Shear imparted on the pellets provides much of the energy to melt the pellets as they travel the length of the screw. If a screw is designed without the correct length/diameter (L/D) ratio or the wrong compression ratio it is possible to generate excess shear, which in turn can lead to degradation of the material, which will create excess gas byproducts that must be vented from the mold. A typical general-purpose screw is shown in Figure 20.3.