High processing temperature ? too large mold compression ratio ? not enough for stabilizer ? the problem of lubrication sysytem ? not stability for pvc resin ? wet Caco3 ? the problem of gap between the screw and barrel ? too large shear ? We need check each step carefully. 

I only talk stabilizer and lubricants as below. Pvc stabilzier and lubricant are main reasons for color changing in pipe. 

For stabilzier : Reason 1: not enough for stabilzier Reason 2: Your stabilzier quality is not good. Reason 3: If it is ca.zn stabilzier system, wet material may also cause color change. 

Solution: add your stabilizer filler and change good-quality stabilizer. 

For lubricant system. Reason 1: too less outer lubricant dosage. It also lead pipe fragile. PLS try to make your lubricantion system keep dynamic balance. 

Solution : add extra 0.1 phr Stearic acid & 0.3 titanium dioxide ( Tio2) , it can makes your pipe very white. If you are not lack of money, i recommend rutile Tio2. It is better than anatase. Because the base color of Rutil titannium is partial red, anatase titannium is partial blue. Rutil Tio2 is better with higher price.

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