For many small-scale enterprises, although the continuous production line of polyurethane flexible foam offers high output, the costs are also very high, and the target market may not require such large quantities. As a result, non-continuous production lines for polyurethane flexible foam have become their preferable choice. The following is an introduction to the non-continuous production line for polyurethane flexible foam:

1. Box Foaming Process Equipment

The box foaming process and equipment have been developed as a new technology to accommodate the needs of small-scale polyurethane foam production facilities. It builds upon laboratory and manual foam production techniques, essentially an upscaled version of laboratory foam methods. This process has gone through three development stages. Initially, all component materials were sequentially weighed and added to a larger container, followed by the addition of TDI. After rapid mixing, the mixture was immediately poured into a large box mold. This method had high labor intensity, emitted high concentrations of toxic gases, and posed significant health risks to operators. Additionally, the splattering of materials during pouring would entrain a large amount of air, leading to the formation of large air bubbles within the foam structure and even causing foam cracking. Furthermore, there was a significant amount of leftover waste, resulting in substantial material waste and high production costs. 

Later on, the process incorporated metering pumps to convey materials to a mixing barrel with an automatically opening bottom. After high-speed mixing, the bottom plate of the mixing barrel would open, and compressed air would swiftly expel the material into the mold for foam expansion. However, this approach suffered from uneven foam pore structures due to the rapid material flow, leading to swirling foam structures and quality issues like crescent-shaped cracks. The third stage of process improvement is the box foaming device that is mostly adopted today. Its fundamental foaming principle is illustrated in Picture

(a) Raw Material Metering and Mixing      (b) Foaming     (c) Foam Rises to Limit Height

1 - Elevatable Material Mixing Barrel; 2 - Assemblable Box Mold; 3 - Floating Box Top Plate; 4 - Foam Body

Picture 1: Schematic Diagram of Box Foaming Principle

The industrial production equipment for box foaming primarily consists of raw material tanks, metering pump units, elevatable mixing barrels, and assemblable wooden box molds. As depicted in the schematic diagram of the box foaming equipment manufactured by Hennecke (Picture 2), the foaming raw materials are stored in tanks and regulated by control devices to attain the required processing temperature range, typically maintained at 23°C ± 3°C. Sequentially, the metering pump injects polyether polyols, catalyst, surfactants, foaming agents, etc., into the mixing barrel for a stirring duration of 30 to 60 minutes. Next, according to the formulation, TDI is introduced, either directly or through an intermediate container with a bottom switch. Immediate mixing follows TDI addition. Depending on the materials and formulation, the stirring speed is usually controlled at 900 to 1000 revolutions per minute (r/min), with a stirring time of 3 to 8 seconds. After stirring, the mixing barrel is swiftly lifted. The lower part of the barrel lacks a bottom and is placed on the mold box's bottom plate upon lowering, utilizing a sealing ring at the barrel's bottom edge to prevent material leakage.

When lifted, the well-mixed slurry can be directly spread and dispersed on the bottom plate of the box mold, allowing natural foam rise. To prevent the formation of a domed surface on the upper part during foaming, an upper mold plate that matches the mold area and allows for upward limit movement is equipped. The mold box primarily comprises rigid wooden panels, with the bottom plate fixed on a movable mold transport carriage. All four side panels are assemblable, featuring quick-opening and closing locking mechanisms. The inner sides of the panels are coated with silicone-based release agents or lined with polyethylene film material to prevent adhesion. After 8 to 10 minutes of forced maturation within the box, the side panels of the mold box are opened, allowing the removal of block-shaped flexible foam. Following an additional 24 hours of maturation, these foam blocks can undergo cutting and other post-processing procedures.

