The vertical foam production process for continuous production of PU soft foam, known as the vertical foam process, is a new technology developed and patented by Hymon Corporation in the United Kingdom in the 1980s. The rapid development and attention garnered by the vertical foam process in the polyurethane industry are closely tied to its outstanding advantages.
Key features of this process include:
1. Significant reduction in floor space, requiring only 600 square meters.
2.Material total flow reduced to 20-40 kg/min.
3.The same equipment can produce both round and rectangular foam blocks, with only partial component replacement needed for alternating production.
4.Regularly shaped foam blocks with waste trim reduced to 4%-6%.
5.Uniform foam physical properties across the same cross-section.
6.Reduced start/stop losses, with defective lengths around 1 meter, making it suitable for small to medium-sized factories with an annual production of 500-4000 tons. Additionally, the investment cost is low, and labor is saved.
The vertical foam process involves several steps, including raw material storage, metering, mixing, input, foaming, curing, foam lifting, cutting, and foam conveying.
The specific operation involves measuring various chemical raw materials with metering pumps, inputting them into a mixing head, and then sending them to the foaming tray to initiate the foaming process. The foaming process occurs inside a bag formed by polyethylene film. As the cross-sectional area increases, the foaming direction along the long axis with foam holes occurs horizontally. In the foaming chamber, the density gradually decreases from bottom to top, creating a density gradient. Due to gravity, the material layers undergoing foaming do not mix between layers. Therefore, the influence of gravity on the same cross-section is equivalent, resulting in consistent physical properties on the same cross-section. The foamed foam enters the curing section, where a heating cylinder with insulation promotes rapid curing. Before foaming, the heating cylinder is preheated to around 65°C. After normal operation, heating can be stopped, and the temperature can be maintained by foaming heat.
The cured foam then enters the lifting section, composed of strips forming a conveyor belt with small needles (approximately 1.5 cm in length) on them. The small needles pierce the outer skin of the foam, continuously lifting it upwards, forming a circle with ten conveyor strips. The cutting saw, according to a predetermined length, influences many factors of the vertical foam PU block foam process. Still, the most important is to maintain the balance and stability of the formulation during the foaming machine’s operation. Continuous monitoring of raw material temperature, curing temperature, ambient temperature, material flow rates, pressures, mixing factors (stirring speed, mixing head pressure, air injection), catalyst usage, etc., is essential. Adjustments and controls should be made in a timely manner during the foaming process to produce high-quality products.
During the operation of the vertical foaming machine, some abnormal phenomena commonly occur and should be promptly addressed to ensure normal production.
The most commonly encountered issues include:
1.Foam “tightening” or contraction: During foaming, if the foam tightens or closes before the foam rises to its highest point, it indicates excessive tin catalyst or too much foam stabilizer, leading to solid gel on the pore walls.
2.Foam rupture: Mainly caused by formula errors, errors in various metering pumps resulting in inaccurate formulations, and insufficient or degraded tin catalyst and foam stabilizer.
3.Foam slipping: Occurs when the needle plate conveyor cannot grip the foam, often during the removal of the “starting ring.” Various factors, such as prolonged rising time, too low curing section temperature, incomplete foam filling of the entire cross-section, high closure of foam, or too fast conveyor belt speed, can cause foam slipping.
4.Compression lines: Refers to distinct white interfaces inside foam blocks, appearing as white lines on the cut cross-section. These white lines indicate poor foam strength in the affected area and result in substandard products. Compression lines can occur when the conveyor belt speed is too slow. To eliminate compression lines, either increase the conveyor belt speed while maintaining a constant discharge rate or gradually increase the conveyor belt speed. The method to eliminate compression lines depends on estimating how far the foam enters the curing section when fully risen. If it enters the curing section soon after full rise, it’s best to increase the conveyor belt speed. If it has entered the curing section far away when fully risen, it’s best to decrease the discharge rate while maintaining the conveyor belt speed constant.