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Why do PVC products precipitate?

Why do PVC products precipitate?

2021-09-03

Many PVC manufacturers will encounter precipitation problems in the production process. Today, Qingdao sainuo polyethylene wax manufacturer will take you to understand the relevant problems and solutions related to precipitation.

1、 What is precipitation?

Precipitation refers to the accumulation of substances in the setting sleeve, mold and other parts in the production process, which affects the surface. No matter what kind of stable system, there are precipitation problems in varying degrees in the PVC extrusion process. According to our analysis, there are lubricants and a small amount of fillers and low molecular polymers in the precipitates. PVC products are often precipitated during and after production, mainly on the die and setting sleeve. In the customer's words, it means spraying cream or oil. In fact, these are two different precipitation types. 

1) In the processing of PVC mixture, the additive components precipitate from the mixture and adhere to the mold and pressing roller to form a harmful film or a longer "moustache" on the die, which is called pressure analysis.

2) During the use of PVC products, if the additive precipitates from the surface of the product, the white or yellow wax (oil) that appears on the die is called spray cream. If it is oil, it is called oil, or sweating.

2、 What are the precipitation mechanisms and factors?

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

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

1) There is a certain amount of low molecular volatiles in the formula components, which has a poor exclusion effect in the process of mixed extrusion.

2) The lubricant with low melting point has poor compatibility with other components in the formula.

3) The extrusion die temperature is low and too close to the setting die.

4) The cooling water temperature of the setting die is too low, and the design of the cooling water channel is unreasonable.

5) Unreasonable production formula, improper selection of raw materials and poor plasticizing effect.

6) The precipitation of setting sleeve is easy to occur after the use of calcium zinc stabilizer. The reasons are complex. Try to use external lubrication with high efficiency and high melting point. On the premise of meeting the processing requirements, try to use less lubricant (including internal and external lubrication), and pay attention to melt strength and demoulding.

The reason for the growing the "moustache" on the die, the longer the moustache. It may be that the material melt strength is insufficient due to lack of lubrication in the later stage, and the demoulding effect is not enough in the later stage.

When the above factors are basically the same, the precipitation in different degrees in the production of various manufacturers should be directly related to the specification, origin and stable system of raw materials used.

ope wax

As we all know, the plasticizing capacity of parallel twin-screw is worse than that of conical twin-screw, that is to say, improving the plasticizing capacity of stabilizer is an important way to solve the precipitation. According to the above analysis, the calcium zinc stabilizer used for pipes, especially the calcium zinc stabilizer used for parallel twin-screw, shall follow the following design concept:

(1) Low molecular materials are rarely used or not used;

(2) The wax with various melting points shall be used together, and the internal and external lubrication shall be balanced;

(3) The materials used shall avoid affecting the plasticization as far as possible;

(4) Improve compatibility.

At present, product manufacturers generally use equipment suitable for lead salt stabilizer to directly replace the environmental protection stabilizer, and the equipment applicability and operation process setting need to be adjusted and changed. In this regard, of course, equipment suppliers and mold manufacturers need to see the business opportunities and invest in the research and development of screws and molds suitable for the processing rheological characteristics of environmental protection stabilizers. A large number of front-line operators also need to learn more and think frequently in the daily operation process to formulate the production process suitable for environmental protection stabilizers. Take pipe extrusion as an example:

(1) Screw speed control is not accurate. Now, due to the continuous progress of variable frequency speed regulation technology, this situation is rare at present;

(2) If the heating system is not controlled accurately and the barrel temperature is high or low, extrusion fluctuation will also occur;

(3) The material conveying is unstable, and the amount of light calcium powder used is too high, resulting in uneven mixing, fluffy mixing, uneven blanking speed and bridging of biological materials in the hopper, which is the most common problem at present;

(4) Insufficient plasticizing capacity and unbalanced internal and external lubrication will also cause uneven discharge;

(5) Too much lubricant, especially too much external lubrication, will cause the material to slip between the barrel and the screw, which can not form a strong solid plug, and there will also be discharge operation fluctuation.

Compared with the composite lead stabilizer, the biggest disadvantage of the environmental protection stabilizer is that the components of the environmental protection calcium zinc stabilizer are complex, resulting in many factors affecting the compatibility between the stabilizer and the resin and the plasticization effect of the resin. The precipitation in the extrusion process is the most difficult to overcome. The precipitates adhere to the mold, setting sleeve and other parts, seriously affecting the surface finish of the products, It will also affect other properties of the product. Each link must be well controlled so that there will be no common problems.

Therefore, in the production process, it is necessary to increase the PVC melt fluidity, significantly improve the demoulding effect, and increase the product surface smoothness and finish, which can effectively solve the problem of precipitation of die and sizing sleeve.

Qingdao Sainuo Chemical Co., Ltd. We are manufacturer of  PE wax, PP wax, OPE wax, EVA wax, PEMA, EBS, Zinc/Calcium Stearate…. Our products have passed the REACH, ROHS, PAHS, FDA testing. 

Sainuo rest assured wax, welcome your inquiry!

Website:https://www.sainuowax.com

E-mail:sales@qdsainuo.com

               sales1@qdsainuo.com

Address:Room 2702,Block B, Suning Building, Jingkou Road, Licang District, Qingdao,China


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