Last updated: 2026-09-04 · 5 min read

Why Do Molded Parts Crack After Shipment With No External Load? — Environmental Stress Cracking Explained

Environmental stress cracking is the failure that shows up months after delivery on a part carrying a fraction of its design load. Three ingredients must meet at the same spot: tensile stress, a specific chemical agent and a susceptible resin. Remove any one and the crack never forms.

Key Facts

ESC is not chemical corrosion and not mechanical overload — it is the physical combination of both, where a mild agent diffuses into stressed polymer and lowers the energy for a craze to open into a crack.

Residual molding stress is the invisible ingredient: frozen-in shear at the gate, packed regions near thickness transitions, and assembly preload from snaps or screws all supply the tensile field that the agent attacks.

The defense is systematic — inventory every chemical the part touches, shrink residual stress through geometry and process, and verify with a stressed-exposure test on real parts rather than trusting a datasheet column.

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

What Three Ingredients Must Meet for ESC to Start?

Five rows, each independently necessary:

Ingredient	Typical Source in a Part	Common Culprit
Tensile stress	Residual molding stress; press-fits; screws	Any sustained tension, even low
Stress concentrator	Sharp corners; gate vestige; knit lines	Raises local stress above nominal
Active chemical	Cleaners; oils; adhesives; gasket plasticizer	Surfactants, oils, alcohols
Time and temperature	Months in service; warm environments	Accelerates diffusion
Susceptible resin	Amorphous plastics most exposed	Grade chemistry decides sensitivity

Cut any row and the failure does not happen. The stress row is where mold makers have the most control and the least visibility: injection molding freezes residual stresses into every part, concentrated where the melt was sheared and packed hardest, and assembly adds more by forcing snaps and tightening screws into bosses — none of which shows on a dimensional report.

The chemical row is the one engineers routinely misjudge. Agents that crack plastics are often mild everyday substances — dish soap, disinfectant wipes, mineral oil, even plasticizer migrating from a neighboring rubber seal — chosen for their ability to wet and diffuse rather than to react. Amorphous resins such as polycarbonate, ABS and polystyrene are the classic victims; semi-crystalline resins resist better because their crystals are hard to invade, but no grade is immune to every agent.

How Do You Test for ESC and Design It Out?

Four steps, in order:

Inventory every chemical the part will touch. Build the list the way a failure analyst would: cleaning agents, lubricants, adhesives, marking inks, gasket and cable-jacket materials it sits against for years, even hand creams for consumer products. Get actual formulation families, because two wipes with the same name can carry different surfactants. This list drives resin selection, test design and service warnings — skipping it is the most common reason ESC programs test the wrong chemicals.
Run a stressed-exposure test on real molded parts. Mount production parts under representative strain, apply the agent, hold at a warm service-like temperature, and inspect for crazes and cracks on a fixed schedule. Compare candidate resins and stress states side by side. This one experiment ranks resins, exposes which assembly stresses matter, and reveals whether the failure needs the chemical, the stress, or both.
Squeeze residual stress out of geometry and process. Round every inside corner, give snaps room to engage without forcing, specify boss sizes that do not preload the plastic, and review the molding process for stress signatures — over-packing at the gate, cold filling that freezes shear in, abrupt thickness transitions that lock shrinkage mismatch in place. Annealing can relieve stress in some resins, but the durable fix is geometry and process that leave less to relieve.
Choose resin against the agent list, then lock the choice in. Select a grade with documented resistance to your specific chemicals — suppliers publish ESC resistance charts for this purpose. Put the approved chemical list into the drawing, warn the service organization against unapproved cleaners, and re-run the exposure test whenever a chemical, supplier or geometry changes.

Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

ESC is not a resin defect — it is a system mismatch. The programs that never see field cracking are the ones that treat the chemical list and the residual-stress budget as living documentation.

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Frequently Asked Questions

How is ESC different from ordinary chemical attack?

Chemical attack is corrosion: the agent reacts with the polymer, and damage appears whether or not the part is stressed — discoloration, swelling, property loss everywhere the chemical touched. ESC is physical, not chemical: the agent diffuses only into regions under tensile stress and lowers the energy for a craze to open into a crack.

That is why the defenses differ. Chemical attack needs a barrier or a different polymer; ESC needs one of its three ingredients removed. The field evidence also differs: attack damage is diffuse and visible; ESC damage is a single clean crack at a stress riser on an otherwise pristine part.

Which resins are most vulnerable?

Amorphous resins lead the list because their disordered chains leave open diffusion paths — polycarbonate, ABS, polystyrene, acrylic and their blends are the classic ESC-sensitive family, each with its own specific nemeses. Semi-crystalline resins resist better because crystal regions are dense and hard to invade, though no resin is immune to every agent and stress state.

Two practical warnings: grade chemistry changes sensitivity, so two polycarbonates can rank differently against the same wipe; and fillers, mold release residue and even paint layers shift the outcome. The only reliable ranking is one you test with your own parts, stresses and chemicals.

Can a part fail ESC with no obvious external load?

Yes, and it is the most confusing case in failure analysis. Residual molding stress is a real stress: a part can carry locked-in tension at the gate, at sharp corners or in frozen shear layers, concentrated enough for ESC once the right agent arrives. The forensic signatures are cracks running in non-loading directions, fracture surfaces starting at a gate or inside corner, and failures clustering in parts from one cavity or process window.

When those appear, the investigation should start in the molding process and assembly stack, not in the load calculations. Lowering residual stress through geometry, gate placement and process parameters is the permanent fix — not switching to a tougher resin that merely raises the threshold.

Takeaways

ESC needs three ingredients at one spot — tensile stress, a specific agent and a susceptible resin — and removing any one prevents the failure.

Residual molding stress is the invisible ingredient; it does not show on a dimensional report but supplies the tensile field the agent attacks.

Verify with a stressed-exposure test on real parts, not a datasheet number — the combination of part geometry, assembly stress and actual chemical is unique to your program.

Environmental stress cracking is preventable once you treat it as a system problem rather than a material defect.
Inventory the chemicals, budget the residual stress, and run the exposure test before the first production lot ships — not after the first field return.