Burn marks are serious defects that can occur during injection molding, and from the viewpoint of the molding engineer, are a problem that urgently needs to be solved. In the present study, a gas collection and component analysis system was used to determine the gas production mechanism during injection molding of PBT polymer. The relationship between the occurrence of burn marks, the injection speed and the gas production mechanism was investigated, and the following results were obtained.<br>1) The principal components in the collected gas were CO<sub>2</sub>, CO, CH<sub>4</sub> and H<sub>2</sub>.<br>2) When burn marks were observed, about 70% of the collected gas was CO<sub>2</sub>.<br>3) It is thought that there are some important chemical reactions occur during the plastic molding process.<br>4) According to the characteristic of chemical reactions, it is inferred that the parts of metal in injection machine and mold have important effects on chemical reactions.<br>5) When the injection speed was higher, the chemical reaction that is related to the methanation was retarded.