Multi-component injection molding

In the past few years, we have seen trends where more complex manufacturing systems are developed that manufacture parts using various materials and components such as coextrusion of multilayer films and sheets, multi-component injection molding, sandwiched parts or hollow products. 

In contrast to multi-color and multi-component injection molding, co-injection molding uses the same gate and runner system. Here, the component that forms the outer skin of the part is injected first, followed by the core component. The core component displaces the first and a combination of the no-slip condition between polymer and mold and the freezing of the melt creates a sandwiched structure as depicted in Fig. 3.51. 

FIGURE 1.68 Multi-component injection mold with spider carrier of the parts... 

Especially in micro injection molded parts, the question of further processing always arises. The smallest parts are often difficult and cost-intensive to further process. Through the use of multi-component injection molding, such assembly operations can be prevented. In this method, two or more different materials can be injected either simultaneously or sequentially and thereby create a composite part. 

FIGURE 1.264 Multi-component injection molded part (source Oechsler AG)... 

The use of the hot runner technology allows only certain component geometries and mold concepts. With hot runner systems, gating positions are achieved that cannot be reached conventionally the active movement or complete avoidance of fill lines can be implemented by sequential injection multi-component injection molding can be performed with up to five materials in a mold high-speed stack molds can be built film and fabric can be back injected gas and water injection technology can be implemented multi-cavity molds can be realized clean room production can be optimized and the co-injection technique can be enabled. 

An example of the latter is the boost in the development of thermoplastic elastomers in the last decade of the Twentieth Century, driven by product applications obtained by multi-component injection molding. The design guidelines for such products originally included recommendations to make the material components mutually enclose each other so as not to rely on adhesion between dissimilar materials only. Later grades of thermoplastic elastomers have been improved with respect to the adhesion to thermoplastics, allowing more design flexibility and processability improvements. 

In principle, all injection molding techniques, by which two or more materials are being processed, represent the multi-injection process. As only two materials are employed quite frequently, that is also known as two-component injection molding. 

Mikell Knights, K 2004 Wrap-Up Injection Molding. Spotlight on Electric and Multi-Component Machines, Plastics Technology Magazine, vol. 51, no. 1, pp. 38-45, 2005. 

FIGURE 2.1 Example of a mold for the multi-component technology and the back injection technology... 

Basically, all types of silicone are processed on conventional injection molding machines in a modified version. Corresponding injection units and cylinder modules to multi-component applications are offered by the manufacturers. 

Transfer molding is used for the manufacture of small components and is particularly useful with multi-cavity tools and where small inserts are to be moulded in. Materials used are polyester and epoxide dough moulding compounds, although a new liquid resin injection technique is reported. 

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