Most sandwich panel defects—voids under the metal facing, density drift near the edges, poor adhesion after thermal cycling—are locked in before the board reaches the saw. The line amplifies what metering, mixing, and laydown have already decided. Understanding the manufacturing sequence is less about watching foam rise and more about controlling the variables that let it rise consistently.
How a Continuous PU Sandwich Panel Line Works
On a continuous line, the process is a moving assembly sequence rather than one large pour. A lower facing of color steel, aluminum foil, or paper honeycomb feeds from a decoiler, passes through roll forming or profiling, and moves through a preheating zone. The liquid polyurethane system is metered, mixed, and laid down as a bead across the lower facing. The panel then enters a heated double-belt laminator, where the foam rises, bonds to the upper facing, and reaches the thickness set by the belt gap. After the core cures enough to hold its shape, the continuous board is cut to length, cooled, and stacked.
In a typical panel line configuration, production speed is adjustable rather than fixed. Haifeng continuous lines operate from 1 to 15 m/min with stepless speed regulation, mix at 10–18 MPa, handle panel thicknesses from 30 to 200 mm, and use dynamic metering accuracy within ±0.3%. Those values matter because the line must stay matched to the foam’s reaction speed. A faster belt does not simply produce more panels; it changes how far the liquid travels before cream time and how much rise occurs inside the laminator.
Raw Material Conditioning and Metering
Before any pour, the A-side and B-side must be at process temperature and the ratio must be verified. For rigid sandwich foam, the B-side typically contains polyether polyol, catalysts, surfactants, blowing agents, flame retardants, and water or other chemical blowing contributors. The A-side is isocyanate, usually polymeric MDI for building and cold-storage panel cores. Weight ratios for rigid panel formulations commonly run between 100:100 and 100:130 polyether to isocyanate, depending on the isocyanate index and the fire, density, and adhesion targets.
Material temperature is controlled in the day tanks and through the metering circuit. Panel lines usually hold both components between 20 and 45°C. If the polyol side runs cold, viscosity rises and the mix head can produce streak lines or poorly mixed edges. If the isocyanate side runs too warm, the reaction accelerates and cream time shortens before the liquid has covered the full panel width.
The formulation and application environment also change what the panel line must handle. <What Is Spray Polyurethane Foam Composition And Application> covers composition variables and where they show up in field application.
Pouring, Rise, and Cure Control
The mixing head traverses across the lower facing in a controlled pattern. Since rigid foam has a short cream time, the discharge rate must be matched to line speed so the liquid covers the full width before rise begins. If discharge is too low, the outer edges can tack off with irregular cells or remain unfilled. If discharge is too high, the bead piles up and the laminator has to redistribute material, which creates density gradients.
High-pressure panel lines rely on impingement mixing. The A and B streams pass through opposing nozzles at 10–18 MPa and collide in the mixing chamber. There is no mechanical stirrer to hide ratio error; mixing quality comes from pressure balance and nozzle condition. The required foam mass flow per linear meter depends on panel width, core thickness, target density, and line speed, so changing any one variable changes the laydown rate the head must deliver.
Once the liquid is on the lower facing, the heated double-belt laminator fixes panel thickness while the foam rises. Belt temperature has a direct effect on surface cure and facing adhesion. If the facings are too cold, the foam can form a weak boundary layer rather than a chemical bond. If the belts run too hot, the skin can over-cure while the core is still reacting, which later shows up as bow or dimensional movement.
If foam rises too early or too late relative to belt travel, adhesion and cell structure both suffer. <The Role Of Rise Time In Polyurethane Foaming> covers how rise time affects fill pattern and final properties.
Thermal and fire performance are not set by density alone. Under EN 14509, self-supporting double-skin metal-faced panels are specified by mechanical resistance, air permeability, water permeability, thermal transmittance, and reaction to fire [1]. Core thermal conductivity is determined by heat flow meter or guarded hot plate methods such as ISO 8301 [2]. Fire classification is established through EN 13501-1, with the facing, core, and joint details evaluated together [3]. These standards matter at the line design stage because they set the density, closed-cell content, and adhesion targets the process must hold.
Getting these windows right is more straightforward on a line configured around your panel range than through trial-and-error changes. If you have panel thickness, facing material, and annual output targets, send them to Info@chinahaifeng.com or WhatsApp 86 13566296633 for a configuration matched to the process window.
