A surge of Chinese plastic supply is threatening to overflow in the face of weak domestic demand, morphing into a fresh trade challenge for the rest of the world.

Parts of the country’s sprawling petrochemicals sector are running at as little as half capacity as producers cut back. But with the industry still expanding, that restraint is becoming harder to sustain.

"This is yet another example — after steel, solar panels — where China’s structural imbalances are clearly spilling over into global markets,” said Charlie Vest, a New York-based associate director at Rhodium Group who looks at U.S.-China relations and Chinese industrial policy.