The foaming density control of polyurethane filter element is a key link in the production process, which directly affects the filtering performance, strength and service life of the filter element. Its core control methods involve raw material ratio, process parameter adjustment, equipment precision optimization and real-time monitoring. The following are specific control methods:
1. Raw material ratio control
The foaming density of polyurethane is mainly determined by the ratio of components such as polyols, isocyanates, foaming agents, catalysts, stabilizers, etc., and the following parameters need to be accurately controlled:
Isocyanate index (R value)
Definition: The molar ratio of isocyanate to polyol directly affects the crosslinking density and foaming structure.
Rule: The higher the R value, the higher the crosslinking degree of the system, and the foaming density may increase (need to be balanced in combination with the amount of foaming agent).
Control: Real-time monitoring of isocyanate content by titration or infrared spectroscopy, and adjustment of the feed ratio.
Foaming agent dosage
Physical foaming agent (such as fluorocarbon compounds, water): Increasing dosage will introduce more bubbles and reduce density, but excessive dosage will cause pore rupture or uneven structure.
Chemical foaming agents (such as azo compounds): The rate and amount of gas generated by decomposition must match the polymerization reaction rate to avoid bubble collapse or density fluctuations.
Example: When producing low-density filter elements (such as ≤30 kg/m³), the proportion of foaming agents can be increased to 5%~8% of the total mass of raw materials; high-density filter elements (such as ≥80 kg/m³) need to be reduced to 1%~3%.
Fillers and additives
Fillers (such as calcium carbonate, silica): Increasing the filler content can increase the density and strength of the foam, but will reduce the porosity, which needs to be balanced according to the filtration accuracy requirements.
Surfactants (stabilizers): Control the size and uniformity of the foam cells. Insufficient use will result in coarse foam cells and uneven density, usually accounting for 0.5%~2% of the raw materials.
2. Process parameter adjustment
Mixing temperature and pressure
Temperature: Increasing the material temperature can accelerate the reaction, but it will cause the foaming agent to volatilize prematurely and the density to fluctuate; low temperature will prolong the curing time and may form an inhomogeneous structure.
Control range: The mixing temperature of polyol and isocyanate is usually 20~40℃, which needs to be precisely adjusted by the temperature control system.
Pressure: High-pressure environment inhibits bubble expansion and can produce high-density filter elements; low-pressure or normal-pressure environment is conducive to the formation of low-density porous structures.
Example: High-pressure foaming (5~10 MPa) is used to prepare high-strength hydraulic oil filter elements, and normal-pressure foaming is used for low-density sponge filter elements for air filtration.
Foaming time and aging cycle
Foaming time: The time from raw material mixing to foaming completion must match the reaction rate. Too short time leads to insufficient foaming (high density), and too long time may cause foam collapse (low density).
Aging cycle: Initial aging (2~4 hours at room temperature) completes the initial cross-linking, and later high-temperature aging (60~80℃, 8~24 hours) improves structural stability. Shortening the aging time may lead to later changes in density.
Foaming method
Molding foaming: foaming in the mold, limiting the foaming volume by the mold size, and directly controlling the density (e.g. increasing the amount of raw materials under a fixed volume can increase the density).
Free foaming: no mold restrictions, the foam rise height is controlled by adjusting the amount of foaming agent and environmental conditions (such as wind speed, temperature), suitable for low-density filter elements.
III. Equipment and production environment control
Metering pump accuracy
Raw material delivery requires a high-precision metering pump (error ≤±1%) to ensure that components such as polyols and isocyanates are accurately mixed in proportion to avoid density fluctuations due to metering deviations.
Stirring speed and uniformity
High-speed stirring (2000~5000 rpm) can promote full mixing of raw materials, but too fast will introduce too much air and affect the uniformity of the bubbles; low-speed stirring (<1000 rpm) may cause uneven mixing and abnormal local density.
Solution: Use planetary stirring or dynamic mixing head to ensure uniform shear force and reduce bubble size differences.
