In the field of modern materials processing, molding and extrusion are two widely applied forming processes. The incorporation of chemical foaming agents has expanded the possibilities for both techniques. These agents release gases through chemical reactions during processing, creating uniform cellular structures within products. This not only reduces product density and raw material consumption but also enhances thermal insulation, soundproofing, and impact resistance. As a result, chemical foaming agents are extensively utilized across industries including packaging, construction, automotive, and electronics.
Chemical foaming agents (CFAs) are substances that decompose under specific temperature conditions to generate gases (such as nitrogen or carbon dioxide) through their own chemical reactions, forming a cellular structure within the polymer matrix. Their core function is to “reduce weight and enhance capabilities” during molding or extrusion, balancing product performance with cost.
Organic CFAs
These foaming agents exhibit high compatibility with most polymer processing temperatures and produce uniform cell structures, making them the most widely used type. Common varieties include:
◦ Azodicarbonamide (AZD): Decomposes at approximately 160-200°C, releasing nitrogen gas. Suitable for processing plastics like polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC).
◦ Diisopropyl azodicarbonate (DIPA): Low decomposition temperature (approx. 100-120°C), suitable for polymers requiring low-temperature processing, such as certain elastomers;
◦ Benzosulfonyl hydrazide derivatives (e.g., BSH): Decomposition temperature 100-150°C, releases nitrogen gas and small amounts of water vapor, commonly used in PVC flexible product processing.
Inorganic foaming agents offer low cost and high safety, but produce fewer decomposed gases and exhibit slightly inferior foam cell uniformity compared to organic foaming agents. They are primarily used in applications with less stringent foam cell requirements. Common varieties include:
◦ Sodium bicarbonate (baking soda, NaHCO₃): Decomposes at approximately 60-150°C, releasing carbon dioxide and water vapor. Often blended with citric acid to enhance foaming efficiency. Suitable for PE, PP, etc.
◦ Ammonium carbonate ((NH₄)₂CO₃): Decomposes at approximately 58°C, releasing ammonia gas, carbon dioxide, and water vapor. Due to ammonia's pungent odor, it is primarily used in industrial products where odor sensitivity is low.
Working Principle of Chemical Foaming Agents
The action process of chemical foaming agents can be divided into three stages:
1. Decomposition Stage: Within the heated environment of molding or extrusion equipment, the chemical foaming agent reaches its decomposition temperature, undergoes a chemical reaction, breaks its own molecular structure, and releases gas.
2. Bubble Formation Stage: The released gas forms microscopic bubble nuclei within the polymer melt. As gas continues to generate, these nuclei gradually expand;
3. Bubble Stabilization Stage: The polymer melt cools and solidifies under the constraints of the mold or extrusion die head. The expanded bubbles become fixed within the product, forming a stable cellular structure.
It is important to note that the decomposition rate of the foaming agent must be matched to the flow and solidification rates of the polymer melt. If decomposition occurs too rapidly, gas may escape prematurely, resulting in insufficient porosity. Conversely, if decomposition is too slow, residual gas may remain trapped within the product after solidification, potentially causing cracking.
Preparations Before Mold & Extrude Operations
Material Selection
Selection of Chemical Foaming AgentsComprehensive evaluation based on polymer type, processing temperature, and product performance requirements:
◦ Processing Temperature: For polymers processed at 180-220°C (e.g., PP), prioritize AZD (decomposition temperature 160-200°C); for processing temperatures below 120°C (e.g., certain PVC soft compounds), select DIPA or blended inorganic blowing agents.
◦ Product Properties: For odorless products (e.g., food packaging), avoid ammonia-containing foaming agents like ammonium carbonate. For high porosity (e.g., insulation materials), select organic foaming agents (typically releasing 150-250 mL/g gas, higher than inorganic agents' 50-100 mL/g).
Polymer Material PreparationSelect polymers with moderate melt flow rate (MFR). — Excessively low MFR increases melt viscosity, hindering bubble expansion. — Excessively high MFR reduces melt strength, causing bubble rupture.Additionally, pre-dry polymer raw materials (such as highly hygroscopic materials like PET or PA), controlling moisture content below 0.05% to prevent moisture reacting with the foaming agent, which could impair foaming efficiency or cause bubble defects in the final product.
Addition of Auxiliary MaterialsDepending on requirements, auxiliary agents may be incorporated: foaming aids (e.g., zinc oxide to lower AZD decomposition temperature), cell stabilizers (e.g., calcium stearate to enhance melt strength and prevent bubble coalescence), and antioxidants (to prevent polymer degradation at high temperatures). Typical addition levels range from 0.1% to 2% (based on polymer mass).
