PVC common problem causes

May 7, 2019

PVC common problem causes
       In philosophy, it is especially important to grasp the main contradictions and see the essence of things. There are various problems in plastic processing. Only by thoroughly understanding the causes of the problems will we be able to achieve targeted and accurate responses.
1, the problem of air holes
     Where does "air" come from? In general, it should be considered from three aspects: pvc compound raw materials, pvc compound formula, equipment technology.
       Moisture problem. The gasification temperature of water is lower than the processing temperature of conventional PVC products. Therefore, water vapor is released during the process. If the water vapor is not completely discharged, it will form pores on the surface of the product. Therefore, first check the moisture of the raw materials to see if it exceeds the standard. This condition is generally manifested in the kneading process and the vacuuming of the extruder.
      Stability and processing technology. PVC is a relatively special polymer material, its decomposition temperature is lower than the processing temperature. If the amount of stabilizer is insufficient, it will cause the decomposition of PVC, accompanied by the release of HCl gas, resulting in "pores". When this problem is serious, it usually comes with a color change. Therefore, the problem of porosity and stability is also an important part of the investigation.
In addition, there is a problem in that the device itself has a problem, and the compression ratio is small, which causes the exhaust gas to be incomplete at the time of granulation, and also causes a pore problem.
2, the surface is rough
     There are two types, one is knot and the other is pitting.
     knot, it can be simply understood as "poor plasticization" or "unplasticized."
     Cause One, the problem of resin raw materials - fish eyes. In the polymerization process of PVC, due to the production process problem, a small amount of high molecular weight PVC particles with a bulk molecular structure are produced, which has low ability to absorb plasticizers, and can only swell plasticizers under normal processing conditions without plasticizing them. It cannot be melted and plasticized, and it is "inclusion" in molten PVC. In the field of PVC cables, the "fisheye" causes the surface to crease and affect its insulation. The cockroaches caused by fish eyes tend to be small.
      Cause 2, partial unplasticization caused by unevenness. The filling powder aggregates, which cannot be plasticized during the extrusion process and is caused by the plasticized PVC material, so attention should be paid to the mixing effect and the surface treatment of the filler.
      Pitting problem
      The problem of pitting is relatively complicated, but its causes can be divided into two aspects:
      1. Volatile residue.
      The plasticizer or lubricant used in PVC is partially volatilized during processing, and this part of the volatilization is very special. It is formed only in the later stage of extrusion. At this time, it cannot be discharged, and it can only be extruded before the material is extruded. And due to volatilization, the compatibility of the two decreases, the generated gas tends to migrate to the surface of the fluid, and to the die, there is no pressure to bind the volatilized gas, and the gas escapes from the surface, leaving "pitting" ”
       2, insufficient lubrication
      In the case of insufficient external lubrication, the friction between PVC and metal is large, especially under the condition of insufficient lubrication at the later stage. Under the extrusion condition, the PVC material adheres to the metal surface, resulting in rough surface and pits.
      3, soft products are brittle
1, PVC resin selection problem
2, the filler is filled high
3, excessive lubrication
4. Poor plasticization caused by low processing temperature
4, scorch
  Phenomenon: The smoke of the die is large, the smell is strong and there is a buzzing sound. The surface of the plastic has a particulate scorch. There are continuous bubbles at the meeting place.
  Reasons and improvements:
  Cause 1: The screw was used for a long time and was not cleaned, and there was accumulated char in the inside, which was taken out during extrusion.
  Improved method: disassemble the screw, clean the screw, and remove the accumulated charred material in the dead corner.
  Reason 2: The heating time is too long, and the PVC is aged and burnt.
  Improvement method: shorten the heating time, check whether the heating system has any problems, and timely repair.
  Reason 3: The parking time is too long and the plastic decomposes and burns.
  Improvement method: change the mold to drive in time when parking, and clean the machine head thoroughly after scorching;
  Reason 4: The mold change is frequent, the glue is not exhausted, and the plastic is burnt.
  Improvement method: the mold should be cleaned
  Reason 5: The confrontation between the auxiliaries used, with the reaction of gas release.
  Improved method: check the additives used, especially the acidity and alkalinity of the additives
5, poor extrusion plasticization
  Phenomenon: The surface of the plastic has a suede phenomenon, the surface is black, and there are tiny cracks or small particles that are not plasticized. There is a clear trace at the meeting place.
  Causes and improvements
  Reason 1: The processing temperature does not match;
  Improved method: increase processing temperature;
  Reason 2: When granulation, the plastic is mixed unevenly or there are particles in the plastic that are difficult to plasticize.
  Improvement method: the mold sleeve can be appropriately matched to increase the pressure of the glue outlet. ;
  Reason 3: The screw and traction speed are too fast, and the plastic is not completely plasticized.
  Improved method: appropriately slow down the speed, prolong the residence time of the material in the screw, and improve the plasticizing effect.
6 "fake fisheye" problem
     “Fake fish eyes” in PVC processing is very common, and “most fish eyes” in PVC processing belong to “fake fish eyes”.
It is not easy to plasticize under normal PVC processing conditions, and when the temperature is high and the shear is strong, such "fake fish eyes" can still be plasticized. The structure of the polymer determines the performance. The essence of the fake fisheye is that the molecular weight distribution of PVC is wide, and some particles cannot be plasticized normally at a specific processing temperature.
Response plan:
A. Increase the mixing temperature and extend the mixing time;
B. Increase the number of mesh screens and increase the pressure on the head;
C. Slow down the speed of granulation and increase the residence time of the material in the extruder;
D. Replace the strong shear screw combination;
E. Secondary granulation of particles containing fish eyes

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