Purging compounds are used in injection molding, blow molding, and extrusion molding machines as a transition material from one product to another. They remove residue from color or resin changes and are used to clean screws, barrels, and hot runners. Purging compounds are available as concentrated liquids, powders, or pellets. Many have a cast acrylic base and can remove residue from thermoplastics such as nylons, polycarbonates polyesters, and polyvinyl chloride (PVC). Some products can be poured directly into a hopper or the throat of a molding machine. Others are combined with a base resin or used as part of a multi-product system. Pre-mixed purging compounds are more expensive than raw materials, but reduce the amount of setup and cleanup time required.