UPVC pipes will expand and contract with heat during use. The national standard quotes "longitudinal shrinkage rate" to indicate. Longitudinal shrinkage refers to the percentage change in the length of the pipe at a specific temperature (23～150oC). GB 6671. 1-86 (Determination of the longitudinal shrinkage rate of unplasticized polyvinyl chloride (UPVC) pipes" clearly stipulates the measurement plan and standard requirements. Controlling the longitudinal shrinkage rate of the pipe is to control the thermal expansion and contraction range of the pipe for the normal In use, the longitudinal shrinkage rate of the pipe must be strictly controlled in accordance with GB 6671. 1-86.
The longitudinal shrinkage rate of UPVC pipes is related to many factors. How to effectively control the longitudinal shrinkage rate is a concern for UPVC pipe manufacturers. In this article, the author analyzes several main factors that affect the longitudinal shrinkage rate of UPVC pipes.

1 Mechanism analysis of influencing factors

1.1 The influence of UPVC pipe formula
1.1.1 The influence of PVC resin
PVC plastic is composed of PVC resin and auxiliary materials, among which the PVC resin component accounts for more than half of the entire formula; besides, the "thermal expansion and contraction" changes of PVC plastic mainly come from the entanglement, twisting, thermal expansion and coldness of PVC resin molecules. Reduced changes.
PVC molecular weight and molecular weight distribution range both affect the agglomeration state of plastics after processing and plasticization. The agglomeration change of PVC resin with large molecular weight is small, and the agglomeration change of PVC resin with small molecular weight is large. Therefore, the large molecular weight of PVC resin has a small effect on the longitudinal shrinkage rate, on the contrary, the small molecular weight has a greater effect on the longitudinal shrinkage rate. The wide molecular weight distribution indicates that the molecular agglomeration change range of PVC plastic is large during processing; the narrow molecular weight distribution indicates that the molecular agglomeration change range of PVC plastic is small during processing. Therefore, the longitudinal shrinkage rate of the PVC resin with a wide molecular weight distribution range is large; on the contrary, the longitudinal shrinkage rate of the PVC resin with a narrow molecular weight distribution range is small.
1.1.2 Filling
PVC plastic processing is generally filled with different amounts of inorganic fillers. It is not only to reduce costs, but when the amount of fillers is added to a certain level, it can also fill the matrix to improve the hardness and related physical properties of the plastic products. In the UPVC pipe processing formula, as the amount of filler (CaCO3) increases, the longitudinal shrinkage rate of the pipe gradually decreases.

1.2 Design of extrusion die
The extrusion die is not only the geometric shape and related dimensions of the shaped product, but also a channel for conveying the uniformly plasticized melt from the barrel. When designing the extrusion die, the rheological behavior of the melt must be fully considered.
1.2.1 Compression ratio of extrusion die
The molten material is extruded into a cylindrical shape from the inner hole of the extruder barrel. The front end of the extrusion die divides the molten material into a tube shape, expands in the direction of the material flow, and then gradually decreases toward the outlet to form a certain expansion and compression. The so-called compression ratio refers to the ratio of the largest cross-sectional area of ​​the pipe die runner to the cross-sectional area of ​​the die.

The compression ratio is large, the shear force of the plastic melt flowing through the die of the die is large, the molecules are entangled, compressed, and agglomerated, and the residual "stress" of the hot bubble tube flowing out of the die is large, so the longitudinal return The shrinkage rate is large; on the contrary, the compression ratio is small, the shearing force of the plastic melt flowing through the die of the die is small, the various reaction forces are small, and the longitudinal shrinkage rate is small.
1.2.2 Stretch ratio of bubble tube
The bubble tube from the extruder head can be formed into a tube after being drawn, stretched and shaped. Therefore, the bubble tube extruded from the die must be stretched. Due to the compression of the extrusion mold, the bubble tube exiting the die will expand without stretching. Moreover, the compression ratio of the mold is different, and the expansion ratio of the bubble tube is also inconsistent.
The stretch ratio of the pipe refers to the ratio of the cross-sectional area of ​​the die to the cross-sectional area of ​​the pipe after forming. The above-mentioned stretching ratio can only reflect the stretching status from the die. If the cross-sectional area of ​​the same die but the compression ratio is different, then its stretching ratio is the same, but the expansion of the extruded bubble is not consistent, and the internal stress of the bubble is different. . The concept of stretch ratio can only explain the relationship between the mold and the pipe. To explain the longitudinal shrinkage rate, the author believes that the concept of "bubble tube stretch ratio" should be introduced. "Bubble stretch ratio" should be the ratio of the cross-sectional area of ​​the extruded bubble to the cross-sectional area of ​​the tube after forming. The "bubble tube stretch ratio" must be obtained through experiments to measure the size of the bubble tube after being extruded from the die without traction and free expansion. When the "bubble tube stretching ratio" is large, the bubble tube molecular orientation is large, and the bubble tube is cooled quickly, the tensile stress in the memory is large, and the longitudinal shrinkage rate is large; on the contrary, when the "bubble tube stretching ratio" is small, the bubble tube molecular orientation Small, the longitudinal retraction rate is small.

