This is a process that creates a skin and core material arrangement in a plastic molded part. The skin material is injected first into the mold cavity and is immediately followed by injection of the core material. As the skin material flows into the cavity, the material next to the cavity walls freezes and material flows down a center channel. When the core material enters it displaces the skin material in the center of the channel by pushing the skin ahead. As it flows ahead it continues to freeze on the walls producing the skin layer.

Machine Based Co-Injection molding
The Co-Injection molding process requires two injection/processing units. The units generally inject material through a manifold located at the end of the injection barrels. The manifold ports the two melt streams into a centrally located nozzle. The machine controls the injection units to achieve a skin-core-skin flow sequence through the manifold into the mold. Last skin flow is needed to clear the short nozzle section of core material and to seal the gate area with skin. This arrangement can be used on any single or multiple cavity, conventional cold runner mold.

Mold Based Co-Injection molding
This same process can be achieved on a hot runner mold by utilizing a hot runner system from Incoe Corporation of Troy Michigan. This system, sometimes identified as “Mold Based Co-Injection”, still utilizes two injection units. The two melt streams are directed into the mod via separate channels. These two channels remain separate until they reach the gate area of the part. At this point they flow through a nozzle arrangement similar to the normal co-injection manifold.
Co-Injection Benefits

Lower Cost PartsHigher Strength CoreSound Absorption CoreReduced Cooling Time for Lower Temperature CoreImproved Aesthetic QualitiesCombined Property Characteristics
Co-Injection Features of a Milacron Machine

Independent Co-Injection Manifold Easy Purging Of Individual Barrels and Manifold.Reversible Manifold for Optimizing Barrel Capacity with Core %.Active Split ScreensTMĀ for Viewing Multiple Screens At Once.Control Sequences for Utilizing in Mold Co-Injection Manifolds.Flexible Manifold Design for Machine or Mold Based Co-Injection.

Manifold for Machine Based
Manifold for Mold Based

Schematic of Incoe Co-Injection Hot


  • Foamed Core for reduced weight and noise transmission
  • Glass filled cores for improved physical properties
  • Low cost core for cost savings.
  • High gloss skin material over structural core material for combination of aesthetic and structural properties.
  • Post Consumer Recycled material in core. Environmental friendly.
  • Post Industrial recycled Material in core
  • Reground painted parts recycled into core.