What Is MIM Sprue? Runner? Gate? Mold Flow System Explained

The flow system in a MIM mold — sprue, runner, and gate — delivers molten feedstock from the injection machine nozzle to the mold cavity. Each element plays a specific role in part quality.

Definitions:

Component	Location	Function	Impact on Part
Sprue	Connects machine nozzle to runner system	Primary flow channel; first material to enter the mold	Too small: restricts fill. Too large: wastes material
Runner	Distributes material from sprue to each gate	Ensures balanced flow to all cavities	Runner size affects fill pressure and material waste
Gate	The restriction between runner and cavity	Controls flow rate into the cavity	Gate size and location are the most influential molding parameters
Cold slug well	A pocket at the end of the runner	Traps cooled or degraded first-shot material	Prevents cold material from entering the cavity

Common gate types for MIM:

Gate Type	Application	Gate Vestige	Self-Trimming?	Cost
Edge gate	General purpose, side-fed parts	0.10-0.30 mm	No — requires manual or automated trim	Low
Tunnel (submarine) gate	Automated high-volume, self-trimming	0.05-0.15 mm	Yes — shears during ejection	Low
Fan gate	Thin-walled parts, large cavities	0.08-0.20 mm	No	Low
Pinpoint gate	Cosmetically sensitive, small parts	0.05-0.12 mm	Partial — may need light trim	Moderate
Tab gate	Thick-walled parts requiring strength at gate	0.15-0.40 mm	No	Low

Why gate design matters: The gate is where the highest shear rate occurs, and where binder degradation is most likely. A gate that is too small causes frictional heating, degrading the binder and creating surface defects near the gate. A gate that is too large leaves a prominent gate vestige and wastes material in the runner. Quick Q: What is the difference between MIM sprue, runner, and gate?

The sprue is the main channel from the injection nozzle, the runner distributes material to each cavity, and the gate is the restriction that controls flow into the cavity itself. Gate design — type, size, and location — is the single most influential mold design parameter for part quality. The sprue, runner, and gate material (typically 15-30% of shot weight) can be reground and reused at limited ratios.