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 internal temperature of foam is as indispensable as vitality is to a person. If the post-cure temperature of the sponge is too low, its physical properties will not be optimal, and there will be significant fluctuations in these properties.



Once the foam is well developed, its internal temperature rapidly rises to over 120 degrees Celsius due to the exothermic reaction occurring under poor heat dissipation conditions, becoming one of the fire hazard risks.



The internal temperature of the foam is crucial for forming its superior properties. Foam matured at specific external temperatures exhibits exceptionally superior physical properties like tensile strength. Some calculate the foam temperature through formulas, while others use software to input formulas and automatically calculate the internal temperature of the foam. So, what factors influence the internal temperature of the foam? Is it significant to know these factors? It's akin to how modern phone cameras are high-resolution, but does that render professional photography useless? Are adjustments like aperture, focal length, and exposure time pointless? To better control things, one must grasp more of the key variables of that thing. Let's start with basic principles to understand the changes in internal foam temperature.



First, let's grasp a few basic rules.



The temperature of a space is directly proportional to the amount of heat energy injected into that space and inversely proportional to its size.

For example, if 10 kilojoules of heat are distributed in an 8-liter space, the temperature of that space is 20 degrees Celsius. If the same 10 kilojoules of heat are distributed in a 4-liter space, the temperature becomes 40 degrees Celsius.

The amount of heat input is directly proportional to the heat input value and the speed of heat input.

For instance, if 100 kilojoules of heat are released at speed "v," the heat input is "A." If the same 100 kilojoules of heat are released at 2v speed, the heat input becomes 2A.

The size of a space is directly proportional to the absolute temperature.

For example, a 1-liter space at 0 degrees Celsius becomes 1.366 liters at 100 degrees Celsius because (273.15 + 100)/(273.15 + 0) = 1.366.

The size of a space is inversely proportional to atmospheric pressure.

The lag in methane vaporization needs to be considered.

Next, let's examine how fine-tuning the formula affects the internal foam temperature.



Since this is fine-tuning, we'll approximate that the surrounding environment remains unchanged before and after the adjustments. Let's consider the effects of adjusting water and methane on the internal foam temperature.

For example, if a formula increases methane by 5%, we can be certain that the internal foam temperature decreases because methane vaporization absorbs heat, reducing the heat input to the foam, and increasing the space to accommodate heat. Similarly, if the water content is increased by 5%, the added water releases heat upon injection into the foam, raising the heat input, and the reaction of the added water generates gas, increasing the space for heat. So, does the internal foam temperature increase or decrease in this case? Experience indicates that the internal foam temperature increases. This suggests that the increased heat input due to this change contributes more to the increase in internal foam temperature than the gas produced by water diluting the temperature.



The changes involving foam index, heat release, and heat dissipation all increasing can make it difficult to intuitively guess whether the internal foam temperature will rise or fall. One might need to insert a probe after foaming to compare internal temperatures or calculate to reach a conclusion.



For calculations, several formulas (algebraic expressions) derived from the earlier basic rules are needed, along with some data: the heat released when water reacts with TDI to form carbon dioxide in kilojoules per mole, the heat absorbed during methane vaporization in kilojoules per mole. To estimate the total foam internal temperature, one must know the heat released when forming amino methyl formate, urea methyl formate, urea, and biuret (polyurea), in kilojoules per mole, and the reaction rate (reaction time).



This also explains why the density calculated from the foam index drastically differs from the theoretical and actual values for foams without fillers at 50 densities. The lower the density, the more closely the theoretical and actual values of foam density match.

Understanding the principles behind foam reactions is crucial. To master foaming, we must strive to establish a foam reaction model in our minds using the following four reaction equations. Through familiarity with the variations within the model, we cultivate sensitivity that allows us to comprehend the entire foam reaction process. This approach helps structure our knowledge base and professional skills in polyurethane foam. Whether actively studying foam reaction principles or passively exploring them during the foaming process, it serves as a vital means for us to deepen our understanding of formulations and enhance our skills.



