Last updated: 2026-09-05 · 5 min read
Brittle Parts in Injection Molding — Why Do Molded Parts Crack or Snap Instead of Bending, and What Resin, Process and Storage Conditions Cause It?
A brittle part breaks without visible deformation — it snaps cleanly instead of bending or yielding. The cause: the polymer chains have lost their ability to absorb energy. Three root causes account for 90% of brittle failures: moisture-induced degradation in hygroscopic resins (PC, nylon, PET), thermal degradation from excessive melt temperature or regrind cycles, and stress concentration at sharp corners or weld lines. Fix by drying resin, lowering melt temperature and adding corner radii.
Key Facts
- Moisture-induced degradation is the most common cause for PC, nylon, PET and PBT. When these resins are molded with excess moisture, the water reacts with the polymer chains at high temperature (hydrolysis) — cutting the chains into shorter segments. Shorter chains = lower molecular weight = lower impact strength. For PC: impact strength drops from 700 J/m (dry) to 200 J/m (wet-molded) — a 70% loss. The part looks fine but shatters on first impact.
- Thermal degradation from excessive melt temperature or too many regrind passes also embrittles the part. Each time the resin is heated above its processing range, the polymer chains break (chain scission). For ABS: melt above 270°C causes chain scission — impact strength drops 40%. For nylon: more than 3 regrind passes reduces molecular weight by 15–20%, making the part brittle. The fix: stay within the resin manufacturer’s recommended melt range and limit regrind to 2–3 passes maximum.
- Stress concentration at sharp corners or weld lines makes an otherwise tough resin behave in a brittle manner. A sharp internal corner (zero radius) has a stress concentration factor of 3.0× — the actual stress at the corner is 3× the nominal stress. Under impact load, the corner cracks before the rest of the part yields. Adding a 0.5mm radius drops the stress concentration to 1.5× — the part bends instead of cracking. For glass-filled resins, the effect is worse — glass fibers amplify the stress concentration by 20–30%.
How Do You Identify Which Cause Is Responsible for the Brittle Failure?
| Symptom | Likely Cause | First Check | Fix |
|---|---|---|---|
| Part shatters on impact, clean fracture | Moisture degradation | Resin moisture level | Dry resin to spec |
| Part cracks at same location every time | Stress concentration | Corner radius at crack | Add radius, reduce notch |
| Brittle after 3+ regrind passes | Thermal degradation | Regrind count, melt temp | Limit regrind to 2–3 passes |
| Brittle at weld line only | Weld line weakness | Weld location vs. load | Move gate, raise mold temp |
| Brittle after months in storage | UV or environmental | Storage conditions | Use UV-stable grade, proper storage |
The fracture surface tells the story. A brittle fracture from moisture degradation shows a smooth, glassy surface with no deformation marks. A brittle fracture from stress concentration shows a crack initiation point at the corner or notch, with a flat fracture surface radiating outward. A ductile failure (which is what you want) shows visible deformation — the material stretches, necks and tears before breaking.
Hydrolysis (in resin processing) — the chemical breakdown of polymer chains by water at high temperature. In hygroscopic resins (PC, nylon, PET, PBT), moisture turns to steam at molding temperature and cuts the polymer chains — reducing molecular weight and impact strength. For PC: every 0.01% moisture above 0.02% reduces impact by 10–15%. The hygroscopic resin drying guide covers moisture targets and drying protocols for each resin family.
What Three Fixes Address the Most Common Brittle Part Causes?
- Dry the resin to the manufacturer’s specification — this eliminates moisture-induced hydrolysis. For PC: dry at 120°C for 4 hours to 0.02% moisture. For nylon: 80°C for 4 hours to 0.1%. For PET/PBT: 130°C for 3 hours to 0.02%. Use a desiccant dryer with dew point below -30°C. Measure moisture at the dryer outlet with a moisture analyzer — if above spec, the part will be brittle regardless of other process settings. For PC: every 0.01% moisture above 0.02% reduces impact strength by 10–15%.
- Lower the melt temperature to the middle of the recommended range — this prevents thermal degradation. If the melt is at the high end (e.g., ABS at 260°C instead of 230°C), polymer chains break during molding — the part has lower molecular weight than designed. Lower melt by 10–20°C and check impact strength. Also check barrel residence time — if melt sits more than 60 seconds (slow cycle, large shot), resin degrades even at correct temperature.
