In an injection mold, molten plastic is injected into a mold, cooled to solidify it, and then the molded product is taken out.
It is necessary to apply pressure in order to inject molten plastic.
How much pressure is required can be predicted in advance using calculations of fluid dinamics.
Actually, molten plastic has some viscosity (resistance to flow), and since the viscosity changes upon time (as the plastic cools, the viscosity increases making it more difficult to flow), the equations of calculations of fluid dinamics become very difficult.

In this issue, we explain the "Bernoulli's theorem" which is the most fundamental principle for discussing the general state of fluids.

In other words, in a fluid, the total energy is determined by the amount of energy based on the velocity, the amount of energy based on the pressure, and the amount position energy.

Since it is difficult to apply the Bernoulli's equation as it is when there are changes in the viscosity or compressibility of the fluid, fluid velocity, it cannot be applied as it is for actual plastic injection molding. However, this is a very important basic equation when considering the principles.