In September 2021, we received an inquiry from Mr. Abdullah in Saudi Arabia regarding a continuous foaming machine. The client was planning to establish a PU foam factory to produce products for the local and Yemeni markets. He had some basic knowledge about machine usage and selection.

The client had no prior experience in foam production before, so he was particularly concerned about after-sales support and technical assistance.

We began by analyzing the client's target market (specific industry) and understanding the local product requirements (such as foam density, hardness, etc.) to confirm the client's production needs.

Through video conferences, we guided the client through our PU foam production process, providing the client with a concrete understanding of foam production and highlighting the convenience and efficiency advantages of our machines compared to those of other manufacturers.

Drawing upon our more than 20 years of experience in foam foaming, we shared insights with the client about using the machine and common challenges in the foam foaming process, addressing any technical concerns the client may have had.

We also provided the client with factory layout plans to expedite the setup of the entire foam production line while maximizing production efficiency.

Due to the client's high level of trust in our professional service, he ultimately chose us as his supplier for foam machinery and later made repeat purchases for a rebonded foam production line and foam cutting machines.

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.

Polyurethane foam(PU foam) primarily consist of polyurethane as their main component. The raw materials primarily include polyisocyanates and polyols, with the addition of various additives, the most crucial of which are a series of foaming agents related to the foaming process. These additives lead to the production of a significant amount of foam within the reaction product, resulting in polyurethane foam products. This article provides a brief overview of the raw materials used in producing PU foam and the foaming agents.

1.Polyisocyanates

The most commonly used polyisocyanates in the industrial production of polyurethane foams include toluene diisocyanate (TDI), polymethylene polyphenyl isocyanate (PAPI), diphenylmethane diisocyanate (MDI), and liquid MDI (L-MDI).

TDI

TDI is mainly used in the production of polyurethane flexible foams. MDI has higher reactivity than TDI, lower volatility, and some modified forms of MDI can be used as substitutes for TDI in the production of polyurethane flexible foams, including high-density polyurethane foam and the manufacturing of semi-rigid or microcellular polyurethane elastomers.

PAPI, also known as crude MDI or polymerized MDI, typically has an average molecular weight ranging from 30 to 400, with an NCO content of 31% to 32%. In the field of foam plastics, PAPI and modified PAPI are primarily used to produce various polyurethane rigid foams, with some also used in the production of high-rebound flexible foams, integral skin foams, and semi-rigid foams. PAPI can be mixed with TDI to manufacture cold-cure, high-rebound foam plastics.

2.Polyether and Polyester Polyols

2.1Polyether Polyols

Polyether polyols used for producing polyurethane flexible foams are generally long-chain, low-functionality polyethers. In the formulation of flexible foams, the functionality of polyether polyols is usually between 2 and 3, with an average molecular weight ranging from 2000 to 6500. Polyether triols are most commonly used in flexible foams, typically initiated with glycerol (propane-1,2,3-triol) and obtained through ring-opening polymerization with 1,2-epoxy propane or copolymerization with a small amount of ethylene oxide, with a molecular weight generally falling within the range of 3000 to 7000.

Polyether Polyols

High-activity polyether polyols are mainly used for high-rebound flexible foams and can be used in the production of semi-rigid foams and other foam products. Some polyether diols can be used as auxiliary materials, mixed with polyether triols in flexible foam formulations. Low unsaturation and high molecular weight polyether polyols are used for the production of soft foams, reducing the amount of TDI required.

Polyether polyols used in rigid foam formulations are generally high-functionality, high hydroxyl value polyether polyols to achieve sufficient cross-linking and rigidity. The hydroxyl value of polyether polyols for rigid foam formulations is typically in the range of 350 to 650 mg KOH/g, with an average functionality of 3 or higher. Rigid foam formulations often use a combination of two types of polyether polyols, with an average hydroxyl value of around 4000 mg KOH/g.

Semi-rigid foam formulations often use some high molecular weight polyethers, especially high-activity polyether triols, and some high-functionality, low molecular weight polyether polyols from rigid foam formulations.

 2.2Polyester Polyols

Low-viscosity aliphatic polyester polyols, such as hexanediol adipate diols with a hydroxyl value of approximately 56 mg KOH/g, or slightly branched polyester polyols, can be used for producing polyester-based polyurethane flexible foams. Polyester polyols have high reactivity. Currently, block polyurethane foam made from polyester is only used in a few fields such as auxiliary materials for clothing.

Polyester Polyols

Aromatic polyester polyols, synthesized from dicarboxylic acids (such as phthalic anhydride, terephthalic acid, etc.) and small-molecule diols (such as ethylene glycol, etc.) or polyols, are used to produce polyurethane rigid foams and polyisocyanurate rigid foams. Lower hydroxyl value polyester polyols derived from phthalic anhydride can also be used for high-rebound flexible foams, integral skin foams, semi-rigid foams, and non-foam polyurethane materials.

 2.3Polymer Polyols

Polymer polyols, including rigid styrene, acrylonitrile homopolymers, copolymers, and grafted polymers, act as organic "fillers" to enhance load-bearing performance. Polymer polyols are used in the production of high-hardness flexible block foams, high-rebound foams, thermoplastic flexible foams, semi-rigid foams, self-skinning foams, and reaction injection molded (RIM) products. They can reduce product thickness, lower foam density to reduce costs, increase foam plastic cell opening, and impart flame retardant properties to the products.

Polymer Polyols

Polyurea polyols (PHD dispersions) are a special class of polymer-modified polyols used in high-rebound flexible foams, semi-rigid foams, and soft foams, but their presence in the market is limited.

There are also some special polyols used for the production of polyurethane foams, such as vegetable oil-based polyols, rosin-based polyester polyols, and polymer polyesters. These are not described in detail in this article.

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.

The amount of foam stabilizer determines the size of the foam structure's cells. More stabilizer leads to finer cells, but too much can cause shrinkage. Finding the right balance is crucial; too little stabilizer and the cells won't support each other, resulting in collapse during forming. Both are catalysts in action.

Polyurethane (Soft Foam) refers to a type of flexible polyurethane foam plastic with a certain elasticity, mostly having open-cell structures.

Polyurethane (Hard Foam) refers to foam plastics that do not undergo significant deformation under certain loads and cannot recover to their initial state after excessive loads. Mostly closed-cell.

Hard Foam Silicone Oil

Hard foam silicone oil is a type of highly active non-hydrolyzable foam stabilizer with a silicon-carbon bond, belonging to a broad-spectrum silicone oil category. It has excellent comprehensive performance and is suitable for HCFC-141b and water foaming systems, used in applications such as boards, solar energy, pipelines, etc.

Product Features:

1. Good emulsification performance: The excellent emulsification performance allows for good dispersion and mixing of the composite materials during the reaction with isocyanate, resulting in good flowability. The produced product has uniform cells and a very high closed-cell rate.

2. Good stability: The special molecular structure effectively controls the surface tension of the cells, stabilizing the cell structure and providing the product with excellent mechanical properties.

Soft Foam Silicone Oil:

A general-purpose siloxane surfactant for polyether-type flexible polyurethane foam plastics, it is a non-hydrolyzable polydimethylsiloxane-polyethylene copolymer, a high-activity stabilizer. It is used as a foam stabilizer in the production of polyurethane soft foam (sponge). It can provide a thin skin. In very low-density foam, it provides strong stability with fine and uniform cells. In medium-depth foam, compared to similar silicone oils, it has better foam opening properties and breathability.