Influence of Material Storage and Mixing on Consistency
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Importance of the Entire Material Chain
The consistency and quality of the final pipe product depend significantly on the whole chain from the storage of single material components and their mixing, through to packaging. For PVC dry blend, which is typically processed on double screw extruders, the consistency of the material flow is crucial.
Dosing and Intake Aspects
The pourability of dry blend is a key factor. It impacts how quickly the material fills the C-ring chambers of the intake section of the extruder. Without a dosing feeder (flood feeding), the maximum intake capacity is restricted to prevent overfilling downstream sections at lower RPM, which can limit maximum output and potentially cause high wear or screw breakage. Two basic dosing feeder types exist: horizontal gravity (starve feeder) and vertical crammer feeders. Crammer feeders pack the material, hindering air escape, and may stall with high CaCO3 loads. High-filled dry blends often have pourability issues requiring hoppers with pneumatic knockers and stirrers to prevent bridging. The design of the powder lock is also critical to match the dry blend properties (stabilizer/lubricants, filler types) and avoid high local pressures and wear upstream.
Dry Blend Formulation and Components
PVC dry blend consists of PVC powder mixed with stabilizers (heat, light), stearates, lubricants (waxes), and colorants. Fillers like CaCO3 can also be added. The characteristics of the dry blend, including K-value of the resin, stabilizer type (Tin, OBS, CaZn), lubricants, filler level, pourability, and gelation speed, all influence processing behavior. Reliable suppliers and homogenizing pulverized material from different sources in a mixing silo are important for using recycled content successfully. Adjustments on line by dosing feeders can also be required.
Can cause crammer feeders to stall; high-filled blends have pourability issues.
Wear Risk
High local pressures at powder lock if not matched to dry blend; absence of dosing feeder at high RPM.
FAQ Section
How does the pourability of PVC dry blend affect the extrusion process?
Dry blend pourability determines how effectively the material fills the extruder intake chambers. Low pourability requires more time to fill and can restrict maximum output if not managed correctly, potentially requiring dosing feeders and risking wear or screw breakage at higher speeds.
What challenges are associated with handling high-filled PVC dry blends in storage and feeding?
High-filled dry blends, particularly those with high CaCO3 loads, often have pourability problems that can cause bridging in hoppers, requiring pneumatic knockers and stirrers. Crammer feeders may also stall with high CaCO3. Additionally, the powder lock design must be optimized for these blends to avoid high local pressures and wear.
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Patrick van Bezouw
