Runners should be sized so there is shear heat in the runner. For most materials, 20ºC is good target for the amount of shear heat. Some materials can have a much higher level of shear heat. Materials like PVC should keep shear heat to a minimum.
Shear heat in runners is advantageous for several reasons because it:
Reduces the pressure to fill the part by increasing the melt temperature entering the part.
Reduces the shear stress in the part because the melt temperature is higher.
The barrel melt temperature can be lower resulting in a longer residence time before the material degrades.
The amount of shear heat is controlled by the size of the runners. The smaller the runner, the more shear heat. Following best analysis practices, the melt temperature is optimized by considering only the part initially. This allows you to design the part to be molded at higher temperatures to achieve the benefits mentioned above. When the runners are sized, the melt temperature entering the sprue is lowered so by the time the material enters the part it is back to the optimized temperature.
Jay Shoemaker has held several positions within Moldflow, including Design Engineer, Customer Services Engineer, and Senior Technical Instructor and Designer, his current role. Jay's primary responsibility is to develop technical training material and deliver training classes for Moldflow Plastics Insight (MPI) products. Learn more.
