Every cracked PC injection molding part is a small murder mystery. The victim: a crystal-clear enclosure that passed every inspection. The scene: your customer’s warehouse, eleven days after delivery. The weapon is almost always the same — stored internal stress, meeting the wrong chemical or the wrong load at the wrong moment.

This article is the autopsy report: six suspects, the forensic toolkit that identifies them, and the prevention protocol we run at Ulite on every polycarbonate injection molding program.

⚗️ The Physics in 90 Seconds — Stress + Agent = Crack

Polycarbonate is tough — until it isn’t. Residual (internal) stress gets locked in during molding by three mechanisms: flow/orientation stress (high shear near gates), thermal stress (uneven or too-fast cooling), and assembly stress (screws, press-fits, inserts). Alone, it may sit silently for weeks. Add a chemical agent — solvent wipes, plasticizers, alkalis, even alcohol — and you get environmental stress cracking (ESC): crazing and cracking far below the material’s normal strength.

The equation to remember: Crack = Locked-in Stress × Chemical/Load Trigger. Remove either factor and the crack never starts.

🕵️ The Six Suspects — Exhibit Cards

EXHIBIT A · Moisture & Hydrolysis

Evidence: Silver streaks/splay, brittle parts, char-free brittleness, MFR shift.

Root cause: PC is hygroscopic; undried resin hydrolyzes in the barrel — water literally cuts the polymer chains, dropping impact strength.

Fix: Desiccant-dry 4 h @ 120 °C, dew point ≤ −40 °C, moisture < 0.02%; verify with a moisture analyzer, not a timer.

Prevention: Dryer dew-point alarms + logged moisture per lot.

EXHIBIT B · Process Stress (Shear & Cooling)

Evidence: Stress fringes near gates under a polariscope; warpage; cracks radiating from the gate.

Root cause: Too-fast injection (high shear orientation), mold temperature too low (PC wants 80–110 °C), unbalanced cooling, over-packing.

Fix: Raise mold temp into spec, slow first-stage speed, decouple packing (scientific molding), balance cooling to ±2 °C.

Prevention: Polariscope audit on first articles + periodic pulls.

EXHIBIT C · Design Stress Concentrators

Evidence: Cracks always at the same sharp corner, wall transition, or boss.

Root cause: Sharp internal radii (<0.4 mm), abrupt wall changes, and metal inserts (steel/brass shrinks differently than PC → hoop stress).

Fix: Internal radii ≥ 0.5–0.8 × wall; uniform walls; pre-heat inserts (~80 °C), reduce interference, add relief.

Prevention: DFM radius map before cutting steel — the cheapest crack fix costs $0 in steel.

EXHIBIT D · Environmental Stress Cracking (ESC)

Evidence: Fine crazing after shipping/storage/cleaning; cracks where a gasket, label adhesive, or wipe touched.

Root cause: Aromatics/chlorinated solvents, ketones, strong alkalis, ammonia cleaners, plasticizers, alcohol wipes on stressed parts, thread-lockers.

Fix: Chemical audit of every touch-point (cleaner, gasket, carton liner, ink, adhesive); switch to ESC-safe agents; lower assembly stress.

Prevention: Solvent craze test as a release audit.

EXHIBIT E · Degradation & Regrind Abuse

Evidence: MFR up 20–30%+ vs virgin; impact strength down; yellow tint.

Root cause: Over-long residence, overheated melt, or regrind > 15–20% re-cooking the polymer.

Fix: Cap regrind (≤10–15% structural, 0% optical/medical); incoming MFR check; right-size the barrel.

Prevention: Lot-level regrind log + MFR trend chart.

EXHIBIT F · Ejection & Assembly Stress

Evidence: Cracks at ejector pins; cracks appearing only after screws are torqued.

Root cause: Parts sticking (low draft, rough cavities) → high ejection force; over-torqued screws adding hoop stress on top of molded-in stress.

Fix: Draft ≥1–1.5°, polish, ejector speed profile, more/larger pins; written torque specs + boss radii.

Prevention: Torque validation on the assembly line, not in the field.

🔬 The Forensic Toolkit — How We Identify the Suspect

🛡️ The Ulite Prevention Protocol (Spec Sheet)

🧭 Crack Pattern → Suspect → Fix (Field Matrix)

🗂 Case Files — Six Cracks, Solved

❓ FAQ (Snippet-Optimized)

Q1: Why does polycarbonate crack after molding?
Locked-in internal stress (from shear, fast/uneven cooling, or design sharp corners) combines with a trigger — a chemical (ESC), a load, or a temperature swing — and crazing/cracking follows, sometimes weeks later.

Q2: What mold temperature prevents PC stress?
80–110 °C; the higher end for clear or structural parts — low mold temp is the #1 process cause of residual stress.

Q3: How do you dry polycarbonate properly?
Desiccant dryer, 120 °C for 3–4 h, dew point ≤ −40 °C, moisture below 0.02% — verified with a moisture analyzer.

Q4: Does annealing really fix internal stress?
Yes — 120–125 °C for 1–2 h (scale with wall) with slow cooling relaxes orientation stress; we verify with a polariscope before/after.

Q5: Which chemicals crack polycarbonate?
Aromatics/chlorinated solvents, ketones, strong alkalis, ammonia cleaners, plasticizers, thread-lockers — and alcohol wipes on stressed parts.

Further Reading： Secondary Operations Guide (annealing station)↗ · Custom Plastic Molding Cost Calculator↗

🎯 CTA — Contact The Ulite Team

Cracking PC Injection Molding parts right now? Ship us 5 samples + your resin grade. Within 72 hours Ulite returns a free crack autopsy report: polariscope stress map, moisture/MFR data, suspect ranked, and a written fix spec. Ulite — your one-stop custom plastic molder.

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