It's been over forty years since Benjamin ignored his uncle's sound advice, opting instead to sample the pleasures of Mrs. Robinson and daughter. I had a similar problem myself, with detours in the shipbuilding and consulting industries, until I finally got smart and entered the plastics industry in 1989. That's when NOVACHEM was founded. We manufacture and market purging compounds to several groups of processors of thermoplastic materials.
We are surrounded by many materials that are known as polymers. A polymer is a substance that is made up of many very large molecules where each molecule consists of very many repeated units, like links in a chain. These units are called monomers. Among the polymers we see every day are wood (cellulose), rubber and all those materials we call "plastic".
Folks who make their living in this industry often refer to polymeric materials as plastic resins. There are two basic kinds of resins: thermosets and thermoplastics. Both are families of polymeric materials, but in thermosets the polymer molecules are interconnected at many points by chemical bonds while in thermoplastics the molecules are physically entangled but not chemically bonded. The interconnection of polymer molecules in thermosets is called crosslinking.
The most important consequence of the difference between thermosets and thermoplastics is this: if you heat a thermoplastic you will "open up" the molecular structure and allow the polymer molecules to move relative to one another. We call this "melting"! The more heat you apply, the more readily the molecules move around. As with lubricating oils and maple syrup, hot means thin and runny while cold means thick and gooey. The technical term for level of gooey-ness is viscosity.
In contrast, the crosslinked structure of thermosets keep the polymer molecules locked together. Heat it all you want -- no melting will occur. You will wind up with a black lump of carbon.
The nice thing about thermoplastics is that you can melt them,
change their shape, and let them cool into a solid state in the
new (presumably more useful) shape. If you want you can then do
it again...and again...and (well, not really forever, but you get
the basic picture).
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Since about the late 1930's engineers have been designing clever systems for the purpose of melting, shaping and cooling thermoplastic resins. (The processing of thermoset resins is pretty much "beyond the scope of this site". That's because of their crosslinked structure, which is immune to the chemical activity of the purging compounds that my business makes.)
Just about all processing of thermoplastics today is done with plasticating screw extrusion equipment. The final useful shape that the processor is seeking will dictate the exact type of process equipment that should be employed. In most cases if you want an object that is complex in three dimensions you will use injection molding; if your end product has an axis of symmetry (like a rod or a tube or a sheet or a film) you will use extrusion; if you want a product that has a void in the middle (like a bottle) you will use some form of blow molding.
These machines are adaptations of rubber processing equipment. In them, a machined metal (usually steel) screw fits rather precisely in a cylindrical barrel. The barrel is very much like a cannon barrel. A motor at the back of the barrel causes the screw to rotate and an opening on top near the back of the barrel allows resin to enter. Most often, the resin is in the form of pellets. (You'll hear Europeans refer to them as "nibs".) As the screw turns, the resin is carried along toward the front of the machine.
There are many screw designs that differ in detail, but in all cases as the material moves toward the front of the screw and barrel, the root diameter of the screw -- the diameter down at the base of the "thread" -- grows larger, subjecting the resin pellets to increased pressure and shear stress. This mechanically imposed shear stress in combination with thermal energy from electric resistance heater bands causes the resin to be plasticated (that's a 50 cent word for "melted"). As it turns out, when a machine is up to operating temperature and running steadily on a properly sized job, the shear energy provides all or most of the heating -- the heater bands may run little or not at all!
Most barrels are arranged in sections called zones that are defined by the screw geometry in that section. At the back, right under the opening where resin pellets enter the barrel, is the throat area. It is usually kept quite cool, and no melting occurs here (we hope!). The screw is usually of constant dimensions here and is acting only to transport material.
Just forward of the throat area is the feed area. This is the first heated area. Here the resin begins to warm up; the first part of the screw's compression zone passes through here, with the root diameter beginning to grow. Forward of the feed area will be one or more independently controllable heating zones as you move toward the front of the machine. The screw will continue to compress and shear the resin more and more through here until the designer feels that adequate plasticating will have occurred. Then, once more the root diameter becomes constant. is the nozzle itself.
