What causes cold slug in injection molding?

Cold slug in injection molding is caused by solidified plastic at the front of the melt flow that reaches the mold before it can be properly packed out. Here's a breakdown of the causes:

Primary Cause

When the injection cycle ends, the molten plastic in the nozzle tip is exposed to the cooler sprue bushing and surrounding air. This front portion of plastic cools and partially solidifies between shots — that cooled chunk is the "cold slug."

Contributing Factors

Machine/process side:

• Nozzle temperature too low, accelerating premature solidification

• Long cycle times giving the nozzle tip more time to cool

• Insufficient back pressure during plasticating

• Nozzle tip diameter mismatch with the sprue bushing

Tooling side:

• No cold slug well (also called a sprue puller well) at the base of the sprue — this is the most common design oversight; a properly sized well traps the slug before it can enter the runner system

• Runner system design that allows the slug to reach part cavities

Material side:

• High-viscosity resins are more prone to cold slugs because they lose flow more readily when temperature drops

• Materials with a narrow processing window

Effects if Unaddressed

• Surface defects (blobs, streaks, discoloration) on the part

• Short shots if the slug blocks flow to a cavity

• Weld line weakness if it breaks apart mid-fill

• Dimensional inconsistency

Solutions

• Add or enlarge the cold slug well (the standard fix in tool design)

• Increase nozzle temperature

• Use a hot tip nozzle to keep the front of the melt molten

• Reduce cycle time or add a nozzle heater band

• Adjust the sprue puller pin to actively pull the slug into the well on mold open

The cold slug well is considered essential in most mold designs precisely because cold slugs are nearly unavoidable — the goal is to trap them harmlessly rather than prevent them entirely.