Polyol and isocyanate ratio:

Polyol has a high hydroxyl value and a large molecular weight, which will increase the crosslinking density and help improve the foam density. Adjusting the isocyanate index, that is, the molar ratio of isocyanate to active hydrogen in the polyol, will increase the degree of crosslinking and increase the density. Generally, the isocyanate index is between 1.0-1.2.

 

Selection and dosage of foaming agent:

The type and dosage of the foaming agent directly affect the air expansion rate and bubble density after foaming, and then affect the thickness of the crust. Reducing the dosage of physical foaming agent can reduce the porosity of the foam and increase the density. For example, water, as a chemical foaming agent, reacts with isocyanate to generate carbon dioxide. Increasing the amount of water will reduce the foam density, and its addition amount needs to be strictly controlled.

 

Amount of catalyst:

The catalyst must ensure that the foaming reaction and gel reaction in the foaming process reach a dynamic balance, otherwise the bubble collapse or shrinkage will occur. By compounding a strongly alkaline tertiary amine compound that has a strong catalytic effect on the foaming reaction and a strongly catalytic effect on the gel reaction, a catalyst suitable for the self-skinning system can be obtained.

 

Temperature control:

Mold temperature: The thickness of the skin will increase as the mold temperature decreases. Increasing the mold temperature will accelerate the reaction rate, which is conducive to forming a denser structure, thereby increasing the density, but too high a temperature may cause the reaction to get out of control. Generally, the mold temperature is controlled at 40-80℃.

 

Ripening temperature:

Controlling the aging temperature to 30-60℃ and the time to 30s-7min can obtain the optimal balance between the demoulding strength and production efficiency of the product.

 

Pressure control:

Increasing the pressure during the foaming process can inhibit the expansion of bubbles, make the foam structure more compact, and increase the density. However, excessive pressure will increase the requirements for the mold and increase the cost.

 

Stirring speed:

Properly increasing the stirring speed can make the raw materials mix more evenly, react more fully, and help increase the density. However, too fast stirring speed will introduce too much air, resulting in a decrease in density, and is generally controlled at 1000-5000 rpm.

 

Overfilling coefficient:

The injection amount of the reaction mixture of the self-skinning product should be much greater than the injection amount of the free foaming. Depending on the product and material system, the overfilling coefficient is generally 50%-100% to maintain a high mold pressure, which is conducive to the liquefaction of the foaming agent in the skin layer.

 

Skin layer leveling time:

After the foamed polyurethane is poured into the model, the longer the surface is leveled, the thicker the skin. Reasonable control of the leveling time after pouring is also one of the means to control the thickness of the skin.