Characteristics and Advantages of Various Composite Injection Molding Techniques
Multi-component injection molding has opened up new possibilities in plastic processing by allowing the combination of different materials or colors in a single molding cycle. Production can be completed in one step, eliminating the need for additional assembly or post-processing steps outside the machine. Multi-component injection molding is a fully automated process with high flexibility, making it particularly suitable for large-scale production.
The final molded parts can possess a variety of functions and characteristics. Using this process, parts with high pressure resistance, heat resistance, or chemical resistance can be produced, even with added coloring.
The concept of multi-component injection molding encompasses multiple independent processes. The common thread among these processes is the simultaneous injection of the respective number of different materials into the mold using two or more injection units, resulting in the final product through a series of steps. The final parts are ready for use without the need for further processing.
Classification of Composite Injection Molding Processes
Based on the number of gates, these processes can be divided into two groups:
① Systems with a single gate, which include the sandwich and alternate injection molding processes.
② Systems with multiple gates, which can be initially divided based on core pulling and transfer processes.
- The transfer processes involve the transfer between two standard machines by a robotic system or transfer within specific multi-component machines through a combination of robotic systems and mold rotation.
- Mold rotation can include the rotation of the movable half-mold using a rotating device, the rotation of internal mold components, and rotation around a vertical axis (as seen in the GRAMTM process).
Advantages of Multi-Component Injection Molding
In multi-component injection molding, the components of molded parts are completely separated. All components are visible on the surface, reflecting the appearance and functionality of the part.
For example, keyboard buttons, labeled switches, or handles with soft areas for added comfort. In addition to the advantage of producing injection-molded parts in multiple colors or materials in a single process without the need for additional assembly or post-processing, continuous improvements in molding technology can lead to ongoing benefits.
Injection-molded parts exhibit resistance to external influences such as mechanical effects, thermal effects, or chemical effects, achieved through appropriate material combinations and high bonding strength. Adhesion between two-component surfaces can be achieved through chemical bonding or mechanical linking. If chemically compatible materials are used, permanent molecular bonding can also be achieved through melting or welding processes.
Two-Component Injection Molding
Fully automatic two-component injection molding uses a two-station mold, where the production of molded parts is completed in another injection stage after pre-injection.
- Prefabricated parts are produced in the first cavity.
- Then, the mold opens, and the entire movable half-mold rotates 180°, positioning the prefabricated cavity for the final injection.
- Subsequently, by introducing the second material, the prefabricated part is transformed into the final part. The mold cavities can rotate in the same direction or alternate directions.
- Once the final part is demolded, the empty cavity is ready for the next prefabrication.
To enable part demolding independently within the production process, a demolding station is integrated into the two-component molding. Then, the mold rotates clockwise. There is an opening on the side of the third station. And a robotic system’s gripper can reach into the closed mold, demolding the part and its gate, placing it on a conveyor for further processing.
Multi-Component Injection Molding with Three or More Components
Molding processes involving three or more components can be achieved through various methods. Below, two feasible methods are described.
(1) Two-Station Mold
Setting up a two-station mold can be accomplished in a manner similar to the three-component mold described above. In the first step, three or more (up to five) components are injected simultaneously to produce prefabricated parts. Then, the entire half-mold rotates 180 degrees to the second position. At this point, additional material is used to inject and seal the prefabricated part, producing the final part.
Another approach involves molds configured in such a way that the part can essentially combine with up to five surface elements made of other materials/colors in a single production step. Therefore, embedded components within the mold can be rotated between three stations using rotating templates and electrically driven rotary devices.
(2) Four-Station Mold
For example, multi-layer plastic parts can be produced using a four-station mold. This method is particularly suitable when recycled materials and barrier layers are used. The innermost layer is produced in the first station. Then, the mold rotates 90° to the next station. At this point, the second component is used to inject and encapsulate the first component. The half-mold continues to rotate to the third station and finally to the fourth station for the final stage of production.
In this process, an outer layer that serves as protection for the part or forms the surface layer of the molded part is injected onto the part. After a cooling phase, the final multi-layered part is demolded from the cavity. In a continuous cycle, each time the mold is opened, a final molded part is produced.
Alternate Injection Molding, as in alternate injection molding, two different colors of the same plastic component are alternately injected into the same cavity.
Before entering the mold, both colors are placed in a special mixing nozzle. The two-color components are mixed to create a color effect. The two colors can be deliberately mixed in various configurations. In the alternate injection molding process, the two injection units are connected with a special alternating injection device (which includes a mixing nozzle).
Sandwich Injection Molding
The sandwich injection molding process involves injecting a core material into an outer layer. This process is carried out within a single cavity in two or three steps.
- First, the material for the outer layer is injected into the cavity’s partial space.
- Then, the core component is injected into its inner core using the first material.
- Finally, the first component is sealed at the gate location.
This prevents the core material from appearing on the surface while purging the second component from the gate system for the next injection of the first component.
Assembly Injection Molding
Using rotating molds, components that need to be assembled after injection molding can be individually molded on two-component machines and then assembled in the mold. An example of assembly in cable conduits illustrates how the assembly process can be achieved within the mold. The two separate components are first molded simultaneously in their respective mold stations.
Then, after opening the mold, the first component is transferred to the second station by rotating inserts and placed on top of the second component core. The assembly of the two parts is achieved through core pin punching. As a result, subsequent steps can be eliminated, and there’s no need for complex automation solutions.
Multi-component injection molding will become increasingly important in the future. Especially in the development of technology for manufacturing functional combinations of hard and soft components, this field is just beginning to flourish. Through assembly injection molding, the rational integration of various functional elements can be achieved in the near future, gradually replacing traditional assembly processes.
Promising technologies for the future include utilizing material shrinkage behavior for directional separation of components or producing integrated circuits through metal-plastic combinations, etc.