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.

Cold Cure

A process for seat foam production, which produces high resilience foam (referred to as HR foam).

During this process, the mold temperature is generally between 50-70 degrees Celsius; the polyether molecular weight is typically between 2500-6500, and the ISO can be TDI/TM/MDI.

This process has high production efficiency, low energy consumption, and is currently widely used.

Pump Capacity

Used to check the stability of the metering pump flow output.

The current method for verifying pump capacity is as follows: at the set flow rate, shoot continuously 35 times, weigh each shot, then calculate the capacity. Based on the pump capacity, determine whether the metering pump needs repair or replacement. Generally, pump capacity is checked every three months.

Pump Linearity

A characterization of the correlation between the metering pump's speed and output.

Usually, five different speeds are selected for flow testing. The output of the metering pump at each speed is then obtained. If these five points align on a straight line, it indicates good linearity between the metering pump's speed and output.

NBT (New Blending Technology)

NBT stands for New Blending Technology.

The previous blending technology involved spraying and mixing one ISO with one POL to react and produce polyurethane foam. When adjusting process parameters with this method, only the POL/ISO mixing ratio and the casting weight could be adjusted, with no other adjustments possible.

NBT involves spraying and mixing one ISO with 2 or 3 groups of POLY materials to react and produce polyurethane foam. (Equipment requires a frequency converter)

NBT can adjust the following variables: formula moisture, formula solids content, formula index, casting weight, and other variables. This allows for greater process tolerance when manufacturing foams of different densities and hardnesses.

TPR (Timed Pressure Release)

TPR stands for Timed Pressure Release, also known as venting or pre-venting.

Typical TPR parameters are: venting starts around 90-120 seconds after mold closure, with the bag dropping down, venting for about 2 seconds, then the bag rising back up.

Common phenomena: Venting too early can result in tender products prone to tearing. Venting too late can lead to stiff products prone to shrinkage after demolding.

Initial Spray

At the start of normal pouring, the ISO and POLY nozzles are opened simultaneously, allowing the materials to mix in the mixing chamber and react to produce polyurethane foam.

If during pouring the ISO and POLY nozzles do not open simultaneously, the one that opens first will cause the material to flow out of the mixing chamber without reacting, resulting in unreacted material at the beginning of the foam. If polyether comes out first, the foam will be sticky and wet at the top (mild initial spraying), while if ISO comes out first, the foam will be crispy, locally thin (mild initial spraying), or have ISO spots (severe initial spraying).

Common phenomena: Another special case is when there is softness at the initially poured area, which could also be a form of initial spraying. This might be due to the component coming out first, causing the foam at the initial pour point to be soft.

Foaming Index

When ISO and POL react, if they react in the exact theoretical amounts, it's called stoichiometric reaction, and the foaming index is defined as 100.

Foaming Index = Actual ISO usage/Theoretical ISO usage * 100. Currently, the foaming index for seat foaming is generally between 90-105.

As the foaming index increases, the foam gradually becomes harder.

Index > 105, the product is prone to being brittle; Index < 85, the product is prone to closed-cell shrinkage.

In modern industrial production, polyurethane flexible foam play an important role in various fields such as furniture, automotive seats, and shoe insoles. However, the key technical control points for producing high-quality polyurethane flexible foam plastic products cannot be overlooked. Here are several key technical points in the production process:

Control of Toluene Diisocyanate (TDI):

The optimal isomeric ratio of TDI is 80/20. If this ratio is exceeded, it can lead to the formation of large and closed cells in the foam, prolonging the curing time. Particularly in the production of large block low-density foam products, an excessive isomeric ratio can delay heat release, potentially causing the foam center temperature to remain high for a long time, leading to carbonization and even ignition. If the isomeric ratio is too low, the foam product's density and resilience will decrease, and fine cracks may appear on the foam surface, resulting in poor process repeatability.

Addition of External Blowing Agents:

External blowing agents (water) not only reduce the density of the foam but also improve the softness of the product and help remove reaction heat. To prevent center carbonization in the foaming process of large block foam products, a certain amount of water is usually added. However, as the amount of water increases, the amount of catalyst should also increase correspondingly; otherwise, it may prolong the post-curing time of the foam. Generally, for every 5 parts increase in water, 0.2 to 0.5 parts of silicone oil should be added.

Catalyst Ratio:

Organic tin and tertiary amine catalysts are commonly used to control the NCO-OH and NCO-H2O reactions. By adjusting the ratio of different catalysts, the growth of polymer chains and the foaming reaction can be controlled. Under certain product densities, choosing the appropriate catalyst ratio can control the foam's open-cell rate, cell size, and void load value. Increasing the amount of organic tin catalyst can generally produce foams with smaller cell sizes, but excessive use may increase the closed-cell rate. It is necessary to determine the optimal catalyst dosage through experiments to achieve the best performance of foam products.

Foam Stabilizers:

The role of foam stabilizers is to reduce surface tension of the material, making the foam film wall elastic and preventing foam wall rupture until the molecular chain growth and cross-linking reactions lead to material solidification. Therefore, foam stabilizers play a critical role in the production of one-step polyether sponge and must be strictly controlled in usage.

Temperature Control:

The foam generation reaction is highly sensitive to temperature, and changes in material and foaming temperature will affect foaming operations and physical properties. Therefore, temperature control is one of the important conditions to ensure stable foaming processes. The material temperature is generally controlled at 20-25°C.

Stirring Speed and Time:

The stirring speed and time affect the amount of energy input during the foaming process. If stirring is uneven, a large number of bubbles may appear on the foam surface, leading to defects such as cracking. During mixing of Component A, the speed is 1000r/min; after Component B is added to Component A, the high-speed stirring speed is 2800-3500r/min for 5-8 seconds.

In summary, the key technologies for producing polyurethane flexible foam include controlling TDI, adding external blowing agents, adjusting catalyst ratios, using foam stabilizers, temperature control, and controlling stirring speed and time. Proper control of these technical parameters can ensure the production of stable quality and high-performance polyurethane flexible foam plastic products.

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.