With fast fill speeds it is more likely that the gas will be trapped rather than escaping through the vents. Often times slowing fill velocity will improve a burning issue; however, this is a compromise on cycle time. Typically, if slowing the fill velocity improves the burn defect improving the venting on the mold will also fix the problem (fix the root cause). If burns are a problem venting should always be reviewed prior to modifying the molding process.