Average Velocity describes the overall speed of the material as it is flowing through a part of the mould which has a constant cross section. It is usually described as inches per second of material travel. Barrel Capacity is the amount of material that a machine will hold with the screw fully retracted. The machine is prepared to deliver that amount of material when it is at its largest shot size setting. Barrel capacity can be described as either the weight or the volume of material that is in front of the screw when it is at its maximum shot size setting. Cavity Pressure describes the force that is operating directly on the moulding material at a specific mould location and time during the moulding cycle. Clamping Force is the force or pressure applied by the machine's clamping unit to the mould during the filling, packing and holding phases of the moulding cycle. Clamping force is described in terms of pressure acting on the mould's projected surface. The units are usually tons / in^2 or kN / m^2. Cushion is the small amount of material that is left in the barrel when the screw has stopped moving forward at the end of the packing phase. The cushion prevents the screw tip from making direct contact with the end of the barrel as the screw finishes its forward movement. Cycle Time is the length of time that it takes to make a part and complete preparations to produce the next one. It is the time required to complete one trip through the moulding cycle. Density and specific gravity are terms that refer to the fact that some materials are relatively heavier than others. For example, polypropylene is not as dense as ABS. This can be shown by weighing equal volumes of both materials.