The final screw section, leading up to the tip, is the metering zone. Here again, the screw's root diameter is constant and the rate at which the now melted resin is transported becomes a predictable function of screw RPM.
Each heated zone, from the feed zone to the nozzle, is
independently thermostatically controlled. The zones are set in a
temperature profile which more often than not increases
from a lower feed zone temperature to a higher nozzle
temperature. This is a typical sloped or ramped profile. However,
humped profiles are not unheard of.
Back to Thermoplastic Processors
Injection Molding was the earliest technology applied to the processing of thermoplastic materials. The earliest injection molding machines were modelled on metal die-casting technology as used in children's toys, carburetor bodies and souvenir models of the Empire State Building.
In these early machines, resin was melted (usually by electric resistance heating elements) in a pot and the liquid was channelled into a reciprocating plunger (or piston or ram) which injected it into a mold cavity -- hence the name injection molding. A few of these old style plunger machines are still in service because of their special usefulness in certain applications or because the owners are too darned cheap to replace them. For the most part, however, this segment of the industry has adopted screw-type extrusion equipment.
In injection molding machines that use screws, after the screw plasticates the resin and moves it to the front of the machine, it is discharged from a nozzle (itself the final controllable heating zone) and into a mold where it assumes the shape of the desired part and is cooled and ejected. The nozzle usually incorporates a non-return valve or check valve -- in any of several designs -- to preclude backflow from the mold to the nozzle.
Obviously, the job requires an intermittent flow. A shot must be discharged from the nozzle periodically, of a quantity just sufficient to fill the mold. Presumably one could do this by starting and stopping the screw but in practice this doesn't work very well. So the clever engineers designed machines where the screw could move in two different ways: in rotation to provide for melting of the resin, and axially, to provide -- just like the earlier plunger -- for injection pressure.
In this new reciprocating screw scheme, between shots the screw rotates, causing resin to be plasticated and transferred to the front of the machine. Pressure builds up at the front of the machine (behind the nozzle) causing the screw to slide all the way to the back of its travel (usually an adjustable amount, to control shot size). When sufficient melted resin to fill the mold cavity accumulates, the rotation stops and the screw is forced forward (by, for example, a hydraulic cylinder), discharging a shot into the mold.
Then the screw resumes rotation (a process called recovery)
while the plastic in the mold cools and solidifies and the mold
is opened and the finished part taken out. Then, the mold is
clamped shut, ready to accept another shot. The total time for
all this to occur -- from shot to shot -- is the cycle time.
Back to Thermoplastic Processors
As noted above, extrusion processing is used when the end product has an axis of symmetry (like a rod or a tube or a sheet or a film). In all extrusion processes, the extruder discharges through a die. There may or may not be intermediate plumbing between the extruder screw/barrel unit and the die.
There are several different types of extrusion systems. They are distinguished by the geometry of the die, i.e.,:
Blown film systems are wonderful Rube Goldberg contraptions where the extruder discharges through an annular die that is rotated to face vertically. The resulting semi-molten tube is pulled up while compressed air is blown up through the middle of the die to inflate the tube -- increasing its diameter and thinning its wall down to the desired film thickness. This is called "erecting the bubble". At a point about halfway to the ceiling, if everything is working right, a freeze line appears as the material fully solidifies. Then the bubble is collapsed onto takeup rollers and taken off for finishing. The blown film process produces good quality film at a prodigous rate, and brings joy to the heart of any mechanical engineer.
Blow molding systems come in two varieties: Injection blow molding and extrusion blow molding. Their names are indicative of their respective family backgrounds.
If you would like to make a thermoplastics processor happy, tell him or her that you want the equipment to run 24 hours per day, 365 days per year, making the same part in the same color out of the same resin type, for the rest of his life. If the equipment is in good condition and properly sized for the job it is perfectly possible that you will retire before anything goes wrong.
But that's not the way the world works. In the real world, the processor gets an order for, oh, let's say cups and saucers. Marketing has sold 100,000 cups and saucers (hooray!). That's 20,000 each in white, yellow, green, red and dark blue (hooray???).
