Basic Components of a Flexible PU Foam Production Line: Mixing System

The mixing head of a foaming machine generally consists of four main parts: the transmission section, material distribution chamber, mixing chamber, and nozzle. The power for the agitator can be directly driven by a motor, such as a DC motor or an AC motor with variable frequency speed control or a belt-driven mechanism. It can also be powered through a hydraulic system using a hydraulic infinitely variable transmission.

For large-scale foaming machines, the mixing head’s stirring speed is mostly controlled by various types of continuously variable transmission systems, allowing for real-time adjustments to meet foaming production needs and select the appropriate rotation speed. In smaller foaming machines, the speed adjustment of the mixing head is typically achieved using a belt-driven mechanism. The commonly used mixing head stirring speeds range from 3,000 to 6,000 r/min. Most foaming machines use fixed mixing heads, and reciprocating mechanisms have become less common.

The mixing chamber of the mixing head is cylindrical, and the agitator is typically designed in a spiked-rod style, with two rows of short rods arranged perpendicular to each other on the stirring shaft. The high-speed rotation of the agitator ensures uniform mixing of materials. The residence time of materials in the mixing chamber (i.e., mixing time) generally ranges from 0.4 to 1.3 seconds, depending on the foaming machine type and the characteristics of the materials.

In low-pressure foaming machines, materials enter the mixing chamber at a lower flow rate, and mixing is performed at high rotational speed and high shear force. These machines have a larger mixing chamber volume, allowing for a longer residence time to achieve better mixing energy. Conversely, in high-pressure foaming machines, most materials are injected into the mixing chamber at high velocity through a nozzle under high pressure, providing higher kinetic energy. Therefore, low-shear agitators are commonly used, the mixing chamber volume is smaller, and the material residence time is shorter. Figure 1 illustrates a typical high-pressure mixing head, characterized by a smaller mixing chamber and agitator.

Schematic Diagram of High-Pressure Machine Mixing Head

Besides formulation, foam cell size in low-pressure foaming machines is primarily controlled by adjusting the mixing head’s stirring speed and the amount of air introduced into the mixing head. In high-pressure foaming machines, in addition to adjusting the stirring speed, mixing chamber pressure is also an important factor in controlling foam cell size. The pressure in the mixing head is mainly adjusted by changing the size of the outlet gap of the mixing chamber.

Nozzle Types in the Mixing Head

Mixing head nozzles can generally be categorized into straight-tube type, horn-type, distribution-plate type, and distribution-pipe type. To prevent material rotation inside the nozzle and reduce splashing on the conveyor belt, various flow-straightening devices can be installed within the nozzle.

Straight-tube nozzles are the simplest type and are mostly used in small-flow reciprocating mixing head systems. The reciprocating movement of the mixing head ensures uniform distribution of material onto the conveyor belt mold.

Horn-type nozzles, an improved version of the straight-tube type (Figure 2), have a flared opening at the nozzle’s end with a metal or nylon mesh to reduce the discharge speed further, expand the distribution area, and prevent splashing on the conveyor belt.

Schematic Diagram of Mixing Head Nozzle

In foaming machines using the balanced pressure plate method, horn-type nozzles allow materials to be evenly distributed onto the conveyor belt mold, eliminating the need for a reciprocating mixing head.

For polyester-based foam plastics, where the initial viscosity of the material system is high, even a horn-type nozzle may not ensure uniform distribution. In such cases, a reciprocating mixing head system is usually required.

A modified nozzle designed for fixed mixing heads includes a flat distribution plate below the outlet to evenly distribute the material, ensuring a stable mixing head position. Fixing the mixing head not only reduces power consumption but also minimizes air bubble entrapment caused by the abrupt direction changes of reciprocating motion.

Another method for fixing the mixing head is to connect a horizontal material distribution pipe to the lower end of the mixing head nozzle outlet, ensuring an even spread of material on the conveyor belt mold. The material distribution pipe can be either a single-pipe or dual-pipe system (Figure 3), with the number, diameter, and spacing of distribution holes carefully designed based on factors such as flow rate and foaming machine width to ensure uniform material distribution.

Schematic Diagram of Horizontal Distribution Pipe

Additionally, in overflow-trough horizontal foaming machines and vertical foaming machines, Y-shaped dual-outlet nozzles are often used to connect two material distribution pipes, allowing the mixed material to be evenly delivered into the foaming trough.