Briefly describe characteristics and advantages of various composite injection molding technologies

Time:2021-06-17 11:55:20 / Popularity: / Source:

composite injection molding technologies 
Multi-component injection molding provides new possibilities for plastic processing, that is, combining different materials or colors in a single molding. Production can be completed in one process without need for other assembly or post-processing steps outside machine. Multi-component injection molding is a fully automatic process with a high degree of flexibility, especially suitable for mass production.
Final molded part can have a variety of functions and characteristics. Using this process, colored parts with high pressure resistance, heat resistance or chemical resistance can be produced.
Multi-component injection molding concept encompasses multiple independent processes. Common point of these processes is that two or more injection devices are used to inject a corresponding number of different materials into mold at the same time, final product is produced through a series of steps. Final parts can be used directly without subsequent processing.

According to number of gates, it can be divided into two groups:

Use of single-gate systems, including sandwich and alternating injection molding processes.
Multi-gate system can be preliminarily divided according to core pulling and transfer process. Transfer process includes transfer between two standard machines by manipulator system, transfer by rotation of manipulator system and mold in a specific multi-component machine. Mold rotation includes rotation of movable half mold through rotating device, rotation of mold inner parts and rotation around vertical axis (GRAMTM process).

Application advantages:

Advantages of multi-component injection molding: In multi-component injection molding, components of molded part are completely separated. All components are visible on the surface, reflecting appearance and function of part.
For example, keyboard buttons, logo switches, or handles with soft areas to increase comfort. In addition to advantage of being able to produce injection-molded parts of multiple colors or materials in one process, without need for other assembly or subsequent processing, continuous improvement of molding technology can also bring continuous growth in benefits.
Injection molded parts are resistant to external influences (such as mechanical effects, thermal effects or chemical effects), which are achieved through appropriate material combinations and high bonding strength. Adhesion of two-component bonding surface can be achieved by chemical bonding or mechanical linkage. 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 mold has two stations. After molded parts are pre-injected, they pass through another injection stage to complete production of parts. Prefabricated parts are produced in the first cavity. Then mold is opened, entire movable half mold is rotated 180°, and prefabricated cavity is rotated to final injection position. Then, by adding a second material, prefabricated part is made into final part. Mold cavity can rotate in same direction or alternately in different ways. After final part is demolded, cavity is ready for next prefabrication.
In order to make part demolding independent of production process, a demolding station was integrated into two-component molding. Then, mold rotates clockwise. There is an opening on the side of third station through which gripper of robot system can reach into closed mold, demold part and its gate, place it on conveyor belt for further processing.

Injection molding with more than three components:

Molding process of more than three components can be realized by a variety of methods. Two possible methods are described below.

Two-stop mold:

Setting of two-station mold can be completed in a manner similar to setting of three-component mold mentioned above. In the first process step, three or more (up to five) components are simultaneously injected to produce prefabricated parts. Then, entire half-face mold is rotated by 180 and moved to second position. At this time, other materials are used for injection molding to encapsulate preform to produce final part.
Another method is in correspondingly configured mold, part can be combined with up to five surface elements composed of other materials/colors in one production step. Therefore, insert in mold can be rotated between three stations by rotating mold plate and electric drive rotating device.

Four-station mold:

For example, multi-layer plastic parts can be produced using a four-station mold. When using recycled materials and barrier layers, this method is easy to implement. Innermost layer is produced at the first station. Then, mold is rotated 90° to next stop. At this time, second component is used for injection molding to encapsulate the first component. Then, mold half continues to rotate to third station, and finally to fourth station, for final stage of production.
At this time, a protective outer layer is injected on the part, or surface layer of molded part. After cooling stage, final multi-layer part is demolded from cavity. In continuous cycle, each time mold is opened, a final molded part is produced.
Alternate injection molding, alternate injection molding injects two different colors of same plastic component into same cavity alternately.
Before entering mold, both colors are placed in a special mixing nozzle. Two-color components are mixed to form a color effect. Two colors can be mixed purposefully. In alternate injection molding process, two injection molding devices are connected together with a special alternate injection molding device (which has a mixing nozzle).

Sandwich injection molding:

Sandwich injection molding process injects a core material into outer surface layer. Process is carried out in two or three steps in a cavity.
First, inject material of stroke surface layer into cavity part of space. Then, core component is injected into its inner center through first material. Finally, use first component to seal at gate location. In this way, core material can be prevented from appearing on the surface, at the same time, second component in gate system is removed to prepare the first component for next injection.

Assembly injection molding:

Using a rotary mold, components that need to be assembled after injection molding can be individually molded on a two-component machine and then assembled in the mold. Example of seal assembled in cable duct illustrates how assembly process can be implemented in mold. Two separate components are first formed simultaneously in their respective mould stations.
Then, after opening mold, first component is transferred to second station by rotating insert, and then placed on top of second component core. Assembly of two parts is realized by shaft core punching die. Therefore, subsequent steps can be omitted, complex automation solutions are not necessary.

To sum up:

Multi-component injection molding will become more and more important in the future. Especially for function of manufacturing hard and soft combinations, development of this technology has just started. Use of assembly injection molding can realize reasonable integration of various functional elements in the near future and gradually replace connection process.
Technologies with potential for development in the future include: using shrinkage behavior of materials for directional separation of components, or manufacturing collective circuits through metal-plastic combinations.

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