Co-injection FAQS

It is the simultaneous injection of two compatible thermoplastic materials where one material engulfs or encapsulates the other. This is accomplished with the use of two separate injection units and a single injection nozzle. The two materials flow into the mold together as skin and core and maintain this configuration throughout the injection.
This process should not be confused with two-shot or two-color molding where two colors and/or two materials are injected separately.

How old is co-injection?

This technology is more than fifteen years old. Imperial Chemical Industries holds the original patents for this molding process. The pioneer developers of co-injection machines were Germany’s Schloemann-Siemag and Italy’s Presma.

Why use co-injection?

• In thin wall parts, it is mostly used for technical reasons such as shielding or combining a soft skin with a rigid core or for using prime material with off-grade or scrap.
• In thick wall parts, all of the above reasons are applicable as well as additional benefits from the physical and mechanical properties and the cost savings possible with a foamed core.

What are the main advantages of co-injection?

• The skin can be hard and shiny with a Class “A” finish, or even provide a different surface such as a “soft touch” material.
• Problems of shrinkage, sinking, warping and internal stress are greatly reduced.
• The inferior materials of the core do not flow in direct contact with the walls of the mold which facilitates their use in the molding. For the same reason, difficult mixes such as material loaded with wood flour and fiberglass can be easily injected as the core with minimal shrinkage problems.
• In some applications, incompatible materials may even be used in spite of the serious processing problems they pose. This is possible because the pressure exerted by the foamed core on the skin is such that an acceptable mechanical bond can be achieved in substitution of a chemical one.

What are the principal cost savings with co-injection?

• As only the skin requires prime material, the core may be off-grade, recycled or otherwise inferior material, as much as 50% by weight.
• Colorant is required only in the skin and not in the core.
• The fact that the core is normally foamed represents a savings in the total weight of the part of approximately 15%.

What are the royalties, fees or licenses required to use this process?

There are none.

What about tools?

Most conventional existing tools can be used as is, or with some minor modifications. Multi-cavity molds must be balanced. Since this is a low pressure system, new tooling is typically more economical as aluminum and less expensive grades of steel are the materials of choice.