1.3 Process control
In extruded pipe production, various mechanical and physical properties of the pipe are related to the process control of the extrusion process. Reasonable process settings can make the PVC mixture form a melt with a certain degree of plasticization and uniform plasticization, so the longitudinal shrinkage rate of the pipe is also related to process control. The author discusses from the following two aspects.
1.3.1 Temperature control of die
The melt flows through the die, and there is very little heat to be compensated, so the temperature of the die should not be too high. If the temperature of the die is too high, the low-molecular compounds on the outer wall of the bubble will volatilize, and the bubble will expand severely, resulting in an increase in the longitudinal shrinkage; if the temperature of the V1 die is too low, the low-molecular compounds on the outer wall of the bubble will not be well obtained At the same time, the bubble tube cannot be "stretched" freely, causing the longitudinal shrinkage rate to increase. Therefore, the temperature of the die should be 10~15~C lower than the highest temperature of the barrel.
1.3.2 The cooling rate of the bubble tube
For processing with general extrusion equipment, the die of the bubble tube is extruded into the vacuum sizing device for rapid cooling. However, in such a processing technology, the process of forming the tube becomes shorter. Rapid cooling is a sharp decrease in the temperature of the pipe, and the internal stress of the pipe is "frozen", so the longitudinal shrinkage rate of the pipe becomes larger.
At present, through experiments, the cooling of the bubble tube is changed to a two-stage type, which makes the cooling time of the tube longer and essentially lengthens the cooling device, which not only meets the needs of rapid production, but also realizes the effective control of the longitudinal shrinkage rate of the tube. Therefore, the longitudinal shrinkage rate of the pipe increases as the cooling rate increases.

2 Experimental analysis
2.1 The relationship between the filler and the longitudinal shrinkage rate of the pipe
2.1.1 Experimental part
(1) Material
PVC resin, SG-5, produced in Qingtongxia meets GB/T 5761-93; light CaCO3, meets HG 2226-91; others: stabilizers, lubricants, modifiers, etc. remain unchanged.
(2) Equipment
Extruder: SJSZ-65, twin-screw counter-rotating pipe extrusion unit, made in Wuxi; φ160 pipe mold, compression ratio 19:1, self-made; experimental oven, 101-3 type, made by Changchun Intelligent Testing Machine Research Institute.
(3) Sample preparation
The dosage of stabilizer, lubricant, and modifier in the test production formula remains unchanged. The dosage of CaCO3 is (a), 10 parts, (b), 20 parts, (c), 25 parts, (d), 30 parts, (e), 35 parts and (f), 40 parts were mixed with 6 materials to test and extrude O160×3.5 pipes. In this way, 6 samples can be obtained.
(4) Process setting
The processing technology is set as follows (see Table 1)

(5) Measurement of longitudinal retraction rate
The test is carried out according to GB 6671. 1-86. Measure three samples each time and take the average value. The measured values ​​are shown in Table 2.

2.1.2 Analysis
The addition of CaCO3 can reduce the expansion and contraction of the plastic melt, so as the amount of CaCO3 added increases, the longitudinal shrinkage rate of UPVC pipes is gradually reduced; when the amount of CaCO3 added reaches a certain amount, it hardly affects The longitudinal shrinkage rate of UPVC pipe.
2.2 The relationship between bubble tube stretching ratio and longitudinal shrinkage
2.2.1 Experiment
(1) Material
PVC resin, SG-5, produced in Qingtongxia meets GB/T 5761-93; light CaCO3, meets HG 2226-91; others: stabilizers, lubricants, modifiers, etc. remain unchanged.
(2) Equipment
Extruder: SJSZ-65, twin-screw counter-rotating pipe extrusion unit made in Wuxi; φl10 pipe mold, compression ratio 19:1, self-made; experimental oven, type 101-3, made by Changchun Intelligent Testing Machine Research Institute.
(3) Sample preparation
The dosage of stabilizer, lubricant, and modifier in the test production formula is unchanged, and the dosage of CaCO3 is 40 parts for the test to extrude 110 pipes. The die gap is φl18×3.5mm, and the wall thickness of the heating bubble is measured to be φ121×4.0mm. Use this die to extrude pipes with four specifications, φl10×3.5, φl10×3.2, φl10×3.0, and φl10×2.8. The stretch ratios of the bubble tubes are 1.11, 1.20, 1.28, and 1.37 respectively.
(4) Process setting
The processing technology settings are shown in Table 3.

(5) Measurement of longitudinal retraction rate
The test is carried out according to GB 6671. 1-86. Measure three samples each time and take the average value. The measured values ​​are shown in Table 4.

Table 4 The relationship between bubble tube stretch ratio and longitudinal shrinkage rate

2.2.2 Analysis
According to the analysis in Table 4, the stretch ratio of the bubble tube of the extruded tube directly affects the longitudinal shrinkage rate of the tube.

3 Conclusion
(1) Through mechanism analysis, there are many factors that affect the longitudinal shrinkage of UPVC extruded pipes. Among the more important factors are the material factor in the formula of the pipe, the structure of the extrusion pipe mold, and the molding process.
(2) The filling amount of the filler in the formula has the greatest influence. Take the amount of CaCO3 added for analysis. As the amount of CaCO3 increases, the longitudinal shrinkage rate of the pipe decreases. When the amount of CaCO3 reaches a certain value , The amount of CaCO3 added is no longer the main factor affecting the longitudinal shrinkage of the pipe.
(3) The stretch ratio of the bubble tube is the main factor affecting the longitudinal shrinkage rate of the tube. As the stretch ratio of the bubble tube increases, the longitudinal shrinkage rate of the tube increases.

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