In December 2021, we received an inquiry from Mr. Hairun in Malaysia. Mr. Hairun is a mattress manufacturer in need of producing rebonded foam. He had limited knowledge about machine usage and selection and had no prior experience with the production process. Therefore, he required guidance from experts who could assist him from the ground up.



We systematically explained the principles of foam production to Mr. Hairun, along with the necessary materials and equipment. We also took him on a tour of our factory to provide a clear understanding of the entire production process.



After understanding Mr. Hairun's preferences for the rebonded foam, including density, softness, and market prices, we offered him the most suitable foam production solution. We also provided him with information on foam production costs and compared raw material prices for his reference.



Based on the client's needs, budget, and existing factory layout, we devised a cost-effective machine configuration and layout plan for his facility, including an assessment of startup costs.



Once the machines were successfully installed, our team of engineers provided Mr. Hairun with one-on-one foam production training. When he successfully produced the foam he desired for the first time, he called us and said, "I am happy with crying, thank you very much!" Afterward, he purchased a batch foam machine from us and continued to reorder foam chemical materials from our company.

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. Basic Reactions



The formation of polyurethane foam involves two basic reactions: foaming reaction and polymerization reaction (also called gel reaction).



Foaming reaction: Isocyanate reacts with water to produce a reaction of di-substituted urea and carbon dioxide. The reaction equation is as follows:



2R-N=C=O + HOH → R-NH-CO-NH-R + CO2↑



The released carbon dioxide acts as the bubble core, causing the reaction mixture to expand, resulting in foam with an open-cell structure.



Polymerization reaction: The hydroxyl group in the polyether undergoes a stepwise polymerization reaction with isocyanate to form an aminoformate. The reaction equation is as follows:



R=N=C=O + R′-OH → R-NH-COO—R′



2. Polyols



Domestic block foam production uses 3-functionality, molecular weight 3000 (hydroxyl value 56) or 3500 (hydroxyl value 48, less commonly used) soft foam polyethers.



3. Polyisocyanates



The main polyisocyanate used is toluene diisocyanate (TDI). There are three main types of TDI industrial products: pure 2,4-TDI (or TDI100), TDI80/20, and TDI65/35. TDI80/20 has the lowest production cost and is the most widely used variety in industrial applications.



The molecular weight of TDI is 174, with two isocyanate groups (-N=C=O) having a molecular weight of 84. Therefore, the isocyanate content in TDI is 48.28%.



The amount of TDI used has a significant impact on foam properties. In foam formulations, the excess of TDI is expressed as the isocyanate index, which is the ratio of actual usage to theoretical calculated amount. When producing soft foam, the index is generally 105-115 (100 is equal to the theoretical calculated amount). Within this range, as the TDI index increases, the foam hardness increases, tear strength decreases, tensile strength decreases, and elongation at break decreases. If the TDI index is too high, it can lead to large and closed cells, long maturation times, and foam burning; if the TDI index is too low, it may lead to cracking, poor rebound, low strength, and significant compression permanent deformation.



4. Blowing Agents



Water reacting with TDI to produce carbon dioxide is the main blowing agent used in soft foam foaming. Increasing the amount of water in the formulation will increase the urea content, increase foam hardness, decrease foam density, and reduce foam load-bearing capacity. However, TDI reacts with water to produce a large amount of heat. If the water content is too high, it can cause the foam to burn or ignite.



Methylene chloride is a physical blowing agent with a boiling point of 39.8°C. It is a non-flammable gas that can vaporize during foaming, reducing foam density and hardness. The amount of methylene chloride added should prevent the foam from burning while ensuring that too much does not remove too much heat, affecting foam curing. The amount of methylene chloride used is limited.



5. Catalysts



The main role of catalysts is to adjust the speed of foaming and gel reactions to achieve a good balance.



Triethylenediamine (A33, a 33% solution of diisopropyl ether or dipropylene glycol) is the most important tertiary amine catalyst in soft foam production. It is 60% effective in promoting the reaction between isocyanate and water, i.e., foaming reaction, and 40% effective in promoting the reaction between hydroxyl and isocyanate, i.e., gel reaction.



