PVC Profiles With Bubbles, Cracks, And Pockmarks: How To Fix Them？
Date:2022-7-07 Author:hcscrew
In the processing of PVC profiles, bubbles, cracks, and pockmarks are common. What can we do to solve these problems?
Several factors cause these defects.
1) Moisture and volatiles
There is a serious problem with moisture and volatile substances in the materials in the various formulations. It is not possible to completely remove them during extrusion and mixing, so the volatiles remain in the material. The result is bubbles appearing inside the PVC profile or on its surface. The heated profiles show bubbles, cracks, and pockmarks.
Solution:
It is imperative to exclude raw material defects when such defects appear in production. It is essential to establish reasonable conditions for high-speed mixing, which can effectively exclude volatiles.
2) Hyperthermia
PVC has poor thermal stability and is heat-sensitive. A lack of heat stabilisation or overheating due to the extrusion process can lead to incomplete discharge of the hydrogen chloride gas. This results in small dense holes in the profile section.
Pores are generated when the heating and cooling system of the extruder is able to control the extrusion temperature uniformly. A high extrusion temperature can lead to over-plasticization and gas residue in the profile or even cracks, while a low extrusion temperature will result in poorly plasticized material, leaving the profile with pockmarks, burrs, and rough surfaces.
Solution:
Controlling the plasticization of materials requires adjusting the process temperature. Extrusion generates heat primarily through the application of heat and friction. Initially, heat comes mainly from external sources, but after the machine starts, the friction heat increases continuously, sometimes even exceeding the material’s requirements. The cooling system must dissipate heat in order to prevent thermal decomposition. The heating and cooling system of the extruder should be able to ensure a stable extrusion temperature and high control accuracy. Screw speed, head pressure, and other factors affect shear friction heat. By adjusting the screw speed and head pressure during production, you can improve the heating and cooling system operating conditions to a certain extent.
3) Venting system function
Bubble generation, as well as the extruder venting system. There are 4 zones in a twin screw: supply area, compression area, venting area, and homogenization area. The feed zone pressure gradually increases, the compression zone pressure drops sharply, and the homogenisation zone pressure rises rapidly. Due to the pressure drop in the venting zone, the volatiles can easily escape from the melt.
Solution:
Extruder barrel venting port discharges them under a vacuum of not less than 0.05 MPA. Also, the material supply and extrusion volume of the extruder should be balanced. Material supply exceeding extrusion volume will result in the excess materials bubbling out of the barrel venting port and blocking the port when under pressure.
Implementation of the solution
PVC profiles with bubbles, cracks, and pockmarks can be eliminated one by one depending on the circumstances. First, check every raw material’s volatile fraction and check for any impurities. As well as adjusting the amount of lubricant and stabilizer in the formula. It is possible to reduce the shear friction heat by increasing the amount of lubricant moderately. To avoid serious decomposition of the material, stabilizer should be added moderately to ensure temperature-bearing capacity and thermal stability. Next, ensure that the temperature, screw speed, and extrusion pressure are set appropriately based on the characteristics of the extruder. Ensure that the venting is smooth and well sealed by checking the vacuum level and the condition of the venting hole. Lastly, make sure there are no dead corners in the mould runners.
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