How Runner Change Pins Redirect Flow in Multi-Cavity Mold Layouts

Why Flow Redirection Is Needed

In simple three-plate molds, the runner channels on the fixed plate and runner plate are aligned — resin flows straight from the sprue into the runner plate channels. But in complex multi-cavity layouts, the runner path often needs to:

• Transfer between plates — The primary runner on the fixed plate must connect to branch runners on the runner plate at different X-Y positions
• Change direction — The runner must turn corners to reach cavities that are not in line with the sprue axis
• Split between levels — Stack molds use multiple runner planes, requiring vertical flow transfers between levels

Runner change pins provide these flow transitions as precision, replaceable components. The pin's internal bore creates a controlled flow path through the plate thickness, connecting runner channels on opposite plate surfaces.

Flow Path Through a Change Pin

When resin encounters a runner change pin, it follows a three-segment path:

• Entry — Resin flowing horizontally in the runner channel enters the pin's bore at a 90° turn
• Transfer — Resin flows vertically through the pin bore (length = plate thickness)
• Exit — Resin exits the bore at another 90° turn into the receiving runner channel

Each 90° turn creates a pressure drop and a flow disturbance. The magnitude of the pressure drop depends on the bore diameter, flow rate, and resin viscosity. According to fluid dynamics principles documented in ScienceDirect's pressure drop reference, a 90° turn in a circular bore adds approximately 0.5–1.0× the straight-bore friction loss for the same bore length.

Pressure Drop Analysis

For most standard applications, a properly sized change pin (bore matching runner diameter, polished bore, radiused entries) adds 5–15% additional pressure drop per transition. This is generally acceptable unless the mold is already pressure-limited.

Impact on Cavity Fill Balance

Runner change pins can introduce fill imbalance if different cavity flow paths have different numbers of transitions. In a balanced H-pattern runner:

• Balanced scenario — All cavity paths pass through the same number of change pins → fill balance maintained
• Unbalanced scenario — Inner cavities pass through 1 change pin while outer cavities pass through 2 → outer cavities fill slower

To compensate for unequal change pin counts, use flow rate adjustment pins to add restriction to the lower-resistance paths, equalizing flow to all cavities.

Design Best Practices

• Minimize direction changes — Each change pin is a flow restriction and a potential degradation site. Use the fewest change pins possible to achieve the required runner layout.
• Match bore diameter to runner — Never use a change pin with a bore smaller than the runner diameter. Oversizing by 5–10% is acceptable for high-viscosity resins.
• Radius bore entries — A 0.5–1.0 mm radius at the bore entry and exit reduces the 90° turn pressure drop by 20–30% and eliminates dead zones where material can stagnate.
• Polish bore surface — Ra ≤ 0.4 prevents material hang-up that causes degradation, discoloration, and black specks in the molded parts.
• Equalize transitions per path — In balanced runner layouts, ensure every cavity's flow path passes through the same number of change pins to maintain natural fill balance.

According to JIS B 5111 mold component standards, change pin bore specifications should be documented on the mold assembly drawing along with the required surface finish and entry radius.