When dyeing at high temperature with dyeing machine, nylon is usually dyed with disperse dyes, neutral dyes and acid dyes. Disperse dyes are mainly dyed by the molecular gravity between dyes and nylon, so it is easy to reduce color. Neutral dyes and acid dyes can not only dye by molecular gravity between dyes and nylon, but also dye by ionic bond with NH2 in nylon. Therefore, the combination fastness between neutral dyes, acid dyes and nylon is relatively good, and color subtraction is relatively difficult.

When dyeing nylon with disperse dyes, it is easier to reduce the color with alkali agent. However, the color light changes greatly and darkens seriously. When nylon is dyed with neutral dyes, soda ash treatment has almost no color reduction ability, and the color light changes. Caustic soda has color reducing ability, and the higher the concentration, the greater the color reducing ability and the greater the change of color light. When acid dyes are used to dye nylon, soda ash has little color reducing ability, but the change of color light is small; Caustic soda has slight color reduction ability, but the color light changes seriously.

It can be seen that alkali agent is used to reduce the color of nylon dyes, either the color reduction rate is low or the color changes seriously (especially darkening). This brings great difficulties to the next color repair, and even can not repair the original color light.

Therefore, when the nylon dye must be reduced (lightened), it can only be stripped to an appropriate extent with stripping agent according to needs, and then proofing and re dyeing (color trimming).