Two shot molding
The cycle and the resin pairs.
Two shot moldingProcess guide
Core Back Molding
Instead of moving the part to a new cavity, core back moves the steel. A core pulls back after the first shot, the space it leaves becomes the second shot cavity, and the part never leaves its seat.
How it works
The cavity changes shape between shots
One cavity set, two injection units, and a core that moves at the right moment.
During the first shot, part of the cavity is blocked by a core. Once the first material has set enough to hold its shape, a hydraulic cylinder or a cam pulls that core back. The empty space it leaves is filled by the second injection unit. The part sits in the same cavity from start to finish.
That keeps the press simple. No turntable, no index plate, and the tool does not need two matching stations. What you give up is freedom over where the second material goes. It has to land in a pocket that one straight moving core can open, which rules out a lot of grips that wrap around a part.
You also pay in time. The second shot cannot start until the core is back, and the next first shot cannot start until the finished part is out. On a rotary tool both shots run at once at different stations. On a core back tool they take turns.
Tradeoffs
Core back next to a rotary platen tool
Same goal, two ways of getting there. The part geometry usually settles it.
| Point | Core back | Rotary platen |
|---|---|---|
| Where the second material can go | A pocket one core can open: a band, a pad, a rim, a window | Almost anywhere the second cavity can reach, including wrap around grips |
| Press | Two injection units, no turntable | Two injection units plus a rotary platen |
| Cycle | Shots run one after the other | Shots run at the same time at different stations |
| Tool | One cavity set with moving cores and their actuators | Two cavity sets and a moving half with identical stations |
| Bond | First shot never moves and stays warm | First shot rides its core and stays warm |
Good fits
Part shapes that suit a retracting core
If the second material could be described as a strip, a ring, or a patch on one face, core back is worth a look.
- A soft strip along the back of a handle, opened by a core that faces that strip
- A seal around the rim of a lid or a housing, where the core pulls back from the sealing face
- A clear window in a darker frame, with the core facing the window
- A two color split that runs straight along the core's direction of travel
Design checks
What the geometry has to allow
Run through these before assuming a part can skip the rotary press.
One core moving in one direction can open the whole second material region
The first shot has enough section beside the core to hold its shape when the core pulls away
The edge where the core stops makes a clean shutoff against the first material
A gate for the second shot can reach the pocket without crossing a show face of the first material
Core travel does not leave a thin, fragile lip of first shot material at the edge of the pocket
Same name
The other core back: opening the mold so the part can foam
The term turns up in foam molding with a different purpose. Ask which one a supplier means.
In foam molding, the cavity is filled with resin carrying a gas or a foaming agent. After a solid skin forms against the steel, part of the mold opens slightly. The still molten center expands into the extra room, which lowers weight and stiffens the wall without adding material.
It is the same mechanical move, a core pulling back mid cycle, used for a completely different result. If core back shows up on a quote or a process list, one question clears it up: is it for a second material, or for foaming?
Keep reading
Related guides
Other ways to put two materials on one part, and the tool features core back relies on.
Rotary platen molding
Moving the part instead of the core.
Rotary platen moldingCo injection molding
A second material hidden inside the wall.
Co injection moldingUndercuts
Slides, lifters, and the moving steel behind them.
UndercutsParting line
Where steel meets steel, and where flash starts.
Parting lineOvermolding
Two materials from two simpler tools.
OvermoldingHandles
Grips, cores, and where the soft material goes.
HandlesProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about core back molding
Is a core back tool simpler than a rotary tool?
The press is simpler, and the tool has one cavity set instead of two. The moving cores, their actuators, and their shutoffs still need careful design and upkeep. It is a different kind of complexity, not an absence of it.
Why is the cycle longer than on a rotary press?
The shots cannot overlap. The core has to finish retracting before the second shot, and the part has to come out before the next first shot. On a rotary press one station is always molding while the other one is too.
Can core back handle three materials?
With more than one moving core, in principle yes. Each extra core adds a shutoff and a step in the sequence, so parts with three materials usually move to rotary tools.
Does the first shot need to be fully cooled before the core pulls?
No. It needs to be stiff enough to hold its shape where the core was. Pulling too early lets that edge sag, and waiting too long costs bond strength and cycle time.
Next step
Mark where the second material sits
Upload CAD with the second material as its own body and note which faces must bond. The review looks at whether an overmold program gets you the same part from two simpler tools. NetProto manufacturing is ISO 9001 certified.