The forming methods of rigid foam mainly include pouring method and spraying method.
Plate production technology and matters needing attention
Rigid polyurethane foam plastic sheet has light weight, high strength and good thermal insulation effect, and is an ideal building material, which is used for roof thermal insulation, cold storage and so on. This kind of board is mostly covered by surface layer, so it is a sandwich board or composite board with hard foam as the core material. The surface layer can be made of soft materials and hard materials. Soft materials include kraft paper, polyethylene coated kraft paper, asphalt paper, glass fiber fabric, asphalt glass fiber fabric, aluminum foil, etc. Hard surface materials include steel plates (flat plates and profiles, wood plywood, rough cardboard, gypsum board, reinforced cement board, mineral wool board, perlite board, gypsum board, etc.
The processing methods of composite plate can be roughly divided into continuous composite forming method, discontinuous composite forming method, hard foam block cutting method and so on.
Continuous method is an advanced method with low labor cost and energy saving.
The production of polyurethane rigid foam sandwich panel is mainly divided into two ways:
1. Intermittent production
2. Continuous production
Most processes of batch production adopt single-hole one-time injection molding.
It is suitable for processing thick, large and complex plates with low productivity.
Production technology: pre-assembly → template entering laminator → template locking → pouring → mold heating curing → mold opening → parts cleaning → transfer to the next cycle.
Product specifications:
Length:1.8m-12m thickness: 80mm—300mm.
Production of continuous building sandwich panel (steel plate sandwich panel)
Production technology: preheating
Open-book molding, pouring PU (pouring raw materials between the upper and lower layers through a distribution pipe), molding (curing and rolling molding with a constant-temperature double-track press), cutting (cutting and molding the cured and molded plate with a cutting unit), and packaging and warehousing.
In the process of continuous production, the process of PU casting is more important. PU casting in different production lines is generally divided into: two-component casting (combined polyol+isocyanate); Four-component casting (polyol+foaming agent+catalyst+isocyanate)
Potential factors affecting foam quality
raw material
Reaction time: too fast, there will be more bubbles on the foam surface, and the adhesion will become worse; If it is too slow, the demoulding time will be too long, the production efficiency will be reduced and the adhesion will become worse.
Fluidity: influences the density distribution of foam and waste gas.
Foaming machine
Measurement: If the measurement is not standard, the foam will be too soft or too brittle.
Feed temperature: too high, the reaction will be faster, the feed fluidity will become worse, and there will be many bubbles; If it is too low, the reaction will be slow and the demoulding time will be prolonged.
Fixture temperature: 35-45℃ is too low, and the adhesion becomes worse; If it is too high, the reaction will be faster, the material fluidity will become worse, and there will be more bubbles.
Ambient temperature: too low temperature, slow reaction and long demoulding time.
Surface material: the surface treatment is not good or clean or there is water vapor, and the adhesion is poor. The surface should be dry and free of frost, ice, grease, oil and loose debris and dust. There must be no solvent on the surface. If the primer has been laid, there must be enough time for it to dry thoroughly.
Demoulding time: too short will cause deformation.
Others: If the exhaust is not good, there will be more bubbles on the foam surface.
Spraying rigid polyurethane foam
The sprayed foam directly adheres to the substrate to form a continuous seamless coating.
Foam quality
When the mixture leaves the nozzle of the spray gun, a fan-shaped atomization effect should be formed. The humidity and temperature of air and the temperature of the substrate itself have great influence on the final quality of foam.
temperature
When the foam is sprayed on the frozen substrate (i.e., below 10℃), the cold surface may play the role of heat dissipation, absorbing part of the heat energy needed to completely evaporate the foaming agent, and reducing the polymerization rate. It leads to the formation of a thick and brittle foam skin on the surface, which leads to poor adhesion and the formation of foam bubbles in the later stage.
If the foam is sprayed on a hot substrate, the foam density will increase due to the loss of foaming agent, and so-called pinholes will be formed. Therefore, for the substrate with a temperature as high as 50℃, it is necessary to choose a special system with a long milky white period and increase the amount of foaming agent if necessary.
Moisture/humidity
The most important thing is to spray polyurethane foam on the dry surface. Moisture will react with isocyanate components, resulting in low adhesion between foam and substrate and brittle skin.
Spraying conditions
Outdoor spraying should not be carried out under the following circumstances:
rain
thick fog
blow
Visible water droplets
Humidity exceeds 80%
The surface water content exceeds 10% (measured by hygrometer)
wind
If the wind speed exceeds 25 km/h, it is difficult to spray a uniform foam layer. The cooling effect of wind will reduce the surface quality of foam, because the surface layer can not be cured normally. Construction in a cold climate may be a problem.
Operator skill
Can produce qualified foam surface texture, that is, smooth surface and uniform thickness. Operators should receive comprehensive training. These two characteristics have a great influence on the cost, and then on the profit.
Adjustment of equipment
If the temperature is slightly lower than the recommended temperature, the pressure of each component can be adjusted. Excessive isocyanate will form brittle or brittle foam, which is characterized by dark black color and smooth and hard structure.
If the polyol is excessive, soft or spongy foam with light color, "coarse orange peel" texture and pinholes will be formed.