Equipment Inspection
Molding Equipment (e.g., injection molding machines, presses)◦ Injection molding machines: Verify the accuracy of the barrel heating zone temperature control system (tolerance ≤±5°C) to ensure foaming agent decomposition within the specified temperature range;Verify screw speed and backpressure adjustment functions to prevent melt overheating from excessive shear or gas escape due to insufficient backpressure. Inspect mold vent channels (depth 0.02-0.05mm, width 5-10mm) to ensure smooth gas evacuation, preventing flash or bubbles in finished products.
◦ Compression molding press: Verify platen flatness (tolerance ≤0.1mm) and pressure control system to ensure uniform cavity pressure, preventing localized density variations in foam cells.
Extrusion equipment (e.g., single-screw extruders, twin-screw extruders)◦ Extruder: Verify barrel section temperatures (feed section, compression section, homogenization section) meet process requirements (e.g., PP extrusion: feed section 150-170°C, homogenization section 180-200°C); inspect screw configuration (prioritize barrier screws or separation screws to enhance melt-foaming agent mixing uniformity);Inspect the flow channel design of the die (e.g., sheet die, pipe die) to ensure uniform melt flow and stable cell formation.
◦ Downstream Equipment: Verify temperature control of the cooling system (e.g., cooling water bath, air ring) (e.g., maintain cooling water temperature at 20-30°C for PE extruded products) to prevent surface cracking from excessive cooling or cell collapse from insufficient cooling.
Safety Measures
1. Operational Protection: Partial decomposition products of chemical foaming agents (e.g., AZD decomposition may produce trace amounts of formaldehyde) are irritating. Operators must wear dust masks, acid/alkali-resistant gloves, and safety goggles to avoid direct contact. When using organic foaming agents, ensure workshop ventilation is adequate (air exchange rate ≥3 times/hour) and install exhaust collection systems if necessary.
2. Equipment Safety: Never insert hands or tools into the mold cavity when molding equipment is closing. Before starting an extruder, run it idle for 5-10 minutes to verify normal screw rotation and prevent equipment damage from material jams.
3. Storage Requirements: Seal chemical blowing agents and store in a cool, dry location (temperature ≤30°C, relative humidity ≤60%). Avoid co-storage with acids or alkalis (e.g., sodium bicarbonate must not contact hydrochloric acid) to prevent premature decomposition.
Step-by-Step Guide to Molding with Chemical Foaming Agents
Molding processes primarily encompass two common methods: injection molding and compression molding. The specific steps for each are outlined below:
Injection Molding
Material Mixing◦ Use a high-speed mixer (rotation speed 800-1200 r/min) to blend polymer pellets, chemical blowing agents, and auxiliary materials:First add polymer pellets and mix at high speed for 1-2 minutes. Then add foaming agent (typically 0.5%-3% by polymer weight) and auxiliary materials, continuing to mix for 3-5 minutes to ensure thorough blending (verify by sampling; no visible foaming agent agglomerates should be present).
◦ When using powdered polymers or high-viscosity materials, pre-form the mixture into masterbatch pellets (via an extrusion pelletizer) before injection molding to enhance mixing uniformity.
Mold Preparation◦ Clean the mold cavities to remove residual impurities or old material, preventing surface defects on the finished product. Apply a release agent (e.g., silicone-based release agent at 0.1-0.2g/m²) to the mold cavity surfaces to facilitate part ejection.
Adjust the mold cavity clearance based on the product dimensions: Since the product will shrink slightly after foaming (shrinkage rate approximately 1%-3%), the mold cavity dimensions should be 1%-3% larger than the target product dimensions (e.g., if the target product thickness is 2mm, set the mold cavity thickness to 2.02-2.06mm).
Injection Parameter Setting & Operation◦ Temperature Settings: Barrel temperature should increase progressively through the “feeding zone → compression zone → homogenization zone” (e.g., for PP+AZD system: feeding zone 160-170°C, compression zone 170-180°C, homogenization zone 180-190°C). Nozzle temperature should be 5-10°C lower than the homogenization zone to prevent premature foaming at the nozzle.Mold temperature should be controlled at 40-60°C (PE products) or 60-80°C (PP products) to ensure slow melt cooling and stable bubbles.
◦ Pressure and Speed: Injection pressure should be 10%-20% lower than non-foaming injection molding (e.g., set to 64-80 MPa for foaming PP molding if non-foaming PP requires 80-100 MPa) to prevent bubble rupture. Injection speed should be set to medium (30-50 mm/s) to avoid uneven bubble distribution caused by excessive melt flow velocity.
◦ Hold Pressure and Cooling: Hold pressure should be 50%-60% of the injection pressure, with a hold time of 5-10 seconds (adjusted based on part thickness) to prevent bubble collapse. Cooling time should be 20%-30% longer than for non-foamed injection molding (e.g., if non-foamed parts require 10 seconds, foamed parts should be set to 12-13 seconds) to ensure complete solidification.
