Black Specks, Identify and Correct

Black specks in tubing or pipe, espe­cially in light-colored or clear plastics— lead to scrap, unscheduled shutdowns, and unhappy customers.  Specks can also cause holes or post-process failures in tube or pipe as it continues on to final product and use.

The screw and barrel seldom cause degradation, though vents or complex mixing or barrier sections may do so.  Degradation more often occurs downstream from the extruder; for example, in plumbing that forces abrupt changes in the polymer flow path or in components such as breaker plates, screen packs, static mixers, and melt pumps.  These can have potential degradation areas, for example, a taper into an adapter fitting that is too abrupt.

Complex dies for multi-layer or multi-lumen products can also be the source if they contain low-flow areas where polymer can stagnate and overheat. Your experience from past teardowns and inspec­tions is the best indicator of whether degradation is devel­oping in areas of slowed or stagnated flow. In tubing and pipe dies, it could be the point where each port is fed or where a splitter feeds a spiral.  Once identified, these areas should be given particu­lar attention in future cleanings. They may require local use of higher
temperatures plus chemical purging com­pounds.  A redesign of the components providing the degradation areas may be required.

 Heat sensitivity

Heat tolerance of the polymer is also part of the situation. Heat-sensitive materials like PVC, ABS, and EVOH, or engineering resins like acetals, PC, nylon, or polyesters are more likely to degrade than more heat-tolerant polyolefins. An extrusion system that processes
LDPE with no prob­lem might degrade heat-sensitive EVA in a matter of min­utes.  Make certain that residence times are not excessive and melt temperatures are below critical levels.

Also, consider throughput rate and shear sensitivity. Obviously, running too hot can lead to degradation. But on occasion, running too cold can, too. Forcing a cool mater­ial to flow can generate excessive shear energy and localized degradation in the screw’s flow channel.

Finally, consider the shutdown schedule and procedures.  Stoppages and shutdowns for adjustments, die changes, or maintenance often extend residence time and cause material degradation.  In a system operated five days a week, small amounts of residual material in the extrusion system acquire substantial heat his­tory as the machine slowly cools
and starts up again.  It is common for a system on five-day operation to begin producing black specks only a few weeks after a complete teardown and cleaning.

 When all else fails

In the real world, hardware design or operating conditions that are overstressing your material may not be immediately identifiable or correctable.  You may know that your material is suscepti­ble to degradation, but changing resin isn’t an option without customer approval.  Or you realize that week­end shutdowns are killing you, but you can’t justify going to 24/7 opera­tion. In such situations, purging compounds can play an important role in managing contamination.

When problems result from hard­ware configuration or condition, die geometry, known hang-up areas, or even slightly worn screws, periodic use of a purging compound as pre­ventive maintenance will attack degradation at an early stage and minimize troubles during startup.  But remember, purging won’t eliminate the root cause of the degradation, so if your system is spewing out black specks at an intolerable rate, that’s not the best time to try a purging com­pound.

If weekend shutdowns are the cause of contamination, use a high-performance
purging compound to remove heat-sensitive materials from the system Friday night, not after the fact on Monday, to avoid startup problems and unscheduled teardowns for cleaning.

Use the right kind of purging compound for your situation.  In general, mechanical purging compounds are best for color and material transitions in relatively small and simple systems. Chemical purging compounds are best for large or com­plex systems or for addressing problem contamination issues like black specks.

Spiral manifold melt flow technology was brought about by
the need to eliminate the historic problem of knit lines (or weld lines) in the
walls of extruded tubular products made using conventional spider-style
extrusion dies. Spider-style extrusion dies require that polymer melt flow
around spider legs of different designs that hold the mandrel pin in the
melt flow channel.

Spiral manifold extrusion dies were first developed by
Egan Corporation (now a part of Davis-Standard) in the early 1960s.
Initially, they were a center-fed design with an arrangement of radial feedports
leading into spiral melt flow channels in the cylindrical die surface.

Egan Corporation adapted the new spiral design into blown film processes, as the
blown film process had come from experiments in inflating pipe performed during
the 1940s.

Soon after, the spiral manifold extrusion die became the
most widely used design for the blown film and pipe industry.  Surprisingly, Egan Corporation never applied for a patent on the invention because Egan had thought in error that the invention fell under an earlier David-Standard patent for a wire-coating extrusion die that used a single spiral melt flow channel. Many competitors began manufacturing spiral manifold extrusion dies erasing the ability to acquire a patent.

