Stack molds
Levels that fill together.
Stack moldsProcess guide
Tandem Molds
On a thick part, most of the cycle is spent waiting for it to cool, and the press injection unit sits idle. A tandem mold keeps it busy by filling a second level while the first one is still cooling.
The idea
Same layout as a stack, opposite timing
The hardware looks similar. The sequence is what makes it a tandem.
A tandem mold has two parting lines on either side of a center section, like a stack mold. The difference is that the levels never fill together. The press injects into one level, and while that level holds and cools, the other level opens, ejects, closes, and waits for its own shot.
Latches lock whichever level is cooling, so the press can open the other. A hot runner with valve gates switches the melt from one level to the other. The clamp only ever resists one level's injection pressure, and the injection unit, which would sit idle through a long cooling time in a normal mold, works for most of the cycle.
Sequence
One tandem cycle
Level A and level B swap roles every time the press opens.
- 1
Fill level A
Valve gates open to level A only. Level B is closed and its parts are still cooling from the previous shot.
- 2
Open level B
A latches shut to hold level A closed. The press opens and only level B parts, now cool, are ejected.
- 3
Close and fill level B
The press closes, the valves switch, and level B takes its shot while level A keeps cooling.
- 4
Open level A
The latches swap. Level A opens and ejects, and the cycle repeats from the start.
Three layouts
Tandem, stack, and family molds compared
All three put more than one part in a cycle. They solve different problems.
| Point | Tandem mold | Stack mold | Family mold |
|---|---|---|---|
| When levels fill | In turn | Together | One level; all cavities together |
| Best suited to | Long cooling times | Short cycles and flat parts | Sets of related parts at moderate volume |
| Different parts in one tool | Easy, since each level has its own shot and timing | Only if they fill and cool alike | Yes, if volumes and sizes are close enough to balance |
| Clamp force | One level at a time | About one level, since faces oppose | All cavities on one face |
| Tool complexity | Highest: latches, valve gates, sequencing | High: linkage, sprue bar, center manifold | Close to a standard mold |
Where it helps
Mismatched parts that a family mold struggles with
A family mold fills every cavity in one shot, so a big thick part and a small thin one fight over pressure and cooling time. Tandem levels do not share a shot.
- A heavy housing and a much thinner cover, one per level
- Thick parts such as handles and structural brackets, where cooling dominates the cycle
- Parts that must ship as matched sets, molded in the same tool at the same rate
- Programs that want more output from an existing press without adding clamp force
Caveats
What makes tandem molds rare
The idea is simple. Running one well is not.
The press controller has to sequence latches, valve gates, and ejection for each level
A latch that fails to hold can open a level with hot parts inside
Each level is ejected every other opening, so part handling has to follow the alternation
Few mold makers and molders run them, so support and second sources are limited
The gain only exists when cooling is long compared with the rest of the cycle
Keep reading
Related guides
Other layouts for more than one part per cycle, and where cycle time goes.
Family injection molding
Related parts in one single level tool.
Family injection moldingCycle time
Why cooling usually takes the most time.
Cycle timeHot runner molding
Manifolds and valve gates.
Hot runner moldingClamping force
What holds the mold shut during a shot.
Clamping forceEnclosures
Housings and covers made as matched sets.
EnclosuresProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about tandem molds
Is a tandem mold a kind of stack mold?
It is often described that way, since the hardware is similar. The alternating fill is what sets it apart, and that changes the controls, the runner, and which parts suit it.
Does a tandem mold need a special press?
It needs enough mold height and stroke for a two level tool, and a controller that can handle the extra sequencing. Many presses need added control outputs rather than a different machine.
Why not just use two molds in two presses?
That is often the simpler answer. A tandem mold makes sense when press capacity or floor space is the limit, not when there are free presses available.
Do both levels have to run the same resin?
In most tandem molds, yes, because both levels are fed by the same injection unit. Color and resin are shared across the tool.
Next step
Molding a matched set of parts?
Upload CAD for every part in the set, with volumes. The review weighs a family mold against separate tools, based on size, wall, and cooling. NetProto manufacturing is ISO 9001 certified.