Demolding & Post-Treatment◦ After cooling is complete, activate the mold opening mechanism and slowly eject the part to avoid deformation or bubble rupture caused by excessive force.
◦ Remove gates and flash from the product (using a trimming knife or ultrasonic trimmer). If minor surface bubbles are present, lightly sand the area (using 400-600 grit sandpaper). For products with higher aesthetic requirements, apply a surface coating (e.g., water-based paint) to enhance appearance.
Compression Molding
Preform Preparation◦ Place the mixed polymer - blowing agent mixture (same mixing ratio as for injection molding) into the preform mold. Prepress for 5-10 minutes at 10-20 MPa pressure and 80-100°C temperature (below the blowing agent decomposition temperature) to form a preform with uniform density (thickness 2-3 times that of the final product). This reduces material loss and uneven bubble distribution during molding.
Molding Operation◦ Place the pre-pressed blank into the compression mold cavity, close the mold, and heat to the foaming agent decomposition temperature (e.g., 170-190°C for AZD systems) while applying pressure (20-50 MPa). Maintain this for 10-20 minutes (adjust based on product thickness). During this period, the decomposing foaming agent generates gas, forming cells within the mold cavity.
◦ After reaching the set time, slowly depressurize (depressurization rate ≤5 MPa/min) to prevent sudden pressure drops causing cell rupture. Then cool the mold to 60-80°C (below the polymer softening point) before opening the mold and removing the product.
Post-Treatment◦ Trim the product to remove excess material from the edges. If minor residual undecomposed foaming agent remains inside the product, bake it in an oven at 80-100°C for 2-4 hours to eliminate residual gas and enhance product stability.
Step-by-Step Guide to Extrusion with Chemical Foaming Agents
Extrusion processing can produce various foamed products such as sheets, pipes, and profiles. The following outlines the specific steps using a single-screw extruder to process PP foamed sheets:
Raw Material Pre-treatment
1. Drying: Place PP pellets (MFR=2-5g/10min) into a hot-air dryer and dry at 80-90°C for 2-3 hours, controlling moisture content below 0.05%.
2. Mixing: Using a twin-screw mixer (rotation speed 300-500 r/min), blend the dried PP pellets, AZD foaming agent (addition rate 1.5%-2%), and calcium stearate (cell stabilizer, addition rate 0.5%) for 5-8 minutes to ensure uniform dispersion of the foaming agent within the PP matrix.
Extruder Parameter Settings
1. Temperature Settings:
◦ Feed Section: 150-160°C (for initial melting of raw material, preventing premature decomposition of foaming agent);
◦ Compression Section: 160-170°C (achieve complete melt and thorough mixing with foaming agent);
◦ Homogenization Section: 170-180°C (foaming agent begins slow decomposition, forming micro-bubble nuclei);
◦ Die Head: 180-190°C (foaming agent rapidly decomposes, bubbles expand, while melt is formed through the die).
2. Screw Speed and Back Pressure:
◦ Screw Speed: 40-60 rpm (Excessive speed may cause melt shear overheating and accelerated foaming agent decomposition; insufficient speed leads to poor mixing);
◦ Back Pressure: 5-8 MPa (Appropriate back pressure enhances melt density, prevents premature gas escape, and ensures uniform cell structure).
Extrusion Molding
1. Startup Procedure: First, raise each section of the extruder to the set temperature and hold for 30 minutes (to ensure temperature stability). Then start the screw at low speed (20 rpm) and feed material. Once the barrel is completely filled, gradually increase the speed to the set value while adjusting the feed rate (to match the screw speed and ensure stable material flow).
2. Die Adjustment: Observe the melt state extruded from the die. If surface bubbles burst or no bubbles appear, adjust the die temperature (increase by 5-10°C if too low; decrease by 5-10°C if too high). If product thickness is uneven, adjust the die lip gap (via die adjustment bolts, precision 0.01mm).
3. Cooling and Setting: Guide the extruded sheet into a cooling water tank (water temperature 20-30°C). Set up forming rollers in the tank (pressure 0.5-1MPa) to ensure sheet flatness. Cooling time: 5-10 seconds (adjust based on sheet thickness; 5 seconds for thicknesses under 2mm, 8-10 seconds for 2-5mm).
Cutting and Winding
Cutting: The cooled sheet is conveyed via a traction machine (traction speed matched to extrusion speed, typically 1-3 m/min) to the cutting unit. It is cut to specified lengths (e.g., 2 m per sheet) or wound into rolls according to customer requirements.
Rewinding: For roll products, a rewinding machine (with winding tension of 50-100N) is used. Maintain stable tension during rewinding to prevent sheet wrinkling or stretching deformation.
Key Factors Affecting Mold & Extrude Quality
Chemical Foaming Agent Dosage
• Insufficient dosage: Inadequate gas release results in low foam cell density, with density approaching that of unfilled products, failing to achieve weight reduction and thermal insulation effects.
• Excessive dosage: Excessive gas release overwhelms the melt's ability to encapsulate bubbles, causing bubble coalescence and rupture, leading to surface depressions or voids in the product.
• Recommended Dosage: Adjust based on product requirements. Generally: - Low porosity products (10%-20% density reduction): 0.5%-1% blowing agent - Medium porosity products (20%-30% density reduction): 1%-2% - High porosity products (30%+ density reduction): 2%-3% (All based on polymer weight)
Temperature Control
• Barrel/Mold Temperature Too Low: Incomplete decomposition of foaming agent, insufficient gas release, resulting in fine and unevenly distributed pores.
• Temperature Too High: Excessively rapid decomposition of foaming agent causes significant gas escape before melt solidification, or polymer thermal degradation leading to yellowing and brittleness in the product.
• Control points: Based on the blowing agent decomposition temperature curve (available in supplier technical manuals), set barrel homogenization section/mold temperature within ±5°C of the blowing agent's peak decomposition temperature. Ensure temperature fluctuations in each section ≤±5°C.
Pressure adjustment
• Excessive injection/molding pressure: Bubbles compress, pores shrink or rupture, increasing product density;
• Excessively low pressure: Gas escapes prematurely, causing pore collapse and surface bubble defects;
• Excessive extrusion backpressure: Prolonged melt residence time leads to premature foaming agent decomposition and gas escape within the barrel, reducing extrusion output;
• Pressure control recommendations: Injection pressure should be 10%-20% lower than non-foaming processes. Molding pressure adjusts based on part thickness (20-30MPa for thicknesses under 2mm; 30-50MPa for 2-5mm thicknesses). Set extrusion backpressure to 5-10MPa to ensure uniform melt mixing without significant gas escape.
Melt Residence Time
• Excessive residence time: Foaming agent decomposes prematurely in the barrel, releasing gas and resulting in insufficient cell structure. Simultaneously, polymer thermal degradation occurs, compromising product performance.
• Insufficient residence time: Incomplete foaming agent decomposition leads to inadequate gas release and uneven cell structure.
• Control method: Match residence time by adjusting screw speed, barrel volume, and feed rate. Typically, control melt residence time at 2–5 minutes (organic blowing agents) or 3–8 minutes (inorganic blowing agents).
Common Issues and Solutions in Molding and Extrusion Processes
Uneven Blistering in Products
1. Causes: Inadequate raw material mixing (agglomeration of foaming agents), significant barrel temperature fluctuations, excessive screw speed causing uneven melt shear;
2. Solutions:
◦ Increase mixing time (extend to 5-8 minutes) or use foaming agents in masterbatch form;
◦ Inspect barrel heating system, replace faulty heating elements, ensure temperature fluctuation ≤±5℃;
◦ Reduce screw speed (e.g., from 60 r/min to 40-50 r/min) while appropriately increasing back pressure (by 2-3 MPa) to enhance mixing uniformity.
Surface Depressions/Pores in Products
1. Causes: Excessive foaming agent dosage, poor mold venting, excessive cooling rate;
2. Solutions:
◦ Reduce foaming agent dosage (e.g., from 2% to 1.5%-1.8%);
◦ Clean mold vent channels or increase vent channel density (add one vent channel per 100mm of mold circumference);
◦ Reduce cooling rate (e.g., increase cooling water temperature from 20°C to 25-30°C, or extend cooling time by 2-3 seconds).
Product dimensional instability
1. Causes: Fluctuations in blowing agent decomposition rate, uneven mold temperature, mismatch between pull-off speed and extrusion speed;
2. Solutions:
◦ Switch to a batch-consistent blowing agent or add a co-foaming agent (e.g., zinc oxide, 0.2%-0.5%) to stabilize decomposition rate;
◦ Inspect the mold heating system to ensure temperature variation across mold zones is ≤±3℃;
◦ Adjust draw speed (e.g., set draw speed to 1.02-1.05 m/min when extrusion speed is 1 m/min) to compensate for product cooling shrinkage.
Decreased mechanical properties of products
1. Causes: Polymer thermal degradation, excessive cell size (leading to fracture under stress), improper additive formulation;
2. Solutions:
◦ Reduce barrel homogenization zone temperature (e.g., from 190°C to 180-185°C), or add antioxidants (e.g., 1010 antioxidant, 0.1%-0.2%);
◦ Reduce
blowing agent dosage (lower foam porosity) or add foam stabilizer (e.g., calcium stearate, 0.5%-1%) to decrease cell size;
◦ Verify compatibility between additives and polymer; avoid reactive additives (e.g., do not mix acidic additives with sodium bicarbonate).
