The main reasons are the amount of Stabilizer is not enough or grade is not good, There should be redness of the phenomenon. It is possible to use a particularly poor quality Stabilizer.
But most of the problems are caused by the problem of Lubrication imbalance, too little Lubrication, especially outside the Lubrication is too little, excessive plasticization causes pipe discoloration, no toughness.
You can add some Titanium Dioxide, to see if it can be covered, if can’t solve the problem, you should add the amount of Stabilizer or change the Stabilizer. If it is Calcium Zinc Stabilizer, pay more attention to the adjustment of the Lubricant.
In addition, Lubricants affect plasticization, check whether Calcium Carbonate is added with Brightener or not，and maybe because of PVC Resin. Other conditions are the same, if you use the poor quality PVC Resin, will also lead to pipe becomes more yellow, then should add more Stabilizer.
Under normal circumstances, will think of the Stabilizer problem at the first time, In fact, in the case of sufficient stability, Lubrication is also a very important factor. In addition, If the materials（Calcium Carbonate、PVC Chemicals） are does not dry，will also affect the plasticization.
Temperature is too high when pipes extrusion, mold compression ratio is too large, Stabilizer addition is not enough, Lubrication system has problems, PVC Resin are not virgin, materials are not dry, the gap between screw and barrel and so on. Many reasons...Have to investigate according to the actual situation.
1. Add 0.1 more Stearic Acid, to promote plasticization.
2. 0.3 extra proper amounts of Titanium Dioxide, mainly to cover up the problem of background color.
3. Increase the amount of PVC Stabilizer, check the quality of PVC Stabilizers.