Dibutyltin dilaurate (A-1) is a general-purpose tertiary amine catalyst for soft foam. It is 80% effective in promoting foaming reaction and 20% effective in promoting gel reaction. It is often used in combination with triethylenediamine.



Improper use of amine catalysts can have a significant impact on the product. Too much amine can cause:



(1) Short reaction time, rapid increase in initial viscosity, and excessive smoking during foaming.

(2) Foam cracking. Too little amine will result in slow initiation speed, affecting foam height.



Dibutyltin dilaurate is the most commonly used organic tin catalyst, which is very easy to hydrolyze and oxidize in the presence of water and tertiary amine catalysts in polyether mixtures.



The lower the foam density, the narrower the adjustable range of dibutyltin dilaurate. The effect of tin dosage on foam is as follows:



Too little dosage: Foam cracking.



Too much dosage: Rapid increase in viscosity, foam forming closed cells and shrinking, forming skins on the top and sides.



6. Foam Stabilizers (also called Silicone Oils)



Foam stabilizers reduce the surface tension of the foam system mixture, thereby stabilizing the bubbles, preventing foam collapse, and controlling the size and uniformity of voids.



Increasing the amount of silicone oil from the minimum amount to an appropriate level can produce well-opened foam plastics. When the amount is too high, the closed-cell rate of the foam increases.



7. Other Influencing Factors



In addition to the formulation, process parameters, and environment also have a certain impact on foam properties.



Raw material temperature: Under relatively normal ambient temperatures (20-28°C), the raw material temperature is controlled at 25±3°C, preferably within a range of ±1°C. It can also be controlled within the range of 28-30°C.



The effect of temperature increase or decrease on the speed of foaming and gel reactions varies. An increase in temperature results in a much greater increase in polymerization reaction compared to foaming reaction. Catalysts need to be adjusted for temperature changes.



For the same formulation, using the same amount of blowing agent, foam density is also related to altitude. In high-altitude areas, foam density noticeably decreases.

In polyurethane flexible foams, dichloromethane (MC) is often used to adjust the density and hardness of the foam. With a boiling point of only 40.4°C, during foaming, the reaction of water and TDI generates a large amount of heat, causing MC to evaporate into gas, thus expanding the foam body and reducing foam density.



The vaporization of MC consumes a lot of heat, which can affect the foaming process of the foam in some cases. The following two figures show the changes in the maximum foam temperature and the time to reach it after adding different amounts of MC to a specific formula.



From the charts, it can be observed that after adding MC, the maximum foam temperature decreases significantly, and the time to reach the maximum temperature also increases.



These are just changes in data, but how do they manifest during the actual foaming process? To understand this, let's briefly look at the polyurethane reaction process.



The main reaction in polyurethane foaming is the reaction of water and isocyanate to produce carbon dioxide and amine, and the reaction of polyether polyol and isocyanate to produce polyurethane. However, there are many secondary reactions, summarized as reactions generating urethane and reactions generating urea.



Secondary reactions change the molecular structure of the polymer from linear to cross-linked. Due to different reaction conditions and raw materials, the structure of polyurethane can vary greatly. In general, the more secondary reactions, the more complex the cross-linked structure, resulting in increased hardness and improved tear strength. Of course, the resistance to yellowing also improves, but that's another topic. Increasing the foaming index will strengthen secondary reactions.



Having said so much, what does this have to do with MC? Secondary reactions are all endothermic reactions, requiring heat absorption. However, the vaporization of MC also requires a large amount of heat, thus creating a competitive relationship. Adding a large amount of MC will significantly weaken secondary reactions, increasing the proportion of linear structures in the foam, making it softer, and decreasing thermal plasticity.



Even in colder temperatures during winter, attention should be paid to this issue. Properly increasing the water content in the formula to generate more heat helps maintain the physical properties of the foam without significant changes.