Overview of Soft Polyurethane Foam Machinery

In laboratories, the simplest method is manual foaming; however, in large-scale industrial production, mechanical foaming is used. Depending on the foaming process employed, such as block foaming, molding foaming, spraying, pouring, and foam foaming, the foaming machinery also varies. The main components of foaming machinery are generally divided into three main parts: material storage tanks, metering systems, and mechanical mixing components.

• Material Storage Tanks

Typically, stainless steel-lined reaction vessels are used, equipped with stirring and jacketed temperature control systems. Some are made of ordinary carbon steel or glass-lined reaction vessels. The purpose of stirring is to prevent the separation of materials with different densities, ensuring uniform mixing. The temperature control system ensures that the material temperature remains consistent, which improves the repeatability of the foaming process.

• Metering Systems

Metering is a key part of foaming equipment, ensuring the accuracy of the foaming formula. The metering system consists of various types of mechanical metering pumps, continuously variable drives, and silicon-controlled rectifier motors.

Viscous materials like polyester, polyether, and prepolymers typically use gear-type metering pumps or annular piston pumps, with metering errors generally required to be less than 1%.

For metering TDI and other low-viscosity components, there are two types of metering pumps. One type is suitable for high-pressure foaming processes with high back-pressure mixing heads, often using a Bosch-style diesel injection pump. Flow can be controlled by adjusting the stroke of the piston or the motor speed, with an adjustment range of approximately 1-10, and an output pressure of 6.89–10.3 MPa. The other type is used for low-pressure foaming processes and typically uses an annular piston pump, where the flow is adjusted by changing the clearance between the sliding ring. Both types of metering pumps can use silicon-controlled variable speed motors or mechanical continuously variable transmissions to control speed, thereby regulating material flow and proportions.

The device is equipped with accurate flow meters for various components, allowing real-time display and recording of flow rates. The connection between the pumps and motors uses direct connection or chain wheels and gear structures to prevent slippage, ensuring metering accuracy.

• Mechanical Mixing Components

The mechanical mixing part is a crucial section of the foaming equipment. Based on the mixing mechanism, it can be divided into two categories. One category is the internal mixing type, where the materials are first mixed in the mixing head and then continuously discharged from the mixing head outlet pipe for foaming into molds, such as in block foaming and molding foaming processes. The other category is the external mixing type, where materials are metered by the metering device into a spray gun, and the materials are sprayed out by compressed air. The materials are then mixed externally and foamed, such as in spray foaming.

The internal mixing type relies primarily on mechanical stirring for uniform mixing, and it must accommodate materials of different viscosities across a wide flow range. Therefore, many different forms of stirring devices have been designed. Broadly speaking, there are two types of stirring devices: one is a low-shear type, consisting of a cylindrical body and a needle-like stirring rod, which is suitable for low-pressure metering systems but is difficult to clean, so it is used for large block foam production. The other is a high-shear type, consisting of a cylindrical body and a spiral stirrer, with very small gaps between the mixing container and the stirrer, allowing for precise tolerance. It is easy to clean with compressed air, making it suitable for molding foaming processes, though it requires a high-pressure metering system due to higher back pressure.

The diameter and length of the nozzle at the mixing head outlet are adjustable. The mixing time (i.e., residence time) within the mixing head can be adjusted by changing the material flow rate from the metering system.

In addition to injecting materials, the mixing head is also equipped with a device for injecting small amounts of air to adjust the foam cell size, particularly in large block foam production equipment.

• Material Composition in Soft Polyurethane Foam Machines

In soft polyurethane foam machines, materials are divided into several components, with 2-6 metering pumps delivering them into the mixing head. Generally, isocyanates, which are prone to degradation when exposed to water and catalysts, are often handled separately as a single component. Polyether (polyester) polyols, blowing agents, foam stabilizers, water, and catalysts can be combined into one or several material streams, with each stream metered into the mixing head in the required proportions. The composition principle is determined by factors such as the potential for material degradation when mixed and the ease of process operation.

Foaming machines also come with other supporting equipment. For example, block foam production systems are equipped with infrared or microwave heating ovens, conveyors, and automatic cutting devices. Molding foaming machines are equipped with automatic injection molding, heating, curing channels, automatic demolding, and mold cleaning devices.

Depending on the foaming process and capacity, foaming machines come in a wide range of specifications and models. In block foam production, the largest machines can produce up to 500 kg/min, with foam product dimensions reaching 1.2 m in height, 2.4 m in width, and length adjustable as needed. The smallest laboratory molding foaming machines have a flow rate of 675 g/min, with a material injection amount of 1.5 g per cycle.