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.

Polyurethane material is a block polymer containing carbamate characteristic groups produced by the reaction of polyisocyanate and hydrogen donor. Because of the various appearance forms of the generated products, its application has entered various fields of the world economy. The following is an overview of the horizontal continuous production line for polyurethane flexible block foam.

1. Hennecke flat top method

The continuous production line equipment for large-scale flexible polyurethane foam blocks was designed and put into production by Hennecke Company in Germany in 1952, which is the basis for continuous production of polyurethane foam blocks. Many companies have successively designed and manufactured continuous production lines for various forms of block bubbles, but only the basic principles designed by Hennecke have been used to this day. The production equipment is shown in Picture 1.

Pic 1 Schematic diagram of Hennecke polyurethane soft foam flat top continuous foaming production line

The polyurethane flexible foam continuous production line produced by Hennecke consists of several main parts: raw material supply section, mixing and pouring section, foaming and curing section, cutting section, post-curing section, and post-processing of the product. This production line has high production efficiency and requires a large supply of raw materials. Therefore, in addition to equipping tanks for polyols and isocyanates, separate systems for raw material storage, process parameters, condition control, and preparation are necessary to ensure a continuous supply of prepared raw materials to the production line during continuous operation (Picture 2).

Pic 2: Metering supply systems and mixing head input systems for 22 components

Temperature has a significant impact on the foaming reaction, and strict control of raw material temperature is necessary during foaming. Typically, the temperature is controlled within the range of 18 to 25°C, with a temperature fluctuation range of around 1°C. High-precision metering pumps are used for the metering and delivery of raw material components, with a general viscosity range of less than 2000 mPas. For high viscosity components such as colorants and flame retardants, gear pumps can be used. To prevent leakage of isocyanate components, the use of magnetic couplings is recommended. For convenience of operation and improved metering accuracy, some additives are now combined to reduce the number of metering pumps. However, it's important to note that certain additives, such as organic tin catalysts, are sensitive to other components and prone to degradation.

The mixing device used in this production line typically employs a low-pressure mixing head, with the agitator driven by a variable-speed motor at a rotational speed of 3000 to 6000 r/min. In modern continuous block foam production enterprises, high-pressure metering, mixing, and foaming equipment have also been adopted, allowing for adjustments in the mixing head's stirring form, flow rate, and nozzle size to enhance product quality. An air input device can also be configured at the mixing head to create gas nuclei and generate a fine and dense cell structure.

Well-mixed material is continuously discharged from the mixing head under certain pressure. To prevent material splashing and the entrapment of a large amount of air causing large voids within the foam body, various measures are taken during the foaming process. Apart from reducing the distance between the mixing head and the bottom plate and minimizing impact force, special designed baffles, horn-shaped or duckbill-shaped deflection tubes, and metal meshes are installed in the front part of the mixing head's outlet to reduce the impact energy of the material. 

Meanwhile, the distance from the material outlet pipe to the bottom plate needs to be decreased to around 10 mm. To ensure the uniformly distributed material on the bottom plate, cross beams are set up on the production line. The mixing head can be adjusted to move left and right in coordination with the bottom plate conveyor belt's movement speed. Alternatively, the material can be divided into multiple conduits to enter distribution slots arranged laterally in the direction of the bottom plate's movement, ensuring the material is evenly distributed on the conveyor belt, as shown in Picture 3.

Pic 3 In order to prevent the spit out material from splashing, the mixing head is equipped with some deflectors

The material ejected from the mixing head exhibits good flowability before the emulsification time. As the reaction progresses, the mixed material gradually initiates and expands. At the front end of the conveyor belt in the ejection section, the conveyor belt should be inclined at an angle of 3° to 9° and equipped with hydraulic or manual adjustment devices. This allows for appropriate adjustments of the incline angle according to process requirements, ensuring the material flows and initiates uniformly in one direction. If the incline angle is too small or the conveyor belt's movement speed is too slow, the foam thickness increases, and initiating the foam becomes difficult. If the incline angle is too large, the ejected material will flow too quickly, reaching the lower part of the foam layer that has already started to rise, causing cracks in the foam body.

Typically, for high-flow-rate units, the conveyor belt's movement speed is controlled at 3 to 10 m/min, while for medium-sized units, it's controlled at 1.5 to 3 m/min. During operation, it's crucial to carefully adjust process parameters such as the ejection rate, conveyor belt angle, and movement speed to maintain a suitable distance of 300 to 600 mm between the ejected distribution line and the milky line formed during foam initiation.

