Thermodynamics in injection molding: what is it and why does it matter?
Thermodynamics is not just a fancy word, it is vital to ensuring an efficient and effective injection molding process.
Thermodynamics: The physics that deals with heat and other forms of energy.
Thermal Conductivity (Tλ): The rate at which heat can be conducted through a material. Table 2 provides a general range of substances commonly found in molding.
Thermal Expansion: How much a material will expand at a given temperature.
Specific Heat (Cp): The amount of heat required to raise the temperature of a unit mass of a given substance by a given amount.
Shrinkage: The amount a thermoplastic shrinks when it cools.
How does thermodynamics work?
The machine is responsible for injecting energy into the material through the rotation of the screw, forcing the particles from solid to liquid, and then reversing the process in the mold.
If too much energy is put into plastic, there is a high chance of degradation. Conversely, if the energy is insufficient, we can expect to see unmelted particles in the molded part. Plastic specialists can vary screw speed and back pressure during recovery, which can affect the amount of energy put into the plastic. They have to understand that not all polymers melt at the same rate, and some polymers are very heat sensitive.
The majority of the molding cycle (nearly 80%) is waiting for the plastic to reach a temperature low enough that it is stiff enough to withstand demolding forces. The main force in this regard is mold.
The thermal conductivity of the mold metal is also critical. Not all metals transfer heat at the same rate, stainless steel is very poor, and copper alloys are very good.
Due to the heat transfer ability of plastic, only about 40% of the energy is released from the part. As the molding cycle progresses, the plastic becomes more like an insulator every moment, making it harder and harder to dissipate heat.
What if there is too much heat?
When large molds run very hot, thermal expansion calculations must be used to ensure adequate clearance between moving parts at operating temperatures. In general, the larger the mold and the higher the temperature, the more worthy of attention. If the mold contains a hot runner system directly connected to the valve gate, thermal expansion at operating temperature must be addressed or serious damage to the mold may occur.
How does cooling work?
It is also important to know how to remove energy from molded parts. One method of heat transfer is through the air around the mold, which is not very efficient. Heat is also carried away through the water channel. It is important to achieve turbulence in the channels to improve the heat transfer capacity of the water. If you calculate the Reynolds number, you can determine whether the water is laminar or turbulent.