Sandwich panel production principles
Manufacturers of polyurethane foam products generally receive liquid components – polyols and polyisocyanates – from their suppliers, and then convert these components into polyurethane through a chemical reaction. The process takes place in an apparatus in which, in addition to the polyol and polyisocyanate, special additives are added according to a specific formula, providing the desired final chemical mixture.
Today, there are different processing techniques, often related to the use of different equipment. One of the most commonly used for large-scale industrial production is called reaction injection molding (RIM), and is based on the use of a high-pressure foaming unit. Through this process it is possible to obtain microcellular, microporous and therefore almost solid foams in extremely short cycle times.
Reaction Injection Molding (RIM) process
Dosing and mixing technology
The two main components, which are polyol, polyisocyanate, and all additives, stabilizers, flame retardants, pigments and the like, which are essential for the foaming reaction, are contained in storage tanks and are transferred just before production to containers called feed tanks (or day tanks). The feed tanks are equipped with an efficient temperature control system, since temperature fluctuation, especially of the polyol temperature, results in viscosity variation which, in turn, can cause problems in the subsequent production phase. For this reason, feed tanks generally have a double-walled structure and are equipped with stirrer and temperature control circuits capable of ensuring that the desired process temperature is maintained constant.
Dosing pumps, supplied by the feed tanks, deliver these components in the desired proportions from the day tanks to the foaming head. This mixture is then dispensed by the foaming head of an open mold or into the cavity of a closed mold. After the reaction time has elapsed, the finished product is removed from the mold.
In a high-pressure foaming unit, prior to each mixing and filling cycle, the components are continuously recirculated in the required proportions and at the required pressure for injection, with electronically controlled actuators capable of diverting the flow from the recirculation circuit to the injection circuit and then back to the recirculation circuit.
Pumps
The dosing of the components in the desired ratios must be reproducible and must take place with a high degree of accuracy through the use of high-precision pumps. Distribution rates of 12÷150 liters/min can be obtained with axial piston pumps, with a low noise level. A noticeable limitation of axial piston pumps is noted when the liquids to be pumped have high viscosities. In addition, there must be no abrasive fillers in the liquid components.
Foaming head
The most important equipment in an injection and reaction molding machine is the foaming head. It should be noted that the extremely advanced method of mixing the components by high-pressure shock has made very short production times possible in the production of polyurethane foams. The velocities of the components through the injection nozzles vary from 100 to 150 m/s. The liquid components are injected through the orifices into the mixing chamber, where they mix intimately with each other due to their kinetic energy.
In addition, the high-pressure foaming heads are equipped with a component recirculation system capable of blocking the distribution of the polyurethane mixture or precisely synchronizing it in a discontinuous production process. When the foaming head is in the recirculation position, the components flow to the feed tanks through the small ducts that are left open by the piston movement.
Once the piston is removed, the mixing chamber opens and the components collide with each other.
When the control piston advances again, mixing is interrupted, recirculation is restored and the remaining reaction mixture is removed.
Production facilities
Essentially, a production system includes the injection molding and reaction machine (feed tanks, dosing/recirculation pumps, foaming head, etc.), a mold and, if necessary, additional equipment. In addition, sandwich panel production systems are classified as continuous or discontinuous (cast-in-mold).
Discontinuous production systems – In-mold molding
In in-mold molding, the panel is made in a closed mold whose dimensions are those of the finished panel. The molds generally have a solid bottom structure and a closing lid. In addition, they must have a robust construction since very high pressures are produced during the solidification phase of the foam.
Before foaming, the metal surfaces (already profiled) are placed in the mold; the lower surface is spread on the bottom of the mold while the upper surface is placed in position and supported by suitable side supports.
The mold lid is then lowered and closed on the lower structure. At this point, an exact amount of foam is sprayed into the cavity through a nozzle on the side of the mold; this operation requires only a few seconds to complete.
