Quality defects of PPR pipe extrusion and the solutionsQuality defects of PPR pipe extrusion and the solutions
PP-R (Poly Propylene Random) pipe, that is, random copolymer polypropylene pipe, is a new type of water pipe material commonly used in developed countries today. PP-R pipeline has been used in Europe for more than 10 years. Due to its excellent performance, its market share has been far ahead of other plastic water pipe materials in recent years. The production and application of PP-R pipes in my country also have several years. As the country vigorously promotes the application of chemical building materials, China's chemical building materials have entered the stage of industrialization development. The development of new plastic pipes has once again reached a new climax. PVC-C, PE-X, aluminum-plastic pipes and PP-R pipes are almost used in the hot and cold water and heating of buildings, and gradually transition to PP-R pipes. PP-R tube is one of the hot spots in production and application today. The characteristics of PP-R pipes are gradually recognized by China's pipeline engineering community. In addition to the general advantages of light weight, corrosion resistance, high strength, non-scaling and long service life, PP-R pipes are also clean and non-toxic , Good low-temperature impact resistance, long-term heat and pressure resistance, good thermal insulation and energy-saving performance, is a true green environmental protection building materials product, also has convenient system connection and installation, firm pipe connection, and a permanent connection Features. In addition, PP-R pipes have excellent performance-price ratio, which makes them stand out from various new types of water delivery pipes. They have developed rapidly in a few years and are widely used. There are currently more than 300 PP-R pipe production plants in China, and there are many kinds of PP-R pipes in the existing building materials market. Due to various reasons, the quality problems of PP-R pipes are still prominent, resulting in the advantages and characteristics of PP-R pipes It can not be fully reflected, which affects the function of pipeline engineering, and also lays hidden safety hazards for the corresponding project quality.
China has formally formulated and promulgated the national standard GB / T18742.2-20002 "Polypropylene Piping System for Hot and Cold Water", which was officially implemented on January 1, 2003. After the implementation of the standard, we conducted a comprehensive supervision and inspection of PP-R pipes in the market in accordance with the technical requirements of the National Standard "Polypropylene Piping System for Cold and Hot Water: Part 2: Pipes". Although the quality of R pipes has been improved to varying degrees compared to the past, the total pass rate of products of production enterprises and distributors is about 70%, and the pass rate of product quality of distributors is only 50%, indicating that the current market sales The proportion of unqualified PP-R pipes is still quite high, and the quality problems found in the inspection are varied. In 2006, the Zhejiang Chemical Building Materials Association entrusted the National Chemical Building Material Quality Supervision and Inspection Center to the results of product sampling tests conducted on some pipe markets in Zhejiang Province. The quality status of plastic pipe products was shocking. The qualified rate of polypropylene PP-R pipe sampling is only 10%.
Now briefly analyze the various factors that affect the quality of PPR pipe:
1. Raw materials for PP-R pipe production
The chemical name of PP-R raw material is: propylene-ethylene random copolymer, which is copolymerized by a mixed gas of propylene and ethylene to obtain a copolymer with propylene and ethylene segments randomly distributed in the main chain. Good quality PP-R raw materials should meet the requirements of GB / T1888742.1 standard, with minimum required strength SMR guaranteed value. The better quality PP-R raw materials are mainly from companies such as Borealis and Hostalen. The raw materials provided have SMR guarantee values, and the relationship between the performance of the raw materials and the performance of the pipe is studied in depth, especially the ethylene content is related to the overall performance of the pipe influences. At present, the grades of PP-R materials on the market are complex, the performance is uneven, and there are great differences in use. Some grades of PP-R materials are not suitable for hot water pipes and pressure pipes. The propaganda is not very practical, and it is easy to cause chaos in the currency market.
2. PP-R tube appearance quality problems and analysis ...
According to the national standard, the appearance of qualified PP-R pipes should be as follows: the color should be basically the same; the internal and external surface ICI should be smooth and flat, without dents, bubbles and other surface defects that affect performance; it should not contain visible impurities.