The mixed material ejected from the mixing head is directly distributed onto the pre-laid liner paper on the conveyor belt. In the foaming section, a conveying and recovery device is assembled, including conveyor belts, a drying tunnel, side guards, and foam liners. In the past, a three-liner system was commonly used, with liner paper on the left and right sides moving synchronously with the foam body along the exhaust duct, while the bottom liner paper moved forward in sync with the conveyor belt. In the past, the upper part of the foam body was not restricted, resulting in a wasteful arched shape. Subsequently, the Hennecke-Planidiock method (see Picture 4) and the Hennecke flat-top foaming method (see Picture 8-5) were invented. The improved Hennecke flat-top method is now widely used.

Pic 4 Hennecke-Planidiock method

Pic 5 Schematic diagram of Hennecke flat-top foaming process

Both of the aforementioned production methods are equipped with mechanical balance pressure plates on the upper part of the rising foam body to reduce the volume of arched waste generated at the top of the foam body. Currently, the equipment for Hennecke's flat-top foaming often employs four liner papers synchronized to move in the upward, downward, leftward, and rightward directions along with the conveyor belt.

The lining materials for the foam body include specialized liner paper and plastic film. The base material of the liner paper is strong and durable kraft paper, treated with release agents such as polydimethylsiloxane or paraffin, or coated with non-adhesive chemicals like polyethylene. In recent years, some production facilities have begun using cost-effective plastic films like polyethylene, but it's important to ensure that the film does not crease during operation. Regardless of the lining material, it must remain flat and free of folds during operation.

In the foaming section's drying tunnel, the foam body expands and foams on the liner paper of the conveyor belt. Depending on the specific production formulation, the heat generated by the material's reaction or external heat sources are utilized to expedite the foam body's reaction, curing, and solidification, achieving the desired strength and performance for the subsequent process. The drying tunnel is equipped with multiple exhaust devices to remove various harmful gases produced by the foam body. After purification, these gases are released into the atmosphere.

The conveyor belt system for the foam body requires an extremely smooth surface and operates very steadily without any vibrations. The spacing between the side guards can be adjusted within a certain range as needed, allowing the production of rectangular foam bodies of different widths. The width can reach up to 2.2 meters, and the height of the produced foam bodies generally exceeds 1 meter.

After passing through the drying tunnel, although the foam body has not yet reached its maximum performance, it has been shaped. To facilitate subsequent stages of work, an online assembly cutting machine is used to cut the foam body into desired lengths. Following this, post-curing is performed to ensure complete reaction before further processing.

2. Maxfoam down-moving foaming method

The Maxfoam method, also known as the downward foaming method, was invented by Norwegian scientist Leader Berg in 1959. It employs a distinctive approach, where the foam foaming bottom plate moves downward. The fundamental principle involves raising the front end of a movable bottom plate to a position approximately 70% of the anticipated final foam height. This allows the entire bottom plate to be inclined downward. As the poured material rises to around 30% of its foam height, the lower bottom plate moves downward at the foam's rate of expansion. This causes the remaining 70% of the foam's height to expand downward, resulting in a foam body with a rectangular cross-section. The principle and equipment can be seen in Picture 6. Leader Berg used this principle to design and develop the renowned Maxfoam downward foaming process, depicted in Picture 7.

Pic 6 Schematic diagram of the principle of bottom plate moving down method

Pic 7 Schematic diagram of Maxfoam down-moving foaming process

In the development of the Maxfoam production apparatus, Leader Berg initially placed a baffle at the discharge point of the mixed material. This gradually evolved into an elongated downward foaming trough, and the flat plate where the material flowed was transformed into a downward-inclined bottom plate. This alteration changed the foam body's upward expansion during initiation to a downward expansion, leading to the creation of the renowned Maxfoam foaming process. Leader Berg's company has been dedicated to the research, development, production, and sales of flexible polyurethane block foam production processes and equipment, becoming one of the most prominent companies in this field. The basic process flow can be seen in Picture 8.

Pic 8 Maxfoam equipment produced by Hennecke

(1)The cross-section of the produced foam body is in a regular rectangular shape, leading to a significant reduction in waste rate and a high yield of finished products. In traditional processes, waste from edge and corner cuts is approximately 15%. In the Draka edge sliding method, it's around 12%. However, the waste generated by the Maxfoam process is less than 8%. With further improvements, such as using rotating forks, traction, and flattening devices covered with polyethylene film to fully envelop the foam body (see Picture 9), and utilizing the heat generated by the reactants to heat the bottom plate to make the lower skin of the foam thinner, waste can be lowered to 1% to 2%.

