Co-Injection Molding

Principle

Co-injection is a molding process for making components of two materials. It is a three step injection process.

1^st Step – Start injection of outer skin into mold.

2^nd Step – Then a core (different material) is injected behind the first material.

 3^rd Step – Finally, the original “Skin” material is injected behind the core material.

This third step is required to purge the system and seal the gate for next shot.

The skin and core material are sequentially injected through the nozzle. The skin material is partially filled in the mold followed by injection of core component and filling of cavity is completed with skin components.

There are 3 options of co-injection

a a) Solid outer skin/foam core.

b b)Solid outer skin/solid core.

c c) Foamed outer skin/solid core.

The gate location is very typical with varying wall thickness. The wall thickness is >4.0mm for foam core/solid skin.

       3-4 mm for solid core/solid skin.

       2mm is minimum.

This technique is specially suitable for EMI shielding and high loading of conductive filler (carbon black) application.

Advantages

·         High strength cores with soft skins

·         Reclaimed core material inside of a high cost skin.

·         Attractive colors.

·         Reduced assembly costs

·         Decorative

·         Innovative solution

Process description and its types:

Co-injection is done in three ways:

·       Machine based

-          Skin and core material is injected through nozzle

·       Mold based

-          Skin and core material is injected through valve gate (hot runner system)

·       Mono sandwich

-          Skin material is extruded into the nozzle and injected into mold

Effects of viscosity

It is always preferable to use a more viscous material as the core. If the core resin is less viscous than the skin resin, it will not be able to push the skin resin ahead and it will “tunnel” through the hot center flow area. This result in low core percentages and inconsistent flow patterns.

Benefits of the Co-injection process

The co-injection molding process is similar to conventional injection molding except for one major difference. Co-injection uses a special valve configuration that enables two separate injection units to inject chemically compatible thermoplastics through the same injection port. This process allows one material, usually the prime material, to form the outer skin of the part while a second material fills the center. Co-injection offers many cost saving and design benefits over the conventional injection molding process. These benefits include the ability to mold larger parts with less clamping pressure, reduced material costs, and eliminating the need to paint glass filled parts.

The best part about this technology is that no special molds are required and existing conventional molds can be used with usually only minor modifications needed.