Cracks in injection molded parts are among the most frustrating defects in plastic manufacturing. They not only damage the appearance but also undermine mechanical strength and reliability. The good news is that most cracking issues are traceable to a few core causes—mainly material degradation, moisture, and internal stress from inserts or process imbalance.

This article breaks down the phenomenon systematically, drawing on field experience and materials science, to help molders and buyers pinpoint root causes and implement long-term prevention measures.

1. Common Symptoms

Cracks often appear around metal inserts, corners, or areas under assembly stress. Some are visible right after demolding; others only develop hours or days later, especially after assembly or heat exposure. The cracks are typically linear or radiating and indicate excessive internal stress or molecular brittleness.

2. Main Root Causes

(1) Material Degradation or Substitution

When the resin supplier replaces the specified grade with regrind or mixed material, or when the batch absorbs moisture, the polymer’s molecular weight drops during processing.
Result: brittle parts that fail under normal load.

Even if the resin label matches, color masterbatch overdose or improper additives can lower impact strength dramatically.

(2) Moisture Contamination

Polycarbonate (PC) and similar hygroscopic materials must be thoroughly dried before molding.
If hot-air drying is used instead of a desiccant system, or if the drying temperature/time is insufficient, hydrolysis occurs inside the barrel—creating invisible microcracks and molecular scission.
The result is “instant brittleness” after molding.

(3) Insert Stress and Assembly Pressure

Cracks forming around inserts often originate from press-fit or overmolding stress.
Metal inserts expand and contract at different rates than the plastic. When the press-fit hole is too small or the assembly force too high, the surrounding resin is permanently strained.
If oil or plating residue remains on the insert, it can even trigger environmental stress cracking (ESC).

(4) Molding Stress

Uneven cooling, improper gate design, or excessive packing pressure all contribute to internal stress.
Localized orientation or cold joints (weld lines) act as weak points that propagate cracks under load or thermal cycling.

(5) Chemical or Handling Factors

Exposure to solvents, packaging pressure, or long-term UV exposure can accelerate surface cracking, especially in stressed zones.

3. Diagnostic Steps

Verify Material Authenticity – Request the resin’s Certificate of Analysis (COA), confirm batch number, and ensure no recycled or mixed content.
Check Drying Process – Inspect the dryer type, temperature, time, and dew point (ideally ≤ –40°C).
Observe Crack Surface – Under magnification, brittle fracture surfaces appear shiny and granular, while stress fractures are dull and fibrous.
Compare Timing – If cracks appear immediately after demolding, focus on molding parameters. If after assembly, focus on insert fit and assembly force.
Laboratory Testing – Conduct FTIR, DSC, and MFR tests to confirm material integrity, and SEM analysis for fracture morphology.

4. Immediate Corrective Actions

Switch to desiccant drying for PC or nylon materials.
Reduce press-fit or assembly pressure on inserts.
Perform 100% visual inspection before assembly.
Document all molding parameters for traceability.

5. Long-Term Preventive Measures

Supplier Control: Require COA and drying records with each batch. Specify “no regrind” and “no formula change” in the contract.
Process Documentation: Record injection parameters (temperature, packing, cycle time, etc.) and verify alignment with material datasheet.
Design Optimization: Add fillets and stress relief features around inserts; increase local wall thickness if possible.
Insert Handling SOP: Clean, preheat, and position inserts consistently to avoid trapped moisture or contamination.
Periodic Testing: Schedule regular FTIR/DSC analysis to detect early signs of resin degradation.

6. Key Takeaway

Cracks in injection molded parts are rarely caused by a single factor. In most real-world cases, material moisture, thermal degradation, and assembly stress interact. A disciplined approach—starting from material verification to molding and assembly control—is the only way to permanently eliminate cracking and ensure product reliability.

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