Introduction to Polyurethane Foam Spray Machine--Mixing Systems

The polyurethane foam spray machine is essential in molding production, especially for mixing systems. Low-pressure polyurethane foam spray machines typically use a mixing head with stirring. Equipped with a material circulation system, the machine switches the material flow status via a circulation/mixing valve adjacent to the mixing head. During spraying, the polyurethane foam spray machine directs the materials into the mixing head for blending. After spraying, the valve switches back to circulation, and the mixing chamber is cleaned with solvent and dried with compressed air. The switching power is usually supplied by compressed air or hydraulic systems. Figure 1 shows a typical low-pressure spray mixing head for a polyurethane foam spray machine.

High-pressure polyurethane foam spray machines generally use non-stirring mixing heads. In a high-pressure mixing system, materials with high potential energy are sprayed into the mixing head through nozzles, where the energy rapidly converts into kinetic energy. The high-speed liquid streams collide and disperse thoroughly, achieving a perfect mix. Depending on the intersection method of the material flows, high-pressure mixing heads for polyurethane foam spray machines are divided into impingement type and parallel flow type, with most being impingement type. Due to increasing demands for molded products, material systems of polyurethane foam spray machines have evolved from direct systems to circulation systems. Figure 2 displays a high-pressure spray gun in action.

Generally, a high-pressure mixing head in a polyurethane foam spray machine consists of several parts: material direction components, material nozzles, the mixing chamber, and cleaning devices.

The material direction component, often a plunger controlled by hydraulics, switches material flow from circulation to mixing mode and back. Material nozzles regulate the material's entry speed and distribution into the mixing chamber, typically in cylindrical or conical shapes.

The mixing chamber of a polyurethane foam spray machine is where all reactants gather and blend. The mixing quality mainly depends on the material's distribution and flow within the chamber. Mixing chambers are usually small, ranging from 0.1 to 10 cm³. A smaller mixing volume shortens the nozzle distance, improving mixing quality. Materials typically inject countercurrently into the chamber, relying on collision and internal static flow to achieve mixing.

Initially, small polyurethane foam spray machines used compressed air for chamber cleaning, but as chamber volumes increased, it became ineffective. Most modern polyurethane foam spray machines use piston-cleaning rods to push leftover materials from the chamber and nozzles after spraying. This is especially effective in multi-piston-controlled spray machines.

Below are several designs of high-pressure mixing heads for polyurethane foam spray machines:

(1) Direct Self-cleaning Mixing Head

This type suits open mold spraying with a polyurethane foam spray machine. Figure 3 illustrates its working principle:

Figure 3 Working Principle of Direct Self-cleaning Mixing Head

It uses a single control piston. In circulation, materials return to tanks through the piston channel. When the piston retracts, materials enter the mixing chamber. When it moves forward again, the flow stops and returns to circulation. As the piston advances, it pushes material out, achieving self-cleaning for the polyurethane foam spray machine.

(2) "L" Type Self-cleaning Mixing Head

Developed independently by Cannon (FPL type) and Krauss Maffei, this design features an L-shaped channel between the mixing chamber and outlet. It uses two hydraulic pistons: one for switching between circulation and mixing, cleaning the chamber afterward; another for positioning a throttling bolt during mixing, stabilizing flow, and preventing splashing during open mold spraying with a polyurethane foam spray machine. Modified "L" type heads also support dual hardness products using two different polyols and isocyanates.

(3) Throttle Valve Type Self-cleaning Mixing Head

The MQ-type developed by Hennecke is a typical throttle valve design. In this polyurethane foam spray machine, material injection is regulated by two switch valves, suitable for mixing challenging systems and multi-component demands for dual hardness products.

(4) Built-in Injection Mixing Head

Besides the three types above, polyurethane foam spray machines designed for RIM processes also use built-in mixing heads mounted directly on molds. Figure 4 shows its principle:

Figure 4 Working Principle of Built-in Mixing Head  

(5) Handheld Mixing Head

For spraying into internal structures, handheld high-pressure heads are common. Figure 5 shows the ML-type from Hennecke:

(a) Mixing Mode (b) Cleaning/Circulation Mode

Figure 5 Handheld ML-type High Mixing Head Diagram

In these polyurethane foam spray machines, the vortex plate and turbulators ensure uniform mixing, and outlet pipes can be adjusted according to operation needs.

(6) Parallel Flow Mixing Head

Unlike impingement types, the parallel flow mixing head does not use collision mixing. It is mainly used in high-speed polyurethane reaction systems like RIM and RRIM for direct mold cavity injection via a polyurethane foam spray machine.

The support structures for high-pressure mixing heads in polyurethane foam spray machines mainly include rotary arm suspension, 3D trusses, or robotic manipulators.