A process for producing higher molecular weight polyester includes heating a polyester to form a melt, and applying and maintaining a vacuum of between about 5 mm and about 85 mm of mercury to the melt while passing bubbles of gas through the melt until molecular weight has increased. The process may involve esterification of a diacid component and a diol component at elevated temperature. After the acid functional groups have essentially reacted, a vacuum of about 5 mm of mercury or less was applied and excess diol stripped off during transesterification, thereby increasing molecular weight.