Batch Manufacturing for Non-Standard Panels
Continuous production is not always the right format. Batch lines mold one panel or a small set of panels per cycle, which suits prototypes, laboratory sampling, special shapes, and non-standard building materials. The panel cavity is filled inside a clamped mold rather than a moving belt, so edge profiles, cutouts, and thickness changes are easier to manage.
Batch panel equipment typically uses a mixing pressure of 8–15 MPa, a single-mold forming cycle of 3–10 minutes per panel, and a thickness range of 20–300 mm. The hydraulic clamping and temperature-controlled mold set the final dimensions. Changeover is simple because the mold, not the entire line, defines the product. The trade-off is output: batch is slower per panel but much less expensive to adapt when the product range is fragmented.
Common Defects and Equipment-Level Causes
Most rejects trace back to the mixing head or the temperature path, not the facings. Voids and hollow corners usually mean the laydown pattern did not cover the width before cream time. A density gradient across the panel can come from uneven nozzle pressure, pump pulsation, or an unmatched belt speed. Poor facing adhesion often appears when the substrate temperature is too low or the reactivity at the surface is too weak. Bowing and shrinkage after cutting are cure-related: the core may still be moving when the panel is cut, or the surface and core have cured at different rates.
When panels come out bowed, delaminated, or undersized, the root cause frequently traces back to cure and dimensional behavior rather than the facings. <Causes Of Shrinkage In Polyurethane Foam How To Control It> covers the mechanisms and control points.
Panel projects in China may also need to meet GB/T 23932 requirements for double-skin metal-faced insulating panels, including dimension, adhesion, and mechanical checks [4]. The most useful process response is to record ratio, temperature, line speed, and pressure for every shift. When a defect appears, the first question is what changed before the pour, not what the board looks like at final inspection.
Equipment Decisions and Line Specification
Start with the product format. If the output is standard width, standard thickness, and high volume, a continuous line is usually the lower-cost route per square meter. If the product range is non-standard or volume is low, batch molding avoids paying for flexibility you do not need.
Then specify the real process envelope: panel width and thickness range, facing material and coil width, target core density, blowing agent constraints, fire classification, and required annual output. The metering and mixing configuration follows from those numbers. For continuous panel production, high-pressure mixing stability under changing line speed usually matters more than nominal throughput. Buy the control range first, not the largest pump.
The best configuration depends on your panel dimensions, facing materials, density and fire-class targets, and annual output. Send those parameters to Info@chinahaifeng.com or WhatsApp 86 13566296633, and Haifeng engineers can help you compare continuous and batch panel production and specify metering, mixing, and lamination around the products you actually run.
Frequently Asked Questions
Can a continuous PU sandwich panel line run both color steel and aluminum foil facings?
Yes. The line must be equipped for the coil widths and material thicknesses you intend to run. Facing changes affect preheating settings and belt temperature. The polyurethane system stays largely the same even though adhesion and surface finish may require different production parameters.
What is the difference between continuous and batch panel production?
Continuous production moves the facings through roll forming, pouring, lamination, and cutting as a single flow. Batch production fills clamped molds panel by panel. Continuous suits high-volume standard panels; batch suits prototypes, non-standard sizes, and laboratory sampling.
How is fire performance of PU/PIR sandwich panels classified?
Fire classification is evaluated according to EN 13501-1 on the assembled product or representative construction, not on the core alone [3]. The facing, core type, joint geometry, and installation details all influence the final class. Buyers should specify the end-use fire requirement before the formulation is locked.
What causes poor foam adhesion to the metal facing?
Low facing temperature, surface contamination, insufficient preheating, low surface reactivity, or low metering accuracy at the edges can all create a weak boundary layer. Adhesion failures are usually process-related before they are formulation-related.
How do you keep panel density consistent across the width?
A stable traversing pattern, matched line speed, balanced mixing pressure, and consistent raw material temperature all help. Calibrate the metering pumps with actual material at process temperature, not just with calibration oil, and check the fan pattern whenever facings or line speed change.
References
[1] European Committee for Standardization. EN 14509:2013, Self-supporting double skin metal faced insulating panels — Factory made products — Specifications. CEN, 2013.
[2] International Organization for Standardization. ISO 8301:1991, Thermal insulation — Determination of steady-state thermal resistance and related properties — Heat flow meter apparatus. ISO, 1991.
[3] European Committee for Standardization. EN 13501-1:2018, Fire classification of construction products and building elements — Part 1: Classification using data from reaction to fire tests. CEN, 2018.
[4] Standardization Administration of China. GB/T 23932-2009, Double skin metal faced insulating panels for building. SAC, 2009.
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