It sounds mystical, ethereal and beautiful. In reality, however, it is anything but.

Angel hair is the term used to refer to long “hair”-like strands of plastic that are created when certain resins move at high-speeds through vacuum-conveying tubing.

Which resins?

Mainly soft materials like polypropylene, polyethylene and, especially, linear low-density polyethylene

Accelerating sometimes to more than 6000 ft/minute, the resin pellets are softened by friction so that it smears along the pipe walls. This phenomenon occurs most often at elbow bends or in the first few feet downstream from the elbows.

After solidifying, these thin strands build up, eventually breaking away and getting carried along with the pelletized material, causing problems throughout the system, from rail car or truck to the final processing machine.

Angel hair from resin conveying

Let’s look at the many ways that angel hair can adversely affect your productivity, quality and profitability.

Ways Angel Hair Affects Plastics Processing

Rail-car or truck unloading

An increasing number of processors, particularly those who run large volumes of a few materials, are buying resin in bulk and unloading it into silos or large surge bins. This is where a lot of angel hair is created.

Truck delivery operators, for instance, often crank up pressures in order to get the unloading job done faster so they can get back on the road. But higher speeds and pressures create angel hair that eventually gets caught in the filter grate at the base of the silo, where it builds up and needs to be removed manually by maintenance personnel.

Conveying from the Silo

Silos are usually located outdoors and may be hundreds of feet away from the first destination inside a facility. Resin in a conventional high-speed, dilute-phase vacuum-conveying line accelerates continuously from the pick-up point to the destination.

Consequently, pellets that may have been picked up at a reasonable 3500 to 4000 ft/min rate will be racing at well over 5000 ft/min (and creating angel hair) by the time they reach the end of an 800- or 1000-ft run from the silo.

At the Destination Receiver

The material receiver is where resin is separated from conveying air. If this separation involves a screen or filter, the lighter weight angel hair is going to get sucked onto the surface of the filter as the air is pulled away and the heavier pellets fall into the receiver body.

As angel hair builds on the filter, air flow is impeded and the pump must generate higher vacuum levels to compensate and maintain adequate throughput within the conveying system. At the very least, the accumulated angel hair is going to hurt your conveying performance and eventually, the needed vacuum levels could get so high that the pump can’t supply them.

In any case, maintenance will be required to clean the filter. A filterless receiver will not suffer the same problem, but it has its angel-hair problems too.

Disrupting Material Flow

In a filterless receiver, angel hair falls into the receiver along with the pellets. Even in a filtered unit, the filter or screen does not remove the angel hair and much of it will remain with the pellets.

Angel hair can weave together among the pellets, causing clumping or bridging in the cone-shaped base of the receiver, preventing the smooth flow of pellets through the receiver. The receiver doesn’t empty properly and, especially when level sensing is used as the trigger for the next loading cycle, the receiver stops calling for more resin and whatever process it is feeding will be starved material, eventually shutting down the operation.

In the Discharge Valve

Another problem angel hair can cause is fouling of the discharge valve. Every receiver has a discharge valve that must be closed so that vacuum builds, drawing material into the line and ultimately to the receiver.

Once the vacuum is shut off, the valve opens to discharge the resin to the processing machine or whatever the receiver is feeding. However, if there is angel hair in the pelletized material, it can get hung up on the valve and prevent it from operating properly.

This can happen regardless of what kind of valve – whether a gravity operated flap, positive discharge valve, knife gate etc. – is involved, and when it does, the valve cannot close properly. This, in turn, creates a vacuum leak that, again, reduces conveying efficiency or prevents conveying totally.

Angel Hair in Blender Hoppers

From the receiver, resin may be delivered to a blender where more problems can occur. Blenders store material in hoppers or bins which, like the receivers, usually discharge via gravity through a valve.

Here there is no vacuum-leak problem because the hoppers are not under vacuum, but angel hair certainly can cause bridging that disrupts the flow of material into the weigh hopper or mixing chamber.

