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.

Cold Cure

A process for seat foam production, which produces high resilience foam (referred to as HR foam).

During this process, the mold temperature is generally between 50-70 degrees Celsius; the polyether molecular weight is typically between 2500-6500, and the ISO can be TDI/TM/MDI.

This process has high production efficiency, low energy consumption, and is currently widely used.

Pump Capacity

Used to check the stability of the metering pump flow output.

The current method for verifying pump capacity is as follows: at the set flow rate, shoot continuously 35 times, weigh each shot, then calculate the capacity. Based on the pump capacity, determine whether the metering pump needs repair or replacement. Generally, pump capacity is checked every three months.

Pump Linearity

A characterization of the correlation between the metering pump's speed and output.

Usually, five different speeds are selected for flow testing. The output of the metering pump at each speed is then obtained. If these five points align on a straight line, it indicates good linearity between the metering pump's speed and output.

NBT (New Blending Technology)

NBT stands for New Blending Technology.

The previous blending technology involved spraying and mixing one ISO with one POL to react and produce polyurethane foam. When adjusting process parameters with this method, only the POL/ISO mixing ratio and the casting weight could be adjusted, with no other adjustments possible.

NBT involves spraying and mixing one ISO with 2 or 3 groups of POLY materials to react and produce polyurethane foam. (Equipment requires a frequency converter)

NBT can adjust the following variables: formula moisture, formula solids content, formula index, casting weight, and other variables. This allows for greater process tolerance when manufacturing foams of different densities and hardnesses.

TPR (Timed Pressure Release)

TPR stands for Timed Pressure Release, also known as venting or pre-venting.

Typical TPR parameters are: venting starts around 90-120 seconds after mold closure, with the bag dropping down, venting for about 2 seconds, then the bag rising back up.

Common phenomena: Venting too early can result in tender products prone to tearing. Venting too late can lead to stiff products prone to shrinkage after demolding.

Initial Spray

At the start of normal pouring, the ISO and POLY nozzles are opened simultaneously, allowing the materials to mix in the mixing chamber and react to produce polyurethane foam.

If during pouring the ISO and POLY nozzles do not open simultaneously, the one that opens first will cause the material to flow out of the mixing chamber without reacting, resulting in unreacted material at the beginning of the foam. If polyether comes out first, the foam will be sticky and wet at the top (mild initial spraying), while if ISO comes out first, the foam will be crispy, locally thin (mild initial spraying), or have ISO spots (severe initial spraying).

Common phenomena: Another special case is when there is softness at the initially poured area, which could also be a form of initial spraying. This might be due to the component coming out first, causing the foam at the initial pour point to be soft.

Foaming Index

When ISO and POL react, if they react in the exact theoretical amounts, it's called stoichiometric reaction, and the foaming index is defined as 100.

Foaming Index = Actual ISO usage/Theoretical ISO usage * 100. Currently, the foaming index for seat foaming is generally between 90-105.

As the foaming index increases, the foam gradually becomes harder.

Index > 105, the product is prone to being brittle; Index < 85, the product is prone to closed-cell shrinkage.

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.

What is Polyurethane Rigid Foam?

Polyurethane rigid foam, often abbreviated as PU rigid foam, is one of the most commonly used polyurethane products, second only to polyurethane soft foam, in polyurethane applications. Polyurethane rigid foam is mostly a closed-cell structure, known for its excellent insulation, lightweight, high strength-to-weight ratio, ease of construction, as well as soundproofing, shock absorption, electrical insulation, heat resistance, cold resistance, solvent resistance, and more. It is widely used in the insulation layers of refrigerator and freezer boxes, cold storage rooms, refrigerated trucks, as well as insulation materials for buildings, tanks, and pipelines. A small amount is used in non-insulation applications such as wood imitation and packaging materials. Generally, lower-density polyurethane rigid foam is primarily used as thermal insulation material, while higher-density polyurethane rigid foam can be used as structural material (wood imitation).