So we set up the cup mold and fill the machine with white resin and mold 120 cups per hour, 24 hours per day for a week. Then we put the yellow resin in. Amazingly enough, we go from white, to real light yellow to light yellow to yellow in about four shots! All Right! A week later, we put in the green resin. Same drill. Then the red. We're getting really good at this! Then, we load the blue resin and run the last 20,000 cups. This is what we call a color cycle. We go from light to dark and it works really well. We go get a quick cup of coffee while the guys with the wrenches mount the saucer mold. Uh-oh. Time to make the white saucers. So we put in the white resin and start making ... not-quite-so-dark-blue saucers (boo!). Lots of 'em. Followed by a lot of a-little-less-dark-blue saucers. Then about half a truckload of still-pretty-blue saucers. Then we fill the rest of the truck with when-are-we-gonna-get-rid-of-this-godawful-blue saucers. And we pile some really-they're-almost-white saucers on top of the truck. Finally after three days of hair-thinning, we're making white ones. Now we can start our color cycle again.
Welcome to the dark-to-light color change. Good processors try to run a light-to-dark color cycle in order to minimize transition losses -- parts that aren't really either color. But sooner or later you have to go dark-to-light. That is when you realize that in all those earlier changeovers you were masking the lighter color with the darker. You also realize that white does not mask black worth a darn.
The process of emptying resin from the machine without injecting it into a mold cavity is called purging. We speak of "purging the machine empty"; we refer to the solidified puddles of resin that result as "purgings". We talk about wanting to "purge" the dark resin out of the machine. We call any material that we hope will help us to do this a "purging material".
The first operator who had to spend a very looong time going from dark to light by purging light colored (or natural, i.e. un-colored) resin through the machine probably stood their saying "There's gotta be a better way!" Well, there is.
A while ago we said: "The more heat you apply (to thermoplastics), the more readily the molecules move around. As with lubricating oils and maple syrup, hot means thin and runny while cold means thick and gooey. The technical term for level of gooey-ness is viscosity." You could imagine that if the machine has thin runny stuff inside (like water) and you feed it thick gooey stuff (like molasses) then the thick molasses-like stuff will push out the thin watery stuff. High viscosity materials will displace low viscosity materials.
This is the functional basis for Physical Purging Compounds.
For example, if our friend with the cups and saucers is molding them from low density polyethylene (LDPE), a relatively low-viscosity resin, and he has on hand some high-density polyethylene (a relatively stiff (i.e., high-viscosity) resin), then he might feed some of the high-density PE (HDPE) through to "push out" his dark blue low-density PE. There is a good chance that this will work quite well. In a few (or a few dozen) shots -- for sure not three days -- the blue color will probably be gone and the white LDPE can be re-introduced.
Note well: One reason we get away with this is that the HDPE is very similar chemically to the LDPE. If, after purging, there's still some HDPE in the machine, it probably won't matter too much. The point is, the white LDPE won't push out all of the HDPE purge material. Low viscosity materials will flow over and around high viscosity materials, leaving them in place.
That which you can say about dark-to-light color changeovers is also largely applicable to high-to-low viscosity material changes.
We've implied that there are different kinds of thermoplastic materials. For example, we just discussed two different kinds of polyethylene -- LDPE and HDPE. In fact there are literally hundreds of different polymers being processed out there. Some of them are very stiff materials that are often -- but not exclusively -- processed at high temperatures. Some are very thin materials that are commonly processed at comparatively low temperatures. And every stop between.
Just as a processor may run many colors, he or she may also run many polymers having different flow characteristics. Suppose, for example, we are processing a polysulfone (a high-temperature engineering thermoplastic) at 640 deg. F. and the next job for that machine is a part in ABS (acrylonitrile-butadiene-styrene copolymer). Here's a dilemma. I can't just put the ABS into the machine. First, at polysulfone temperatures it would have no stiffness at all. To make matters worse, that level of heat would quickly damage the ABS molecule leading to the presence of degraded material in the system.
