1. Filling Pressure

Among the plastic injection molding conditions, the filling pressure, also called the primary pressure, is the pressure required for filling the inside of the mold with plastic. The unit of filling pressure is MPa or kgf/cm². That is, it is the unit of pressure. There are also injection molding machines that adjust the condition by changing what percentage of the maximum filling pressure of the machine is to be used as the filling pressure (for example, 35%).

Filling defects (short shot) occurs when the filling pressure is too low. When the filling pressure is too high, the parting surface opens instantaneously due to the pressure, which may cause flashes (burrs) to appear on the periphery of the molded item. Further, there may be defective releasing of the molded item from the mold.

2. Injection Speed

The injection speed is the speed of the molten plastic when it is being injected into the mold. The unit of this speed is mm/s.

This is also indicated as the injection rate which is the injected volume per unit time. The unit in this case will be cm³/s.

Filling defects can occur when the injection speed is too low, and burrs and jetting (the defect in which corrugated pattern appears on the surface of the molded item) can occur when the filling speed is too high.

3. Dwelling Pressure

The dwelling pressure is the pressure that is applied during the period after the entire cavity has been filled with molten plastic until the gate has been sealed (the gate has solidified) so that the plastic does not flow back from the gate. Its unit is MPa or kgf/cm².

When the dwelling pressure is too low, the plastic flows back from the gate causing the surface of the molded article to get dented (sink), or the shrinkage becomes too large making the dimensions smaller.

When the dwelling pressure is too high, the dimensions may become too large, or burrs can occur, or there may be defective releasing from the mold.

The dwelling time is very important in making the dwelling pressure have the appropriate effect, and it is very important to understand that the dwelling pressure must not be released until the gate has been sealed.

4. Injection Time

Generally, the sum of the fill time and the dwelling time is called the injection time. The dwelling time is adjusted by changing the injection time. The unit is s (seconds).