The national standards for pipe appearance quality standards, although only a simple qualitative description of the text, but all its qualitative indicators can reflect the important inherent quality of the pipe. Whether the appearance of the pipe is qualified is closely related to the raw materials and processing technology of the product. Therefore, the appearance quality of the pipe not only affects the appearance of the pipe, but also affects the physical and chemical properties of the pipe, often determining the overall quality of the pipe. Therefore, the appearance inspection of the pipe is very important, and the overall quality of the product can be preliminarily judged "from the surface and inside".
2.1 Appearance and color
According to national standards, the appearance and color of qualified PP-R pipes should be basically the same. Regarding the quality requirements of the appearance and color of the pipe, the standard is only a stipulation of simple principles, but the problems of the appearance and color of the PP-R pipe in our daily inspection are various. Some pipes are locally discolored or have spots on the surface. The appearance and color cannot be done To be uniform; some surfaces are dull and dull or there are bright and transparent blocks on the outer surface. All the pipes with local discoloration, spots on the surface, dull and dull surface, and bright and transparent blocks on the surface have unqualified appearance and color. Most of the physical and mechanical performance indicators do not meet the standard requirements, and they are easily broken during use. Although there are a few pipes with bright and bright appearance or colorful colors, these pipes are mostly fake and inferior products, and their performance indexes are far from the national standard requirements.
2.1.1 Local discoloration, resulting in uneven color appearance
(1) Causes. PP-R pipe is made by extruding PP-R resin. The role of the extruder can make the PP-R resin plasticize from the crystal to the high elastic state, and then replasticize to the melt viscous flow state. If the PP-R raw material is subjected to inappropriate heat and pressure during extrusion molding , Will make the molecular configuration change when the raw material state changes. Too high a plasticizing temperature destroys the molecular structure of PP-R and causes discoloration. At the same time, the extrusion speed is too fast, which generates high friction heat in the barrel, causing the temperature of the material to rise and decompose
(2) Prevention measures. In view of the special properties of PP-R resin, it is required to avoid excessively high plasticizing temperature and excessive extrusion speed during the extrusion processing of PP-R pipe, causing changes in the molecular structure of PP-R resin. So for extruder screw production of PP-R pipe
The design of the rod requires the use of advanced computer models, so that it has a good low-temperature plasticizing ability and the best extrusion speed. It is required that the structure of the screw can meet the PP-R material can be extruded at a lower temperature and the best extrusion speed, and to ensure the quality of plasticization without reducing the amount of extrusion.
The screw barrel of the extruder used for the production of PP-R plastic pipes must be accurately calculated using advanced computer models to determine the pressure distribution and melting rate of each section, so that it has good low-temperature plasticizing ability.
2.1.2 Surface spots, so that the appearance of color is uneven
(1) Causes. The cause of the spots on the surface of the pipe may be caused by the processing raw materials, or it may be caused by defects in the extrusion process.
The presence of moisture in the PP-R resin raw materials used to produce pipes may cause spots on the surface of the extruded pipes.
When using vacuum tank sizing in the process of extruding and adding soil, if there are bubbles on the pipe in the water tank, it will also cause spots on the surface of the extruded pipe.
(2) Prevention measures. Remove moisture from raw materials or eliminate air bubbles on the tubes in the sink. In order to ensure that there are no air bubbles on the sink pipe, it must be done: to prevent the circulation pump of the sink from generating cavitation, keep the water surface in the sink above the water outlet, and increase the circulation of water in the sink.
2.1.3 The surface is dull and dull
(1) Causes. The cause of such problems may come from raw materials, or may be due to inappropriate processing parameters.
There is moisture in the raw materials used to produce pipes, which may cause the surface of the pipes to be dull and dull.
The melt temperature is not suitable for extrusion processing, the molten material extruded by the extruder is not uniform, the sizing sleeve is too short, the die forming section is too short, etc. may cause the surface of the pipe to be dull.
