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processing feature | Top tips for twin-screws


escape easily, while prevent- ing large amounts of filler being lost out of the vent. The best configuration for this is to have a top vent in the barrel


immediately upstream of the side stuffing barrel. Sometimes a small half-slot vent insert can also be used in the


top of the side stuffing barrel. Feeder drop height: Ideally the feeder should be


positioned as close as possible above the side stuffer, to minimize the drop height. If a fluffy material is allowed to drop through air, it becomes aerated to the point where the bulk density is significantly reduced. This can have a net effect of limiting the throughput rate of the entire line. Feeder agitation type: Make sure the agitator in the


Figure 4: A “blaster” arrangement of air jet nozzles can be used to break up any caking


Figure 5:


Insulating heat pipes can


deliver several important benefits


filler feeder isn’t aerating the material, reducing the bulk density. Many feeder manufacturers have special agitator designs for powders, to preserve bulk density. Make sure the stuffer hopper is vented: Along with


the filler, the stuffer also introduces a lot of air into the extruder. This air has to come from somewhere. If you have an open top on the side stuffer chute, then venting is already taken care of. If you have a solid cover on the chute with a round stub-up and flex connector to the feeder, it’s important to also have a vent opening. Ground all hoppers/chutes to drain static electric-


ity: Some materials generate static electricity from friction. If static is present, it can cause powder to cling to the inside surfaces of hoppers and chutes, leading to problems such as caking. If you think this could be happening, an easy fix is to run a ground wire (10 gauge wire is recommended) from the chute to a known good ground on the machine frame. Compressed air blaster: If caking persists, some-


times a special solution is needed. Hopper vibrators can be used, but are tricky to size and mount. Another device which can be used is a “blaster” (Figure 4). This consists of air jet nozzles strategically placed within the wall of the chute, directed to break up any cakes before they get too large. The air jets get connected to a solenoid valve using poly tubing, and the solenoid valve is actuated by a repeat cycle timer. This way you can set both the blast period and dwell time in between blasts. Just upstream of the solenoid valve it’s best to have a small air accumulator tank, to provide a sharp pulse of air.


0 4 Insulate melt pipes and dies


Very few compounders insulate their melt pipe adapters and dies. The reason is because the machine didn’t come from the factory with insulation, it takes some


30 COMPOUNDING WORLD | May 2012


extra effort, and it is difficult to see the benefits. But if your control system has temperature trending capabil- ity, the benefits can easily be demonstrated. As an example, take a heat zone actual temperature which is cycling up and down all the time. The trend looks like a sine wave. Now if you wrap the melt pipe section with insulation, you’ll find the temperature trend is much more like a straight line. The reason is because you’ve greatly slowed heat transfer from the melt pipe to the environment, so the temperature stays the same all the time. If the temperature doesn’t fall over time from heat loss, the heater contactor doesn’t have to come on. If you can prevent the melt pipe from cooling and then needing heat cyclically, the tempera- ture trend is going to be much smoother. An exposed, heated steel part like a melt pipe loses


a large quantity of heat to the surrounding environment via radiation and convection. This can easily be demon- strated by standing 0.5 m or so away from a heated melt pipe or die. You can feel heat emanating from the melt pipe, even though you are not touching it. There are some other benefits to insulating melt


pipes (Figure 5). Because it slows heat loss, insulation will tend to make all portions of the melt pipe the same temperature[2]


. This has the effect of eliminating cold


spots from uneven heater coverage. Another side benefit is faster initial heat-up times. One very important benefit is that insulation makes the


equipment safer for operators. Burns are the single most frequent injury around plastics equipment. If an operator accidentally leans against a melt pipe which is wrapped with insulation, he or she will probably not be injured.


0 5


Make the most of an under appreciated tool: the variable-


depth melt thermocouple Polymer flow through a melt pipe rarely has a uniform temperature throughout the cross-section. There is


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