For many small-scale enterprises, although the continuous production line of polyurethane flexible foam offers high output, the costs are also very high, and the target market may not require such large quantities. As a result, non-continuous production lines for polyurethane flexible foam have become their preferable choice. The following is an introduction to the non-continuous production line for polyurethane flexible foam:

1. Box Foaming Process Equipment

The box foaming process and equipment have been developed as a new technology to accommodate the needs of small-scale polyurethane foam production facilities. It builds upon laboratory and manual foam production techniques, essentially an upscaled version of laboratory foam methods. This process has gone through three development stages. Initially, all component materials were sequentially weighed and added to a larger container, followed by the addition of TDI. After rapid mixing, the mixture was immediately poured into a large box mold. This method had high labor intensity, emitted high concentrations of toxic gases, and posed significant health risks to operators. Additionally, the splattering of materials during pouring would entrain a large amount of air, leading to the formation of large air bubbles within the foam structure and even causing foam cracking. Furthermore, there was a significant amount of leftover waste, resulting in substantial material waste and high production costs.

Later on, the process incorporated metering pumps to convey materials to a mixing barrel with an automatically opening bottom. After high-speed mixing, the bottom plate of the mixing barrel would open, and compressed air would swiftly expel the material into the mold for foam expansion. However, this approach suffered from uneven foam pore structures due to the rapid material flow, leading to swirling foam structures and quality issues like crescent-shaped cracks. The third stage of process improvement is the box foaming device that is mostly adopted today. Its fundamental foaming principle is illustrated in Picture



(a) Raw Material Metering and Mixing (b) Foaming (c) Foam Rises to Limit Height

1 - Elevatable Material Mixing Barrel; 2 - Assemblable Box Mold; 3 - Floating Box Top Plate; 4 - Foam Body

Picture 1: Schematic Diagram of Box Foaming Principle

The industrial production equipment for box foaming primarily consists of raw material tanks, metering pump units, elevatable mixing barrels, and assemblable wooden box molds. As depicted in the schematic diagram of the box foaming equipment manufactured by Hennecke (Picture 2), the foaming raw materials are stored in tanks and regulated by control devices to attain the required processing temperature range, typically maintained at 23°C ± 3°C. Sequentially, the metering pump injects polyether polyols, catalyst, surfactants, foaming agents, etc., into the mixing barrel for a stirring duration of 30 to 60 minutes. Next, according to the formulation, TDI is introduced, either directly or through an intermediate container with a bottom switch. Immediate mixing follows TDI addition. Depending on the materials and formulation, the stirring speed is usually controlled at 900 to 1000 revolutions per minute (r/min), with a stirring time of 3 to 8 seconds. After stirring, the mixing barrel is swiftly lifted. The lower part of the barrel lacks a bottom and is placed on the mold box's bottom plate upon lowering, utilizing a sealing ring at the barrel's bottom edge to prevent material leakage.

When lifted, the well-mixed slurry can be directly spread and dispersed on the bottom plate of the box mold, allowing natural foam rise. To prevent the formation of a domed surface on the upper part during foaming, an upper mold plate that matches the mold area and allows for upward limit movement is equipped. The mold box primarily comprises rigid wooden panels, with the bottom plate fixed on a movable mold transport carriage. All four side panels are assemblable, featuring quick-opening and closing locking mechanisms. The inner sides of the panels are coated with silicone-based release agents or lined with polyethylene film material to prevent adhesion. After 8 to 10 minutes of forced maturation within the box, the side panels of the mold box are opened, allowing the removal of block-shaped flexible foam. Following an additional 24 hours of maturation, these foam blocks can undergo cutting and other post-processing procedures.





1 - Raw Material Tank; 2 - Metering Pump Unit; 3 - Control Cabinet; 4 - Mixing Barrel with Elevating Device; 5 - Foaming Box; 6 - Foam Finished Product; 7 - Floating Plate

Picture 2: Box Foaming Equipment Manufactured by Hennecke (BFM100/BFM150)

Box foaming process and equipment exhibit characteristics such as simple operation, compact and straightforward equipment structure, low investment, small footprint, and convenient maintenance. These features make it particularly suitable for small enterprises engaged in intermittent production of low-density block foam. However, its drawbacks are also quite evident: lower production efficiency, less favorable production environment, high concentration of emitted toxic gases on-site, necessitating the use of highly effective exhaust and toxic gas purification systems.