Pic 9 Laying polyethylene thin turning fork (a) device (b) and flattening device (c)

(2) The equipment is well-designed, precisely manufactured, accurately controlled, with a long lifespan, low production costs, and typically requires only 3 to 4 personnel for operation, with low maintenance costs.

(3) The unique foaming process ensures that the produced foam body has a uniform and consistent density, fine cell structure, and excellent quality.

(4) A typical control panel or an enhanced computer system monitors the entire production process with precision.

(5) The range of applicable raw materials is extensive, including both polyether and polyester types. Various types of foam bodies can be produced, including standard flexible foam as well as high-resilience foam, flame-retardant foam, filled foam, viscoelastic foam, and foam produced using carbon dioxide foaming. 

In 1960, Leader Berg established his own company, Laader Berg AS, dedicated to the research and production of continuous polyurethane foam production equipment. The key components of the basic MaxformTM foaming machine are the Multi Trough (Picture 10) and the drop plate. As shown in the equipment schematic in Picture 11, the mixed materials are conveyed through multiple pipes to the bottom entry of the multi-trough. The material begins to react in the multi-trough and flows onto the bottom liner paper sliding on the inclined drop plate just before the emulsification of the mixed liquid. The foam from the multi-trough evenly overflows and spreads between the two side walls of the drop plate. The overflow volume of the multi-trough can be adjusted based on the foaming formula and production volume, and its outlet height is set at 70% of the final foam height. 

Simultaneously, the angle, quantity, length, and width of the inclined drop plate can be adjusted according to the formula and production volume, ensuring that the foam body completes its full expansion process when it reaches the horizontal conveyor belt. During the downward flow of the foam body in the foaming channel of the drop plate, the friction between the foam body and the side walls is eliminated by downward gravity, resulting in a more uniform and smooth foam structure on both sides of the foam body. The foam body discharges the waste gases produced during production in the foaming channel, completes the maturation of the foam body, and can then proceed to the cutting process.

Pic 10 Multiple slots for Maxfoam foam machine multi trough

Pic 11 Basic MaxfoamTM Schematic

Our company also produces this kind of production line on the basis of this foaming method. The introduction reference is as follows (see Picture 12)

Technical parameters of SAB-CF02 automatic horizontal continuous foaming production line produced by Sabtech Technology

1. Main machine specification: total length 42m × width 6m × 4m

2. Foam sponge width: 915mm ~ 2350mm

3. Foaming height: below 1300mm

4. Foaming speed: 1500rpm ~ 7000rpm

5. Maximum output: 350kg / min

6. Spraying mode: The way of trough device, with inverter controlling

7. Foam box specification: L21m * W4.5m * H3m

8. Oven inner conveyer line (standard): L27m * W2.6m * H0.8m

9. Oven side links(standard) L21m * H1.3m

10. Drop frame: 7 sections of electric adjustment height / 0.2KW deceleration motor chain is used to drive rack adjustment between each section of plate

11. Side paper lifting device: front and rear electric movement, lift lever height electric adjustment, left and right-side independent control.

12. Side film collect and release system: the side film and lifting film releasing device is equipped with motor drive, the side film adopts magnetic powder clutch device to automatically reel in.

13. Bottom paper storage system.

14. Exhaust fan: 3kw * 2 sets (excluding exhaust pipe). 

15. Constant temperature system: 20HP air-cooled cold and hot thermostat. Proportional valve is installed at the front inlet of tank coil, and raw material temperature is controlled and set.

16. Powersupply:3phrase 380V 50HZ

Figure 12 Sabtech Technology Limited horizontal continuous foaming unit3

3. Vertical Foam Method

In 1971, the UK-based company Hyman Development Corporation developed a unique vertical foam process technology and equipment. The apparatus mainly consists of a material storage tank system, metering conveying system, mixing injection system, barrel-shaped foaming device, heating and foam lifting device, as well as a cutting mechanism (see Picture 13).