Polyurethane rigid foam is typically foamed at room temperature, with a relatively simple molding process. It can be categorized into manual foaming and mechanical foaming based on the degree of construction mechanization; high-pressure foaming and low-pressure foaming based on the foaming pressure; and casting foaming and spraying foaming based on the molding method.

What is Polyurethane Soft Foam?

Polyurethane soft foam, also known as PU soft foam, is a type of flexible polyurethane foam with a certain degree of elasticity. It is the most extensively used polyurethane product among all polyurethane products.

Polyurethane soft foam is mostly an open-cell structure, characterized by low density, good elastic recovery, sound absorption, breathability, and insulation properties. It is primarily used as cushioning materials for furniture, mattresses, vehicle seat cushions, and also finds industrial and domestic applications as filter materials, soundproofing materials, shock absorption materials, decorative materials, packaging materials, and insulation materials. Based on the degree of softness and load-bearing capacity, polyurethane soft foam can be divided into ordinary soft foam, super soft foam, high load-bearing soft foam, high resilience soft foam, etc. High resilience and high load-bearing soft foams are generally used for manufacturing seat cushions and mattresses. According to the production process, polyurethane soft foam can be divided into block foam and molded foam. Block foam is produced by continuous process to form large volume foam which is then cut into required shapes, while molded foam is produced by direct injection of the mixture into molds to form foam products of desired shapes.

After understanding polyurethane rigid foam and polyurethane soft foam, the question arises: how do we differentiate between the two?

In fact, the classification can be based on the degree of hardness, dividing them into soft foam plastics and hard plastics. Soft foam plastics have a matrix polymer component above the crystalline melting point, or, if it is an amorphous polymer, it is above the glass transition temperature; hard foam, on the other hand, has its matrix polymer in a crystalline state or an amorphous state but below the glass transition temperature. Semi-rigid foam is a foam plastic between soft and hard foam. It is similar to soft foam, with an open cell rate above 90℃, but semi-rigid foam has higher density and higher compression strength. After compression deformation, semi-rigid foam takes much longer to recover, and its crosslinking density is much higher than soft foam but lower than hard foam.

Based on this classification of softness and hardness, most polyolefin foams, unplasticized polyvinyl chloride (PVC) foams, phenolic foams, polycarbonate foams, and polyphenylene ether foams are all classified as hard foams, while elastic polyurethane foams and some polyolefin foams and plasticized PVC foams are classified as soft foams.

According to the national standard, soft foam plastics are those that are flexible, have low compression hardness, return to their original state after stress is relieved, and have minimal residual deformation. On the other hand, hard foam plastics are inflexible, have high compression hardness, deform when stress reaches a certain level, and do not return to their original state after stress is relieved.

The American Society for Testing and Materials (ASTM) specifies that to differentiate between soft and hard foam plastics, at a temperature of 18-29℃, a rod with a diameter of 2.5 cm is rotated around it for one full rotation within 5 seconds. If it does not fracture, it is classified as soft foam plastic; otherwise, it is classified as hard foam plastic.

According to ISO standards, when the compression deformation reaches 50% and is then released, if the thickness decreases by no more than 2% compared to the original thickness, it is classified as soft foam plastic. If it decreases by more than 10%, it is classified as hard foam plastic. If the decrease is between 2-10%, it is classified as semi-rigid foam plastic.

If we use elastic modulus as the criterion, in a standard environment of 23℃ and 50% relative humidity, a foam plastic with an elastic modulus greater than 686 MPa is classified as hard foam, less than 68.6 MPa is classified as soft foam, and between 68.6-686 MPa is classified as semi-rigid foam. Although the elastic modulus of semi-rigid foam is higher than that of soft foam, its stress-strain behavior is closer to soft foam and significantly different from hard foam. Generally, soft foam plastics mostly have an open-cell structure, while hard foam plastics mostly have a closed-cell structure, but there are exceptions.

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.