(2) Prevention measures. Remove moisture from raw materials or adjust process parameters of pipe processing.
Among them, adjusting the processing parameters of pipe processing includes: selecting the appropriate melt temperature, improving the uniformity of the molten material extruded by the extruder, using a long sizing sleeve, and increasing the length of the die forming section.
2.1.4 There are bright and transparent blocks
(1) Causes. The head temperature is too high and the cooling is insufficient.
(2) Prevention measures. Lower the head temperature and the cooling water is opened.
2.1.5 Bright or bright colors
Regular PP-R pipes should be matt gray or other colors. However, we found that a few PP-R pipes have bright and bright appearance or colorful colors during the market random inspection. These pipes are typical and inferior products. These pipes are not produced by PP-R resin. Compared with the regular PP-R, this kind of fake and inferior product also has some other prominent features: its density is slightly larger, the sound of landing is relatively crisp, and the feel is smoother.
This kind of pseudo PP-R pipe is very harmful, and the service life is only 1 to 5 years, while the true PP-R pipe life is more than 50 years.
2.2 Surface defects
According to national standards, the internal and external surfaces of qualified PP-R pipes should be smooth and flat, without pits, bubbles and other surface defects that affect performance. We found in the daily inspection that the surface defects of PP-R pipes are also various, among which the surface roughness is not
Smooth, with bubbles and pits on the surface.
Defects on the surface of the pipe often affect the mechanical properties of the pipe, thereby leaving hidden dangers in the pipeline engineering. If there are depressions and bubbles on the surface of the pipe, under high-temperature and high-pressure conditions, stress concentration will occur in the depressions and bubbles, causing the pipeline to rupture.
2.2.1 The surface is rough and not smooth
(1) Causes. In the extrusion process of PP-R pipe, the tube blank extruded at high temperature needs to be vacuum-shaped by a vacuum sizing groove and cooled and shaped in time with appropriate cooling means and temperature to obtain a product with excellent appearance quality. The cooling and setting operation of the pipe is mainly to control the two parameters of vacuum degree and cooling rate. The correctness of the vacuum control of vacuum setting will directly affect the surface quality of the pipe. Too high a vacuum will make the surface of the pipe rough; too fast cooling rate will easily cause stress in the pipe and cause the surface of the pipe to be rough.
In addition, the moisture contained in the PP-R resin raw material will also cause the inner and outer surfaces of the pipe to be rough.
(2) Prevention measures. Adjust and control the vacuum degree and cooling rate of the vacuum sizing tank to the best state.
Remove moisture from PP-R resin raw materials.
2.2.2 Bubbles appear on the surface
(1) Causes. Extrusion temperature has a very important influence on the quality and output of pipe products. If the temperature is too high, the polymer will decompose and bubbles will form on the surface.
The raw material contains moisture, which may cause bubbles in the melt and bubbles on the surface of the pipe.
(2) Prevention measures. Correctly control the extrusion temperature of the pipe; remove moisture from the PP-R resin raw materials.
2.2.3 Table ICI appears speckled pits
(1) Causes. The tube blank extruded at high temperature has a certain influence on the performance of the product when it is cooled and molded. If it is cooled too quickly, it will easily cause stress in the tube and reduce the appearance quality. If the flow of cooling water is too large, the surface of the pipe will be rough, and there will be spots and pits. The raw material contains moisture, which may cause pits on the inner wall of the pipe.
(2> Prevention measures.
Adjust and control the cooling rate of the vacuum sizing tank to the best state. Remove moisture from resin raw materials
3. The problem of opaque quality and its analysis ...
According to national standards, qualified PP-R pipes should be completely opaque. In the opaqueness detection of PP-R pipes, some pipes were found to be slightly or semi-transparent.
The light-transmitted or semi-transparent pipes found in the test are found to be fake and inferior products through further inspection. The resin components of the pipes are all miscellaneous PP-R resins. This kind of situation is mostly due to a small number of companies. Profit, reduce costs, and choose low-energy PP-R resin or other materials to pretend to be PP-R raw materials. All physical and chemical performance indexes and system adaptability of such pipes are far from meeting the standard requirements. If it is used in engineering, the harm is self-evident.
