Signals that point to procedure
- Slugs only on first shots after restart
- Slugs disappear after defined purge count
- Same material runs clean after hopper and nozzle check
- Fix path: startup purge, idle management, documented discard shots
Injection molding defects
Cold slug
A cold slug is a solid or semi-solid plug of resin that travels into the cavity and appears as a hard inclusion, incomplete melt region, or fish-tail at the gate. It signals that melt temperature, homogenization, or shot preparation failed before injection.
Defect troubleshooting
What is a cold slug in injection molding?
Cold slugs form when material in the nozzle, sprue, or front of the barrel is too cold or insufficiently mixed to flow as a uniform melt. The plug enters the cavity and may show as a hard spot, surfac
Cold slugs form when material in the nozzle, sprue, or front of the barrel is too cold or insufficiently mixed to flow as a uniform melt. The plug enters the cavity and may show as a hard spot, surface dimple, or weak knit downstream.
They often appear after startup, after long idle with heat on, or when back pressure and screw recovery are set too low for the grade. Document whether the slug is at the gate only or embedded along a flow path.
Startup and shutdown procedures belong on the setup sheet for every program. DFM during instant quote review helps teams align gate and cold-slug-prone geometry before steel is final.
Root causes
Cold slug causes mapped to floor signals
Use this table to decide whether the next change belongs in material prep, process, mold steel, or part design.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Cold nozzle or sprue after idle | First shots after downtime show plug at gate | Purge before production; verify nozzle heater and tip contact |
| Insufficient back pressure during recovery | Unmelted pellet core in part; random hard spots | Increase back pressure in supplier window; verify screw wear |
| Melt temperature below supplier recommendation | Slugs with splay or poor knit nearby | Confirm barrel profile and thermocouple calibration |
| Small shot size on large barrel | Slugs on short-shot programs or small parts | Use appropriate press size or extended purge protocol |
Diagnosis
Startup hygiene versus equipment wear
Splitting ownership tells you whether material, process, or tooling is the honest fix path.
Signals that point to machine condition
- Slugs persist through purge on multiple lots
- Recovery time erratic with unmelt at screw tip
- Slugs worsen as back pressure increases toward wear limit
- Fix path: screw and check ring inspection, barrel maintenance
Troubleshooting
Cold slug troubleshooting order
Document what you see, prove a fair process window, then escalate to steel or design when the limit is proven.
- 1
Save the slug witness
Sample
Cut or micro-scope the inclusion if spec requires. Note gate vs mid-fill location.
- 2
Run startup purge protocol
Discipline
Apply supplier startup steps and record shots discarded before good parts.
- 3
Verify melt and nozzle heat
Temperature
Confirm setpoints and actuals within supplier range; check for open thermocouple alarms.
- 4
Tune recovery
Homogenization
Adjust back pressure and screw speed together in small steps while watching cushion stability.
- 5
Inspect screw and check ring
Maintenance
When slugs persist, schedule wear inspection before blaming the mold.
Prevention
Checklist to prevent cold slugs
Most recurring defects trace to setup discipline and design margin rather than a single out-of-spec shot.
Define startup purge and discard count on every setup sheet.
Avoid long idle with heat on without periodic purge shots.
Match press barrel capacity to shot size when possible.
Verify nozzle body and tip heaters before color or material changes.
Log recovery time and cushion to catch drift early.
Upload CAD for gate and wall thickness review during quoting.
Program context
Startup discipline on production tools
Cold slugs are often eliminated with documented startup, not mold rework.
- NetProto delivers one hardened steel tool per program; floor startup ownership stays with the production team using supplier resin guides.
- Instant quote DFM highlights thin sections and gate sensitivity while revisions are still low cost.
FAQ
Cold slug injection molding FAQ
Is a cold slug the same as a short shot?
Short shot is incomplete cavity fill. Cold slug is a discrete unmelt inclusion that may still allow mostly full fill elsewhere.
Can cold slug cause weak knit lines?
Yes. A solid plug can disturb melt front fusion downstream. Fix homogenization before tuning hold for knit cosmetics alone.
How many startup shots should we throw away?
Use the resin supplier startup guide and your documented history for that press and barrel size.
Do cold slugs mean the mold is too cold?
Mold temperature affects flow but slugs originate in the melt preparation path. Prove barrel and nozzle first.
How can NetProto help before cold slug shows up on T1 shots?
Upload CAD for an instant quote and DFM feedback. Engineers can review gates, vents, wall transitions, and ejection risk while the hardened steel tool is still in planning, depending on part size and complexity. There is no minimum order quantity once the tool is running.
Ready to quote
Qualify melt before cosmetic release
Share CAD and material data through instant quote for DFM aligned with your fill and cosmetic targets.