Two shot molding
The cycle and the resin pairs.
Two shot moldingProcess guide
Rotary Platen Molding
A two shot tool has to carry a warm, freshly molded part from one set of cavities to the next without dropping it or marking it. The rotary platen is the usual way to do that. It is not the only way, and the choice shows up in the part design.
The turntable
A mold half that turns between shots
The rotation belongs to the press, not the mold. That one fact explains most of the design rules.
On a rotary platen press, a turntable sits on the moving platen and the moving half of the mold bolts to it. After the first shot the mold opens, the turntable turns the moving half, and the parts it carries line up with the second shot cavities in the fixed half. Some machines swing half a turn and back again. Others keep turning in the same direction.
Because the moving half turns, both of its stations have to be identical: the same cores in the same places. The fixed half is where the stations differ, with first shot cavities on one side and second shot cavities on the other. A designer who forgets this ends up with a core that fits one cavity and crashes into the other.
Keeping the rotation in the machine keeps the mold simpler than a tool with its own rotating mechanism. The catch is that the tool only runs on presses fitted with a matching turntable, and there are far fewer of those than standard presses.
Options
Four ways to get the first shot to the second station
Each method works. Each also puts a different demand on the part.
| Method | How the part moves | What it asks of the part |
|---|---|---|
| Rotary platen | The whole moving half turns on a turntable built into the press | The part rides on its core, so both stations need identical cores |
| Index plate | A plate lifts the parts off their cores, turns, and seats them on the second station | The part has to release from its core cleanly and keep its shape while it is lifted |
| Robot transfer | A robot picks the parts from one cavity set and places them in another, in the same mold or a second one | Needs something to grip. The part cools while it travels, which can weaken the bond |
| Core back | Nothing moves except a core, which retracts to open space in the same cavity | The second material has to sit where one straight moving core can open a pocket |
One cycle
A rotary cycle from close to close
Both stations work at the same time, which is where the output comes from.
- 1
Close and inject both
The mold closes. Station one gets the first shot while station two, holding parts from the last cycle, gets the second shot.
- 2
Cool together
Both stations cool at once, so whichever shot needs longer sets the cycle for both.
- 3
Open and eject one side
Finished parts eject from station two. Ejection at station one is held back, so its parts stay on their cores.
- 4
Turn
The turntable carries the fresh first shots to station two and brings the empty cores back to station one.
Part design
Why the first shot stays on its core
The core is doing more than shaping the part. It is carrying it.
- The core holds the part's shape while it is still warm, so it does not warp between shots
- A light undercut or a textured face helps the part grip the core so it does not shift during the turn
- The second shot cavity seals against the first shot, not bare steel, so first shot dimensions matter more than on a single material part
- Anything that would stick up out of the parting plane during the turn has to clear the fixed half
Vertical presses
The same turntable idea in insert molding
Here the rotation hides load time. There is no second material.
Vertical presses often carry a rotary or shuttle table with two or more lower mold halves. While one half sits under the press being molded, an operator or a robot loads inserts into the other. When the shot finishes, the table turns and the loaded half moves under the press.
It pays off on parts where placing the insert takes longer than molding around it: metal pins, threaded inserts, wire leads, and terminals. Gravity helps too, since loose inserts sit in a horizontal lower half without falling out. Insert molding is one of NetProto's core molding processes, and the insert molding page covers how those programs are quoted.
Choosing
What usually decides the transfer method
Nobody picks a method in the abstract. The part and the supply base pick it.
How big the part is compared with the turntable and the press
Whether the first shot can come off its core without distorting
How much the bond depends on the first shot still being warm
Whether the second material sits in a pocket that one core could open
How many presses in the supply base have the right platen or table
Keep reading
Related guides
The processes that depend on part transfer, and the design rules that come with them.
Core back molding
Two materials without turning the mold.
Core back moldingMulti shot molding
More stations, more shutoffs.
Multi shot moldingInsert molding
Metal and preformed parts molded in place.
Insert moldingMolded in inserts
Holding an insert in the tool and in the part.
Molded in insertsOvermolding vs two shot
When two separate tools are the better route.
Overmolding vs two shotProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about rotary platen molding
Is the rotary platen part of the mold?
No. It is fitted to the press, and the mold is built to bolt onto it. That is why a rotary tool can only move to presses with a compatible turntable.
Why not use a robot every time?
A robot adds a pick and place step, and the part cools while it travels. That is fine for some material pairs and hurts the bond on others. Robots make sense when the first shot cannot stay on its core, or when the two shots run in separate molds.
Does the rotation slow the cycle down?
It adds the time the turn takes. Both stations mold during the same cycle, though, so every cycle still ends with a finished part.
Is a rotary table on a vertical press the same thing?
Same mechanical idea, different purpose. On a vertical press the table usually exists to load inserts while the other half is molding, not to add a second material.
Next step
Show both shots, or every insert, in the CAD
Model each material or insert as its own body. The review will say whether an overmolding or insert molding program gets you the same part. NetProto manufacturing is ISO 9001 certified.