1 - Raw Material Tank; 2 - Metering Pump Unit; 3 - Control Cabinet; 4 - Mixing Barrel with Elevating Device; 5 - Foaming Box; 6 - Foam Finished Product; 7 - Floating Plate

Picture 2: Box Foaming Equipment Manufactured by Hennecke (BFM100/BFM150)

Box foaming process and equipment exhibit characteristics such as simple operation, compact and straightforward equipment structure, low investment, small footprint, and convenient maintenance. These features make it particularly suitable for small enterprises engaged in intermittent production of low-density block foam. However, its drawbacks are also quite evident: lower production efficiency, less favorable production environment, high concentration of emitted toxic gases on-site, necessitating the use of highly effective exhaust and toxic gas purification systems.

To enhance mixing efficiency, some companies have added several vertical and equidistant baffles to the inner walls of the mixing barrel. These baffles, combined with high-speed spiral-type agitators, facilitate high-speed mixing. This approach can to a certain extent reduce laminar flow effects in the mixing liquid and improve mixing efficiency. An example of this is our product, the SAB-BF3302. For the product's appearance and technical specifications, please refer to Picture 3.

Picture 3: Fully Automatic Box Foaming Machine (Sabtech Technology Limited)

This production line comes with both fully automatic computer control and manual control modes. It's suitable for producing flexible polyurethane foam with densities ranging from 10 to 60 kg/cm. Maximum foam output: 180L. Foam height: 1200mm. Mixing power: 7.5kW. Total power: 35kW.

2. Equipment for Open-Cell Foam Preparation

Open-cell polyurethane foam is a functional foam product developed in the 1980s. It possesses a high porosity, a distinct network structure, softness, breathability, and good mechanical strength. It finds wide application as excellent filtration and shock-absorption material in transportation, instrumentation, medical material filtration membranes, and as catalyst carriers in the chemical industry. Filling it into aircraft fuel tanks can suppress oil agitation and reduce the risk of explosions. Impregnating it with ceramic slurry and high-temperature sintering results in a novel open-cell ceramic filter material used in the metallurgical industry.

The preparation of open-cell polyurethane foam involves methods such as steam hydrolysis, alkaline soaking, and explosion. In industrial production, the explosion method is predominantly used. Initially, polyurethane foam of a specific pore size is prepared using the box foaming process. Subsequently, it's placed in dedicated explosion network equipment, filled with explosive gas, and ignited after completely filling the foam body. By utilizing the impact energy and high-temperature heat generated by the explosion parameters, the cell walls of the polyurethane foam are ruptured and fused onto the cell walls, forming a distinct network structure, as shown in Picture 4.

Picture 4: Clearly Networked Open-Cell Foam

Methods like steam hydrolysis or alkaline soaking are used to prepare open-cell foam. However, there are issues of low efficiency, poor quality, and environmental pollution with these methods. They are mainly employed for small-scale production such as laboratory sample testing. Large-scale production primarily uses the explosion method.

ATL Schubs GmbH, a German company, specializes in the research and development of polyurethane reticulated foam and manufactures the ReticulatusTM foam explosion machinery. The explosion chamber of the reticulated foam explosion equipment comes in two forms: cylindrical and rectangular. The former is suitable for cylindrical foam, while the latter is more versatile. It can be used not only for square foam but also for processing reticulated foam from cylindrical foam, as shown in Picture 5. The explosion chamber is constructed from high-quality 100mm-thick steel plates. Operation is controlled by a computer modem, offering features like automatic opening and closing, automatic locking, automatic operation, and automatic alerts. Additionally, remote program design and modification can be facilitated through data transmission sensors.

Picture 5: Polyurethane Foam Reticulation Processing Equipment (ATL Schubs)

During production, foam bodies measuring 3 to 6 meters in length that are intended for reticulation are pushed into the explosion chamber. The chamber's door is closed hydraulically, and the air inside the chamber is evacuated using a vacuum pump. Under computer control, a precise proportion of oxygen and hydrogen gases is introduced, and the gas mixture's ratio is mechanically adjusted based on factors such as foam sample type and network size requirements. 