During the 1960s and 1970s the blown film dies had only a
few spiral channels. An 8 or 12 inch die might have had 4 channels. Later, in
the 1980s, die designers began using melt flow analysis and learned that six channel
overlaps produced the best product quality and throughput.

In the 1990s the manifolds became shorter and more
streamlined in order to reduce polymer residence times. Die assembly and product
sizes became smaller, mainly driven by the medical device industry, where burst
pressure and product aesthetics became major design considerations.

In the 2000s, scaled down versions of the multi-spiral
melt flow manifolds were applied to smaller extrusion processes for medical
device and other small tube manufacturing processes where greater burst pressure
capability was needed.

Usually, the final one or two downstream bar­rel zones are set reasonably close to the desired exit melt temperature.  However, the proper set­tings for other zones depend a great deal on the particular polymer being extruded and the screw design being used.  Generally speaking, we can describe different barrel profiles as being flat, reverse, or normal.

A normal profile reflects a situation where the rear zone temperature is set significantly below the exit melt temperature.  The temperatures of the intermediate zones gradually taper up to match the temperature of the final downstream zone.  A reverse temperature profile describes the opposite situation: the rear zone is hotter than the final downstream temperature zone (which is sometimes set below the exit melt tem­perature).  A flat temperature profile reflects a sit­uation where all barrel zones are set at approxi­mately the same temperature.

Resin supplier technical bulletins of­ten suggest barrel temperature profiles.  How­ever, these recommendations may not apply to all screw designs.  To establish a good barrel tem­perature profile, start with these recommenda­tions but be prepared to compare the results af­ter making adjustments.  Pay particular attention to the effect of changing the rear zone tempera­tures.  A Du Pont nylon extrusion study noted that exit-melt-pressure fluctuations were seven times more extreme after the temperature of the two rear zones was dropped 50 deg F.  A Mobay processing handbook indicates that a 10-deg-F change in a 390F rear barrel zone can affect screw power by as much as 20 percent in extru­sion of Texin 355D polyurethane.

Some polymer/screw combinations are rela­tively insensitive to small changes in rear zone barrel temperature.  However, with some com­mercial screw designs I have seen major output increases for polystyrene and low-density poly­ethylene occur after the rear zone temperature was lowered.  I have also seen situations where a reverse temperature profile gives better output with a high rear zone temperature, especially with polypropylene.  Results of such experi­ments vary widely depending on the polymer, screw design, and downstream barrel tempera­ture settings, as well as the rear zone tempera­ture setting.

If rear zone temperature changes have little effect on output or pressure stability, either a flat or normal barrel temperature profile will be ac­ceptable in most cases.  Avoid barrel tempera­tures so low that they affect stability and melt quality.  Also avoid high barrel temperatures that needlessly lead to degradation or a downstream cooling problem.

For different extruder models with different zone configurations and thermocouple loca­tions, the barrel temperature settings may need to be modified even if the same screw design is used.  Assuming no difference in screw and bar­rel wear, we want to obtain the same temperature at the inside barrel wall of each extruder—not merely the same temperature reading at the ther­mocouple locations.

Sophisticated instruments may do an excel­lent job of keeping the metal in the immediate vi­cinity of these instruments at a constant temper­ature.  However, unless we also consider the pro­cess and surrounding hardware, we may end up with fine-tuned control over only the metal that immediately surrounds the thermocouple.

Location of the thermocouples may be ex­tremely important in interpreting what tempera­ture environment actually exists at the inside barrel wall.  If the thermocouple for each zone is imbedded halfway into the thick metal barrel wall (a common practice), the inside barrel wall will have a similar temperature if the zone operates largely without heating or cooling.  But the inside wall will be hotter than indicated if the zone calls for cooling, and colder than indicated if the zone calls for heating.

Some extruders have thermocouples located deep inside the barrel wall, and others have very shallow thermocouples. Some use a combina­tion of deep and shallow thermocouples.  With modem sophisticated control instruments, well-engineered extruders can be run properly under steady-state operation with any of these thermo­couple locations.  However, it is important to recognize that the same inside barrel wall tempera­ture may require widely different temperature settings for two different extruders.  Recognize also that any of these systems can malfunction.  Suppose, for example, that water-flow passages become fouled in one aluminum block of a barrel zone, but not in the other blocks.  The metal tem­perature surrounding the thermocouple may rep­resent a compromise between a heavily cooled block and an uncooled block.  Temperature differences of more than 100 degrees F between blocks in the same zone have been recorded after fouling occurred.

