A well-designed screw using a barrier flighted melting half can current larger fees — as a lot as 20% — and reduce discharge temperatures as compared with a regular flighted melting half. When blended with a downstream Maddock mixer, the usual and cost of the extrudate could possibly be optimized for the making use of, maximizing productiveness.
This experience was the first of its kind to utilize the concept of separating the compacted sturdy mattress from the soften pool as a result of it progressed down the transition a part of the screw. A cross a part of a barrier melting half perpendicular to the first flight edge is confirmed in Decide 1. The compacted sturdy mattress is adjoining to the trailing side of the first flight. Melting occurs by dissipating energy throughout the skinny soften film positioned between the sturdy mattress and the barrel wall.
FIG 1 Schematic for a barrier melting half. The cross half is perpendicular to the first flight. Provide: Mark Spalding
The dissipated energy conducts into the sturdy mattress and melts resin on the interface. The molten resin is then conveyed over the barrier flight and into the soften channel. The barrier flight is undercut by about 0.5 to 1.5% of the diameter from the first flights. Thus, for a 6-inch diameter screw, the undercut will be about 0.030 to 0.090 inch. The barrier flight supplies a level of dispersive mixing and prevents huge sturdy resin fragments from stepping into the soften pool.
There are numerous barrier melting half designs which is perhaps used commercially. One in every of many highest performing designs makes use of lead lengths equal to the diameter throughout the feed half and throughout the meter half. The lead measurement of the first flight and the barrier flight throughout the barrier half are elevated to 1.25 to 1.40 events the diameter. The lead measurement is elevated to permit an even bigger entire width partially. The width of the solids channel is usually about twice the width of the soften channel. The solids channel is the deepest on the entry and it tapers to a shallow depth on the exit of the half. The soften channel is the choice with the channel being shallow to start with and the deepest on the exit. As beforehand acknowledged, there are numerous variations of the design practiced commercially.
The Worth Controlling Course of
For smooth-bore plasticating extruders, the metering channel must be the pace controlling course of. A poorly designed barrier half, nonetheless, could possibly be the pace limiting step. When this happens, the extruder will run at low specific fees and should set off degraded resin to look throughout the product. The exact cost is the pace divided by the screw tempo — lb/(hr rpm). The pace restriction might trigger elements of the metering half to operate partially crammed and at zero pressure. For polyethylene (PE) resins, degradation gels can kind throughout the partially crammed portion of the screw. When the gels separate from the screw, they might set off defects throughout the product (as confirmed by Decide 2) for a strong film.
FIG 2 Crosslinked PE gel in a strong film.
For instance, a 3.5-inch diameter extruder with a screw designed with a barrier melting half was working at a selected cost of three.6 lbs/(hr rpm). The calculated specific cost for the metering half, nonetheless, was 5.5 lbs/(hr rpm). Proper right here, the metering channel was not controlling cost. In its place, some operate upstream of the metering half was controlling cost. The extruder was stopped, and the screw was eradicated scorching.
The metering a part of the screw is confirmed in Decide 3. It reveals freshly melted resin on the pushing side of the channel. When the screw was first put in, the trailing side of the channel was void of resin, nevertheless after continued operation, this side of the channel grew to turn into stagnant with resin. This pattern of degraded and up to date resin is confirmed schematically in Decide 4. It took about 13 days of operation to develop the steady-state diploma of degradation confirmed in Decide 3. Minor course of instabilities would set off a small portion of the degradation to interrupt away from the screw. The downstream Maddock mixer would set off the fragment to interrupt into many smaller fragments, inflicting a gel “bathe” throughout the film product.
FIG 3 {{Photograph}} of the metering a part of a screw with a poorly designed barrier flighted entry. The white supplies is freshly melted LDPE, whereas the darkish supplies is degraded and crosslinked resin.
The exact cost was lowered from 5.5 to 3.6 lbs/(hr rpm), a reduction of 35%. When the screw was far from the extruder, compacted solids had formed on the entry to the barrier half. It was these compacted pellets that lowered the flow into of material into the barrier half. Moreover, the low cost specifically cost was enough to set off the metering channel to operate partially crammed, inflicting the resin to degrade and the eventual gels throughout the film product.
FIG 4 Schematic of {{a partially}} crammed metering channel.
The restriction on the entry of the barrier melting half was attributable to a reduction throughout the width of the channel for solids to flow into. For instance, a 2.5-inch diameter screw with an entrance design confirmed in Decide 5 was cost restricted. For this case, the solids channel width (W2) throughout the barrier half was 15% narrower than the upstream solids conveying half (W1).
Though the lead measurement was elevated throughout the barrier half, the addition of the soften conveying channel lowered the width on the market for the flow into of compacted solids. This barrier flight introduction can prohibit flow into to the downstream sections of the screw, inflicting parts of the metering channel to operate partially crammed as confirmed in Decide 3. This restriction can occur even with a lead measurement throughout the barrier half that is 1.4 events the diameter.
On the entry, the compacted sturdy mattress ought to rearrange because it’s transferred from the solids conveying channel to the solids channel of the barrier flighted half. If the channel design permits rearrangement, then the extruder will operate on the designed cost. If rearrangement is restricted, the channel pressure will enhance upstream of the entry.
The extreme pressure will set off the subsequent diploma of energy dissipated from the friction between the ground pellets and the barrel wall, creating an energy flux that conducts via the barrel wall. Usually this energy flux is extreme enough that the barrel zone positioned over the entry will overshoot the set degree temperature. All through the troubleshooting course of, it is on a regular basis an excellent suggestion to see if this zone is overshooting set degree or has the subsequent diploma of cooling as compared with the solids conveying zone.
FIG 6 Photographs of barrier flight entries: A) customary entry that had a cost restriction, and B) modified entry the place the barrier flight was eradicated proper right down to the idea of the soften channel for the first 1.5 turns of the half.
The pace restriction can usually be mitigated by eradicating metallic on the doorway of the soften channel such that some solids are intentionally handed down the soften channel. {{A photograph}} of a modified screw is confirmed by Decide 6b. For reference, an equivalent screw with out the modification is confirmed by Decide 6a. The barrier flight is eradicated on the entry proper right down to the core of the soften channel for in regards to the first 1.5 turns. Subsequent the barrier flight must be tapered from the idea of the soften channel to full peak over the next flip. This mitigation course of could possibly be tough for some industrial designs. Screw designers and fabricators are aware of the difficulty and the therapy to mitigate it.
In regards to the Creator: Mark A. Spalding is a fellow in Packaging & Specialty Plastics and Hydrocarbons R&D at Dow Inc. in Midland, Michigan. All through his 39 years at Dow, he has centered on progress, design and troubleshooting of polymer processes, notably in single-screw extrusion. He co-authored Analyzing and Troubleshooting Single-Screw Extruders with Gregory Campbell. Contact: 989-636-9849; maspalding@dow.com; dow.com.