Sensors continuously monitor the process, ensuring that all process parameters are within the specified conditions before controlled detonation is initiated. The explosive force and flame intensity generated by the explosion penetrate through the entire foam body, creating a distinct network structure. After forming, the foam body is cooled, residual materials and waste gases are purged using nitrogen, and the pressure chamber can then be opened to retrieve the reticulated foam. The entire process takes approximately 8 to 10 minutes. The pore diameter of the reticulated foam falls within the range of 10 to 100 pores per inch (ppi) (Note: ppi refers to the number of pores within one inch).

The above provides some insight into the non-continuous production process of polyurethane flexible foam. I hope this information proves helpful to you.

There are many reasons for the cracking of polyurethane foam, including both chemical and mechanical factors. During production, it is important to stop and inspect, making sure to eliminate the phenomenon of cracking. Below are some insights we have summarized based on years of production experience, hoping to inspire everyone.

1. Sudden Changes

Sudden changes in the speed of the conveyor, variations in catalyst usage, or irregular operation of the conveyor can cause the foam to crack. Carefully and gradually adjust the above situations. A significant change in the speed of the conveyor can lead to large cracks in the foam blocks.

2. Polyethylene Film

If the film stops moving for some reason, the foam will come into contact with the static surface, leading to cracking. If this situation occurs, check the winding roller of the rewinder and examine the separation of the film in the curing area.

3. Foam Forming Around Certain Substances

Foam tends to form around certain substances, especially in the feed chute, which can easily cause cracking.

Before production, ensure there are no residues or debris left in the assembly of the mixing head, feeding hose, and discharge chute.

4. Excessive Amine

When there is insufficient stannous octoate, excessive amine can lead to the same effect and accelerate the rise time. Therefore, the dosage of amine must be reduced.

5. Insufficient Stannous Octoate

Due to the faster foaming reaction compared to the polymerization reaction, some foam may flow, leading to cracks. Therefore, the amount of stannous octoate must be increased.

6. Silicone Oil

Foaming becomes unstable and prone to rupture, potentially forming "depressions" on the foam, so it is necessary to increase the dosage of silicone oil.

7. Structure of Small Air Bubbles

Thin foam walls due to foaming can cause cracking. To address this comprehensively: reduce the air content, decrease the speed of the mixing head, or increase the pressure of the mixing head. It is also necessary to change the dosage of stannous octoate catalyst.

Testing Conditions:

1. Fast foaming is taken from the center of the foam, while molded foam samples are taken from the central part or for whole sample testing.

2. Newly made foam should be matured for 72 hours in its natural state before sampling. Samples should be placed in a constant temperature and humidity environment (as per GB/T2918: 23±2℃, relative humidity 50±5%).

Density: Density = Mass (kg) / Volume (m3)

Hardness: Indentation Load Deflection (ILD), Compression Load Deflection (CLD)

The main difference between these two test methods is the loading area of the foam plastic. In the ILD test, the sample is subjected to a compressed area of 323 cm2, while in CLD the entire sample is compressed. Here, we will only discuss the ILD test method.

In the ILD test, the sample size is 38*38*50mm, with a test head diameter of 200mm (with a round corner of R=10 on the bottom edge), and a support plate with 6mm holes spaced 20mm apart. The test head loading speed is (100±20) mm/min. Initially, a pressure of 5N is applied as the zero point, then the sample is compressed to 70% of its thickness at the zero point, and unloaded at the same speed. This loading and unloading is repeated three times as pre-loading, then immediately compressed at the same speed. The compression thicknesses are 25±1% and 65±1%. After reaching the deformation, hold for 30±1s and record the relative indentation value. The recorded value is the indentation hardness at that compression level.

Additionally, 65% ILD / 25% ILD = Compression Ratio, which is a measure of foam comfort.

Tensile Strength, Elongation at Break: Refers to the maximum tensile stress applied during the tensile test until fracture, and the percentage elongation of the sample at fracture.

Tensile Strength = Load at Fracture / Original Cross-sectional Area of Sample

Elongation at Break = (Fracture Distance - Original Distance) / Original Distance * 100%

Tear Strength: Measures the material's resistance to tearing by applying specified tearing force on a sample of defined shape.

