If the screw in the machine is of wrong design for the material being molded, it is possible to have gas entrapped in the melt stream. A key to verify the screw design is the compression ratio which is the ratio between the feed fight depth versus the metering fight depth. If the machine has a low compression ratio screw (under 2.5) there may be issues trying to run materials such as PMMA, acetal, nylon, etc.