The essence of PU spray foam is that component A (isocyanate) and component B (polyol blend) are instantaneously impinged and mixed under high pressure, atomized and sprayed onto the substrate surface, then rapidly expanded and cured by exothermic chemical reaction to form a dense closed-cell structure.

The quality of the entire process is determined by three core technical dimensions, and any out-of-control dimension will directly lead to engineering defects:

1.1 High-Pressure Metering and Impingement Mixing (Core of the Process)

This is the basis for determining foam performance, and industrial spray process requirements:

• Dosing Accuracy: Servo or hydraulic driven metering pumps are adopted, the volume ratio of A/B materials is strictly controlled at 1:1, and the dosing accuracy deviation is ≤±0.5%. A ratio deviation of more than 1% will cause softness, incomplete curing and insufficient strength.
• High-Pressure Impingement Mixing: A/B materials achieve 150-200bar high-pressure impingement in the mixing head, with mixing uniformity ≥99%, ensuring sufficient molecular-level reaction and avoiding local uncured dead spots with low closed-cell rate.
• Pressure Stability: The output pressure difference between A/B materials is ≤5bar. Excessive pressure difference directly leads to uneven mixing, which is the core cause of foam density drift.

Figure 2: High-precision servo metering pump for ±0.5% dosing accuracy

1.2 Atomization Spraying and On-Site Reaction

The uniformly mixed raw materials are highly atomized after leaving the spray gun, and rapidly nucleate and expand after contacting the substrate. The core control points of this process:

• Atomization Effect: Spray pressure matches fan width to ensure uniform coverage of the substrate without sagging or missing spray.
• Reaction Time Control: Match raw material temperature with ambient temperature, control cream time at 3-5 seconds and gel time at 8-12 seconds. Too fast foaming leads to insufficient adhesion, too slow leads to sagging.
• Closed-Cell Structure Forming: Qualified spray foam has a closed-cell rate ≥90%, which can be increased to more than 95% with supercritical technology, and thermal conductivity ≤0.024 W/(m·K).

1.3 System-Level Environmental Adaptability

Spray foaming is a reaction process completely exposed to the natural environment. The equipment must have full-chain temperature control capability to offset environmental interference:

• Raw Material Temperature Control: The main heater heats A/B materials to the optimal reaction temperature of 40-60℃ to ensure the fluidity of high-viscosity materials.
• Pipeline Temperature Control: The heated hoses maintain constant temperature throughout the process to avoid viscosity changes caused by raw material cooling in the pipeline.
• Environmental Compensation: The equipment can dynamically adjust parameters according to on-site temperature and humidity, adapting to the construction environment of 5-35℃.

Figure 3: Stainless steel gear pump and full-chain constant temperature control system

According to GB 50404, ISO 10292 and ASTM C1029 standards, zero-defect spray projects must strictly follow the following four-step standardized process, all steps have clear quantitative control requirements:

2.1 Substrate Treatment and Construction Environment Preparation

This is the most easily overlooked step, and also the core cause of project peeling and hollowing, which must meet the following requirements:

• Substrate Requirements: The surface is absolutely dry, dust-free, oil-free, free of floating rust and loose attachments. The moisture content of concrete substrate is ≤8%, and steel substrate must be sandblasted to Sa2.5 grade.
• Environmental Requirements: Construction environment temperature 5-35℃, relative humidity ≤85%. It is strictly prohibited to construct in open air on rainy, snowy days or with wind force ≥5 levels.
• Protection Preparation: Shield doors, windows, pipelines and equipment that do not need spraying to avoid raw material pollution.
• Equipment Self-Check: Confirm that the main hydraulic/pneumatic system is normal, the high-pressure gun filter is unblocked, and the raw material batch and specification meet the design requirements.

2.2 Equipment Temperature and Pressure Debugging and Parameter Calibration

Equipment debugging must be completed before formal spraying to ensure all parameters are within the qualified range:

• Temperature Setting: Set the temperature of the main heater and heated hoses according to the ambient temperature and raw material characteristics of the day, usually set at 45-55℃ under normal environment.
• Pressure Balance Test: Start the equipment for no-load operation, the dynamic output pressure difference of A/B materials is ≤5bar, and the pressure fluctuation is ≤±2bar.
• Test Spray Verification: Perform test spray on the test board to verify the foaming time, curing state, density and appearance of the foam, and confirm that the parameters meet the requirements before formal construction.

2.3 Precise Spraying Operation Specifications

Spraying operation must strictly follow standardized methods to avoid defects caused by human error:

• Spray Gun Posture: Keep the spray gun perpendicular to the substrate surface, the distance is controlled at 60-80cm, and the gun moving speed is stable at 0.5-0.8m/s.
• Spraying Technique: Adopt the "crisscross" multi-pass coverage method to avoid missing spray and uneven thickness.
• Thickness Control: Strictly implement the principle of "thin spray, multiple passes", the thickness of single pass spray is ≤15mm. It is strictly prohibited to spray too thick in a single pass, which will lead to poor heat dissipation in the center, causing "core burn" and internal cracking.
• Continuous Operation: Large-area construction requires continuous operation, the interval between two sprays is ≤30 minutes to ensure interlayer adhesion.

2.4 Curing Maintenance and Quality Acceptance

Curing and inspection after spraying is the last line of defense for project quality:

• Curing Maintenance: 24 hours after spraying is the initial curing period, and 7 days is the full curing period. It is strictly prohibited to step on or apply heavy pressure during the curing period to avoid the rupture of internal microscopic cell walls.
• Quality Inspection:
  1. Thickness Inspection: Use needle probing method or ultrasonic thickness gauge, the average thickness meets the design requirements, and the minimum thickness is ≥90% of the design thickness.
  2. Density Inspection: Apparent density ≥35kg/m³ (insulation scenario), ≥50kg/m³ (anti-corrosion scenario).
  3. Adhesion Inspection: Pull-off adhesion strength ≥0.3MPa (concrete substrate), ≥5MPa (steel substrate).
  4. Appearance Inspection: The surface is flat, without hollowing, cracking, peeling and sagging defects.