To enhance mixing efficiency, some companies have added several vertical and equidistant baffles to the inner walls of the mixing barrel. These baffles, combined with high-speed spiral-type agitators, facilitate high-speed mixing. This approach can to a certain extent reduce laminar flow effects in the mixing liquid and improve mixing efficiency. An example of this is our product, the SAB-BF3302. For the product's appearance and technical specifications, please refer to Picture 3.



Picture 3: Fully Automatic Box Foaming Machine (Sabtech Technology Limited)

This production line comes with both fully automatic computer control and manual control modes. It's suitable for producing flexible polyurethane foam with densities ranging from 10 to 60 kg/cm. Maximum foam output: 180L. Foam height: 1200mm. Mixing power: 7.5kW. Total power: 35kW.

2. Equipment for Open-Cell Foam Preparation

Open-cell polyurethane foam is a functional foam product developed in the 1980s. It possesses a high porosity, a distinct network structure, softness, breathability, and good mechanical strength. It finds wide application as excellent filtration and shock-absorption material in transportation, instrumentation, medical material filtration membranes, and as catalyst carriers in the chemical industry. Filling it into aircraft fuel tanks can suppress oil agitation and reduce the risk of explosions. Impregnating it with ceramic slurry and high-temperature sintering results in a novel open-cell ceramic filter material used in the metallurgical industry.

The preparation of open-cell polyurethane foam involves methods such as steam hydrolysis, alkaline soaking, and explosion. In industrial production, the explosion method is predominantly used. Initially, polyurethane foam of a specific pore size is prepared using the box foaming process. Subsequently, it's placed in dedicated explosion network equipment, filled with explosive gas, and ignited after completely filling the foam body. By utilizing the impact energy and high-temperature heat generated by the explosion parameters, the cell walls of the polyurethane foam are ruptured and fused onto the cell walls, forming a distinct network structure, as shown in Picture 4.



Picture 4: Clearly Networked Open-Cell Foam

Methods like steam hydrolysis or alkaline soaking are used to prepare open-cell foam. However, there are issues of low efficiency, poor quality, and environmental pollution with these methods. They are mainly employed for small-scale production such as laboratory sample testing. Large-scale production primarily uses the explosion method.



ATL Schubs GmbH, a German company, specializes in the research and development of polyurethane reticulated foam and manufactures the ReticulatusTM foam explosion machinery. The explosion chamber of the reticulated foam explosion equipment comes in two forms: cylindrical and rectangular. The former is suitable for cylindrical foam, while the latter is more versatile. It can be used not only for square foam but also for processing reticulated foam from cylindrical foam, as shown in Picture 5. The explosion chamber is constructed from high-quality 100mm-thick steel plates. Operation is controlled by a computer modem, offering features like automatic opening and closing, automatic locking, automatic operation, and automatic alerts. Additionally, remote program design and modification can be facilitated through data transmission sensors.



Picture 5: Polyurethane Foam Reticulation Processing Equipment (ATL Schubs)

During production, foam bodies measuring 3 to 6 meters in length that are intended for reticulation are pushed into the explosion chamber. The chamber's door is closed hydraulically, and the air inside the chamber is evacuated using a vacuum pump. Under computer control, a precise proportion of oxygen and hydrogen gases is introduced, and the gas mixture's ratio is mechanically adjusted based on factors such as foam sample type and network size requirements.

Sensors continuously monitor the process, ensuring that all process parameters are within the specified conditions before controlled detonation is initiated. The explosive force and flame intensity generated by the explosion penetrate through the entire foam body, creating a distinct network structure. After forming, the foam body is cooled, residual materials and waste gases are purged using nitrogen, and the pressure chamber can then be opened to retrieve the reticulated foam. The entire process takes approximately 8 to 10 minutes. The pore diameter of the reticulated foam falls within the range of 10 to 100 pores per inch (ppi) (Note: ppi refers to the number of pores within one inch).

The above provides some insight into the non-continuous production process of polyurethane flexible foam. I hope this information proves helpful to you.

There are many reasons for the cracking of polyurethane foam, including both chemical and mechanical factors. During production, it is important to stop and inspect, making sure to eliminate the phenomenon of cracking. Below are some insights we have summarized based on years of production experience, hoping to inspire everyone.

