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.

The stability of polyurethane soft foam sponge foaming refers to whether the foam breaks, closes pores, collapses, and also includes product hardness, density, elasticity, tensile strength, pore size, and other aspects that meet customer requirements. To achieve these, it is necessary to standardize raw materials, formulations, and operating parameters, and to control the complex and diverse chemical reactions in different environments.

Density: Density is measured in kilograms per cubic meter or grams per cubic centimeter. For irregularly shaped small products, it is not easy to measure the cross-sectional area. One can use graph paper with small squares (such as graph paper with 2-millimeter square sides) to draw the cross-sectional area of the product being measured and calculate the density by counting squares. During the production process, the formulation density, flow rate, conveyor belt speed, and foam width have been determined. Measuring the foam height will reveal the foam density. For example, if a sponge reaches a height of 95 centimeters, the density is 20 kilograms per cubic meter. Density is related to the formulation and is also affected by the reaction rate. There is a density difference between the top and bottom of the same foam.

Hardness: Sponge hardness can be divided into two types. One reflects the surface hardness of the product, used for shoe materials, while the other reflects the overall hardness of the product, used for furniture sponges. The hardness of the foam is related to the hard segments, heat, and raw material content during the reaction, corresponding to the materials TDI, MC, and POP. The hardness of the foam is also affected by the degree of cross-linking. As the density of the sponge decreases, it becomes difficult to increase the amount of POP. For low-density, high-hardness foam, the focus is on how to increase POP and TDI in the formulation to reduce MC. For medium-high-density, high-hardness foam, the focus is on maximizing the hardening effect of POP and TDI.

Elasticity: Elasticity is primarily related to the molecular weight of polyether. The higher the molecular weight, the higher the product's resilience. Secondly, it is related to the formation of side chains during the sponge reaction; the fewer the side chains, the better the elasticity. Reducing the TDI index can reduce the formation of side chains, and reducing the heat inside the foam can also reduce the formation of side chains. However, if there are too few side chains, the tolerance of the formulation is not high, and the foam is not stable. Sponge elasticity is also related to the balance of the formulation. When ordinary foam sponge closes its pores, the elasticity drops sharply. High-hardness foam does not have good elasticity, but foam that is too soft also does not have high resilience.

Tensile Strength: Furniture sponges are mainly used for sitting and leaning, so the tensile strength requirements are not too high. The tensile strength of the sponge is related to the NCO content and cross-linking degree in the meridians. Increasing the TDI index and increasing the heat inside the foam can strengthen the NCO content and cross-linking degree. Increasing MC reduces the increase in tensile strength in many cases. The amount of TDI that a formulation can "accommodate" is related to the foaming method, such as high-pressure machines, low-pressure machines, and manual foaming, which are different. A sponge with a high elongation rate does not necessarily have a high tear strength. For products that emphasize tensile strength, adding a small amount of stone powder can greatly reduce the tensile strength without losing the original.

Pores: Foam with very good pores often becomes mid-to-high-end foam, and the price also rises significantly. Pore formation is a comprehensive problem, and to obtain uniform, delicate, and defect-free pores, one must have a deep understanding of the machinery, raw materials, formulations, and parameters. The formation of pinholes and pockmarks is generally caused by excessive air entrainment in the raw materials during stirring at the machine head or during the movement of the raw materials. It may also result from poor raw material quality or contamination. The theory that air leaks in pipes causing pinholes is not tenable. During foaming, the pressure inside the pipe is higher than the atmospheric pressure outside the pipe. Only the raw material flows out of the pipe, and air from the outside cannot enter.

The compressive resistance of a foam is related to many factors such as the structure of various chain segments composing the foam, the chemical bonds between molecules, the crystallinity of polymers, the degree of phase separation, the structure of isocyanates, and the proportion of isocyanates used.

1. Slow rebound foam is formed by the reaction of high molecular weight polyols and low molecular weight polyols with isocyanates. The soft segments formed by high molecular weight polyols have large volumes, low crosslink densities, and high activity. They are easy to compress and quickly recover after pressure is removed. The hard segments formed by low molecular weight polyols have small volumes, high crosslink densities, and low activity. They are difficult to compress and also difficult to recover after external forces are removed. This characteristic gives foams their slow rebound feature and is the basis for manufacturing slow rebound foams.

Because the properties of the soft and hard segments in slow rebound foams are different, there is a certain degree of phase separation between them. If there is no phase separation between the segments, the foam body is a tightly bound whole on a macro scale, leading to the phenomenon of "move one hair and the whole body moves," meaning it shrinks as a whole when compressed and expands when pressure is released. However, the microstructure of the foam determines that this situation cannot be achieved completely. Especially in slow rebound foams, various chain segments have different molecular structures, uneven molecular weight distributions, and unavoidable phase separation. Slight phase separation causes some hard segments, due to their low activity, to have difficulty recovering during the recovery process after external forces are removed. These "escapees" more or less restrain the recovery of soft segments, ultimately leading to shrinking.

