Voids / Bubbles / Porosity
Causes, Solutions & Prevention in Injection Molding
Two types surface bubbles from moisture vs internal vacuum voids same mechanism as sink marks core shrinks creating vacuum pocket.
Below you will find general information about this defect. To get a pinpoint solution tailored to your specific machine, material, and process parameters using the world’s most comprehensive injection molding troubleshooting database — sign up and access AI-powered troubleshooting, 7 engineering calculators, and detailed project tracking.
🔍Symptoms & Visual Identification
⚠️Root Causes by Category
⚙️ Process
- SURFACE BUBBLES Undried material moisture
- INTERNAL VOIDS Holding pressure too low
- INTERNAL VOIDS Holding time too short
- INTERNAL VOIDS Insufficient screw cushion
🔲 Mold
- Gate runner sprue too small cannot fill thick cores
- No cooling near thick section
- Foam test extrude shot into air bubbles confirm moisture
🧪 Material
- Undried material universal cause of surface bubbles
- High shrinkage material large internal voids in thick walls
🔧 Mold Condition
- Tunnel gate diameters are inconsistent.
📋 General
- Worn check valve inconsistent pack internal voids
- Chemical blowing agent can solve internal voids but reduces impact strength
✅Solutions & Corrective Actions
⚙️ Process
- Surface bubbles dry material per TDS
- Internal voids increase holding pressure
- Internal voids extend holding time to gate freeze
- Internal voids increase screw cushion 3-5mm
🔲 Mold
- Enlarge gate runner sprue to improve packing reach
- Move cooling channel closer to thick section
- S=a/2 rule redesign rib and boss thickness
🧪 Material
- Chemical blowing agent should be tried. It eliminates voids by creating max 200µ micro-cells inside the part, but causes flow marks on surface and reduces impact strength.
🔧 Mold Condition
- During production, tunnel gates are sometimes enlarged with a file to fix short shots or voids. This disrupts in-mold flow. Gate size changes must be done on a CNC machine.
📋 General
- Verify check valve with plus minus 5mm fill variation test
- Chemical blowing agent for thick-wall applications evaluate impact trade-off
🛠Step-by-Step Troubleshooting
Check Process Parameters
Surface bubbles dry material per TDS
Inspect the Mold
Enlarge gate runner sprue to improve packing reach
Verify Material Condition
Chemical blowing agent should be tried. It eliminates voids by creating max 200µ micro-cells inside the part, but causes flow marks on surface and reduces impact strength.
❓Frequently Asked Questions
What causes voids / bubbles / porosity in injection molding?
Voids / Bubbles / Porosity is caused by several factors including: SURFACE BUBBLES Undried material moisture; INTERNAL VOIDS Holding pressure too low; Gate runner sprue too small cannot fill thick cores; No cooling near thick section; Undried material universal cause of surface bubbles; High shrinkage material large internal voids in thick walls.
How do you fix voids / bubbles / porosity in injection molding?
Start by checking key parameters: Holding pressure, Gate size, Cooling rate, Drying protocol. Surface bubbles dry material per TDS
Which plastics are most affected by voids / bubbles / porosity?
Common materials include: PA6, PA66, ABS, PC, PP. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent voids / bubbles / porosity in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Holding pressure, Gate size; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between voids / bubbles / porosity and similar defects?
Bubbles visible on surface or in transparent parts. Air pockets on X-ray or cross-section. Part sounds hollow. This distinguishes it from related surface defects during visual inspection.
📐Related Calculators
Run these engineering calculators to verify your process parameters and prevent this defect.
🔗Related Defects
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