1. Sudden Changes

Sudden changes in the speed of the conveyor, variations in catalyst usage, or irregular operation of the conveyor can cause the foam to crack. Carefully and gradually adjust the above situations. A significant change in the speed of the conveyor can lead to large cracks in the foam blocks.

2. Polyethylene Film

If the film stops moving for some reason, the foam will come into contact with the static surface, leading to cracking. If this situation occurs, check the winding roller of the rewinder and examine the separation of the film in the curing area.

3. Foam Forming Around Certain Substances

Foam tends to form around certain substances, especially in the feed chute, which can easily cause cracking.

Before production, ensure there are no residues or debris left in the assembly of the mixing head, feeding hose, and discharge chute.

4. Excessive Amine

When there is insufficient stannous octoate, excessive amine can lead to the same effect and accelerate the rise time. Therefore, the dosage of amine must be reduced.

5. Insufficient Stannous Octoate

Due to the faster foaming reaction compared to the polymerization reaction, some foam may flow, leading to cracks. Therefore, the amount of stannous octoate must be increased.

6. Silicone Oil

Foaming becomes unstable and prone to rupture, potentially forming "depressions" on the foam, so it is necessary to increase the dosage of silicone oil.

7. Structure of Small Air Bubbles

Thin foam walls due to foaming can cause cracking. To address this comprehensively: reduce the air content, decrease the speed of the mixing head, or increase the pressure of the mixing head. It is also necessary to change the dosage of stannous octoate catalyst.

1. Polyether

Polyether, as the main raw material, reacts with isocyanate to form urethane, which is the skeletal reaction of foam products. When the molecular weight increases with the same functionality, the tensile strength, elongation, and resilience of the foam increase, while the reaction activity of similar polyethers decreases. With the same equivalent value (molecular weight/functionality), an increase in functionality accelerates the reaction, increases the cross-linking degree of polyurethane, raises foam hardness, and reduces elongation. The average functionality of polyols should be above 2.5; if it is too low, the recovery of the foam body after compression is poor.



If the amount of polyether used is high, equivalent to a reduction in other materials (TDI, water, catalysts, etc.), it is easy to cause foam products to crack or collapse. If the amount of polyether used is low, the foam product tends to be hard, with reduced elasticity and poor touch.



2. Foaming Agent

Generally, when producing polyurethane blocks with a density greater than 21, only water (chemical foaming agent) is used as the foaming agent. Low-boiling compounds such as methylene chloride (MC) are used as auxiliary foaming agents in low-density formulas or ultra-soft formulas.



Auxiliary foaming agents reduce the density and hardness of the foam. Since they absorb some of the reaction heat, curing is slowed down, requiring an increase in the amount of catalyst. By absorbing heat, the danger of core burning is avoided.



The foaming capacity can be expressed by the foaming index (the number of parts of water or equivalent number of water used for 100 parts of polyether):

IF = m (water) + m (F-11) / 10 + m (MC) / 9 (100 parts of polyether)



Water, as a foaming agent, reacts with isocyanate to form urea bonds and releases a large amount of CO2 and heat. It is a chain-growth reaction. Excess water reduces foam density and increases hardness. However, it also reduces the size and strength of foam pores, reducing their load-bearing capacity, making them prone to collapse or cracking. Increased TDI consumption leads to more heat release and a higher risk of core burning. If the water amount exceeds 5.0 parts, physical foaming agents must be added to absorb some of the heat and prevent core burning. Less water means a corresponding reduction in the amount of catalyst used, but it increases density.



3. Catalyst

Amine: A33 is generally used, which promotes the reaction between isocyanate and water, adjusting foam density, bubble opening rate, etc., mainly promoting foaming reactions.

Too much amine: The foam product cracks, and there are holes or bubbles in the foam; Too little amine: The foam shrinks, closes pores, and the bottom of the foam product becomes thick.



Tin: Typically, Tin(II) octoate (T-9) is used; Tin(IV) oxide (T-19) is a highly active gel reaction catalyst, mainly promoting the gel reaction, i.e., the later stage reaction.