Pic 13 Schematic diagram of vertical foaming equipment

The material storage tank system consists of five main components: raw material tanks (equipped with temperature control and stirring devices) for PPG, with TDI as the primary raw material, mixed with water, oil, amine catalyst, additives, MC foaming agent, and organic tin catalyst. Their metering and conveying systems generally use gear pumps driven by stepless speed-regulated motors, and flow meters can also be added to enhance metering accuracy. Low-pressure, agitating-type mixing heads are typically chosen. Once the materials are mixed, they are injected through pipelines from the bottom into the conical foam bucket. The foam bucket is pre-fitted with continuous sheets of polyethylene film. As the mixed materials react and foam, they initially move horizontally, filling the conical cross-section and gradually rising as the cross-section expands, eventually filling the polyethylene film-lined bucket and moving upward into the heating section. An electric heating system surrounds the heating section to expedite the foam maturation process.

The foam's ascent is facilitated by vertical conveyors equipped with fine needles (10-15mm in length). Multiple such conveyors are arranged around the entire foam body, with their fine needles embedded in the foam of a certain strength. As the conveyor belt rotates, the foam is gradually lifted. The upper part of the equipment is equipped with a cutting machine and a linked clutch mechanism that activates the cutting machine when the foam body reaches the designated height. The cut foam pieces are transported along an inclined slide to the post-maturation chamber. 

This process can produce foam bodies with either square or circular cross-sections, simply by changing the shape of the foam bucket. During continuous production, the foam's color can be changed online, with a transition zone of only 150mm. This not only facilitates easy color changes but also maintains a high yield of finished foam products. The density and hardness performance on the foam's cross-section are consistent, and the foam's skin thickness at the edges is thin, resulting in low waste rates. Importantly, vertical foaming equipment occupies a smaller footprint, only a quarter of that of traditional horizontal foaming equipment, making it suitable for small and medium-sized enterprises. The products are not only suitable for general soft foam products but also the sliced circular foam bodies are particularly suitable for use as clothing lining materials.

The vertical foaming process imposes stricter requirements on aspects such as raw materials, formulations, and production process adjustment and control, compared to the production process of horizontal block foams. Precise control of various process parameters such as raw material temperature, formulation ratios, foam discharge rate, air injection rate, mixing speed, maturation section temperature, and traction speed is necessary to produce high-quality foam. In actual production, the following issues are prone to occur and must be addressed:

1.High Foam Closed Cell Rate or Shrinkage:

This can result from excessive use of organic tin catalyst, leading to rapid gelation during foaming and excessive growth of pore wall strength. Additionally, an excess of foam stabilizer can hinder the formation of open-cell foam structure due to its excessive stability.

2.Foam Body Cracking:

Foam body cracking is often due to errors in formulation or metering. Insufficient amounts of organic tin catalyst and foam stabilizer can lead to decreased reactivity. Mechanical factors, such as the presence of impurities, oil contamination within the foam body, and fluctuations in traction speed, can also contribute to extensive foam body cracking.

3. Large Bubble Cavities in Foam Body:

When large bubble cavities appear in the foam body, it is important to thoroughly inspect the following aspects: When there is a regular distribution of air bubbles, check whether there are any air leakage issues in the mixing chamber, feed pipes, and other equipment. If there are a few conical large bubbles present, it could be due to excessively high raw material temperature, causing the foaming agent to vaporize more easily. When the foam body exhibits irregularly distributed large air bubbles, the main cause could be excessive mixing speed resulting in a higher amount of entrapped air. Typically, with a well-sealed mixing head, the mixing speed should be controlled within the range of 2500 to 3000 rpm. If large perforations or interconnected bubbles appear in the foam sheet without a clear network structure, it might be due to excessive air input into the mixing head.

4.The Foam Body Sliding Downward：

This issue should be considered from several aspects, including formulation errors, excessive foaming time, insufficient foaming, excessively low maturation temperature, and improper coordination of the traction conveyor. It is a problem that can easily occur in the initial stages of equipm

Pic 14 Schematic diagram of the production process of the polyurethane flexible foam pressure-swing foaming continuous production line device

(1) Open the middle chamber door 3a and close the exit chamber door 3b. Activate the pressure control system 4a4b to bring the pressure in the entire channel to the set pressure value. The typical pressure range is 50 to 150 kPa (0.5 to 1.5 atm).

(2) Start the foaming machine, and the mixed material enters the overflow trough in the enclosed channel and flows to the drop plate for foaming under the set pressure environment.

(3) After the foam body is preliminarily cured and shaped to a certain length, the cutting machine operates to cut it.

(4) The cut foam body enters the post-area of the channel. Close the middle chamber door, adjust the pressure in the rear area to be equal to the ambient pressure, open the exit chamber door, and transport the foam body to the curing area to complete the curing. At the same time, the exit chamber door should be closed immediately, and the pressure regulation device should be activated immediately to equalize its pressure with the pressure in the entire channel. Then, open the middle chamber door to accommodate the next cut foam body.

