Why do PVC products precipitate?

Why do PVC products precipitate?

A.     What is precipitate?

Precipitation refers to the accumulation of material in the forming sleeve, mold and other parts in the production process, so that the surface is affected. No matter which kind of stable system has different degrees of precipitation problem in PVC extrusion process, according to our analysis, there are lubricants in the precipitation and a small amount of filler and low molecular polymer. PVC products in the production process and after production, commonly used precipitation, mainly appear in the mouth mold and shaper. In the words of the client, it's a frosting, or an oiling. There are actually two different types of precipitation.

1） PVC mixture processing, additive components from the mixture of precipitation, adhere to the mold, the roller, the formation of harmful film layer or mouth mold on the longer and longer "moustache", known as pressure analysis.

2）PVC products in the process of use, additives precipitate from the surface of the product, if the powder out of the solid, white or yellow wax (oil) appear on the mouth mold, known as frost, if it is oil, it is called oil, also known as sweating.

Mouth mold precipitation	Inwall precipitation
Mould precipitation	Sizing set precipitate

B.      What are the mechanism and factors of precipitation? 

The organic and inorganic solid components with low compatibility with PVC resin are transferred to the melt surface when thermal degradation, temperature and melt pressure change, and then attached to the metal surface of equipment and mold. Can be divided into "hot pressing" and "cold condensation".

The precipitation factors are multifaceted and very complex, related to equipment, raw materials and technology, etc., which are summarized as follows:

1）There is a certain amount of low molecular volatiles in the formula components, which is not good to remove in the process of mixed extrusion.

2）Low melting point lubricants have poor compatibility with other components of the formula.
3）The extrusion die temperature is low and too close to the mold die.
4）Mold cooling water temperature is too low, cooling water channel design is unreasonable.
5）Unreasonable production formula, improper selection of raw materials, bad plasticizing effect.

6）The precipitation of the set is easy to produce after the use of calcium and zinc stabilizer precipitation, the reason is more complex, try to use high efficiency and high melting point of external lubrication, and in the premise of meeting the processing requirements, try to use less lubricant (including internal and external lubrication), while paying attention to melt strength and release.

7）The longer and longer "moustache" on the mouth mold, this reason is more complicated, the author believes that due to the lack of lubrication in the late material melt strength is insufficient, the late demoulding effect is not enough.

Under the condition that the above factors are roughly the same, the different degrees of precipitation exist in the production of various manufacturers, the reason should be directly related to the specifications of raw materials used, origin and stable system. The most extreme case we encountered was that the stabilizers could be produced continuously for 7 days in the conical twin-screw equipment, and precipitates could be produced for half an hour in the parallel twin-screw equipment.

C.      Measures to reduce precipitation

1. Extruder Selection

1）The extrusion models match reasonably;

2）High fill formulation should be equipped with high shear screw；

3）Use vacuum, improve vacuum degree, reduce volatile components of materials;

4）Effective heating system cooling, prevent material later over plastic；

2. Matching of extrusion dies

1）The amount of extruder to match；

2）The proper compression ratio of the splitter cone at the extrusion die head reduces the dilatation；

3）Temperature setting of die forming plate；

4）The entrance grooving and temperature gradient design of vacuum forming section；

5）The auxiliary module design of the first vacuum setting mold；

3. Processing technical control

1）Reasonable mixing process temperature setting, adapt to seasonal temperature and humidity change；

2）Dry mixture curing time control；

3）The mixing pot exhaust smoothly；

4）Reasonable extrusion process temperature, avoid too high process temperature；

5）Try to avoid setting the mold melting pressure gradient of large changes in the process；

6）Reasonable control of mold temperature change.

D.       Case

There are different degrees of precipitation in the production of various manufacturers, the reason should be directly related to the specifications of raw materials used, origin and stable system. The most extreme case we encountered was that the stabilizers could be produced continuously for 7 days in the conical twin-screw equipment, and precipitates could be produced for half an hour in the parallel twin-screw equipment.

It is well known that the plasticizing ability of parallel twin screw is worse than that of conical twin screw, that is to say, improving the plasticizing ability of stabilizer is an important way to solve the precipitation. According to the above analysis, the calcium zinc stabilizer used for pipe, especially the calcium zinc stabilizer used for parallel twin screw should follow the following design concept:

1). Low molecular materials are used sparingly or not；

2). All kinds of melting point wax collocation, internal and external lubrication to balance；

3). The materials used should avoid affecting plasticization； 

4). Improve electromagnetic compatibility.

At present, the manufacturers generally adopt the equipment directly formula of lead salt stabilizer to replace the environmental protection stabilizer for production, and the equipment applicability and operation process Settings need to be adjusted and changed. In this aspect, of course, equipment suppliers and mold manufacturers need to see the business opportunities, invest in the research and development of screws and molds that adapt to the rheological characteristics of environmental stabilizer processing, and also need a large number of front-line operators to learn more and think more in the daily operation process, and develop production processes that adapt to environmental stabilizer. Taking pipe extrusion as an example, it may be:

a. Screw speed control is not accurate, now due to the continuous progress of frequency conversion speed regulation technology, this situation is rare now; 

b. The heating system control is not accurate, and the temperature of the barrel will fluctuate when it rises or falls;  

c. Material transport is not stable, the weight of light calcium powder is too high, causing uneven mixing, mixing is fluffy, causing uneven feeding speed, material bridge occurs in the hopper, which is the most common problem at present; 

d. Plasticizing ability is not enough, internal and external lubrication imbalance will also cause uneven discharge;

e. The amount of lubricant is too much, especially when the external lubrication is too much, causing the material to slip between the barrel and the screw, unable to form a strong solid plug, will also appear discharge operation fluctuation.

E.        Conclusion

Environmental stabilizer compared with lead composite stabilizer, the biggest drawback is that environmental protection components of complex calcium zinc stabilizer, stabilizer and the compatibility of resin, resin plasticizing effect is the influence of many factors, the precipitation is the most difficult to overcome shortcomings, in the process of extrusion deposition adhesion in the mold, molding, in areas such as the set of seriously affect the products surface finish, It will also affect the other properties of the product. Every link must be controlled so that common problems do not arise.

Therefore, in the production process to increase the flow of PVC melt, improve the demoulding effect, and increase the product surface smoothness, can effectively solve the problem of mouth die, diameter sleeve precipitation.