Too much tin: Fast gelation, increased viscosity, poor resilience, poor air permeability, leading to closed-cell phenomenon. Properly increasing its dosage can obtain good open-cell foam plastics with relaxation, further increasing the dosage makes the foam gradually become denser, leading to shrinkage and closed cells.

Too little tin: Insufficient gelation, resulting in cracking during foaming. There may be cracks on the edges or tops, with burrs and poor consolidation. Reducing amine or increasing tin can increase the strength of the polymer foam film when a large amount of gas is generated, thereby reducing hollow or cracking phenomena.



Whether polyurethane foam plastics have an ideal open or closed cell structure mainly depends on whether the gel reaction rate and gas expansion rate are balanced during foam formation. This balance can be achieved by adjusting the types and amounts of tertiary amine catalysts and foam stabilizers in the formula.



4. Foam Stabilizer (Silicone Oil)

Foam stabilizers are a type of surfactant that disperses polyurea well in the foaming system, acting as "physical cross-linking points" and significantly increasing the early viscosity of the foam mixture, preventing cracking. On the one hand, it has an emulsifying effect, enhancing the miscibility between foam material components. On the other hand, the addition of organic silicon surfactants can reduce the liquid's surface tension, reduce the free energy required for gas dispersion, make the dispersed air in the raw materials nucleate more easily during stirring and mixing, facilitate the production of fine bubbles, adjust the size of foam pores, control the foam cell structure, and improve foaming stability. It prevents collapsed or burst bubbles, makes the foam walls elastic, controls the pore size and uniformity of the foam. Generally, the more foaming agent and POP used, the greater the amount of silicone oil used.



High usage: Increases the elasticity of foam walls in the later stage, making them less likely to burst, resulting in smaller pores and closed cells.

Low usage: Foam bursts, collapses after foaming, larger pore size, and easy co-foaming.



5. Temperature Influence

The foaming reaction of polyurethane accelerates with an increase in material temperature, which can pose a risk of core burning and ignition in sensitive formulations. Generally, the temperatures of the polyol and isocyanate components are kept constant. When foaming, the foam density decreases as the temperature of the material increases. With the same formula, if the temperature remains the same but the ambient temperature is high in summer, the reaction speed increases, leading to a decrease in foam density and hardness, an increase in elongation, and an increase in mechanical strength. In summer, the TDI index can be appropriately increased to correct the decrease in hardness.



6. Influence of Air Humidity

With increasing humidity, the isocyanate in the foam reacts with the moisture in the air, causing a decrease in hardness. Therefore, when foaming, the TDI amount can be appropriately increased. Excessive humidity can cause the curing temperature to rise too high, leading to core burning.



7. Influence of Atmospheric Pressure

With the same formula, foaming in high-altitude areas results in lower foam product density.

Beginners are concerned that if the settling plate is not adjusted properly, the liquid flowing out of the nozzle may cause front surging or back surging, affecting the foaming process. Within two minutes after starting the machine, the reaction speed gradually increases, sometimes requiring adjustments to the settling plate. Adjustments to the settling plate are more critical in low-density and high-moisture-content (MC) formulas.



TDI (Toluene Diisocyanate) flow rate can be calculated to correspond to the scale value, but it is recommended to actually measure the TDI flow rate during the first foaming. Flow rate is too important; if the flow rate is not accurate, everything else will be a mess. It's best to rely on the simplest and most intuitive method of measuring the flow rate.



When mixing powders, the mixed stone powder should be left overnight and production should start the next day. For ingredients containing melamine and stone powder, it is recommended to first mix melamine with polyether for a period of time before adding the stone powder.



Foam machine formulas with long mixing chamber in the machine head or more teeth on the stirring shaft usually have less amine and lower material temperature. Conversely, foam machine formulas with short mixing chamber in the machine head or fewer teeth on the stirring shaft usually have more amine and higher material temperature.



For the same formula, when switching between dual-spray swivel heads and single-spray swivel heads with similar nozzle cross-sectional areas, the requirements for mesh thickness and layers are similar.



For the calibration of minor material flow, one method is to measure the return flow of the minor material, and the other is to calibrate it by dividing the total amount used by the foaming time. When there is a significant difference between the two calibration methods, rely on the data from the second calibration method.



Formulas for high-quality soft foam are usually within an unstable range, such as a low TDI index, low water-to-MC ratio, low T-9 dosage, and low silicone oil dosage.