Reaction 1

TDI + Polyether → Urethane



Reaction 2

TDI + Urethane → Isocyanurate



Reaction 3

TDI + Water → Urea + Carbon Dioxide



Reaction 4

TDI + Urea → Biuret (Polyurea)



01: Reactions 1 and 2 are chain-growth reactions, forming the main chain of the foam. Before the foam reaches two-thirds of its maximum height, the main chain rapidly elongates, with chain-growth reactions predominating inside the foam. At this stage, due to relatively low internal temperatures, reactions 3 and 4 are not prominent.



02: Reactions 3 and 4 are cross-linking reactions, forming the branches of the foam. Once the foam reaches two-thirds of its maximum height, the internal temperature rises, and reactions 3 and 4 intensify rapidly. During this stage, reactions 1 to 4 are vigorous, marking a critical period for the formation of foam properties. Reactions 3 and 4 provide stability and support to the foam system. Reaction 1 contributes to foam elasticity, while reactions 3 and 4 contribute to foam tensile strength and hardness.



03: Gas-producing reactions are termed foaming reactions. The generation of carbon dioxide is a foaming reaction and the primary exothermic reaction in polyurethane foam. In reaction systems containing methane, the vaporization of methane constitutes a foaming reaction and an endothermic process.



04: Reactions leading to the formation of foam constituents are known as gelation reactions, encompassing all reactions except for gas-producing reactions. This includes the formation of urethane, urea, isocyanurate, and biuret (polyurea) from reactions 1 to 4.

Polyurethane foam often encounters various accidents and problems during actual foaming production, each of which is caused by multiple factors. In the analysis of accidents caused by complex factors, it is generally difficult to list all influencing factors and the main factors that actually play a role. Below are 15 frequently encountered problems and their causes, let's take a look together!



1. High closed-cell content

a. Polyether polyols: high proportion of ethylene oxide, high activity, often occurs when switching to polyether polyols with different activities.

b. Process formulation: excessive tin octoate usage, high isocyanate activity, high crosslinking degree, rapid crosslinking speed, excessive amines and physical blowing agents causing low internal pressure of foam, inability to open cells when foam elasticity is high, and high TDI index can also result in high closed-cell content.



2. Shrinkage (gelation speed greater than foaming speed)

a. High closed-cell content, shrinkage during cooling.

b. Process conditions: low air temperature, low material temperature.

c. Process formulation: excessive silicone oil, excessive physical blowing agent, low TDI index.



3. Internal cracking

a. Process conditions: low air temperature, high reaction center temperature.

b. Process formulation: low TDI index, excessive tin content, high early foaming strength.

c. High activity of silicone oil, small usage.



4. Top cracking (unbalanced gasification gelation speed)

a. Process conditions: low air temperature, low material temperature.

b. Process formulation: insufficient catalyst usage, small amine usage, poor quality silicone oil.



5. Bottom corner cracking (excessive amine usage, too fast foaming speed)

Large pore surface: excessive physical blowing agent, poor quality silicone oil and catalyst.



6. Poor low-temperature performance of foam

Poor intrinsic quality of polyether polyols, same hydroxyl value, low functionality, high unsaturation, low TDI index with the same tin usage.



7. Poor air permeability

a. Weather conditions: low air temperature.

b. Raw materials: high polyether polyols, high activity silicone oil.

c. Process formulation: excessive tin or same tin usage, low water and amine content, high TDI index.



8. Poor resilience

a. Raw materials: high activity polyether polyols, low relative molecular weight, high activity silicone oil.

b. Process formulation: large amount of silicone oil, excessive tin content, more water at the same tin usage, high TDI index.



9. Poor tensile strength

a. Raw materials: excessive low molecular weight polyether polyols, low hydroxyl value functionality.

b. Process formulation: insufficient tin causes poor gelation reaction, high TDI index at the same tin usage, low crosslinking degree with less water.



10. Smoking during foaming

a. Excessive amine causes a large amount of heat to be released from the reaction of water and TDI, evaporating low boiling point substances and causing smoking.

b. If not charring, the smoke is mostly composed of TDI, low boiling point substances, and monomeric cycloalkanes in polyether polyols.



11. Foam with white streaks

Fast foaming and gelation reaction speed, slow transmission speed in continuous foaming, local compression to form a dense layer, resulting in white streaks phenomenon. The transmission speed should be increased promptly, or the material temperature should be reduced, and catalyst usage should be decreased.



