The different pressure for injection molding

1. The Details of Injection Pressure

Injection pressure is generated by the hydraulic mechanism of the plastic injection molding system. This pressure is transmitted from the hydraulic cylinder to the molten plastic via the molding screw, forcing the melt to flow through the injection machine nozzle into the mold sprue (also termed the main runner in certain molds), primary runners, and sub-runners, ultimately entering the mold cavity through the gate.

This phase is known as the injection molding process or filling stage. The primary role of this pressure is to overcome resistance during melt flow; conversely, the resistance encountered during flow must be counteracted by the pressure exerted by the injection machine to ensure unobstructed filling.

During injection, the highest pressure occurs at the injection nozzle to overcome cumulative flow resistance. As the molten plastic advances, pressure gradually decreases from the nozzle to the melt front. If the mold cavity is well-vented, the final pressure at the melt front approximates atmospheric pressure.

Injection pressure on the melt is influenced by multiple factors, broadly categorized into three types:
(1) Material factors, including material type, viscosity, etc.;
(2) Structural factors, such as runner system type, quantity and location, mold cavity geometry, and product thickness;
(3) Molding process parameters.

2. Holding Pressure Value and Duration

As the injection molding process nears completion, the screw ceases rotation but continues forward movement, transitioning into the holding pressure phase. During this period, the injection nozzle continuously feeds material into the cavity to compensate for voids caused by product shrinkage.

Without post-filling pressure holding, the product may shrink by approximately 25%, particularly forming sink marks near ribs due to significant shrinkage. Typically, the holding pressure is set at around 85% of the maximum injection pressure, though adjustments depend on practical requirements.

3. Back Pressure

Back pressure refers to the resistance the screw encounters during retraction after material injection. Higher backpressure enhances pigment dispersion and plastic melting but prolongs screw retraction time, shortens plastic fiber length, and increases machine internal pressure. Thus, backpressure is typically maintained at no more than 20% of the injection pressure.

Some injection machines allow backpressure programming to offset reduced screw stroke, which may decrease heat input and lower temperature. However, predicting the variable outcomes of such adjustments is challenging, making corresponding machine tuning a complex task.