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

The metering pump is one of the key components of foaming machinery.

Low-Pressure Foaming Machines

Most low-pressure foaming machines use gear metering pumps, which adjust the discharge volume by changing the pump speed through various speed control devices, such as gear chain stepless transmission, thyristor speed regulation, or frequency conversion. Some low-pressure foaming machines use annular piston pumps, which adjust the discharge volume by changing the eccentric distance between the annular piston cylinder and the drive shaft. The use of annular piston pumps eliminates the need for complex speed regulation systems, making the metering system more compact. The structural diagram of an annular piston pump is shown in Figure 1.

Figure 1: Structural Diagram of an Annular Piston Pump

High-Pressure Foaming Machines

The working pressure of metering pumps in high-pressure foaming machines is generally between 1.0 and 15.0 MPa. The commonly used multi-plunger metering pumps come in two types: vertical plunger pumps and axial plunger pumps. Both types operate at a constant drive speed, typically 1000-1500 r/min, and use a multi-cylinder structure (usually with 4 to 10 piston cylinders). This design ensures that the output liquid flow has only minor pulsations. The precise metal-to-metal fit between the plunger and the cylinder body allows for high output working pressure.

Vertical Plunger Pump

In a vertical plunger pump, the plunger moves vertically driven by a camshaft, and the discharge volume is adjusted by changing the plunger stroke. Typically, vertical plunger pumps have 4 to 10 pistons, require a feed pressure of 0.15-1.0 MPa, operate at a camshaft speed of 1000 r/min, and have a maximum output capacity of 25 L/min. The working principle is illustrated in Figure 2.

Figure 2: Working Principle of a Vertical Plunger Pump

For material flow rates ranging from tens to hundreds of kilograms per minute, axial plunger pumps are generally used. The A2VO-type axial plunger pump operates as shown in Figure 3. During rotation, the plungers slide axially within the cylinder, completing a suction and discharge cycle per rotation. By adjusting the axial angle between the rotor and the drive shaft, the plunger stroke inside the cylinder can be changed, thereby controlling the discharge volume.

Axial plunger pumps typically have 7 plungers, offering precise metering, adjustable flow, low noise, and long service life. In large foaming units, multiple axial plunger pumps can be used in parallel to meter the same component, ensuring the required discharge volume.

Figure 3: Working Principle of an Axial Plunger Pump