4. Specification and size quality problems and analysis ...
The national standard specifies the nominal outer diameter, the average outer diameter of the PP-R pipe and the wall thickness corresponding to the pipe series S.
The specifications and size indexes of pipes are important quality indexes of pipes.
During the inspection of the specifications and dimensions of PP-R pipes, the nominal outer diameter, the average outer diameter and the wall thickness of the pipes all failed. The qualification of the nominal outer diameter and the average outer diameter will affect the application and construction quality of the pipe. The pipes with unqualified specifications and size indicators cannot be well connected with the pipe fittings, and may reduce the compressive strength of the connection, leaving the pipe broken The hidden danger of leakage. The quality of the wall thickness will also affect the physical and mechanical properties of the pipe.
PP-R pipe in the process of extrusion molding and soil addition, in vacuum setting, whether the vacuum control is appropriate or not will directly affect the dimensional accuracy of the pipe. The vacuum degree is too low, the suction force of the tube blank is insufficient, and the tube blank is easily deformed, which causes errors in the nominal outer diameter, average outer diameter and wall thickness of the pipe.
The extrusion and traction speeds directly affect the wall thickness and dimensional tolerance of the pipe. Extrusion and traction speeds are too fast, resulting in negative errors in pipe wall thickness, and extrusion and traction speeds are too slow, causing positive errors in pipe wall thickness.
(2) Prevention measures
The vacuum degree of the vacuum setting is adjusted and stabilized at the optimal value. The control of the vacuum degree is closely related to the sealing degree of the vacuum tank and the performance of the vacuum pump.
The control eliminates the instability of the extrusion molding system and ensures the precise and constant extrusion and traction speed.
5. Physical and chemical quality problems and their analysis ...
National standards stipulate the physical and mechanical properties and chemical performance indexes of PP-R pipes: including longitudinal shrinkage rate, simply supported beam impact test, liquid net pressure test, melt mass flow rate, and thermal stability test under net hydraulic state.
In the examination of the physical and mechanical properties and chemical properties of PP-R pipes, we found that some pipes produced cracks and leaks during the liquid net pressure test. The liquid net pressure test project is the key physical and mechanical performance index of PP-R pipe, which directly affects the performance and life of the pipe.
The main reasons for the failure of the liquid net pressure test are not only that the raw materials of the pipes do not meet the requirements of the standards, but also that the processing equipment and processing parameters of extrusion molding are not standardized.
Raw material problems caused cracks and leakage during the liquid net pressure test
The reasons are: (1) According to the national standard, the production of PP-R pipes should use regular PP-R resins that meet the requirements of GB / T1888742. 1 standards, and the raw materials should have the minimum required strength SMR guaranteed value. However, some PP-R pipes use miscellaneous PP-R resins of unqualified quality. Such PP-R resins have too many branches in the molecular chain and lack SMR data. The pipes produced with them have poor high temperature resistance; (2) Use regular PP-R resin that meets the requirements of GB / T1888742.1, but due to chaos in production management, impurities or moisture are mixed in the production process, so that there are visible impurities or bubbles on the surface of the pipe, and stress concentration will occur at the visible impurities or bubbles Therefore, the hydraulic pressure test of the pipe fails.
Extrusion processing equipment and processing process parameters are not standardized. The surface of the pipe has spot pits and bubble defects (as mentioned earlier in this article). Similarly, spot defects such as spot pits on the surface of the pipe will also cause stress concentration. Cause the hydraulic pressure test of the pipe to fail.
(2) Prevention measures
Use regular PP-R resin that meets the requirements of GB / T1888742. 1 standards to strengthen management to prevent impurities or moisture from being mixed into the raw materials. Use processing equipment and processing technology parameters that meet the requirements of extrusion molding to prevent spot pits and bubble defects on the surface of the pipe.
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