When using a batch foam machine for polyurethane soft foam foaming, have you encountered the following situations?

1.Uneven and numerous foam pores,

2. Rough foam texture.

3. Chaotic pore sizes across the entire foam surface, with slight signs of large pores.

Issues like these are quite common. The main reason for the first issue is that the distance between the mixing impeller of the foam machine and the bottom of the mixing barrel is too great; the second issue is that the mixing blades are too short and narrow: the third issue is that the angle of the mixing blades is too large.

Many manufacturers who design and produce foam machines only understand the principles during the design process, without understanding the significant relationship between a different design in foam production and product quality. A reasonable and perfect mechanical design can only be gradually improved in actual work, and only experienced foamers can achieve this.

Here are some experiences we have had with machine modifications and upgrades, hoping they will be helpful:

First, the installation position of the mixing wheel should be as low as possible, closer to the bottom of the mixing barrel is better. In general, the distance between the lowest point of the mixing blade and the bottom of the mixing barrel should be around two centimeters

Second, the shape of the mixing blade should be fan-shaped, with a moderately wide edge. The advantage of being wide is that it increases the contact area with the liquid material, providing sufficient power and also balances the liquid material.

Third, the length of the mixing blade should also be as long as possible, leaving about three to four centimeters from the baffle inside the mixing barrel.

Fourth, the two edges of the mixing blade should be sloped, with the angle of inclination based on the width of one end and two centimeters difference on both sides. After the mixing blade is modified, proper operation is also crucial, especially the mixing speed. Most batch foam machines nowadays are equipped with high-speed timing frequency conversion devices. However, in actual production, this device is often unnecessary. The operating speed mainly depends on the amount of material in the mixing barrel. If there is a lot of material, the speed should be appropriately faster, and if there is less material, then the speed should be lower.

Polyether Polyol: Hydroxyl Value 36, Primary Hydroxyl > 65%, 60%.

Polymeric Polyol: Hydroxyl Value 28, Copolymer 20%, 40%.

Water: 3%.

80TDI and Polymeric MDI (Viscosity 300mpa): 80:20.

T12: 0.025%.

A33: 0.4%.

HR-3 Silicone Oil: 1%.

HA-1 Crosslinker: 6%.

Di(b-dimethylaminoethyl) Ether: 0.15%.

Manufacturing polyurethane filter foam typically involves two key stages. The first stage involves preparing open-cell or partially open-cell polyurethane foam according to the desired porosity. If the foam is closed-cell, it needs to be subjected to roller compression to rupture the cell walls, thereby creating the necessary porous structure. The second stage is to remove all the cell membranes to form a reticulated structure.



To produce polyurethane filter foam, the size of the pores and the structure of the network of the foam are largely determined by the catalyst, foaming agent, and surfactant.



In practical operations, there are two common methods for forming the network:



First is the alkali hydrolysis method. The steps of this method involve immersing the soft polyester-type polyurethane foam obtained from the first stage in a 10% sodium hydroxide solution at 50 degrees Celsius for about 10 minutes. Then it goes through processes such as washing with water, neutralization with acetic acid, another round of water washing, and drying, resulting in the final product of polyurethane filter foam.



Another method is the combustion method, also known as the explosion method. This method requires placing the soft polyether-type or polyester-type polyurethane foam obtained from the first stage into a sealed container. The container is then evacuated to 13.3 Pa, followed by the introduction of oxygen and natural gas (in a volumetric ratio of 2:1), bringing the internal pressure of the container to a certain level (which increases with the porosity). Next, the gas inside the container is ignited using a spark plug. The heat generated from the combustion process will burn off or melt the cell membranes without damaging the cell struts. Finally, the resulting product after combustion is cleared with air, and the filter foam is removed from the container.



Both of these methods are effective ways to prepare polyurethane filter foam, and the specific choice between them depends on the material of the foam and the desired structural characteristics.

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.

Have you ever wondered how polyurethane plastic foam is formed? In the previous article, we revealed the basic reactions behind it: isocyanates, polyether (or polyester) polyols, and water, all work together to create this magical substance. So, does this mean that in actual production, we only need these three raw materials? The answer is far from it. In our actual production process, in order to more precisely control the reaction rate and produce products with excellent performance, we often need to harness the power of various additives. These additives not only have wide-ranging applications but also can play a huge role in making our production process more efficient and stable.



Surfactants / Silicone Oil

Surfactants, also known as silicone oil, are also called foam stabilizers. In the production process of polyurethane foam, its role is crucial. The basic duty of silicone oil is to reduce the surface tension of the foaming system, thus improving the miscibility between components, adjusting the size of bubbles, controlling the bubble structure, and enhancing foam stability. Furthermore, it also bears the responsibility of preventing foam collapse. Therefore, it can be said that silicone oil plays an indispensable role in the production of polyurethane foam.



Catalysts

Catalysts play a crucial role in the synthesis process of polyurethane, mainly by accelerating the reaction between isocyanates, water, and polyols. This reaction is a typical polymerization reaction. Without the presence of catalysts, this reaction may proceed very slowly or even not at all. Currently, catalysts on the market are mainly divided into two types: amine catalysts and organic metal catalysts. Amine catalysts are compounds based on nitrogen atoms, which can effectively promote the polymerization reaction of polyurethane. Organic metal catalysts, on the other hand, are compounds that particularly affect the reaction between polyols and isocyanates in the formation of polyurethane, usually organotin compounds. The characteristic of these catalysts lies in their ability to precisely control the reaction process, resulting in a more uniform and stable final product.



Blowing Agents

Blowing agents are substances that generate gas during the polyurethane reaction and help form foam. Depending on the way gas is generated, blowing agents are usually divided into chemical blowing agents and physical blowing agents. Chemical blowing agents refer to substances that undergo chemical changes during the reaction, generate gas, and promote foam formation. Many common substances in our daily lives are actually chemical blowing agents, such as water. Physical blowing agents, on the other hand, are substances that generate gas through physical means. For example, dichloromethane (MC) is a common physical blowing agent.



Other Additives

Relying solely on basic raw materials is far from enough to make products have outstanding performance. In order to meet various needs, other additives are cleverly incorporated into the production process, and their roles should not be underestimated. For example, flame retardants can add flame resistance to products, crosslinking agents can enhance their stability, colorants and fillers can give products a more colorful appearance and texture, and various other additives with different functions are also playing their roles. It is these carefully selected additives that comprehensively improve the performance of the products and bring users a better user experience.