IMPORTANCE OF VACUUM IN PVC TWIN SCREW EXTRUSION
As dry blend from hopper enters the twin screw extruder & moves ahead, the hard skin of 90 - 250 micron secondary particles is torn apart due to shear & heat, exposing the hierarchical structure of PVC like sub grains, agglomerates etc for interaction. Interestingly, hard skin does not break during high speed mixing. Due to compression & heat, PVC sub grains, agglomerates etc, begin to loosely attach together. At vacuum port, PVC is neither powder nor fused. This is Gelation, a state prior to fusion At least, 600 mm Hg vacuum is applied to suck the gases but not PVC grains. This facilitating extraction of
1. Occluded air containing oxygen - Responsible for oxidative degradation
2. HCl generated during degradation of PVC - Responsible for accelerating thermal degradation.
3. Moisture & volatiles Responsible for the voids & less impact strength of extruded products.
If PVC at vacuum port is over gelled or is blocked, the extraction of gases will be difficult. Compound Lubrication & temp in BZ1 & 2 needs to be adjusted.
If screws and barrel wears out, powder gets sucked. Correct use of degassing reduces the dose of stabilizers. Output of fluxed & void free PVC from degassing is input for fusion between vacuum port and adapter.
As dry blend from hopper enters the twin screw extruder & moves ahead, the hard skin of 90 - 250 micron secondary particles is torn apart due to shear & heat, exposing the hierarchical structure of PVC like sub grains, agglomerates etc for interaction. Interestingly, hard skin does not break during high speed mixing. Due to compression & heat, PVC sub grains, agglomerates etc, begin to loosely attach together. At vacuum port, PVC is neither powder nor fused. This is Gelation, a state prior to fusion At least, 600 mm Hg vacuum is applied to suck the gases but not PVC grains. This facilitating extraction of
1. Occluded air containing oxygen - Responsible for oxidative degradation
2. HCl generated during degradation of PVC - Responsible for accelerating thermal degradation.
3. Moisture & volatiles Responsible for the voids & less impact strength of extruded products.
If PVC at vacuum port is over gelled or is blocked, the extraction of gases will be difficult. Compound Lubrication & temp in BZ1 & 2 needs to be adjusted.
If screws and barrel wears out, powder gets sucked. Correct use of degassing reduces the dose of stabilizers. Output of fluxed & void free PVC from degassing is input for fusion between vacuum port and adapter.
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