Last updated: 2026-09-02 · 5 min read

What Causes Internal Stress in Injection Molded Parts — and How Do You Detect It?

Internal stress in injection molded parts comes from two sources: flow-induced molecular orientation during filling and differential cooling through the wall — and when the combined stress exceeds the material’s resistance, the part cracks, warps or fails in service, often weeks after molding. The stress is invisible at ejection; it shows up during assembly, after chemical exposure or under sustained load — which is why detection before shipment matters.

Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. See also related guide.

Key Facts

  • Flow-induced stress occurs when polymer chains align in the flow direction and freeze before relaxing — highest near the surface, lowest at the core.
  • Thermal stress comes from differential cooling: the surface solidifies first while the core continues shrinking, locking in tensile stress at the core and compressive stress at the surface.
  • Solvent crack testing (immersion in isopropanol or acetone) reveals stress cracks within minutes — the part’s resistance to chemical exposure in service correlates with its residual stress level.

    • Gate vestige — The visible mark left on the part surface where the gate was trimmed. Edge gate: 0.5–2.0mm vestige. Pin-point gate: 0.2–0.5mm. Valve gate: 0.05–0.1mm (flush). Vestige size affects cosmetic appearance and may require secondary trimming. The related guide covers adjacent topics.

    Where the Stress Lives in the Part

    Stress Type Location Root Cause
    Flow-induced Near surfaces, along flow paths High injection speed, small gates, long flow lengths
    Thermal Core in tension, surface in compression Uneven wall thickness, high mold temperature differential
    Ejection Near ejector pins, undercuts Insufficient draft, excessive ejection force, premature ejection
    Insert-related Around metal inserts Differential shrinkage between metal and polymer

    How to Detect Residual Stress Before Shipment

  • Solvent immersion: immerse the part in isopropanol (for ABS, PC) or acetone (for amorphous resins) for a specified time; surface cracks reveal stress concentration zones — the test is destructive but definitive.
  • Polarized light: birefringence analysis under crossed polarizers shows stress distribution as color fringes — non-destructive, suitable for transparent resins like PC and PMMA.
  • Dimensional monitoring: parts with high residual stress warp over time or after machining; tracking dimensions at 24 and 48 hours after molding catches stress-relaxation drift.
  • Environmental stress cracking: expose parts to the actual chemical environment they will see in service — cleaning agents, lubricants, fuels — and monitor for cracking over an accelerated time period.

    • What the Mold and Process Can Change

    Mold design controls the thermal stress component: balanced cooling channels, uniform wall thickness and adequate draft reduce differential cooling. The process controls the flow component: lower injection speed, higher melt temperature and longer packing time allow molecular chains to relax before freeze.
    For amorphous resins like PC and ABS, a post-mold anneal at 10–20°C below the heat deflection temperature relieves 60–80% of residual stress — but adds cycle time and cost, so the quotation should specify whether annealing is included or whether the process is designed to avoid it.

    Frequently Asked Questions

    Why do stress cracks appear weeks after molding?

    Residual stress relaxes slowly at room temperature — the polymer chains continue to rearrange after ejection. When the part encounters a chemical agent (solvent, adhesive, cleaning spray) during assembly or service, the stress concentration at the surface accelerates crack initiation. Parts that pass visual inspection at ejection can fail days later in the field.

    Which resins are most sensitive to internal stress?

    Amorphous resins — PC, ABS, PS, PMMA — are most vulnerable because their molecular chains do not relax as easily as semi-crystalline polymers. PC is particularly sensitive to solvent stress cracking; ABS is sensitive to both flow and thermal stress. Semi-crystalline resins like PP, PA and POM are more resistant but not immune — they show stress as warpage rather than cracking.

    Does annealing fix all stress problems?

    Annealing relieves the bulk of thermal stress but does not eliminate flow-induced orientation. If the root cause is a small gate or high injection speed, the molecular orientation remains even after annealing. The correct sequence is: fix the mold design and process first, then use annealing as a safety margin for critical applications — not as a band-aid for a bad process.

    Key Takeaways

    1. Internal stress comes from flow-induced orientation and differential cooling — both are invisible at ejection but cause cracking, warping and chemical sensitivity in service.
    2. Solvent immersion and polarized light detection catch residual stress before shipment; dimensional monitoring at 24/48 hours tracks stress relaxation.
    3. Mold design and process parameters are the primary controls; annealing is a safety margin, not a substitute for correct gating, cooling and injection speed.

    Send aCatMould the resin grade and the part’s service environment, and the DFM review identifies the stress risk zones — with solvent test criteria, process parameters and annealing requirements specified in the quotation, so stress-related failures are caught at trial, not in the field.

    Sources
    Plastics Technology — residual stress in injection molding
    SPE — stress cracking and solvent testing guidance
    MoldMaking Technology — mold design for stress reduction

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet