Process Overview

Compounding & Modification (Single-Screw Extrusion)
PP resin + nucleating agent/antioxidant/color masterbatch → plasticization & mixing by single-screw extruder → pelletizing (strand or underwater cutting) → drying & screening → base pellets

High-Pressure Impregnation (Autoclave)
Base pellets + medium (water/dispersion agent) → autoclave injection of physical blowing agent (commonly CO₂, also N₂ or small amount of hydrocarbons) → absorption under controlled pressure & temperature

Pressure Release & Pre-Expansion
Controlled pressure release curve → gas nuclei form inside pellets → hot water/steam pre-expansion → EPP pre-puff beads suitable for molding

Aging & Packaging
Fluidized drying and equalization silo → screening and grading → antistatic/surface treatment → automatic weighing & packaging (bags or drums)

Output: Ready-to-mold EPP pre-puff beads with customizable bulk density.

Main Equipment & Units

• Feeding, Metering & Mixing System: Vacuum loader, weighing scale, pre-mixer
• Single-Screw Extrusion & Pelletizing Line: L/D 30–36 with venting section; strand or underwater pelletizer, centrifugal dryer, hot-air dryer, vibrating sieve
• Intermediate Storage & Buffer: Material silos, dosing hoppers
• High-Pressure Autoclave System: Agitator-equipped, lined, and insulated vessel; inlet/outlet lock hoppers, dispersant dosing, temperature/pressure interlocks & safety system
• Blowing Agent Supply & Recovery: CO₂ storage tank, metering/booster pump, filters, safety valves, condensation recovery & purification system
• Pre-Expansion & Pressure Release: Hot water/steam loop, pressure release valves, gas capture & separation, silencers
• Drying, Aging & Classification: Fluidized bed, cyclone separator, aging silos, classification screens
• Automatic Packaging & Palletizing: Weighing & bagging machine, sewing/heat sealing, palletizer
• Utilities: Steam/thermal oil, cooling water, compressed air, nitrogen blanketing, DCS/PLC centralized control

Key Process Control Points

(Typical ranges – actual settings depend on PP grade and formulation)

• Extrusion: Melt temperature 200–230 °C; gentle shear to ensure uniform dispersion; underwater pelletizing recommended for uniform pellet size and smooth surface
• Autoclave Impregnation: CO₂ commonly operated at medium-to-high pressure (tens of bar) and temperature near but below PP softening point; establish a “pressure–temperature–time–agitation” window for consistent gas uptake and uniform cell structure
• Pressure Release & Nucleation: Stepwise pressure release; combined with hot water/steam to control nucleation and expansion ratio, avoiding excessive foaming or pellet cracking
• Density & Size Control: Adjusted by base pellet size distribution, gas uptake, release rate, and pre-expansion temperature to cover 15–200 g/L density range
• Aging: Equalize internal pressure and moisture content, improve dimensional & mechanical stability, reduce molding shrinkage

Advantages

• Wide Formulation Flexibility: Flame retardant, antistatic, colored, high-heat-resistant grades can all be achieved during extrusion
• Consistent Bead Quality: Autoclave process creates fine, uniform closed-cell structure, excellent rebound & impact resistance
• Green & Safe: CO₂ preferred as physical blowing agent, VOC-free, closed-loop recovery system minimizes consumption
• Expandable Capacity: Scalable by adding autoclave vessels or extrusion lines, enabling modular plant expansion

Typical Capacity Configuration (Example)

1 single-screw extrusion line (350–400 kg/h base pellets) + 1 autoclave (effective volume ~3 m³, alternating operation)
Depending on shift schedule & formulation, production covers 1.5–3.5 t per shift of EPP pre-puff beads (engineering reference only, final design to be validated per resin, density spectrum, and plant operation plan).

Quality Indicators (Reference)

• Particle Size Distribution: e.g., 0.8–1.2 mm / 1.2–1.8 mm / 1.8–2.5 mm
• Density Grades: 15 / 20 / 25 / 30 / 35 / 45 / 60 g/L …
• Closed-Cell Content: ≥95% (depending on formulation)
• Mechanical Performance: Compression recovery, water absorption, drop impact as per automotive/packaging standards
• Residual Volatiles: Meet food-contact, automotive, and packaging safety limits

Utilities & Safety Considerations

• Pressure Vessel Compliance: Autoclave, storage tanks, valves designed & certified per local pressure vessel codes (material, inspection, hydrotest)
• Gas Handling System: Safety valves, rupture disks, double-block isolation, interlocked E-stop & emergency vent; CO₂/N₂ pipelines labeled & gas detectors installed
• Temperature Control & Insulation: Redundant steam/thermal oil/hot water systems, closed-loop cooling water circuit
• Static & Dust Control: Proper grounding of conveying & packaging areas, antistatic additive dosing, humidity control
• Recovery & Environmental Protection: CO₂ recovery & purification, process water recirculation, wastewater treatment to meet discharge standards

Optional Deliverables

• Process Package: Material & energy balance, PFD, P&ID, control & interlock matrix
• Plant Layout: General arrangement, pipe racks, autoclave area safety distances, line class & stress considerations
• Commissioning: Cold/Hot/Material commissioning procedures, density calibration curves, SOP & training manuals
• Trial Production: Particle size & density grading strategy, molding trials & optimization guidance