2. The crystallinity of hard segments, which is stronger than that of soft segments, is also a reason for poor recovery. Materials have similar compatibilities, which also apply in slow rebound foams. Because the hard segments have closer cross-linking points and higher crosslink densities, the small molecules formed are more likely to aggregate together. Due to the presence of hydrogen bonds, these aggregated hydrogen-containing substances enhance the crystallinity of the material, leading to greater cohesive forces. After compression, external forces change the aggregation state of the chain segments, making it easier for polar groups to fuse together. When the external force is released, the new aggregation state, due to strong cohesive forces, is difficult to return to the pre-stressed state, resulting in shrinkage of slow rebound foams.

3. The structure of isocyanates is also a factor affecting the compression resistance of slow rebound foams. TDI is usually used to produce slow rebound foams. Because the two NCO groups in the TDI molecule are at the 2,4- and 2,6- positions, they have a certain angle between them, making them prone to deformation under stress. Especially under hot pressing conditions, significant deformation and heat loss occur, particularly evident in bra cup foams, making recovery from these deformations difficult.

4. The low NCO index of isocyanates used in the preparation of slow rebound foams is also a reason for poor recovery. The NCO index of ordinary foams is usually above 100, while in slow rebound foams , the NCO index is generally between 85-95. This means that 5-15% of the hydroxyl groups do not participate in the reaction. Therefore, although the surface of the foam appears to be a single entity, internally there is a considerable portion of chain segments that are independent of each other.



Solutions for Improving Compression Resistance of Slow Rebound Foams:

1.Use high EO polyether (so-called blowing agent polyether) to replace some slow rebound polyether.

A. High EO polyether has a low hydroxyl value and a large molecular weight. After reacting with isocyanates, the segments formed have molecular weights greater than or close to those formed when ordinary polyether reacts with isocyanates, reducing the degree of phase separation and crystallinity.

B. High EO content polyether has soft and smooth segments, which can provide good slow rebound effects. Additionally, the addition of high EO polyether can effectively improve the low-temperature resistance of slow rebound foams.

2.Add a small amount of polyether-modified polyester to increase the material's cohesive force.

The polyester segments, due to the presence of ester groups, have high internal cohesive forces and good tensile and compressive properties, significantly improving the compressive resistance of slow rebound foams.

3.Use a small amount of high-functionality and high molecular weight polyether as a crosslinking agent, and replace some ordinary polyether with high-activity polyether for slow rebound.

This disrupts the distribution of chain segments, reduces the degree of phase separation, and increases the reaction degree, reducing crystallinity.

4.Use MDI or add MDI to TDI.

MDI has a different structure from TDI and produces foams with better compression resistance and less heat loss. If using MDI, it is best to use modified MDI (with high branching and easy closure of cells); liquid MDI can also be used, as it is intramolecular cyclization and more resistant to compression. Slow rebound foams made with all MDI have much better compression resistance than pure TDI, and many manufacturers are already using this.

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.

The reaction of PU foam is based on two main chemical components: polyether polyols and isocyanates, along with other additives including water, dichlorodifluoromethane, foam stabilizers, and catalysts. These materials are instantly and vigorously mixed, reacting to form foam, a process that generates a considerable amount of heat.



Foam plastic is a porous material with a large surface area. While the heat generated at the edges of the foam can dissipate, the heat in the central part, due to the insulation effect of the foam, is more difficult to remove. In a typical reaction, the heat released raises the temperature of the center of the foam block to achieve curing. It has been observed that within 2 to 6 hours after foaming, temperatures can rise to 140-160°C, and sometimes even higher, around 180°C. If the temperature continues to rise, it can lead to core burning, smoking, and even spontaneous combustion.



Additionally, prolonged exposure of polyurethane foam to sunlight can trigger an auto-oxidation reaction, causing polymer degradation, discoloration, embrittlement, and a decrease in physical properties, rendering it unusable. Since the industrialization of polyurethane, core burning and aging have been hot topics of research and concern in the polyurethane industry.



Antioxidants are crucial additives in polyurethane foam production. Proper antioxidants prevent the decomposition of polyols, reduce the formation of by-products, decrease the risk of core burning, and can delay thermal oxidative aging during product use, thereby extending its lifespan. Commonly used antioxidants for PU foam are typically liquid and fall into three categories: aromatic amines (such as 5057), hindered phenols (such as 1135), and phosphite esters (such as PDP). For applications with low color requirements, a combination of aromatic amines and hindered phenols is generally used, while applications with higher color requirements may use a combination of hindered phenols and phosphite esters.



Furthermore, if products are frequently exposed to sunlight, a certain amount of UV stabilizers should be added to improve lifespan and resistance to yellowing. UV stabilizers mainly consist of UV absorbers and hindered amine light stabilizers (HALS). UV absorbers, such as benzotriazoles, benzophenones, and triazines, absorb harmful UV radiation and convert it into heat through intramolecular hydrogen bond transfer or cis-trans isomerization. HALS refers to amines with two methyl groups on each α-carbon atom, which, after photooxidation, transform into nitroso radicals. These radicals are considered stable components that can capture free radicals, regenerate nitroso radicals by reacting with peroxide radicals. UV blocking agents include carbon black, zinc oxide, titanium dioxide, and other pigments, which are used as colorants. These agents utilize their high dispersibility and covering power to reflect harmful UV radiation, protecting the polymer.