After a tentative barrel temperature profile has been established, it is often useful to estimate how much heating or cooling is required in each zone.  It may help to shut off both heating and cooling for one zone at a time for brief peri­ods after the extruder reaches steady-state oper­ation.  Then note how long it takes the zone to change temperature up or down about 20 degrees F.  This can indicate whether some zones are being subjected to unusual heating or cooling rates.  If any zone requires extreme cooling, the extrusion process may be generating excessive localized polymer heating in that particular zone.  If veri­fied, this condition could justify a change in bar­rel temperature profile or modification in the screw design.

Use of a high-compression screw or applica­tion of a high crammer force to the feed may lead to intensive heat development in the barrel wall, even in the rear barrel zones.  By contrast, if cold uncompressed powder feed brushes the barrel wall through the rear barrel zone, this can cool the barrel wall effectively.  You should consider all of these factors when selecting process con­ditions, including barrel zone temperatures.

BASIC SAFETY PRECAUTIONS

The extruder looks innocent enough, but it’s always potentially dangerous.  Loose-fitting clothing or a necktie are not appro­priate for working on the line. Safety shoes and glasses are a must for everyone.  A small die may weigh only 30 pounds, but dropped on a foot unprotected by safety shoes, it could do serious damage.

The extreme heat of the die represents another serious risk. I’ve seen workers adjust dies without gloves, but it only takes one little touch to get a burn that may take three to four weeks to heal. Die tempera­tures range from 350 to 600F.

Electricity also deserves respect. Every piece of auxiliary equipment should be grounded, including the hopper dryer, hopper loader, preheater, extrud­er, pumps, puller or conveyor, cutoff saw or knife, marking machine, takeup reel for flexible profiles (and pipe), refrigerator unit, and any other unit draw­ing power. This grounding should be done only by a qualified electrician.

The extrusion line can be especially dangerous with water on the floor, water is a great conductor of electricity. Starting up a line or a break in the line will often result in spilled water. To minimize the risks of a wet floor around the cooling trough, a wood platform alongside the trough and floor drains are helpful. A wet/dry vacuum cleaner and floor squeegees can be a useful item. Keep in mind that workers walking on a wet floor will have wet shoes, and they could slip easily anyplace they go.

Next to the cooling trough is usually a belt puller or a conveyor. It is very easy to injure a hand or fin­gers on a puller.  In a startup, many times the product is hand fed through the water trough and onto the puller.  A safer alternative to this practice is to use a starting line. For example, a small plastic pipe or rod cal be threaded through the puller belts while stopped and threaded up to the die head.  As the hot extrudate comes from the die, it is attached to the starter line. In this way, the line worker does not have to lead the new product through the cooling trough or puller.  Puller belts should always have guards.

Care must be exercised when the extruder is started.  Workers should always wear glasses and special gloves to handle the hot extrudate.  No one should be standing directly in front of the machine on start­up.  Air and gas can be trapped in the cylinder or die and, as hot material is moving through, the barrel will occasionally spit out hot plastic.  If it’s a vented bar­rel, the operator should never look into the vent.  On startup, the die may need adjustment, which al­ways requires gloves.

In  production,  problems  inevitably pop up.   A pressure buildup at the head may go unnoticed.  If the pressure is great enough, the clamp bolts will break or the rupture disk will blow.  In replacing ei­ther, workers may fail to use the correct item. It is also important to check that the pressure gauge is working.  Newer extruders usually have a signal light or buzzer to indicate abnormal pressure rises.

Beyond the puller belt or conveyor belt, proper guards must be in place on the cutoff saw or knife.  Cutoff-saw shavings accumulate quickly.  A shallow box under the saw reduces the hazard of slipping, but constant sweeping is also necessary.

Many lines have marking or stamping units.  Some use ink or a hot stamp.  Ink can make a mess on the floor that is quite slippery.  This hazard can be mini­mized by providing a catch pan under the marker (above the floor).  Hot stamps are cleaner, but if not properly guarded they can burn fingers.