12. Brittle foam

The formula has excessive water, resulting in many unreacted urea formations that are not dissolved in silicone oil, poor tin catalyst usage, insufficient crosslinking reaction, high content of low relative molecular weight polyether polyols, excessively high reaction temperature, and ether bond breakage which reduces foam strength.



13. Foam density lower than set value

Foaming index is too large due to inaccurate metering, high air temperature, low air pressure.



14. Foam with skin, edge skin, bottom voids

Excessive tin and insufficient amine, slow foaming speed, fast



15、High elongation at break

a. Raw materials: high activity polyether polyols, low functionality.

b. Process formulation: insufficient crosslinking due to low TDI index, excessive tin, and high silicone oil content.

Polyurethane flexible foam, also known as sponge, usually undergoes a liquid-to-solid transformation process within two minutes. Compared to other fields of polyurethane, the short and intense reaction time of flexible foam increases the difficulty in achieving fine and orderly cell structure, as well as molecular arrangement.



Research and development (R&D) involve introducing new materials to give products new properties. New materials can be categorized into two types based on their reaction characteristics: one type consists of raw materials that conform to the chemical reactions of polyurethane, such as polyether, isocyanate, silicone oil, and catalysts with different properties and activities. These materials do not fundamentally disrupt the chemical reaction process. The other type includes raw materials (fillers) that do not conform to the chemical reactions of polyurethane, such as adding non-foaming substances like special metal powders, specially formulated inorganic or organic powders with unique properties, ultrafine inorganic powders (with a fineness of over 5000 mesh), special antibacterial agents, etc. These materials sometimes completely disrupt the reaction process, leading to new reaction patterns. Some fillers cause the original formula to collapse completely, while others cause a significant change in the density of the finished product by 30% to 50%. Some fillers drastically alter the rise time of the reaction in the formula, and some fillers cause severe damage to the foam structure after being added.



R&D usually keeps certain raw materials on hand to deal with common problems encountered during the process. These raw materials typically require a wide range of physical and chemical properties. For example, amines such as A33, A-1, A-210, A-230, A-260, 33-LV, CS-90, 9717, and 9727 are commonly used in production but may not all be kept on hand for R&D purposes because their reactivity does not have a fundamental difference. In extensive R&D, the above-mentioned amines can be classified as materials in the same category. For instance, if adding a particular substance causes significant collapse when using A33 for foaming, then the remaining eight types of amines might be deemed ineffective.



One common issue encountered in R&D is foam collapse, which is the least efficient result. It is often said that analyzing and adjusting can only be done effectively once the foam structure is stable. Many cases of foam collapse cannot be resolved by adjusting the formula alone; either the foam properties change drastically, rendering it impractical, or the foam remains unstable. To address this, crosslinking agents are introduced. Crosslinking agents, commonly used as additives in R&D, generally come in two types: linear structured agents, where 1,4-butanediol represents the raw material and urea formation occurs through linear chain growth, and bulk structured agents, such as diethanolamine and glycerol, which are closer to polyether chain growth patterns.



For example, in one R&D case, adjusting the formula could not prevent foam collapse. Experience revealed that strong amine activity caused imbalance. The amine content was reduced from 0.2 parts per hundred polyether to 0.05 parts, and the high cyanate content in the formula seemed odd. The overall reaction rate also slowed significantly with the reduction in amine content, but the collapse persisted. Later, switching to a weaker amine resolved the issue. Hence, sometimes the activity of an amine cannot be compensated for by its concentration alone. It should be noted that the addition of certain substances may result in significant differences in performance between small-scale and large-scale foam production. In such cases, the performance of the last large-scale foam produced should be the main reference point for adjustment.



Sunlight, air, and water—the most ordinary and often overlooked things—are the most precious. Mastering foam production, which seems simple and lacking content at first glance, is actually quite challenging. As a colleague in R&D often says, "I have too little knowledge (one regrets having too little knowledge when it's needed)" and "How long will we keep doing this? (There's pressure)." R&D and learning foam production involve using known points to understand unknown areas, while production and teaching foam production involve using known areas to operate unknown points. Although these two aspects seem similar, bridging the gap between them is quite difficult.