Control of Slow Rebound Sponge Foaming Process

Having the right formula is essential, but equally important are good equipment and strict control of the process conditions to complete production.

1. Type of Mixing Head

For conveyor belt foaming machines, the mixing heads are mostly of the wolf tooth rod type, which generates sufficient shear force to meet mixing requirements, so we won't elaborate on that. For manual foaming machines, I recommend using a suction cup type mixing head. This involves drilling 4-5 symmetrical circular holes (3-5 cm in diameter) in a circular steel plate that fits the mixing barrel. This type of mixing head is less likely to incorporate air during rotation, and TDI is less likely to be thrown out (which can happen with propeller-type mixers).

2. Mixing Speed

For manual foaming machines, the mixing speed should be controlled at around 300 rpm before adding TDI. After adding TDI, increase the speed to about 900 rpm within 0.5 seconds, and avoid deviating too much from this speed. If the speed is too low or too high, the resulting sponge pore size will be coarse. For conveyor belt foaming machines, the mixing head speed should be controlled at 4500-5000 rpm. Some operators insist on using speeds of 6000-7000 rpm, but this is not necessarily better. The slow rebound polyether in the formula has a relatively low molecular weight and low viscosity, so high shear forces are unnecessary. Excessive speed leads to over-mixing, which is detrimental to the cell structure.

3. Material Temperature

For manual foaming, control the material temperature at 25±1°C. For conveyor belt foaming, control it at 22±1°C. In the south, the material temperature should be relatively lower due to higher ambient temperatures for extended periods. In the north, it should be slightly higher due to lower ambient temperatures. Material temperature control is particularly crucial for manual foaming due to numerous uncontrollable factors during the process. If the operators do not coordinate well and extend the operation cycle, the material temperature in the mixing barrel will increase due to frictional heat when the material temperature is equal to room temperature. If the ambient temperature is lower than the material temperature and the frictional heat is insufficient to compensate for heat loss, the material temperature will decrease. Conversely, if the ambient temperature is higher than the material temperature, it will increase significantly. Deviations from the set temperature can impact foaming, as the formula is designed for a specific temperature.

Generally, the lower the material temperature, the better the pore size and overall performance of the sponge. Higher material temperatures result in coarser, more scattered pore sizes and poorer product performance. Some operators claim they can foam at temperatures above 30°C, but this increases the chemical reaction rate by twofold for every 10°C rise, making the process harder to control.

We monitored the material temperature at the foam nozzle on a conveyor belt. The material entered at 22°C, stayed in the mixing head for one second at a speed of 5000 rpm, and reached 32°C when it exited the nozzle, even though the room temperature was only 25°C. This demonstrates the significant impact of mixing on material temperature.

4. Additive Preparation

Manual foaming does not require additive preparation; simply measure and add them accurately according to the feeding coefficient. For conveyor belt foaming, some additives need to be diluted with polyether to match the pump's operating characteristics. The concentration of additives should be appropriate. Amines should be diluted to 30% concentration with polyether, and D22 should be diluted to 1% concentration. The concentration of the cell opener should be 50%.

If adjustments to the additive amounts are needed during foaming, too high a concentration results in excessive adjustment ranges, leading to overcompensation. Too low a concentration causes severe delays in adjustment, both of which are detrimental to production. Some operators dilute tin to a 1/400 concentration and find it challenging to achieve the desired adjustment effect immediately.

5. Chain Speed

Chain speed is not an issue for manual foaming. For conveyor belt foaming, controlling the chain speed is crucial. In general, the chain speed for slow rebound foaming should be lower than that for regular foaming. If it is too fast, the foam will collapse. Slow rebound foam uses less water, rises slowly, generates less heat, and gels much slower than regular foam. Thus, the chain speed should be around 3.8 meters per minute. Experienced operators may adjust the chain speed based on their expertise.

6. Gas Injection

To ensure open cells in slow rebound foam, gas injection into the mixing head may be necessary. Manual foaming does not require gas injection, but it must be considered for conveyor belt foaming. The gas injection volume should correlate with the foam density; higher density requires more gas, and lower density requires less. Sometimes, the equipment's gas flow meter range may limit the gas injection volume. This should be addressed in the formula design and process determination by increasing the cell opener amount, reducing the tin amount, decreasing the silicone oil amount, or increasing the material temperature.