Cracks / Brittleness
Causes, Solutions & Prevention in Injection Molding
Cracks occur from excessive internal stress improper drying wrong material or ESC from chemical contact.
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
- Undried material DRYING: ABS 80C 2h ABS-PC 100C 3h PA6 80C 4h PC 120C 3h POM 110C 3h PMMA 80C 3h
- Holding pressure too high residual stress ESC
- Gate broken by hand crazing
- Cooling time too short hot part distorts at ejection
🔲 Mold
- Draft angles too small part grips mold stress at ejection
- Ejector pins too small sink into part
- Sharp corners in mold stress concentration
- Air valve timing wrong suction during ejection
🧪 Material
- Wrong material high MFI lower impact resistance
- Metal-plastic assembly cold environment differential expansion cracks
- ESC residual stress plus chemical contact cleaners lubricants
🏭 Machine
- Barrel temperature control inadequate
📋 General
- Faulty thermocouple uncontrolled melt temp degradation brittleness
- Sharp corners in part design
- Poor gate separation technique
✅Solutions & Corrective Actions
⚙️ Process
- Dry material per drying table most common fix
- Reduce holding pressure to minimum needed
- Increase cooling time slightly to avoid hot ejection
- For transparent gates use hot blade separation
🔲 Mold
- Increase draft angles
- Use blade plate ejectors instead of small pins
- Add radii to all sharp corners in mold
- Adjust air valve timing
🧪 Material
- Even with the same material, different MFI specifications cause cracking. Higher MFI grades have lower impact strength. Plastic parts assembled with metal are especially prone to cracking in cold environments — material selection is critical.
🏭 Machine
- Barrel thermocouples should be serviced every 6 months. If a thermocouple loses accuracy, barrel temperature rises uncontrolled — degrading material and causing brittleness. Check melt temperature before each production start.
📋 General
- For ESC reduce residual stress AND identify avoid chemical contact
- Service thermocouples every 6 months
- Select correct MFI for impact requirements
🛠Step-by-Step Troubleshooting
Check Process Parameters
Dry material per drying table most common fix
Inspect the Mold
Increase draft angles
Verify Material Condition
Even with the same material, different MFI specifications cause cracking. Higher MFI grades have lower impact strength. Plastic parts assembled with metal are especially prone to cracking in cold environments — material selection is critical.
Check Machine Settings
Barrel thermocouples should be serviced every 6 months. If a thermocouple loses accuracy, barrel temperature rises uncontrolled — degrading material and causing brittleness. Check melt temperature before each production start.
❓Frequently Asked Questions
What causes cracks / brittleness in injection molding?
Cracks / Brittleness is caused by several factors including: Undried material DRYING: ABS 80C 2h ABS-PC 100C 3h PA6 80C 4h PC 120C 3h POM 110C 3h PMMA 80C 3h; Holding pressure too high residual stress ESC; Draft angles too small part grips mold stress at ejection; Ejector pins too small sink into part; Wrong material high MFI lower impact resistance; Metal-plastic assembly cold .
How do you fix cracks / brittleness in injection molding?
Start by checking key parameters: Drying protocol, Mold temperature, Draft angle (°), Ejection speed. Dry material per drying table most common fix
Which plastics are most affected by cracks / brittleness?
Common materials include: ABS, HIPS, PC, PPO, SAN. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent cracks / brittleness in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Drying protocol, Mold temperature; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between cracks / brittleness and similar defects?
Part cracks at ejection during assembly or in service. Crazing near gate. Breaks under low load. 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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