Detailed Explanation of Extrusion Foaming Process

Extrusion foaming is a molding technology to manufacture foamed plastic products. It mixes resin with chemical or physical foaming agents and completes foaming & shaping during extrusion to form uniform cellular structures. The core production equipment includes foam extruders and complete foam sheet extrusion lines.Foshan Gaoming Keli Machinery Co., Ltd. has developed a full range of extrusion foaming equipment that can cover all production procedures and meet mass production demands for various foamed products.

1.Basic Principles

The whole extrusion foaming process is divided into three stages:

(1) Cell Nucleation: Foaming agents decompose or vaporize under heat inside the extruder, dissolving gas into polymer melt to form bubble nuclei.

(2) Cell Growth: The melt undergoes sharp pressure drop after exiting the die, expanding supersaturated gas to enlarge and merge bubble nuclei.

(3) Foam Curing & Shaping: The foam cools and solidifies in shaping molds to stabilize cellular structures.

Based on whether foaming occurs right at the die outlet, two mainstream processes are classified:

One-step foaming: The melt foams directly when exiting the die (e.g. PVC foam pipes).

Two-step foaming: The material is plasticized and pre-foamed inside the barrel first, then heated outside the machine for secondary foaming (e.g. cross-linked PE foam). Mainstream PE foam extruders and PS foam sheet extruders in the industry are compatible with both two processes.

2. Main Process Technologies

(1) Chemical Extrusion Foaming

Chemical foaming agents (such as azodicarbonamide and 4,4′-oxydibenzenesulfonyl hydrazide) are blended with resin. The agents decompose under extrusion heat to generate gas, which dissolves in the melt and triggers foaming once the material exits the die.

(2) Physical Gas Injection Extrusion

Inert gases (nitrogen, CO₂) or low-boiling-point liquids are injected into the melt to form a homogeneous gas-polymer blend, which foams after exiting the die.

(3) Chemical Crosslinking Extrusion

Crosslinkers and foaming agents are added to crystalline resins (PE, PP etc.). Crosslinking and foaming are realized simultaneously during extrusion.

3. Typical Process Flows

Case 1: Chemical Extrusion Foaming (PVC Foam Profiles)

• Raw material preparation: Mix resin powder with chemical foaming agents and auxiliary additives.
• Plasticization and mixing: Heat and homogenize materials in single-screw or twin-screw extruders.
• Extrusion molding: Sudden pressure drop at the die initiates foaming.
• Cooling shaping: The melt solidifies in shaping molds to lock the cellular structure.
• Post-processing: Traction, cutting, winding or stacking.

Case 2: Physical Gas Injection Extrusion (PE Foam Sheets & Films)

• Foaming agent injection: Inject inert gas or low-boiling liquid at designated screw sections.
• Melting and blending: Gas disperses evenly in the melt under high temperature and pressure.
• Die foaming: Rapid pressure reduction expands gas to form uniform cells.
• Cooling and traction: Cool and solidify the foam before traction and cutting.

4. Critical Process Parameters

• Extrusion Pressure: Higher pressure generates smaller and more foam cells.

• Extrusion Temperature: Must be controlled near resin melting point; excessive heat ruptures cells while insufficient temperature prevents full gas dissolution.

• Residence Time: Overlong residence weakens nucleation and reduces bubble quantity.

• Foaming Agent Type & Dosage: Determines cell size, density and uniformity.

5. Equipment Characteristics

Single-screw extruder: Simple structure and low investment, yet inferior mixing uniformity.

Twin-screw extruder: Outstanding shear mixing performance, ideal for high-precision foaming production.

Continuous Supercritical CO₂ Extrusion Foaming System: Two-stage screws fully blend gas and melt, applicable to PP, PET and other polymer materials.

Conclusion:

The complete extrusion foaming workflow covers raw material preparation, plasticization mixing, extrusion molding, cooling shaping and post-processing. The core lies in controlling the generation, growth and stabilization of bubble nuclei. Proper selection of chemical/physical foaming routes and optimized process & equipment configuration are essential to produce premium foamed products. Clients with mass production demands can choose complete extrusion foaming turnkey equipment from Keli Machinery.