Setting Injection Pressure, Back Pressure and Injection Speed

Injection pressure

Choose injection pressure according to part shape and wall thickness and the material being molded. Where production conditions and quality standards allow, a lower-temperature, lower-pressure process is recommended.

Back pressure

Back pressure — also called plasticizing pressure — is the pressure the material is under during plasticizing.

• Higher back pressure improves color mixing.
• It helps expel gases from the melt, reducing silver streaks and bubbles.
• Appropriate back pressure prevents material hanging up in the barrel, which is why back pressure is often increased when purging.

Injection speed

Low-speed filling gives a steady flow, stable dimensions with little variation, and low, uniform internal stress. Its drawbacks are weld lines, delamination and flow marks.

High-speed filling allows lower injection pressure, improves gloss and smoothness, eliminates weld lines and delamination, reduces sink marks and gives more uniform color. Its drawbacks are jetting (hesitation or turbulence), excessive temperature rise and yellowing, poor venting and occasionally difficult ejection. With high-viscosity materials, melt fracture can occur, causing surface haze and increasing the tendency to warp from internal stress and to crack along weld lines in thick parts.