Oil Resistance of Reinforced PC: Which Media Cause Parts to Crack

Mechanism: stress plus medium

Environmental stress cracking requires two conditions together: tensile stress inside the material — residual or applied — and a medium that lowers surface energy or swells the skin. Neither alone usually cracks the part.

The amorphous structure of PC makes it sensitive to many organic solvents, especially esters, ketones, aromatics and some plasticisers. Mineral and vegetable oils are relatively mild at room temperature, but cleaners with active additives carry high risk.

Which situations warrant caution

Self-tapping screws introduce sustained tensile stress, and with protective or anti-rust oil present radial cracks can appear within weeks. Parts with residual stress can also crack directly from ink solvents after painting, printing or labelling.

Validate with 48-168 hours of constant-strain immersion in the actual medium and actual assembled state, rather than relying on the supplier's chemical resistance chart alone.

How to improve

Annealing after moulding — 120 °C for 2-4 hours for PC — is the most effective way to remove residual stress. Design-wise, reduce sharp corners and interference fits, and add metal inserts to spread load at screw bosses.

Where the chemical environment is severe and stress cannot be designed out, move to PC/ABS+PBT or PPS, which resist chemicals far better.