Production Process of Flexible Polyurethane Slabstock Foam Products

The production of slabstock foam using slabstock foaming equipment is characterized by high output, making it suitable for large-scale production. The maximum output of the largest equipment can reach 500 kg/min. The length of the products can be adjusted freely based on factory conditions and requirements. After forming rectangular slabstock foam, it can be further cut into various sheet or block products using slicing machines.

Slabstock Foaming Production Flowchart

Slabstock Foaming Machine Workflow Diagram

The materials are delivered from material tanks to a mechanical mixing head through a metering system. After uniform mixing, the materials are poured onto a conveyor belt lined with kraft paper. The materials foam on the conveyor belt within seconds after being discharged from the mixing head. The foam passes through a heating and curing tunnel equipped with infrared (or other heat sources) and exhaust devices to mature and form. The foam is continuously output from the other end. Cutting tools at the exit can slice the foam into specific lengths to create semi-finished products. These semi-finished products are left at room temperature for a certain period for further curing to reach final strength. The surface skin is trimmed off, and the foam is cut into the desired shape, resulting in the final product.

Due to the trimming of surface skin, there is a certain amount of material loss. Smaller foam dimensions lead to higher edge waste. To improve the yield rate, large rectangular slabstock foams have been commonly produced in recent years.

Metering System of Slabstock Foaming Equipment

The metering system typically involves five components:

1. Polyether polyol: Combined with foam stabilizer and foaming agent.
2. TDI (Toluene diisocyanate).
3. Water and triethylenediamine.
4. Organotin catalyst and polyether polyol.
5. Pigments and polyether polyol.

Some systems combine these five components for simplified metering, prioritizing ease of control and maintaining material stability after mixing.

Mixing Head and Stirring System

Slabstock foaming devices commonly use low-pressure needle-type stirring rods in the mixing head. When low-shear needle-type stirrers are used, the stirring speed is typically 3,000–6,000 rpm, and the residence time in the mixing head is about 1 second. High-shear stirrers are also employed in some systems.

The foam's final pore size can be adjusted by the energy input into the mixing head. Two factors influence this:

1. The linear velocity of materials entering the mixing head.
2. The stirring speed and shear force.

In low-pressure, high-shear foaming machines, injecting a small amount of dry air into the mixing head improves pore size. However, this effect is less pronounced in high-pressure, low-shear foaming machines. The outlet size and back pressure of the mixing head also influence pore size. Typically, reducing back pressure and enlarging the outlet diameter leads to finer foam pores.

The distance between the mixing head outlet and the material pouring plane should be as short as possible to avoid splashing, which can cause air bubbles to form in the foam, resulting in pinholes or large surface pores. The bottom liner paper must be flat; wrinkles may cause foam cracking and increased edge waste during trimming.

Conveyor Belt Requirements

The conveyor belt near the material injection point should have an inclination angle, usually between 4°–9°, to direct material flow in a single direction. Conveyor belt speed is generally controlled at 1.5–3.0 m/min, depending on foam output and product dimensions.

Key Foaming Parameters

The whitening time (the interval between material discharge from the mixing head and the start of foaming) is typically 5–12 seconds. The foaming time (the interval between material discharge and maximum foam height) is about 80–160 seconds. Foaming time can be adjusted by the amount of organotin and amine catalysts used.

Example Data:

The distance from the mixing head outlet to the whitening line (start of foaming) is typically 30–60 cm. This can be adjusted by modifying the pouring rate, conveyor belt angle, and speed. The foam is heated in a curing tunnel until the surface is non-sticky, with curing time ranging from 5 to 30 minutes, depending on the formulation.

Open-Cell Formation

In prepolymer-based foaming processes, some semi-finished products require mechanical compression to achieve open-cell structure. However, in the one-step foaming process, natural open-cell formation is typically achieved without mechanical compression.

Typical One-Step Foaming Recipe for flexible polyurethane slabstock foam

• Polyether triol (Molecular weight 3000): 100
• Silicone-based foam stabilizer: 1.0
• Organotin catalyst: 0.45
• N-Ethylmorpholine: 0.4
• Triethylenediamine: 0.05
• Water: 3.5
• Foaming agent: 1.5
• TDI 80/20: 42.5 (TDI index = 104)