The explosion happened at around 7:00 Friday night in a "polishing plant" thought to be "at the end of the production process after devices are assembled," according to the Journal.
Foxconn has another facility in Shenzen, China that produces iPads as well. Fortune says, "Some believe the majority of iPads were being manufactured at Chengdu; others say that Chengdu was having trouble ramping up, and that the majority of iPads were still being manufactured in Shenzhen."
The cause of the explosion at the Chengdu plant is currently unknown and production has been suspended. Foxconn has drawn criticism in the past after one employee reportedly died of exhaustion from working a 34-hour shift and several others committed suicide.
Source: techland.time.com