Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Back pressure in injection molding refers to the resistance or pressure applied against the rear (non-injection) side of the rotating screw during the plasticizing (or recovery) phase of the process. This occurs as the screw rotates to shear and melt the plastic resin in the barrel, building up the shot volume of molten material needed for the next injection cycle.

How It Works:

In a typical injection molding machine with a reciprocating screw:During the recovery phase, the screw rotates backward while pushing molten plastic forward through a check valve toward the nozzle.

Back pressure is generated by restricting this backward movement of the screw (often via hydraulic or electric controls), forcing the screw to work against resistance.

This compresses the melt, promoting better mixing, venting of trapped air (degassing), and uniform heating of the resin.

Without sufficient back pressure, the screw might recover too quickly, leading to poor melt quality; excessive back pressure can overheat the material, causing degradation or extending cycle times.

Why It's Important

Improves melt homogeneity: Enhances shear and compression for consistent density and temperature. 

Aids in color dispersion and venting: Helps eliminate air bubbles and streaks in the final part. 

Optimizes viscosity: Slightly raises melt temperature to reduce viscosity, improving flow into the mold. 

Typical back pressure settings range from 50–150 psi (3.5–10 bar) for most resins, but this varies by material, machine, and part design—always optimize via trial runs to balance quality and efficiency.