[Science Stickers] Cause Analysis of the Brittleness of PVC-U Plastic Pipe (Part 2)

The brittleness of plastics has always been a factor that plagues the normal operation of some companies. The brittleness of pipes has more or less affected the market share and user reputation of these pipe companies in terms of cross-sectional appearance and installation approval. The brittleness of pipes is basically It is fully reflected in the physical and mechanical properties of the product.

This article discusses and analyzes the reasons for the brittleness of PVC-U plastic pipes from the formula, mixing process, extrusion process, mold and other external factors.

The main characteristics of PVC pipes becoming brittle are: cracking and rupture during cold punching during blanking.

There are many reasons for the poor physical and mechanical properties of pipe products, mainly as follows:

Unreasonable extrusion process

(1) Excessive or insufficient plasticization of materials . This is related to the process temperature setting and the feeding ratio. If the temperature is set too high, the material will be over-plasticized, and some of the components with lower molecular weight will decompose and volatilize; if the temperature is too low, there will be no molecules in the components. Completely fused, the molecular structure is not strong. Too much feeding ratio will increase the heated area and shear of the material, and increase the pressure, which will easily cause over-plasticization; too small feeding ratio will cause the heated area and shear of the material to decrease, which will cause under-plasticization. Either over or under plasticization will cause pipe cutting and chipping.

(2) Insufficient head pressure , on the one hand, is related to the mold design (this is described separately below) on the other hand is related to the feeding ratio and temperature setting. When the pressure is insufficient, the density of the material will be poor, which will result in loose organization When the tube material is brittle, the metering feeding speed and the extruding screw speed should be adjusted to control the head pressure between 25Mpa and 35Mpa.

(3) The low-molecular components in the product are not discharged . There are generally two ways to produce low-molecular components in products. One is produced during hot mixing, which can be discharged through dehumidification and exhaust systems during hot mixing. The second is part of the remaining water and hydrogen chloride gas generated when the extrusion is heated and pressurized. This is generally forced to exhaust through the forced exhaust system of the exhaust section of the main engine. The vacuum degree is generally between -0.05Mpa and 0.08Mpa. If it is not opened or is too low, low molecular components will remain in the product, resulting in a decrease in the mechanical properties of the pipe. .

(4) The screw torque is too low . The screw torque is the value of the reaction machine under stress. The setting value of the process temperature and the feeding ratio are directly reflected in the screw torque value. Screw torque Too low reflects low temperature or small feed ratio to some extent, so that the material cannot be fully plasticized in the extrusion degree, and it will also reduce the mechanical properties of the pipe. According to different extrusion equipment and dies, the screw torque is generally controlled between 60%-85% to meet the requirements.

(5) The traction speed does not match the extrusion speed . Too fast hauling speed will cause the mechanical properties of the pipe to be thinned, and too slow hauling speed will result in high resistance to the pipe, and the product will be in a high-stretch state, which will also affect the mechanical properties of the pipe.

Unreasonable mold design

(1) The section design of the die is unreasonable, especially the distribution of internal ribs and the treatment of the angle of the interface . This will cause stress concentration to exist. Need to improve the design and eliminate the right and acute angles at the interface.

(2) Insufficient die pressure . The pressure at the die is directly determined by the compression ratio of the die, especially the length of the straight section of the die. If the compression ratio of the die is too small or the straight section is too short, the product will not be dense and the physical properties will be affected. Changing the pressure of the die head can adjust the flow resistance by changing the length of the straight section of the die on the one hand; on the other hand, different compression ratios can be selected to change the extrusion pressure during the die design stage, but it must be noted that the compression ratio of the die The compression ratio of the extruder screw is compatible; the melt pressure can also be changed by changing the formula, adjusting the extrusion process parameters, and adding a porous plate.

(3) For the performance degradation caused by poor confluence of the diversion ribs , the length of the ribs and the outer surface, the length of the ribs and the confluence of the ribs should be appropriately increased, or the compression ratio should be increased.

(4) The die is not uniformly discharged, resulting in inconsistent pipe wall thickness or inconsistent density. This also caused the difference in the mechanical properties between the two sides of the pipe. In our experiments, we sometimes cold-punched one side as qualified and the other side failed, which just proved this point. As for the thin wall and other non-standard pipes, I won’t say more here.

(5) The cooling rate of the shaping mold. The cooling water temperature often does not attract enough attention. The role of the cooling water is to cool and shape the stretched macromolecular chains in time to achieve the purpose of use. Slow cooling can give enough time for the molecular chain to stretch, which is conducive to shaping. In rapid cooling, the difference between the water temperature and the temperature of the extruded tube is too large, and the rapid cooling of the product is not conducive to the improvement of the product's low temperature performance.

From the explanation of polymer physics, the PVC macromolecular chain undergoes a process of curling and stretching under the action of temperature and external force. When the temperature and external force are withdrawn, the macromolecular chain does not return to the free state in time and is in the glass state. The disorderly arrangement results in low low-temperature impact performance of the macroscopic products.

From the perspective of plastic processing technology, it is explained that after the extrusion of PVC pipe, the product has a stress relaxation process after the temperature and external force are removed. Appropriate cooling water temperature is conducive to this process. If the cooling water temperature is too low, the stress in the product has not had time to eliminate, resulting in a decline in product performance. Therefore, the pipe cooling adopts a slow cooling method, which can prevent the warpage, bending and shrinkage of the molded product, and can prevent the impact strength of the product from being reduced due to the internal stress. Generally, the water temperature is controlled at 20°C.

In order to cool the parison softly without quenching, the water pipe connected to the cooling sizing sleeve is connected to the back of the sizing, and the water flowing in the sizing sleeve is opposite to the movement direction of the parison and discharged from the sizing sleeve . This will not cause rapid cooling of the parison due to too low water temperature, excessive internal stress, embrittlement of the pipe, and reduction of the impact resistance of the profile. Adding or reducing fillers, and adding fillers directly affects its flexibility index. If there is too much filler, the cold flushing of the pipe will not meet the standard.

If the filler is too small, the pipe will have a large rate of dimensional change. The same is that to increase or decrease the flexibility index, it is necessary to increase or decrease the impact modifier or processing aid, and the increase or decrease of the processing aid directly affects the rigidity index.

If there are too many processing aids, the rigidity index of the pipe will decrease; if the processing aids are too little, the rigidity index of the profile will increase . In the formula, the two are contradictory and unified mutually restrictive factors. It is unreasonable to increase the filler without principle while maintaining the flexibility index. Therefore, an optimal bonding point must be determined in the formulation system to achieve a balance between rigidity and flexibility.

The influence of extrusion process on pipe rigidity and flexibility index

The setting of the extrusion temperature is one of the factors that affect the degree of plasticization of the material. The low-molecular polymer in the material that is over-plasticized is decomposed and volatilized, resulting in intermolecular structural changes that will increase the rigidity index and reduce the flexibility index. Insufficient plasticization of the material and insufficient fusion of the molecules of each component in the material will reduce the rigidity index, and at the same time the flexibility index cannot be fully displayed.

Screw torque and extrusion pressure are directly proportional to the rigidity index of the profile, and increase with the increase of torque and pressure.

The flexibility index is inversely proportional to it, and decreases with increasing torque and pressure. What needs to be added is that when the extrusion is just started, it is accidentally found that individual profiles have no cracking phenomenon, but it is found that there are slight bubbles in the inner ribs, which is another new problem.

This article comes from the Internet, only for learning and communication, no commercial purpose.

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