The "precipitation" problem often encountered in PVC production is not complicated at all, and you can fully understand it with a daily life analogy.
Just like making sugar water, if you add too much sugar with insufficient water, there will be a layer of undissolved sugar settled at the bottom. The precipitation in PVC production follows the exact same logic. PVC itself is a base resin, and various additives such as plasticizers, lubricants, stabilizers and fillers need to be added during production. If a certain additive is added excessively, or it is inherently incompatible with PVC and cannot be fully integrated into the system, it will gradually migrate out of the material over time, float to the surface of the finished product, and form white frost or oily sticky layer — this is what everyone calls "precipitation".
This situation can be seen almost every day in the workshop, all of which are real and common cases. When producing soft PVC wire sheaths, to make the sheath more flexible and easy to bend, the plasticizer content may accidentally exceed the standard. In the hot and poorly ventilated workshop in summer, after the finished products are stored in the warehouse for half a month, an oily layer will appear on the surface of the sheath, which feels sticky — this is the typical plasticizer migration. When producing PVC water supply pipes, to make the pipes easier to demold from the mold, the content of lubricants such as stearic acid and paraffin is excessive. Only 2 hours after startup, a layer of white wax will form on the die of the extruder. The white frost on the produced pipes will reappear not long after wiping, which is the lubricant precipitation. Sometimes, to reduce costs, a batch of cheap calcium-zinc stabilizer is used. Only half an hour after startup, a layer of yellow greasy scale will form on the sizing tank, which will reappear soon after wiping — this is the precipitation of incompatible small molecular components in the stabilizer. Many factories add excessive calcium carbonate to the material to cut costs. Finally, the produced profiles will shed a layer of white powder when wiped, and no more powder will fall off after wiping — this is the filler precipitation.
When encountering the precipitation problem, there is no need to guess randomly. There are several practical tips in the workshop that can be used immediately, without laboratory testing, and you can find the root cause in half a day. At the beginning of startup, take a clean white cotton cloth to wipe the die of the extruder. If what you wipe off is oily sticky substance, it is most likely plasticizer migration; if what you wipe off is waxy white powder, it can basically be confirmed as lubricant precipitation. Increase the temperature of each section of the extruder from the feeding section to the die by 5℃ in turn, and keep stable production for 10 minutes after each adjustment. If the precipitation increases sharply after the temperature of a certain section is raised, it indicates that the temperature of this section is too high, which "heats" the additives inside to migrate out. Stop production temporarily for 15 minutes and then restart, observe the first precipitates coming out of the die. If they are yellow-brown greasy substances, it is basically the precipitation of small molecular components in the stabilizer. First use a batch of qualified stabilizer you are familiar with for trial production. If the precipitation disappears directly, it indicates that the previous batch of stabilizer has quality problems. Then replace the plasticizer for trial, and you can quickly find the problematic raw material. Finally, cut several pieces of fresh PVC samples, put them in a 40℃ constant temperature box for 24 hours. If the precipitation on the surface increases significantly after taking them out, it proves that the problem lies in the formula itself, and the compatibility of additives in the formula is too poor.
In the final analysis, PVC precipitation is never a "difficult and complicated problem". It is essentially a small deviation in one of the links of formula, raw material and process. Without relying on complex instrument detection, you can quickly locate the root cause of the problem through these practical on-site investigation methods, no need to blindly change materials and repeatedly adjust the machine to test. It can not only reduce the loss of waste products by avoiding detours, but also make the production line return to stable operation quickly.
