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.

Phase One: Gas Nucleation Process

The raw materials react in the liquid phase or rely on the generation of gas substances and gas volatilization during the reaction. As the reaction progresses and a large amount of heat is generated, the amount of gas substance generated and volatilized continuously increases. When the gas concentration exceeds the saturation concentration, fine gas bubbles begin to form in the solution phase and rise. As the reaction nears its end, a milky phenomenon appears in the liquid polyurethane material, known as the "milky time."

Phase Two: Self-nucleation Process

In this stage, the gas concentration continues to increase and reaches a certain level. After that, the gas concentration gradually decreases, and new bubbles no longer form. The gas in the solution gradually reaches an equilibrium saturation concentration. During this stage, the viscosity of the liquid material gradually increases, and the gas continuously merges and expands in the gradually viscous liquid phase. The volume of the bubbles continues to expand. The viscous liquid phase forming the outer wall of the bubbles gradually thins. Due to the surface tension relationship between the gas and liquid interfaces, the bubble volume increases from small to large, gradually transforming from a spherical shape into a three-dimensional geometric shape composed of polymer thin films, finally forming an open network structure of three-dimensional micropores. In the synthesis process of polyurethane foam, this stage exhibits polymer volume expansion and foam rising.

Phase Three:

After the gas concentration drops to a certain level, bubbles no longer form. With the permeation of the gas, the concentration continues to decrease, reaching the final saturated equilibrium in the process of the polymeric foam wall transitioning from a viscous liquid state to a non-flowing solid state.

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.

PLC (Programmable Logic Controller)

It is an automatic control device with instruction memory, digital or analog I/O interfaces; primarily used for logical, sequential, timing, counting, and arithmetic operations with bit operations; used to control machines or production processes.

Variable Frequency Drive (VFD)

A VFD is a control device that transforms power frequency from one frequency to another using the on-off action of power semiconductor devices.

The main circuits of a VFD can generally be divided into two types:

- Voltage type: Converts DC voltage from a voltage source to AC in the VFD, with capacitor filtering in the DC circuit.

- Current type: Converts DC current from a current source to AC in the VFD, with inductor filtering in the DC circuit.

Photoelectric Switch

It utilizes the obstruction or reflection of an infrared light beam by a detected object, detected by the synchronous circuit, to determine the presence or absence of the object. It can detect any object that reflects light, not limited to metals.

A mirror-reflective photoelectric switch is used on the vacuum perforating machine.

Heat Exchanger System

Controls the temperature of raw materials in the system to meet requirements.

As the temperature of the raw material rises after passing through the heat exchanger, its viscosity increases. To ensure the normal operation of the high-pressure pump, a special feeding pump is required. Specific requirements are calculated based on flow rate and raw material viscosity.

The temperature control of the heat exchanger should be near the mixing head, correlating the raw material temperature with the switch of the cooling water to automatically control the flow of cooling water to cool the raw material.

Perforating Machine

There are roller perforating machines, vacuum perforating machines, and brush perforating machines, with roller machines having the best control effect, followed by vacuum perforating machines, and brush perforating machines being the worst. Currently, brush perforating machines are rarely used.

The purpose of perforating is to prevent product deformation.

The roller perforating machine controls the size of the gaps. If the gaps are too large, the perforating effect is not good; if the gaps are too small, there will be obvious pressure marks on the product.

There are two methods of perforating: 1. Chemical method - using perforating agents, 2. Mechanical method - using perforating machines.

Products must be perforated as soon as they come out of the mold. Some products may expand after being demolded, and at this time, they should be left for a period before perforating.

TPR

It can prevent product shrinkage and collapse of bubbles; its most basic function is effective perforating to facilitate demolding. However, it can also lead to fluctuations in ILD (Indentation Load Deflection); TPR directly affects the rise speed of the foam.

Loop Pressure Regulating Valve

It is crucial for balancing system pressure in the control system and should be placed as close to the nozzle as possible. If it is far from the nozzle, pressure fluctuations may occur, leading to system instability and unstable products.

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.