During the injection molding process, a polymer is heated in different ways, beginning inside the machine cylinder. Electric heater bands around the cylinder create heat which is transferred through convection from the cylinder to the polymer. Then, the rotating screw creates shear heat which transfers to the polymer on top of the convection transfer.
In addition, when the material is rapidly injected through the machine nozzle body, nozzle tip, runner system and gate, more shear heat is generated. As the shear heat energy increases, the temperature of the polymer continues to increase, driving up the temperature of the material inside the cavity.
The two most important material properties are specific heat capacity and thermal conductivity.
Understanding the concepts of specific heat capacity and thermal conductivity is key to understanding how they affect part and mold design. Specific heat capacity is how much heating energy is required to bring a polymer to its process temperature range. Thermal conductivity is a property that will indicate how quickly heat will move through the polymer material during the heating and cooling phase.
By understanding these behaviors, decisions can be made that will influence the overall quality of the molded part, from wall thickness versus flow distance capability to location and number of gates to overall cycle time.