Sample size: 150*25*25mm (GB/T 10808), with the sample thickness direction as the foam rise direction. A 40mm long incision is made along the thickness direction (foam rise direction) at the center of one end of the sample. Measure the thickness along the sample thickness direction, then open the sample and clamp it in the test machine fixture. Apply load at a speed of 50-20mm/min, using a blade to cut the sample, keeping the blade at the center position. Record the maximum value when the sample breaks or tears at 50mm.

Tear Strength = Maximum Force Value (N) / Average Thickness of Sample (cm)

Usually, three samples are tested, and the arithmetic mean is taken.

Resilience: Measures the foam's rebound performance by allowing a given diameter, weight steel ball to freely fall onto the surface of the foam plastic sample from a specified height. The ratio of the rebound height to the steel ball's drop height indicates the foam's resilience.

Test Requirements: Sample size 100*100*50mm, the ball drop direction should be consistent with the foam usage direction. The steel ball size is ∮164mm, weight 16.3g, and it drops from a height of 460mm.

Resilience Rate = Steel Ball Rebound Height / Steel Ball Drop Height * 100%

Note: Samples should be horizontal, steel ball should be fixed before dropping (static), each sample is tested three times with 20s intervals, and the maximum value is recorded.

Compression Permanent Deformation: In a constant environment, the foam material sample is maintained under constant deformation for a certain period, then allowed to recover for a period of time, observing the effect of the deformation on the sample's thickness. The ratio of the difference between the initial thickness and final thickness of the sample to the initial thickness represents the foam plastic's permanent compression deformation.

Compression Permanent Deformation = (Initial Thickness of Sample - Final Thickness of Sample) / Initial Thickness of Sample * 100

Fire Resistance

VOC (Volatile Organic Compounds)

I. Advantages of Polyurethane On-site Foaming Technology:

The method of on-site foaming, spraying (or pouring) polyurethane foam insulation layer, has the surface as a whole without seams, reducing heat loss, with high construction efficiency, easy to meet quality requirements, reducing construction procedures, and eliminating the need for anti-corrosive coatings on pipe surfaces.

II. Polyurethane On-site Foaming Construction Process Principle:

The principle of polyurethane foam plastic foaming and spraying, pouring process is that polyether isocyanate can undergo a polycondensation reaction to form amine methacrylate, which can generate the required polyaminomethyl ethyl, commonly known as polyurethane foam plastic. Catalysts, crosslinking agents, foaming agents, foam stabilizers, etc., are simultaneously added during the reaction to promote and perfect the chemical reaction.

These raw materials are divided into two groups, fully mixed, and then pumped into a special spray gun by metering pumps in proportion. They are fully mixed and sprayed on the surface of pipelines or equipment in the spray gun or pouring mixer, react, foam, and form foam plastic within 5-10 seconds, which then cures and solidifies.

III. Polyurethane On-site Foaming Construction Methods:

Spraying Method: According to this formula, two groups of solutions are stored in two barrels respectively. The materials are filtered to the metering pump, driven by a pneumatic motor, and input into the gun body through the material tube. Compressed air regulates the material into the mixing chamber, mixed, and then sprayed onto the pipeline or equipment to foam and form.

Pouring Method: The prepared two groups of solutions are stored in barrels, filtered to the metering pump, driven by a pneumatic motor, and input into the pouring mixer through the material tube. Compressed air is introduced into the pouring motor, driving the stirring shaft to mix the two groups of materials, which are then injected into the mold for foaming and forming.

Precautions for Polyurethane On-site Foaming Construction:

Stir the material at room temperature to mix and react, then quickly pour it into the space that needs to be formed. During construction, control the reaction foaming time so that the mixed material after stirring is in a liquid state when poured into the gap. During the foaming process, significant expansion forces will be generated, so proper reinforcement should be made to the pouring interlayer or mold.

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.