What is Purging and How Can it Improve Dried Resin Handling?

It is not uncommon to visit a customer or receive a call about a suspected problem with material moisture. Whether the processor is using a brand new dryer or a system that is decades old, the problems they report are often very similar:

Splay or streaking,
reduced intrinsic viscosity,
bubbling,
or a myriad of other issues that can indicate some sort of moisture problem in the material.

Some processors can confirm the moisture content of material leaving the dryer using a moisture analyzer, while others don’t have one. Both are puzzled.

They’re even more puzzled when, after closer examination, it is often found that drying systems are working as expected and material is sufficiently dried when it leaves the dryer.

So what gives? Why are the problems occurring?

In many of these cases, the cause involves the purging – or lack of purging – of conveying lines.

What is purging?

In a resin material handling system, purging refers to clearing the conveying line of material between each cycle. Instead of leaving pellets sitting in the conveying line – between the source and destination – a “purge” valve is use to shut off the material flow and introduce 100% air into the conveying system for the last few seconds of the cycle. This air-only purge ensures that all the material pulled from the source in a conveying cycle makes it to the destination receiver before the cycle ends.

Why would processors need to purge conveying lines?

There are several very good reasons why processors may find it necessary to purge lines:

If materials do not flow easily, or the conveying path is complex (i.e., many bends, large elevation changes), purging after each cycle keeps the line clear. Each new conveying cycle starts fresh, without having to pull a large “head” or line-full of material left over from the previous cycle.
If a manifold is used many times, purging is recommended so when the operator changes materials, the line is clear.
Lastly, and most important to this article, purging can benefit plastics processing when materials are dried centrally, then conveyed to the processing machine.

The main reason to purge conveying lines of dried materials is to maintain the consistently low moisture content in the resin leaving the drying hopper. As noted earlier, most processors are drying their materials successfully, leaving them with a low moisture content and ready for conveying to the processing machine. However, these low-moisture pellets are now thirsty, so just as dry air has pulled moisture from them, they want to start absorbing moisture from surrounding air from the moment they leave the dryer hopper. Just how much moisture they regain depends on the type of material, the ambient conditions, and the exposure time.

So, if you are not purging your conveying lines, “thirsty” pellets are left sitting in the line between the dryer hopper and the processing machine, exposed to ambient air in which they can cool down and regain moisture. Let’s put that into a practical example: If you have 2.5-inch conveying line feeding a machine located 200 ft. away at a rate of 50 lbs/hr – and you’re not purging that line – you may have a full hours’ worth of material sitting in the line. Despite all you’ve invested in drying it properly, it’s now creeping closer and closer to the minimum allowable moisture limit.

What if you’re not sure you need purging?

It’s possible that, given your conveying system or your ambient conditions, purging doesn’t seem to be strictly required. Maybe the resin you’re processing could tolerate an hour outside of the hopper. But what about unplanned downtime or weekend shutdowns? These could leave material sitting in the header for hours or days. In that case, you’ll need to drain/clear your conveying system before restarting, scrap your initial shots after restart, or risk molding/flashing problems when processing resumes. With purging, all of these issues would have been self-corrected or eliminated.

Sometimes, processors require that pellets have a specific, elevated material temperature when entering the machine throat. This consistency helps improve process stability and cycle times. And, while it’s true that conveying material from a central dryer will allow it to cool somewhat, purging the line between conveying cycles minimizes the temperature loss, providing material to the machine hopper with a more uniform temperature overall. Without purging, material left in the conveying line loses heat much more rapidly due to the high thermal conductivity of the aluminum tubing that is typically used. When it finally arrives at the machine hopper, it’s going to be much cooler and moister.

By now, it should be clear that purging can making a huge difference in drying and processing performance. In some cases, it can be the difference between success and failure.

But what if you don’t have a purging capability? The good news is that adding purging to an existing conveying system is usually straightforward and fairly inexpensive.

What is needed to add purging to a system?

A purge valve: This is the valve that will sequence between the normal conveying cycle, and the purging cycle, which provides the air-only purge of the line.
A purging-capable of material handling control: This is fairly common now. Your control may already have this ability but could require some additional outputs to be added.
Installation: The installation of a purge valve is very straightforward. The purge valve is attached to the vacuum take-off tube via slip-fit or a coupling. Then, a set of wires is run from the control to the solenoid of the purge valve and the valve cycle is programmed into the control.

Are there alternatives to purging?

Another solution that has been used over the years is “dry-air conveying,” rather than purging. Unlike regular conveying, which moves material within a column of ambient/facility air, a dry-air conveying system uses low-dew point air similar to the air being used to dry. It is not uncommon to see a customer deploy dry-air conveying without purging. The logic is that by using dried air while conveying, you expose the material to less moisture while it’s moving through the line.

That’s true, but dry air alone is not always enough.

What’s the problem with dry-air conveying?

The problem with dry-air conveying is that, without purging, the conveying line is not cleared of material. When the conveying cycle ends, it leaves some dried material in the line, sitting between the dryer and the machine hopper. As this material sits, it loses the consistency in moisture content and temperature that it had leaving the dryer and that you want in the machine hopper. When that material finally cycles into the hopper, this inconsistency may be enough affect the quality of the molding process.

Think of dry-air conveying not as a substitute for purging, but as “the cherry on top.” It can be very helpful for highly hygroscopic materials or in humid environments, but what we find to be the best bang for the buck is just simply purging the lines. Purging should always be step one in operating a good material handling system for dried resins. If you need the added consistency and control, you can always add dry-air conveying later.

Are there any situations where you would not choose to purge a conveying line?

There aren’t many situations. The only ones I can think of would involve conveying systems that are cycling very rapidly or moving material over very short distances:

Example: Conair recently delivered a conveying system that could move 5,000 lbs per hour. We chose not to purge that line for two reasons:

Because the extra time required to purge (e.g. 10 sec) after each cycle (e.g. 30 sec) would have substantially reduced the throughput of the pump and the system, and
We determined material was moving through the system so quickly that it was never in the line for more than a few minutes at a time. The line in question also runs 24/7 so if a shutdown is imminent, the customer will manually purge the system.

Example: Another example are mobile desiccant dryers, like the Conair Carousel Plus™ dX models, which combine desiccant drying and conveying capabilities. Because these units are designed to handle relatively small amounts of material and convey relatively short distances, they aren’t equipped with purge valves as standard. Instead, they utilize closed loop, dry-air conveying to deliver relatively small amounts of material direct to machines on a frequent basis. The idea is “just-in-time” loading. The systems are designed to keep a minimal amount of material outside of the dryer at any time. However, even in small systems like this, users find that adding a purge valve, and avoiding any buildup of material within the line, can be helpful.

Summary

In summary, adding a purge capability to conveying systems that move dried resins is one of the best and simplest “bang for the buck” things you can do to protect the quality of your drying process and ensure processing consistency. If you have any questions about the process, or would like to learn more, contact Conair.