Stop valve-the extremely complex & difficult mold design-section 1

1. Part analysis

It is stop valve of solar water heater. The plastic is POM material, and see figure below:

Stop Value

There are external threads outside, and it could make slider for it. The difficult is there is internal thread, what is more, the center part can’t rotate.


It is very difficult for mold release, as center of internal thread can’t rotate. It needs rotate by machinery. The center of internal thread must be cold as it is too thick. It is some contradiction each other. It is typical rotate thread mold.

2. Gate design

It is better make 1 cavity with directly gate, bases on complex structure. The key is where put the gate location.


The gate location in the center is best solution, after mold flow analysis. The runner is 10mm length, from Φ2.5mm to Φ3mm. It is easy for molding, mold release and cut after molding. It is simple for cavity design.

3. Thread insert design

We make gear with thread insert in solid, it could make it more strong as it is empty inside. There is guide thread outside too.

Thread insertThread insert

The steel should be HRC48-52, such as SKD61. It makes two part and fit as an internal thread, as it is very difficult for machine.

Thread insert

The thread is machined by EDM, and the copper is machined by CNC lathe.

4. Mold action principle

It is very difficult to understand mold release. Let’s clarify it.

First, the thread insert rotates by motor moving before mold open, it moving by guide thread. It makes the thread of part release. The inside of thread insert is not rotate, as it is stationary.


Second, the back plate and support plate open 15mm. The 4 sliders moving, then make stationary inside thread insert release.  After that the mainP/L open by angle pin, as it make two big sliders moving.


Third, the hydro-cylinders pull the core inserts.


Forth, the robot hand takes the molding part out. All four actions are coherent.

It is clear to know what the mold open now. The key is how to design core? Pls keep care for the next section.