This production line is monitored by highly automated computers, with segment control of the channel, cycle switching, and pressure adjustment system. Depending on the sealed channel, whether it's a vacuum or pressure vessel, it can produce foam bodies with rectangular or circular cross-sections. Based on this continuous production line, intermittent production lines with box-type variable pressure foaming have also been developed. Although the production efficiency is high, the control system is complex, and the equipment is bulky, with sealed channel lengths often exceeding hundreds of meters, resulting in significant investment. 

The above provides an introduction to the horizontal continuous production line for flexible polyurethane foam blocks. Hope it can help you how to choose polyurethane flexible foam continuous production line. Welcome to leave a comment and discuss with me more about polyurethane foam.

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 production of block-shaped soft foam typically utilizes the batch foam machine foaming process, a gap-type production method. This method evolved from manual foaming in laboratories. The process involves immediately pouring the mixed reaction materials into an open mold resembling a wooden or metal box, hence the name "boxed foam." The molds (boxes) for boxed foam can be rectangular or cylindrical. To prevent the foam block from forming a domed top, a floating cover plate can be placed on the top of the foam during foaming. The cover plate stays closely attached to the top of the foam and gradually moves upward as the foam rises.

The main equipment for boxed foam production includes: 1) Electric-mechanical stirrer, mixing barrel; 2) Mold box; 3) Weighing tools such as scales, platform scales, measuring cups, glass syringes, and other measuring devices; 4) Stopwatch for controlling mixing time. A small amount of mold release agent is applied to the inner walls of the box to facilitate easy removal of the foam.

The advantages of producing soft foam using the boxed foam method include: low equipment investment, small footprint, simple equipment structure, easy and convenient operation and maintenance, and flexible production. Some small and underfunded domestic and township enterprises use this method to produce polyurethane soft foam. Boxed foam molding is a non-continuous production method for soft foam, so the production efficiency is lower than continuous methods, and the equipment is mostly manually operated, resulting in higher labor intensity. Production capacity is limited, and there is a greater loss in cutting foam plastics. The process parameters for boxed foam should be controlled within a certain range because even with the same formula, the foam properties may not be the same when different process parameters are used. The raw material temperature should be controlled at (25±3) degrees Celsius, mixing speed at 900 to 1000r/min, and mixing time at 5 to 12 seconds. The mixing time of the polyether and additives mixture before adding TDI can be flexibly adjusted depending on the situation, and after adding TDI, a mixing time of 3 to 5 seconds is sufficient, with the key being thorough mixing after TDI addition.

During boxed foam molding, attention should be paid to the following aspects:

1) Prepare before production, including material temperature and machine equipment inspection;

2) Measure as accurately as possible;

3) Control the mixing time appropriately;

4) Pour the mixed material liquid quickly and steadily, avoiding excessive force;

5) Ensure the box is placed steadily, with the bottom paper flat, to avoid uneven material flow during pouring;

6) When the foam rises, gently press the cover to ensure the foam rises smoothly;

7) Additives should be used as specified, and pre-mixed materials should not be left for too long.

Three types of foam equipment have emerged in boxed foam molding. Initially, various raw materials were weighed into a container according to the formula, mixed with a high-speed mixer, and poured into the box mold for foaming and shaping. This method often resulted in residue in the mixing container. An improved method used a metering pump to transport the raw materials to the mixing barrel for uniform mixing. A mechanical device automatically closed the bottom of the barrel, and compressed air was used to press the material into the foaming box for shaping. Both of these methods could create eddies due to the rapid influx of materials into the box, which might cause defects or depressions in the foam products. The most reasonable boxed foam device is to place a bottomless mixing barrel directly in the center of the foaming box. A metering pump delivers the various raw materials needed for foaming into the mixing barrel. After mixing for a few seconds, the lifting device raises the mixing barrel out of the foaming box, allowing the foaming material to flow smoothly over the entire box bottom. This prevents foam cracking due to material eddies, and ensures relatively uniform height throughout the foam.

A pressure device can be added to the expanding foam material to produce flat-topped foam, reducing waste during cutting. This device is suitable for the production of polyether-type polyurethane soft foam and high rebound soft block foam. For polyvinyl acetate polyurethane blocks, this method cannot be used due to the high viscosity of the material, and continuous methods are generally employed.

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.