It is also useless to bring the temperatures down first. By the time you got down to a level that the ABS could tolerate, the polysulfone would be so stiff that dynamite wouldn't push it out.
Maybe you could use a very, very stiff polyethylene -- which has excellent heat tolerance -- to push out the polysulfone. But then, as you lowered the temperatures you'd have this super-stiff PE in there and no way to get it out.
If you think that there doesn't seem to be a way out of this -- you're right! With the advent of very high temperature engineering plastics, traditional purging techniques based on differences in melt viscosity were found wanting. About all that a processor could do was schedule the machine for three shifts of down time and call the guys with the wrenches to pull the screw and power wire brush everything. Lost revenue! Labor cost! Damage to equipment! Arrrgghhh!!!
Just a bit ago, we referred to ABS as a resin that could not be heated to very high temperatures. It is a heat sensitive resin. In point of fact, the ABS formulation has a rubbery element in it -- it's the "B" part which stands for butadiene. These rubbery molecular elements have "double bonds" in them which can be damaged by too much energy input. When they crack, they look around for a new place to latch on and it just might be a site on a different nearby molecule. Presto -- a crosslink!! If this happens once, no big deal. If it happens many times, you get (drum roll, please) degradation which usually takes the form of black specks. Now, there are various temperature sensitive resins which will degrade in a variety of ways -- but pretty much always resulting in black specks, sometimes referred to as carbon.
It is important to keep in mind that degradation is not always the result of cooking a temperature sensitive resin -- they are just the touchiest cases. In fact, any resin, if kept hot enough for long enough, will degrade. It is all about heat history. This is why a processor tries not to make an itty bitty part on a great big machine. There is too large an inventory of hot resin experiencing an extended residence time in the barrel.
Degradation results when thermoplastics receive excessive heat input as a result of exposure to excessive temperatures or exposure to moderate temperatures for an excessive period.
For example: Every system has lots of little places where small amounts of residual material remain relatively stagnant. These can be discontinuities in the resin flow path such as expansions, contractions or 90 deg. turns. There can be pockets in check valves. Even pits or scratches (remember that wire brush?) in the screw or barrel can retain a small amount of material. All of this "hung up" material is vulnerable to degradation.
Another example: It's late on Friday night. The weekend being upon us, the operator purges a bucket of HDPE through the system, moves the Big Red Switch to "OFF", turns out the lights and goes home. As he locks the door behind him, the machine is still at about 450 deg. F. It will finally get down to room temperature around five hours later. All this time, the little particles of resin residual in the system accumulate heat history.
At midnight Sunday night the third shift guys come in, light the lights, move the BRS to "ON" and go make coffee. After two cups and a cribbage game, around oh-dark-thirty, the heats are up to processing temperatures. The same little bits have been quietly crosslinking in a warm dark place.
So they start making parts, and first thing ya' know, Little Black Things appear in the product. After four hours of cussing and running scrap it clears up and good parts are being made by the middle of first shift. Just a typical weekend at the molding shop!
In days of yore, they might run some acrylic scrap (short for polymethyl methacrylate, a.k.a. PMMA or PlexiGlass/Perspex) which is an abrasive resin. As it would go through the system it would scrape carbonized particles along and do some good. Sometimes you'd get by for another week before the guys with the wrenches...
This is the functional basis for Abrasive Purging Compounds.
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The more difficult purging tasks associated with changeovers and degradation removal, particularly when engineering thermoplastics are involved, are often beyond the capability of physical or abrasive purging compounds. They must be addressed either by mechanical teardown and manual cleaning of the equipment, or by the use of a high-performance chemical purging compound such as (fanfare, please) NOVACHEM's SuperNova Chemical Purging Compound.
Unlike either physical or abrasive purging compounds, chemical purging compounds have the ability to break down polymer chains, thus reducing the melt viscosity of the residual resin. This makes it comparatively easy to flush the residue from the barrel.
Many processors use these products to facilitate color and material changeovers. They are also highly effective in controlling degradation, especially if used for preventive maintenance at shutdown. For more information on this product line, visit NOVACHEM's home page.