Once in a while, the line will break.  This is the time when the machine operator needs help.  When working on a line, place a panic button at the puller.  When it was pushed, a loud horn would go off and help would come.  It is a simple device to install.  Panic buttons could be located anywhere along the line. There could even be more than one connected to the same horn.

On an extrusion line, there are many places for accidents.  It is common to see plastic pellets and oil on the floor in a plant.  Those pellets are like small  ball bear­ings—it doesn’t take many to have someone slip; and oil is a great slipping agent.  Line operators must constantly be on the lookout for hazardous condi­tions such as these.  They should never use an air hose to blow away dust or pellets.  Instead, a broom and dustpan should be kept at each end of the line for fast cleanup.  Well-stocked first-aid kits for treat­ing minor injuries should also be at each line.

Newer pieces of equipment have numerous safety signs regarding the machine.  They are not there for decoration, and they should never be re­moved.  Experienced line operators may know most of the areas that can be unsafe, but what about that new operator who has never seen this kind of equipment before?

Although OSHA has been doing a good job in making plants conform to safe practices, there are many small operations that remain lax in this area.  Proper safety guards are not always in place; control-cabinet doors and cylinder enclo­sures are left open.  Keep in mind that if a worker is injured due to negligence on management’s part, there are always repercussions.  Management could be held legally responsible, and even in less serious accidents the worker’s job performance could be affected.

When a melt pump is the method selected for die pressurization and metering, the extruder and all upstream equipment will be contributing to the polymer melt properties.

When an extrusion system’s output is not completely driven by the extruder, the functions of the polymer feeder and extruder are not constrained as they are when the extruder determines the rate and quality of output.  Melt conditioning becomes crucial, in­cluding preheating of the resin, controlled feed of the material to the extruder, melt homogeneity of the polymer in the extruder, and delivery at the re­quired pressure and rate to the melt pump to avoid cavitation.  All these system elements are discussed below.

Heating and drying the resin.

The incoming plas­tic material, in the form of pellets or powder, needs to be heated to a consistent temperature while in the feed hopper.  In the case of hydroscopic materials such as acrylic and ABS, the equipment includes a hop­per dryer unit that heats the material and re­duces the moisture by a recirculated airstream with very low humidity, produced by either a desiccant bed or a refrigeration unit.  To improve the output rate in the most effective way, the mate­rial should be at the highest temperature that does not cause bridging in the feed throat or other operating prob­lems.  When working with non-hydroscopic mate­rials such as polyolefins and rigid PVC, drying is not required.  The heat input will, however, con­tribute substantially to the output capability and stability of the process.

Probably one of the simplest ways to handle the preheating is to use a hopper dryer unit without the dehumidifying device.  Some such units can be operated with the dehumidifier off.

The energy load on the extruder is substan­tially reduced by using the hopper preheating.  For example, rigid PVC can be brought to a tempera­ture of 175F from a room temperature of 75F.  With a specific heat of 0.35, this represents a heat input of 35 Btu/lb, which is 20 percent or more of the total needed to melt the mate­rial.  For materials with higher melting points, especially crystalline ones that soften close to the melt temperature, the heat input is a larger fraction of the total needed.  Since the remainder of the melting heat is generated by the mechani­cal working of the plastic, the result is lower horsepower consumption or much higher melting rates for the extruder.

Powders represent a somewhat different problem.  To preheat powders, a fluidized-bed heating unit is usually needed.  Another situation exists with oxygen-sensitive materials that may be degraded by the heated air.  These can be dried with an inert gas such as nitrogen.  Since the heated gas is recirculated, little is needed, and it does not represent a significant produc­tion cost.

The beneficial effect of the hopper preheating is to make the Inlet-resin feed temperature uniform and to reduce the energy load on the extruder.  The energy-load reduction occurs in single-screw extruders, twin-screw extruders, and var­ious other melting extruder devices.

In normal operation of single-screw extrud­ers, it is uncommon to use a weigh feeder or volu­metric feeder to supply the resin, because the machine is often operated in a starve mode and the normal melt-bed configuration is dis­turbed, resulting in erratic output.  The feed may be controlled, but surging and other output in­consistencies frequently occur. However, when the extruder’s function is to deliver plasticated material to the melt pump this condi­tion does not apply.  It is possible to use feeders, both of the weigh and volumetric type, to regu­late the throughput of the extrusion device, in twin-screw as well as other types of extrusion machines.

The extruder is the most important element in the melt-delivery system.  Practically speaking, the vast majority of machines in use are single-screw extruders.

