Parameters for Plastic Injection Molding
Understanding process parameters for plastic injection molding requires a little science edu-muh-cation, as the kids say. When considering the perfect process parameters for thermodynamic plastic, I like to work my way through the plastic injection molding process in collaboration with the thermodynamic equation.
The first law of thermodynamics states that “the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system.”1 When we consider the steps that occur during injection molding alongside the laws of thermodynamics, we can get a really thorough picture of manipulating parameters to get the highest quality outcome.
ΔU = Change in Internal Energy
The internal energy during plastic injection molding refers to the change in the kinetic and potential energies on the molecular level that occur when the plastic goes through chemical changes. In this case, the change in internal energy is referring to the chemical change seen when applying heat and pressure. During injection molding, plastic changes from solid to molten, back to solid, indicating a change in internal energy. When discussing process parameters, it’s important to understand how each parameter interacts with another, affecting the internal energy mechanically and chemically.
Why Heat Effects Plastic Molding Parameters
q= Net Heat Transferred Into the System
Since heat is an interaction between the mass of a substance, its specific heat, and a change in temperature, for thermodynamics in relationship with plastic injection molding we are mostly concerned with the change in temperature. For each substance, like ABS or nylon, as an example, the specific heat and substance mass are previously defined.
In relation to heat, during the plastic injection molding process the parameters you will want to consider include:
- Melt temperature
- Mold temperature
- Cooling temperature
The change in temperature that occurs for the plastic corresponds to the change in internal energy. The melting point is the temperature reached by the plastic at the end of the barrel. The mold temperature is the temperature during the fill and pack phases inside the mold cavity. Lastly, the cool temperature refers to the temperature at which the plastic returns to its solid. Important to keep in mind, cooling temperature affects the mold process duration the most.
How Work Effects Plastic Molding Parameters
W= Pressure x the Change in Volume
The work that occurs within the plastic injection molding system is expansive. We can consider the different parameters like pressure, speed, and shrinkage. The volume of the plastic from start to finish is practically the same. However, during the molten phase, as you would expect due to molecular spread, the volume increases. It then contracts in the solid phase and changes, ever so slightly, by the shrinkage rate. Shrinkage occurs after it has returned to the solid form.
Pressure times the change in volume gives us a better understanding of the work involved within the system. These parameters include:
- Injection Pressure
- Holding Pressure
- Injection Speed
- Shrinkage Rate
Setting appropriate process parameters is the difference between a successful and an unsuccessful finished product. Adjusting the parameters to best match the mold cavity, the design elements, and the material characteristics is best approached by a plastic engineer or expert. However, a simple understanding of the plastic injection molding process and thermodynamics can give you a better understanding of setting perfect process parameters for plastic.
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