In order to understand the term “shear rate,” we have to use our imagination and visualize a plastic flowing through a pipe as consisting of minute layers parallel to each other in the direction of flow. The layer that is next to the wall sticks to it and does not move. The next layer moves and slides over the layer adhering to the wall. The remaining layers move at an increasing rate as the distance from wall to center increases. This imaginary layer movement is known as shearing. The change in speed of movement of layers per unit perpendicular distance is called the shear rate. The force per unit area that is exerted on the fluid and brings about the shearing action is called the shear stress. The ratio between shear stress and shear rate is the viscosity of the flowing material; qualitatively, viscosity is defined as the internal resistance of a material to flow. Increased pressure will decrease viscosity, therefore increasing ease of flow. 

          Shear-rate-sensitive materials respond to flow by having their molecules readily shifted  and aligned with the direction of flow. The molder’s  concern  is  with  the  shear-rate-insensitive  plastics, which consist of long-chain molecules (polycarbonate, e. g.) so intertwined that an increase in shear stress will only cause greater entanglement. The net result is that the viscosity will not change, and the danger exists that the entanglements created in polymerization can be disturbed and the polymer properties damaged. In practical terms, gates cannot be too small, and back pressures  should be low, passages  for material to the cavity from the cylinder rather large, and the speed of screw rotation rather low. 

          The melt index (a quantity used mainly for polyethylene) indicates how much material can be pushed through a set orifice with other conditions controlled. It expresses the “flowability” of a material. Larger values indicate easier flow of the compound (Chap. 12). 

           Some specified physical conditions for processing are worth noting. 

           A range of  temperatures is given within which adjustments can be made in order to obtain favorable fluidity of a material.    

           A range of values isagain given, within which adjustments may bemade if pyrometer readings indicate that sucha step will improve quality and productivity. 

           More accurately, this means the pressure needed in the cavity to produce consistent quality of  parts. It is a very important processing datum. The reading on the “injection pressure” gauge is a pressure that is composed of several incremental pressure drops-within  the heating cylinder,  through the nozzle, through the sprue bushing and runners, through the gate, and then through the cavity-together  with the pressure required at the end of flow to produce a dense part with a smooth surface. The pressure at the end of the flow in the cavity need only be 2,000 psi for many materials, and this value may only be : to of the gauge pressure reading, depending on the size of pressure drops that were listed. The most important reading is the one that determines the quality of the part, which is made at the end of the material flow and is in many cases about 2,000 psi. 

            Process control devices are made that limit the cavity pressure to a specified predetermined value, and they have proved very suc-cessful in minimizing rejects. The consistency of injection pressure in the cavity is an essen-tial element in producing uniform parts. Thevalues shown on processing sheets refer to gauge readings and are intended to indicatewhether or not the material flows easily andis readily compressible.