China’s factories continued their expansion in December, according to a key manufacturing index released Monday. HSBC said its Chinese purchasing managers’ index, or PMI, rose to a 19-month high of 51.5 in December from 50.5 last month. The reading was above 50, meaning that manufacturing is now in a state of accelerated expansion. A preliminary reading of 50.9 was published by HSBC earlier in December. “Momentum is likely to be sustained in the coming months when infrastructure construction runs into full speed and property market conditions stabilize,” said Hongbin Qu, an economist at HSBC.