Co injection
- Materials stack through the thickness of the wall.
- The skin covers both faces, so the core never shows when things go right.
- Often a conventional mold on a specialized press.
Process guide
Co Injection Molding
Two shot puts materials side by side. Co injection puts one inside the other. The skin you see and touch is one resin, and the core underneath can be recycled, foamed, or filled material that would never pass as a show surface.
How it works
One gate, two materials, layered through the wall
Also called sandwich molding. The layers form because plastic freezes from the outside in.
Skin material goes in first. As it flows, it freezes against the cold steel, so what keeps moving is a molten channel down the middle. The core material follows through the same gate, pushes into that channel, and spreads out behind the flow front. A short last shot of skin material closes off the gate. Cut the finished part in half and you see the same resin on both faces with something else sandwiched between them.
The press does the unusual work here. It needs two injection units feeding one nozzle that can switch between them, or run both at a set ratio. The mold can be close to an ordinary single material tool. That is the reverse of two shot, where the tool carries most of the complexity.
You find it where a part needs a good surface but most of its volume could be something else: crates and pallets with recycled content hidden inside, housings with a stiff filled core under an unfilled skin, and thick parts with a foamed core to keep the surface from sinking.
Not two shot
Layered through the wall versus placed side by side
Both processes use two injection units. Where the second material ends up is completely different.
Two shot
- Materials sit next to each other on the surface.
- Both materials are meant to be seen or touched.
- A rotating or core back tool does the hard work.
Pairings
Common skin and core pairs, and why people use them
General industry practice. Skin and core usually come from the same resin family so the layers stay bonded.
| Skin | Core | Reason |
|---|---|---|
| Virgin resin in the part color | Recycled or regrind of the same resin | Recycled content goes where nobody sees it, and the face keeps its color |
| Unfilled grade | Glass filled grade of the same family | Stiffness from the core without glass fibers showing on the surface |
| Solid resin | Foamed version of the same resin | Less weight and less sink on thick sections, with a solid skin outside |
| PET | A barrier resin such as EVOH | Layered preforms and containers where the middle layer slows oxygen getting through |
Failure modes
The core does not always stay where it belongs
Most co injection trouble comes down to where the layer boundary ends up. Sectioned sample parts are the only reliable way to see it.
- Breakthrough: near the end of flow, or in thin areas, the core reaches the surface and shows as a streak or a patch of the wrong color
- Uneven skin: skin runs thin where flow is fast and thick where it slows, so thin sections limit how much core the part can hold
- Gate blemish: if the final skin shot is too short, core material is left showing at the gate
- Delamination: skin and core from different families may not bond, and can peel apart under impact
- Viscosity mismatch: a core much runnier or much stiffer than the skin spreads unevenly behind the flow front
Is it worth it
When co injection pays, and when it does not
The process only makes sense when the core is a big share of the part and does something the skin cannot.
It earns its place when the core material is much less expensive than the skin, or more useful, and makes up most of the part's volume. Hiding recycled content under a colored face is the most common reason today. Small, thin parts leave little room for a core, and the extra process control buys very little.
The process window is also narrow. Skin to core ratio, injection speed, and the switch point all move the layer boundary. Expect more sampling than for a single material part, and ask whoever runs it how they check layer distribution. The honest answer usually involves a saw and a stack of cut parts.
Keep reading
Related guides
Sibling multi material processes, the resins that go into cores, and the defects that show up when layers misbehave.
Two shot molding
Two materials side by side, molded in one cycle.
Two shot moldingMulti shot molding
Three or more materials in one tool.
Multi shot moldingRecycled PET (rPET)
Recycled resin behavior and where it fits.
Recycled PET (rPET)Glass filled vs unfilled
What glass adds to stiffness, and what it does to the surface.
Glass filled vs unfilledDelamination
Why molded layers peel apart and how to stop it.
DelaminationSink marks
The thick section problem a foamed core is often used to hide.
Sink marksContainers
Molded tubs and jars, and where their walls get thin.
ContainersProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about co injection molding
Is co injection the same as sandwich molding?
Yes. Sandwich molding is the older name for the same process. You will also see it written as coinjection or skin and core molding.
Can the core be a different resin family from the skin?
It can, but layers that do not bond can separate under load or impact. Most production parts pair a skin and a core from the same family for that reason.
How do you know the core stayed inside?
Cut sample parts at several points along the flow path and look at the layers. Surface inspection only tells you about breakthrough after it has already happened.
What can a standard single material tool do instead?
Core out thick sections, add ribs for stiffness instead of mass, and blend recycled content into the resin where the color allows it. The DFM review will show which of those fits the part.
Next step
Tell us what the core was supposed to do
Upload CAD and say whether the goal is recycled content, weight, stiffness, or sink on a thick section. DFM feedback comes back before any steel is cut. NetProto manufacturing is ISO 9001 certified.