What is Coinjection?

Coinjection is a sequential type of multi component technology. It involves a second component (core) being injected into the first component (skin). A special coinjection nozzle controls the ratio between the first and second components using differential pressure. This method can save costs, as it allows a cheaper core material, such as regranulate, to be used. It is also possible to enhance the quality of a component by giving it a fibre-reinforced core and an unreinforced skin. Sink marks can be prevented by using expanding agents in the core, which can also lead to a reduction of clamping force required – an additional positive side effect.

 

In the packaging industry specifically, coinjection is applied in barrier injection molding. When polyolefins are processed, a gastight material like EVOH is used as a barrier layer in the core. Another special application of coinjection is interval injection molding, used to produce a marbled appearance in molded parts. Because of repeated switching between the two components during the injection process, a reproducible marbling effect is created when molding parts with complex profiles, and a tiger stripe pattern in the case of simple profiles.

Coinjection is a special part of the multiple-component technology

Benefits of coinjection:

  • cost reduction
    • cheaper material for the core component
    • ability to use recycled material
    • foamed core material for lighter parts
  • increase of part properties
    • higher mechanical strength despite high surface quality
    • combination of galvanized skin – with reinforced core components
    • improved part quality – fewer sink holes
  • marbling
 


Physical Basics

 


Process sequence

Coinjection nozzle with hydraulic shut-off and melt pressure controlled sealing bush

 


 

   

Applications


Bumper bar:  skin material TPE, core material PP
Screw cap:  skin material PP, core material recycled PP
Cup with barrier layer:  skin material PP, core material EVOH

 

 


Marbling