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.

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.

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.

The internal temperature of foam is as indispensable as vitality is to a person. If the post-cure temperature of the sponge is too low, its physical properties will not be optimal, and there will be significant fluctuations in these properties.

Once the foam is well developed, its internal temperature rapidly rises to over 120 degrees Celsius due to the exothermic reaction occurring under poor heat dissipation conditions, becoming one of the fire hazard risks.

The internal temperature of the foam is crucial for forming its superior properties. Foam matured at specific external temperatures exhibits exceptionally superior physical properties like tensile strength. Some calculate the foam temperature through formulas, while others use software to input formulas and automatically calculate the internal temperature of the foam. So, what factors influence the internal temperature of the foam? Is it significant to know these factors? It's akin to how modern phone cameras are high-resolution, but does that render professional photography useless? Are adjustments like aperture, focal length, and exposure time pointless? To better control things, one must grasp more of the key variables of that thing. Let's start with basic principles to understand the changes in internal foam temperature.

First, let's grasp a few basic rules.

The temperature of a space is directly proportional to the amount of heat energy injected into that space and inversely proportional to its size.

For example, if 10 kilojoules of heat are distributed in an 8-liter space, the temperature of that space is 20 degrees Celsius. If the same 10 kilojoules of heat are distributed in a 4-liter space, the temperature becomes 40 degrees Celsius.

The amount of heat input is directly proportional to the heat input value and the speed of heat input.

For instance, if 100 kilojoules of heat are released at speed "v," the heat input is "A." If the same 100 kilojoules of heat are released at 2v speed, the heat input becomes 2A.

The size of a space is directly proportional to the absolute temperature.

For example, a 1-liter space at 0 degrees Celsius becomes 1.366 liters at 100 degrees Celsius because (273.15 + 100)/(273.15 + 0) = 1.366.

The size of a space is inversely proportional to atmospheric pressure.

The lag in methane vaporization needs to be considered.

Next, let's examine how fine-tuning the formula affects the internal foam temperature.

Since this is fine-tuning, we'll approximate that the surrounding environment remains unchanged before and after the adjustments. Let's consider the effects of adjusting water and methane on the internal foam temperature.

For example, if a formula increases methane by 5%, we can be certain that the internal foam temperature decreases because methane vaporization absorbs heat, reducing the heat input to the foam, and increasing the space to accommodate heat. Similarly, if the water content is increased by 5%, the added water releases heat upon injection into the foam, raising the heat input, and the reaction of the added water generates gas, increasing the space for heat. So, does the internal foam temperature increase or decrease in this case? Experience indicates that the internal foam temperature increases. This suggests that the increased heat input due to this change contributes more to the increase in internal foam temperature than the gas produced by water diluting the temperature.

The changes involving foam index, heat release, and heat dissipation all increasing can make it difficult to intuitively guess whether the internal foam temperature will rise or fall. One might need to insert a probe after foaming to compare internal temperatures or calculate to reach a conclusion.

For calculations, several formulas (algebraic expressions) derived from the earlier basic rules are needed, along with some data: the heat released when water reacts with TDI to form carbon dioxide in kilojoules per mole, the heat absorbed during methane vaporization in kilojoules per mole. To estimate the total foam internal temperature, one must know the heat released when forming amino methyl formate, urea methyl formate, urea, and biuret (polyurea), in kilojoules per mole, and the reaction rate (reaction time).

This also explains why the density calculated from the foam index drastically differs from the theoretical and actual values for foams without fillers at 50 densities. The lower the density, the more closely the theoretical and actual values of foam density match.

Polyurethane foam often encounters various accidents and problems during actual foaming production, each of which is caused by multiple factors. In the analysis of accidents caused by complex factors, it is generally difficult to list all influencing factors and the main factors that actually play a role. Below are 15 frequently encountered problems and their causes, let's take a look together!

1. High closed-cell content

  a. Polyether polyols: high proportion of ethylene oxide, high activity, often occurs when switching to polyether polyols with different activities.

  b. Process formulation: excessive tin octoate usage, high isocyanate activity, high crosslinking degree, rapid crosslinking speed, excessive amines and physical blowing agents causing low internal pressure of foam, inability to open cells when foam elasticity is high, and high TDI index can also result in high closed-cell content.

2. Shrinkage (gelation speed greater than foaming speed)

  a. High closed-cell content, shrinkage during cooling.

  b. Process conditions: low air temperature, low material temperature.

  c. Process formulation: excessive silicone oil, excessive physical blowing agent, low TDI index.

3. Internal cracking

  a. Process conditions: low air temperature, high reaction center temperature.

  b. Process formulation: low TDI index, excessive tin content, high early foaming strength.

  c. High activity of silicone oil, small usage.

4. Top cracking (unbalanced gasification gelation speed)

  a. Process conditions: low air temperature, low material temperature.

  b. Process formulation: insufficient catalyst usage, small amine usage, poor quality silicone oil.

5. Bottom corner cracking (excessive amine usage, too fast foaming speed)

  Large pore surface: excessive physical blowing agent, poor quality silicone oil and catalyst.

6. Poor low-temperature performance of foam

  Poor intrinsic quality of polyether polyols, same hydroxyl value, low functionality, high unsaturation, low TDI index with the same tin usage.

7. Poor air permeability

  a. Weather conditions: low air temperature.

  b. Raw materials: high polyether polyols, high activity silicone oil.

  c. Process formulation: excessive tin or same tin usage, low water and amine content, high TDI index.

8. Poor resilience

  a. Raw materials: high activity polyether polyols, low relative molecular weight, high activity silicone oil.

  b. Process formulation: large amount of silicone oil, excessive tin content, more water at the same tin usage, high TDI index.

9. Poor tensile strength

  a. Raw materials: excessive low molecular weight polyether polyols, low hydroxyl value functionality.

  b. Process formulation: insufficient tin causes poor gelation reaction, high TDI index at the same tin usage, low crosslinking degree with less water.

10. Smoking during foaming

  a. Excessive amine causes a large amount of heat to be released from the reaction of water and TDI, evaporating low boiling point substances and causing smoking.

  b. If not charring, the smoke is mostly composed of TDI, low boiling point substances, and monomeric cycloalkanes in polyether polyols.

11. Foam with white streaks

  Fast foaming and gelation reaction speed, slow transmission speed in continuous foaming, local compression to form a dense layer, resulting in white streaks phenomenon. The transmission speed should be increased promptly, or the material temperature should be reduced, and catalyst usage should be decreased.

12. Brittle foam

  The formula has excessive water, resulting in many unreacted urea formations that are not dissolved in silicone oil, poor tin catalyst usage, insufficient crosslinking reaction, high content of low relative molecular weight polyether polyols, excessively high reaction temperature, and ether bond breakage which reduces foam strength.

13. Foam density lower than set value

  Foaming index is too large due to inaccurate metering, high air temperature, low air pressure.

14. Foam with skin, edge skin, bottom voids

Excessive tin and insufficient amine, slow foaming speed, fast  

15、High elongation at break

a. Raw materials: high activity polyether polyols, low functionality.

b. Process formulation: insufficient crosslinking due to low TDI index, excessive tin, and high silicone oil content.