Bubbles or Voids in Molded Parts: What to Review

Bubbles and voids may appear as visible surface bubbles, internal gaps, cloudy areas, or hollow spaces after cutting or testing the part. The cause can be material moisture, trapped air, insufficient packing, feeding instability, wall thickness, or flow restrictions around inserts.

Quick Answer

Separate bubbles from voids first. Gas or moisture-related bubbles often connect to material condition and venting. Shrinkage voids often connect to wall thickness, packing, and cooling or curing. Insert-related voids may come from flow interruption or air trapped behind the insert.

What You May See

• Clear or cloudy bubbles appear near the surface.
• Internal voids appear after cutting, testing, or inspection.
• Bubbles appear near the flow end, gate, thick section, or insert edge.
• The defect changes after drying, material change, or feeding interruption.
• The part weight or surface finish changes together with the bubbles.

Likely Cause Areas

• Moisture or volatile content may create gas during plasticizing or filling.
• Air may be trapped because flow wraps around an insert or reaches a dead area.
• Packing behavior may not compensate for shrinkage in thick sections.
• Feeding inconsistency may create shot variation and unstable pressure response.
• Mold venting, gate condition, or runner restriction may trap air in the cavity.
• Material mixing, contamination, or changeover residue may create unstable flow.

Safe Checks Before Adjustment

Stop automatic operation if movement, alarm, guarding, or operator safety is uncertain. Do not place hands in the mold, table, injection, or guarded area while movement is possible. Use photos, controller screenshots, cycle logs, and safe observation before changing several settings.

Diagnosis Path

1. Identify whether the defect is surface bubble, internal void, or shrinkage gap.
2. Cut or inspect samples consistently if internal voids are suspected.
3. Check material drying record, hopper condition, feeding continuity, and material batch.
4. Map void location against gate, flow end, insert position, and thick sections.
5. Review vent condition and whether the defect appears near trapped-air areas.
6. Check whether part weight varies together with the defect.
7. Record temperature, recovery, and pressure behavior before changing settings.
8. If the defect follows material feeding interruptions, review the feeding path first.

Evidence to Collect

Check	Why It Matters	Evidence to Collect
Defect type	Bubble, gas mark, and shrinkage void require different checks	Surface photos, cut samples, inspection notes
Material condition	Moisture and contamination are common contributors	Drying record, batch, hopper and feeding notes
Location map	Location links the defect to flow end, insert, gate, or thick section	Marked part photos and simple flow notes
Part weight trend	Weight variation shows whether shot consistency is involved	Sample weights by cavity and time
Process history	Changeover or stoppage can create temporary instability	Startup record, purge notes, alarm history

What Not to Change Too Quickly

• Do not assume every bubble is caused by wet material.
• Do not ignore feeding instability when bubble rate changes by cycle.
• Do not adjust packing without checking wall thickness, gate, and material condition.
• Do not inspect inside the mold area unless the machine is in a safe stopped state.

When to Contact Deboge

Contact Deboge when bubbles or voids remain after material, venting, and feeding checks, or when they connect to machine alarms or shot variation. Send defect photos, cut samples if available, material records, drying notes, machine model, and recent changes.

Related Reading

• Molding Defects
• Materials
• troubleshootingprocess issuesmaterial feeding problems
• Troubleshooting Center
• Machine Support