The problem is especially acute in volumetric blenders, where accuracy depends on the consistent flow of material over time. But even gravimetric blenders, which weigh each dispense for accuracy and automatically try to compensate for variations in the amount of resin dispensed, can still suffer performance and blend-uniformity issues.

If the amount angel hair in the mix varies randomly – as it usually does – the system has difficulty finding an equilibrium that ensures consistent, accurate blend.

Mixing Chamber Maintenance

Another blender problem occurs when angel hair makes it into the mixing chamber. Here, it can get tangled around the mixing shaft and blades, accumulating until it begins to affect mixing performance and potentially reduce the volume of the mixer.

When the chamber can no longer accommodate the material being fed to it, the system alarms and eventually shuts down, and maintenance personnel need to physically clean out buildup of angel hair.

Drying Issues

Dryers are set up to handle pelletized material, and critical to end product quality are the even flow of resin through the hopper, along with precisely controlled temperature, air flow and dewpoint.

If angel hair is mixed in with the pellets, it can cause material flow problems–just as it does in blenders–so that some pellets move faster through the hopper and drying time is inconsistent. Angel hair can fill in the spaces between pellets, disrupting the flow of hot and dry air that is needed to dry the plastic properly.

Finally, the lower-bulk-density angel hair can be overdried or even burned in the hopper.

End of the Line

Ultimately, the resin being conveyed, and possibly blended or dried, is fed to an injection-molding machine, extruder or blow-molder where it is melted and mixed to form a finished product.

Plasticating screws–as well as the processing parameters (temperatures, pressures and speeds)–are all optimized to process pellets. With its lower bulk density and different melting characteristics, the thin angel hair strands can cause inconsistent processing and quality issues like gels or black specks of burnt material.

Overcoming Angel Hair Problems

Anytime polyethylene or another softer polymer is conveyed at high speeds, angel hair can develop, whether it is between a railcar and silo, on the way into the processing plant, or during internal handling. The problems it can cause are clear and, for the most part, well-known.

Trapping Angel Hair

The traditional way of preventing the problems angel hair can cause has been to remove it, using aptly named angel-hair traps. These devices can be positioned almost anywhere downstream from any point where these strands develop. However, grates, filters and angel-hair traps all need to be cleaned frequently to prevent process disruptions and this requires manpower, which can be costly and, with increasing frequency, less than readily available.

Wave Conveying

Preventing Angel Hair

Seems like it would be much better to prevent angel hair from every happening, doesn’t it? Well, you may have noticed repeated references to “high-speed” conveying.

That’s because, as was explained at the beginning of this article, speed is the culprit that is responsible for angel hair. You don’t see angel hair being created when resin is being conveyed from a machine-side Gaylord or bin to the feed throat of a molding machine or extruder. That’s because distances are short and speed is lower.

So, why can’t you simply convey more slowly everywhere in your plant? The answer to that requires a short lesson in physics, but the simple fact is that conventional dilute-phase conveying – which has been the only practical technology available for decades – must move pellets at high speeds in order to transport them over longer distances.

But all that changed a few years ago with the introduction of Conair’s patented Wave Conveying™ system.

Using an ingenious arrangement of valves, variable-frequency vacuum pumps and control algorithms, Wave Conveying can control the speed of the conveyed pellets very precisely and can vary that speed easily to match the polymers being moved (whether they are hard or soft or something in between) and the distances over which they have to travel.

Depending on how the system is set up, it can move resin:

  • at conventional dilute-phase speeds up to 5000 ft/min
  • at a very slow Wave Pulse rate between 300 and 1000 ft/min, or
  • at a Wave Stream rate that hits the sweet spot between 1000 and 2800 ft/min.

And it does this without limiting the range over which material can be transported.

In fact, installed Wave Conveying systems are moving high volumes of resin over distances that high-speed, dilute-phase systems cannot touch.

At the low speeds of Wave Conveying, you can say good bye to angel hair, traps and filters, and all the problems that they can cause.

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On-Demand Webinar: Wave Conveying

Understand more about the history of resin conveying, how Wave Conveying was developed, and how variable speed conveying works.