To plasticate material in most extrusion equipment, using typical materials, heat genera­tion is done largely by means of the shear work done on the material by the screw in addition to heat trans­ferred through the barrel wall.  When used to feed a melt pump, the operating mode can be quite different.  For example, the use of a preheated feed will materially reduce the amount of shaft power needed for the shear to plasticate the ma­terial.  Also, the equipment can be set up so that a larger percentage of heat is transferred into the plastic through both the barrel wall and the screws.

One approach to supplying additional heat is to place a cartridge heating element into the core hole of the extruder screw.  Barrel heaters are then used to increase the amount of transferred heat.  Operating in this mode, an extruder is able to deliver much greater than the normal output, because the only func­tion of the extruder is to supply the melt pump at a pressure needed to prevent cavitation.

The same sort of changes can be done to intermeshing co-rotating and counter-rotating twin-screw machines. The machines can be operated with high barrel temperatures and with internal heat supplied to the screws. Adding the screws as heat-transfer surfaces approximately doubles the available heat-transfer area.  Be aware that overheating the screw will cause a non-conveyance situation.  The feedscrew pumps because the polymer pellets are sticking to the barrel and slipping on the screw.  If the screw is overheated, the pellets will stick to the screw and not be conveyed, stalling the process.

Note that there is one very important consid­eration in operating the extruder in this mode.  Since shaft power is reduced, the amount of shear imparted to the material is substantially reduced.  A significant degree of shear mixing is essential to achieve effective melting and mixing of most plastics.  Mixing elements in the machine must supply the shear.  In twin-screw machines kneading-type elements are typically incorpo­rated into the screw to supply the shear.  Straight and spiral Maddox-type mixers or other mixers such as the Egan and Rapra can be incor­porated in the pin mixers of single-screw ma­chines to intensify the shear levels for proper plastication.

The screw design must be considered for its effect on plastication, although the design is not as critical in the operating mode for the ma­chines introducing the melt into a melt pump.  Special screw designs are available for use specifically with the melt pumps.  Head pressures must be low for screw operation.  To prevent cavitation in the en­try port of the melt pumps, the typical pressure required is in the range of 500 to 1000 psi.  It is possible to retrofit the melt pump and conditioning units on machines that have worn barrels and screws.  In most cases, loss of head pressure and variations in output are the first In­dicators of screw and barrel wear.  The melt pump can operate with low inlet pressure.  Low inlet pressure will smooth out variations in output from the ex­truder. If the machine suffers from excessive wear, it will not plasticate the material, but ma­chines that would normally need barrel and screw replacement can be operated for a long time past this point when used in conjunction with the melt pump.

The control interface between the melt pump and the extruder unit is important.  The pres­sure to the inlet side of the melt pump must be controlled to ensure high enough levels to pre­vent cavitation and low enough levels to prevent excessive pressure buildup, which can damage the equip­ment.  The pressure is sensed with a melt-pres­sure transducer that feeds back a signal through an appropriate control system to the extruder deliv­ery system.

This can be done in two ways.  The drive units of most extruders are SCR controlled.  With the pressure signal properly set and conditioned, the screw speed can be changed in response to the pressure readings. This will have an effect on the melt-bed pattern, which would be a problem in normal operation; however, in this case the melt pump controls the output.

For equipment having a weight or volumetric feeder, the pressure signal can also be used to change the rate of delivery of material to the ex­truder.  This is a slow-response control loop, but it can be used in conjunction with the screw speed control for long-term stability of head pressure.  Commercially available electronics packages can do the feedback control.

In summary—to operate a melt-pump/melt-conditioning system, a plasticating device must be used.  It can be a single-screw extruder, a twin-screw extruder, or one of the less commonly used continuous plasticating machines.  The unit is equipped with hopper preheating (either with or without drying elements) to introduce the resin into the extruder with as much heat content as is practical.  The extruder can be equipped with weight or volume feeder control useful in the overall control.  The extrusion device is oper­ated as an efficient plasticating device without regard to normal output-stability requirements, since the output is completely controlled by the melt pump.

Since this arrangement involves much lower shear than normal, the extruder will have addi­tional shear elements Incorporated.  Feedback control is used to maintain proper head pres­sures by screw and/or feed control.  The reduced output-stability requirements permit the system to be used on equipment with somewhat worn screws and barrels.