Once foaming is complete, the panel is left in the mold for about 40 minutes, and then the mold can be removed and prepared for the production of the next panel.
The advantage of this method is that complicated shaped panels can be produced, the external appearance of the surfaces is improved and alternative starting materials can also be used; the main disadvantage lies in the fact that the process is relatively slow, which negatively influences the production rate.
Continuous production systems
For large-scale production, automatic continuous foaming lines are used. Two metal sheets, which will form the panel surfaces, are obtained from the rotating movement of the so-called “coils” and pass through the forming rollers capable of reproducing the surface profile and edge details.
These are then heated to a temperature of ~40°C, which is a prerequisite for optimum adhesion of the foam to the metal surfaces. The reaction mixture, produced by the high pressure foaming machine, is distributed by an oscillating movement on the bottom surface, before the sheet enters the double belt press.
The double belt press is a movable mold that withstands the pressures that develop during the hardening phase of the foam and keeps the two surfaces at the required distance (Fig. 3.14). It is in the double tape press that the foam is adhered to the profiled metal top surface. Indeed, PUR and PIR foams are very active in terms of adhesion and adhere strongly to the surfaces with which they come into contact.
When the continuous panel emerges from the double band press, the foam is hardened and can be cut to the required length with a band saw.
Each of the panels is sent to the stacking machine (Fig. 3.15), where they are overlapped in the best possible configuration to have a package with an acceptable height.
In general, the cover panels, which are characterized by a strongly profiled outer surface, are stacked so that their outer surface is in contact with the outer surface of a panel on one side, and their inner surface is in contact with the inner surface of another panel on the opposite side
The stacked panels are then packaged for shipment using a polyethylene stretch film, which is wound around the panel bundle with a rotating ring winder.
This facility can produce approximately 500,000 m2 of sandwich panels in a single shift at an average speed of 6 m/min.
Production speeds vary in general from 2 to 15 m/min; the production speed depends on the final thickness of the sandwich panel to be produced, since the higher the required thickness of the panel, the longer the panel must remain inside the double belt press for a complete adhesion of the foam, and therefore the lower the speed of the production line must be.
When the insulating material consists of mineral wool (or another type of rigid resin), the composite panel is produced following essentially the same process used for the production of polyurethane and polyisocyanurate foams. In this case, a simple solution is to cut the slab into strips (sheets) at right angles to the orientation of the fibers and having a width equal to the desired thickness of the panel. These strips are then rotated 90 degrees so that they can be assembled to form panels with the fibers oriented normally on the outer surfaces, as shown in Fig. 3.20 and 3.21. This technique provides the composite panel with the desired final compressive and tensile strength characteristics. In any case, a new technique currently being developed and used is based on the use of the entire mineral wool slab as an insulating material.
The mineral wool sheets are worn off-center on the metal surfaces. The offset is necessary to avoid having the shear planes of each of the sheets all in the same cross section of the panel, which would result in a significant deterioration of the shear strength characteristics of the panel itself.
The mineral wool sheets and the surfaces are then bonded with a suitable adhesive substance, which is chosen taking into account the composition of the materials to be bonded and the production process.
Two different types of adhesives are used:
solvent-based adhesives are applied to the two surfaces to be bonded with sprayers. These have good adhesion characteristics and the curing time can be further reduced by applying a slight pressure and temperature range. These adhesives have the advantage of being easy to handle. Its disadvantage is that the relative position of the bonded layers cannot be corrected;
adhesives based on epoxy or polyurethane resin, are obtained from two components mixed in situ. After a preset time, they improvisedly react and harden rapidly. The advantage of these adhesives is the possibility of correcting the position of the layers to be bonded. The disadvantage lies in the fact that they must be held in the chosen position under pressure for a certain period of time.
The bonding phase is carried out in the passage of the continuous panel through the double belt press. Sandwich panels produced in this way are also characterized by significant fire resistance.