- Add corner radii at every internal corner — this eliminates stress concentration. A sharp internal corner has a stress concentration factor of 3.0× — stress at the corner is 3× nominal. Under impact, the corner cracks first. Adding a 0.5mm radius drops the factor to 1.5×; a 1.0mm radius drops it to 1.2×. For glass-filled resins, minimum radius is 0.5× wall thickness — notch-sensitive and crack at any sharp corner. The DFM review should flag every sharp corner before the mold is built. The delamination guide covers how moisture degradation also causes brittle failure.
Chain scission — the breaking of polymer chains into shorter segments caused by excessive heat or repeated thermal cycling. Each pass through the barrel (virgin or regrind) breaks some chains — after 3–5 regrind passes, molecular weight drops 15–25% and impact strength drops proportionally. The corner radius guide shows how sharp corners amplify the effect of chain scission on part brittleness.
Brittle parts: break without deformation. 3 causes: moisture hydrolysis (PC impact drops 70%), thermal degradation (melt too hot or too many regrind), stress concentration (sharp corner = 3× stress). Fix: dry resin to spec, lower melt temp, add corner radii ≥0.5mm.
- Resin dried to spec before molding — PC <0.02%, nylon <0.1%. Measure with analyzer.
- Melt temperature in middle of range — too high causes chain scission and embrittlement.
- Regrind limited to 2–3 passes — each pass breaks chains; track pass count.
- Corner radii ≥0.5mm at all internal corners — sharp corners = 3× stress concentration.
- Test bar vs. part comparison — brittle test bar = resin; brittle part only = design.
Frequently Asked Questions
Why does my PC part look fine but shatter when dropped?
The part was molded with excess moisture. PC is extremely sensitive to hydrolysis — at 280°C, water molecules cut the polymer chains, reducing molecular weight. The part looks normal — surface finish and dimensions are unaffected. But impact strength has dropped from 700 J/m (dry) to 200 J/m (wet-molded) — a 70% loss. At 200 J/m, the part cannot survive a 1m drop onto concrete. The fix: dry PC to 0.02% moisture before molding. Measure with a moisture analyzer at the dryer outlet — not by time alone.
Can regrind cause brittle parts even if the resin is dried properly?
Yes — regrind that has been through the molding process multiple times has lower molecular weight than virgin resin. Each pass through the barrel breaks polymer chains (thermal degradation). After 3–5 regrind passes, the molecular weight drops by 15–25%, and the impact strength drops proportionally. For ABS: virgin impact is 250 J/m; after 5 regrind passes, it drops to 150 J/m — a 40% loss. Fix: limit regrind ratio to 15–20% (regrind to virgin ratio) and track the number of passes. For critical structural parts, use zero regrind.
How do I know if the brittleness is from the resin or from the part design?
Test the same resin in a standard test bar (ISO 179 Charpy impact bar) — if the test bar has normal impact strength but the part is brittle, the cause is the part design (stress concentration at sharp corners, thin sections or weld lines in high-stress areas). If the test bar is also brittle, the cause is the resin (moisture, thermal degradation or contamination). The test bar isolates the resin quality from the part geometry — it tells you whether the problem is in the material or the design.
Key Takeaways
- Brittle = breaks without deformation. 3 causes: moisture, heat, stress.
- PC moisture hydrolysis: impact drops 70% if wet-molded.
- Sharp corner = 3× stress. Add radius ≥0.5mm to prevent cracking.
- Limit regrind to 2–3 passes. Track melt temp and residence time.
Send the resin lot certificate, the drying log and the fractured part — the review identifies whether the brittleness is from moisture, thermal degradation or stress concentration, and specifies the fix needed to restore impact strength.
Sources: Plastics Technology — Brittle Failure Analysis in Molded Parts · Plastics Technology Online — Hydrolysis Effects in Engineering Resins · Society of Plastics Engineers (SPE) — Impact Strength and Brittle Failure Technical Papers · BASF — Processing Guidelines for Engineering Resins · MatWeb — Impact Strength Data by Resin Family
— Arthur, Tooling Engineer at aCatMould