Back Pressure in Injection Molding: Principles, Adjustment, and Effects

1. Formation of Back Pressure

2. Benefits of Proper Back Pressure Adjustment

2.1 Enhances Material Compaction and Stability

• Compacts the molten material in the barrel, increasing its density.

• Improves the stability of shot volume, product weight, and dimensions.

2.2 Reduces Gases in Molten Material

• Expels air trapped in the molten material, reducing surface blisters, internal bubbles, and improving gloss uniformity.

2.3 Promotes Uniform Plasticization and Mixing

• Slows down the screw retraction speed, allowing sufficient plasticization of the molten material in the barrel.

• Enhances the uniformity of mixing between color powders, color masterbatches, and the molten material, preventing color mixing defects.

2.4 Improves Product Surface Quality

• Properly increasing back pressure can reduce surface shrinkage and improve material flow at the product edges.

• Raises the temperature of the molten material, improving plasticization quality and flowability during mold filling, eliminating cold lines on the product surface.

3. Problems Caused by Excessively Low Back Pressure

3.1 Poor Material Density and Air Entrapment

• The screw retracts too quickly, resulting in low-density (loose) molten material at the front of the barrel, with excessive air entrapment.

3.2 Inconsistent Plasticization and Dimensions

• Causes poor plasticization quality and unstable shot volume, leading to significant variations in product weight and dimensions.

3.3 Surface and Internal Defects

• Product surfaces may exhibit shrinkage, blisters, cold lines, and uneven gloss.

• Internal bubbles are likely to form, and material flow may be insufficient at edges and ribs, resulting in short shots.

4. Problems Caused by Excessively High Back Pressure

4.1 Reduced Plasticization Efficiency

• Creates excessive pressure and high temperature in the molten material at the front of the barrel, reducing viscosity.

• Increases backflow in the screw grooves and leakage between the barrel and screw, lowering plasticization efficiency (amount of material plasticized per unit time).

4.2 Thermal Degradation Risks

• For heat-sensitive plastics (e.g., PVC, POM) or colorants, elevated temperatures and prolonged residence time in the barrel cause thermal decomposition or increased discoloration of colorants, deteriorating product surface color and gloss.

4.3 Extended Cycle Time

• Slows down screw retraction, prolongs preplasticization and material recovery time, increasing cycle time and reducing production efficiency.

4.4 Nozzle Issues

• High molten material pressure leads to drooling from the nozzle after injection; cold material in the nozzle channel may block the nozzle or cause cold spots in the product during the next injection.

• Excessive back pressure often causes material leakage from the nozzle during preplasticization, wasting raw materials and potentially burning out the heater bands near the nozzle.

4.5 Increased Mechanical Wear

• Accelerates wear of the preplasticization mechanism and screw barrel.

5. Adjustment of Back Pressure

• Increase back pressure appropriately when the product has minor surface blisters, color mixing, shrinkage, or significant variations in dimensions and weight.

• Reduce back pressure when experiencing nozzle leakage, drooling, molten material overheating/decomposition, product discoloration, or slow material recovery.

6. Types of Back Pressure

6.1 Hydraulic Back Pressure

• Generated by the hydraulic cylinder used for injection, acting on the screw during the material storage stage to slow down its retraction.

• Higher hydraulic back pressure results in longer screw reset time; the pressure from the molten material in front of the screw must exceed the hydraulic back pressure to move the screw backward.

6.2 Melt Back Pressure

• Created by the accumulating molten material at the front of the injection cylinder, which pushes the screw backward.

• It has a direct relationship with hydraulic back pressure, determined by the machine's structure (e.g., screw diameter and injection cylinder piston diameter). Generally, hydraulic back pressure is designed to be one-tenth of the generated melt back pressure.

6.3 Back Pressure in All-Electric Machines

• Most injection molding machines are hydraulically powered, making back pressure adjustment during material storage straightforward (even allowing different back pressure values at various screw positions).

• For all-electric machines, back pressure control is more complex. The back pressure setting (via a load device or converter) creates resistance on the pressure bearing during screw rotation.

• This resistance is a function of the AC servo motor's rotation speed: higher back pressure settings increase resistance, reducing the servo motor's rotation speed. In all-electric machines, back pressure is also referred to as resistance-sensing back pressure.