When plastic is overheated often a gas will be given of from degradation of the components of the plastic. Much like moisture these gases can form bubbles that can be carried in the melt stream and injected into the cavity of the mold. When troubleshooting an issue such as bubbles it will be beneficial to conduct a melt temperature check. If the melt temperature is above the recommended value for the material being molded it will be time to check the following: