected using the same injection machine. After the second type of plastic is formed, a final encapsulation process is performed, completing a basic two-color injection molding operation.
This technology is relatively simple to use and operate. Workers with minimal training can generally operate it freely, and it greatly enhances the design freedom of plastic products. Furthermore, it can be processed using simple tools. 1.2 Shrink Mold Core-Type Two-Color Injection Molding Technology
This technology primarily utilizes a hydraulic system to compress the mold. First, under the control of the hydraulic system, a movable core, like a piston, is pushed to the top, and plastic material is injected. After the first material solidifies, the movable core is lowered, and then another type of plastic material is injected. The hydraulic system is then controlled to raise the core again, pressing it down until it solidifies and forms the final shape.
The plastic product produced using this technology is initially completed. The molded part is then removed for further processing. This technology is relatively simple to operate, but precise timing of the hydraulic system's movement is crucial.
1.3 Rotary Ejector Plate Two-Color Injection Molding Technology Principle
This technology requires the injection of the first type of material before operation. During injection, the rest of the mold is closed. During injection molding, the mold is gradually cut and separated, and the later inserts gradually retract, but still remain on the ejector plate. The mold dynamically retracts, passing through the ejector pins and pull rods of the injection molding machine before cooling and detaching. This completes the first part.
The next step is the injection molding of the second part, which is largely the same as the first part. However, the second injection uses two nozzles to inject simultaneously. After solidification, both parts are ejected together, completing one injection cycle.
1.4 Core Sliding Two-Color Injection Molding Technology
The working principle of core sliding two-color injection molding technology is to classify the core for use in two stages. During the first injection, the core is slid to the designated position for mold closing and injection molding, injecting the first type of plastic material into the mold. After cooling, the mold is opened all at once.
The transmission device slides the first core out and the second core into the cavity, then injects plastic a second time. After cooling and solidification, one injection cycle is completed. The finished product, synthesized from both injections, is then ejected. This technology is more suitable for the production of large, two-color injection molded plastic parts.
2. Characteristics of Two-Color Injection Molding Process
2.1 The Two-Set Structure of a Two-Color Injection Molding Machine
Due to the intended use of the products, a two-color injection molding machine requires two identical plasticizing injection units. This is not only a process requirement but also necessitates the ability to rotate and switch between them.
These two identical injection units, besides having the same structure, must also have the same function. During injection molding, they must be able to consistently adjust the raw material temperature, injection pressure, and injection rate in both machines to ensure that the various properties of the molded plastic product remain at a consistent level, facilitating fusion and welding. If the parameters of the two machines are inconsistent, they must be adjusted to be as consistent as possible.
2.2 Differences in Machine Structure Between Two-Color Injection Molding Components and Ordinary Injection Molding Components
Due to the unique characteristics of plastic products produced by two-color injection molding components, the machine's structural setup differs significantly from that of ordinary injection molding machines.
Firstly, two-color injection molding components use a higher melt temperature compared to ordinary injection molding components, requiring a higher temperature to maintain flowability within the mold. Secondly, the injection pressure is also higher, not only because two different plastic raw materials are used, necessitating strong pressure for fusion during pressing and welding, but also because it is designed to overcome the significant resistance to melt flow within the mold, ensuring smooth injection molding.
2.3 Material Parameter Settings for Two-Color Injection Molding Technology
Two-color injection molding technology uses two different materials to produce plastic products. Therefore, material selection is crucial before injection molding, considering not only the quality standards of the selected materials but also the control of parameters such as temperature and pressure during injection.
Because the molds used in two-color injection molding differ from those used in ordinary injection molding, the material parameters for two-color injection molding require higher melt temperatures to ensure the melt does not solidify prematurely before injection. Simultaneously, the injection device must be used to control the pressure of the raw material, ensuring it reaches all parts of the mold smoothly, thus achieving the desired injection molding effect.
3. Key Design Considerations for Two-Color Injection Molded Products and Molds
3.1 Structural Characteristics of Two-Color Injection Molded Products
The structure of two-color injection molded products differs significantly from that of ordinary plastic products. Due to the use of two different materials, the product possesses two distinct properties, fundamentally different from ordinary injection molded products. Therefore, meticulous structural design of the two-color injection molded product is necessary before injection molding, fully considering the compatibility of the two materials.
The mixing ratio and connection method of the two materials should be designed based on the intended function and usage environment of the plastic product. A detailed analysis of the process complexity is required, along with consideration of strength, to ensure the two-color injection molded product meets relevant material standards and requirements.
3.1.1 Selection of Two Materials
Two-color injection molding technology uses two different materials for injection molding. Therefore, the selection of these two materials is a crucial process. Generally, two different colored plastics are chosen, which greatly improves the strength and durability of the injection-molded product and makes it easier to fuse. However, products for special purposes may require the use of two materials with significantly different properties, necessitating the solution to the challenge of fusing these two highly different materials.
The main problem is delamination and detachment, which can be fatal to injection-molded products. Therefore, shrinkage rate and different interfacial conditions need to be carefully considered, and the material ratio must be adjusted precisely.
3.1.2 Design of Internal Structure and Shape of the Product
The design of the product's structure and shape must first consider its intended use and purpose, involving detailed research on its size and internal structure.
Generally, increasing the contact area between the two materials enhances strength. This can be achieved by designing numerous small grooves and protrusions inside the product for embedding and stitching, thus increasing the contact area between the two materials. This improves the product's strength and lifespan during injection molding, enhancing its practicality.
3.2 Rotary Dual-Speed Injection Mold
This type of mold requires different considerations in the molding section. Firstly, the grooves of the two injection molds need to be precisely designed to ensure a tight fit during assembly. Secondly, the demolding mechanism requires a second injection before demolding. Different injection methods also require careful differentiation. In cases where demolding is performed on a vertically rotating injection molding machine, the product is ejected, completing the process. In cases where demolding cannot be performed using an injection molding machine, a hydraulic device is needed.
4. Conclusion
In summary, dual-color injection molding technology has undergone many years of development, but it has only recently gained attention and application. This is because the finished products produced have excellent adaptability to the usage environment, extremely high production efficiency, and can well meet various practical requirements. Therefore, it is gradually being widely used, showing good economic benefits in the automotive industry and other fields, and is a technology with great development potential.
