Controlling Temperature, Pressure and Speed Parameters in Injection Molding

Melt Temperature

Melt temperature is of primary importance in the injection molding process. It can be measured at the machine's nozzle or estimated approximately using the "air shot" method. Barrel temperature settings must account for the plastic's melting point, screw speed, back pressure, injection volume and cycle time. If the operator has no experience with a particular material, trial runs should begin at the minimum allowable temperature. For more precise control, the barrel is usually divided into several independently controlled heating zones. During long injection times or high-temperature processing, it is recommended to set the first zone's temperature somewhat lower — to prevent premature melting and material delamination. Before starting the process, it is essential to confirm that the hydraulic oil, hopper seal, mold and barrel have all reached their set temperature values.

Injection Pressure

Injection pressure is the pressure under which molten polymer is delivered into the mold cavity. It is measured by a sensor at the nozzle or within the hydraulic system. This pressure is not fixed: it should be increased if the mold is difficult to fill.

Injection pressure can be broken down into several stages. During the fill phase, the first and second stages may require elevated values to maintain a high injection speed. Once the mold is fully filled, pressure can be reduced. However, when processing certain semi-crystalline thermoplastics — such as polyamide (PA) or polyacetal (POM) — an abrupt pressure change can cause undesirable structural changes in the molded part, so staged pressure profiles may not be applicable in these cases.

Clamping Force

To counteract injection pressure, the mold must be held under a certain clamping force, but this force should not be excessively high — prolonged operation under high load can damage the mold and equipment. Clamping force is calculated based on the projected area of the molded part — the largest area of the part visible in the direction the force is applied. If the part has significant height, the area of its side walls is also included in the calculation.

Back Pressure

While increased back pressure helps distribute pigments and other fillers more evenly and promotes more complete melting of the material, it also increases screw recovery time and can cause fiber breakage in reinforced polymers. Back pressure values should not be too high — under no circumstances should they exceed 20% of the maximum injection pressure allowed for the given injection molding machine.

Injection Speed

When molding thin-walled parts, a high injection speed is necessary to ensure the mold fills completely before the melt solidifies in the gate. However, the speed should not be excessive. A staged injection speed profile is recommended, as it helps avoid defects such as melt splay and air entrapment.