Staying in Control of the Extrusion Process

Many tools and techniques help extruders meet demanding requirements.

As medical devices get smaller and tolerances get tighter, it has become more important than ever for tubing
manufacturers to maintain good control over the extrusion process. But smaller dimensions and tighter tolerances
also make process control more difficult than ever for extruders.

How are manufacturers meeting today’s process-control challenge? Strategies includes a complex mix of factors
involving extrusion equipment, practices, and personnel. The approaches include actions taken before, during, and
after the extrusion process, both on and off the production line. Not surprisingly, however, all the extras that help
ensure tight control also run up the price of extrusion, so medical-device firms should be sure that they’re not buying
more process control than they really need.

Elements of Control

A review of the elements of good process control starts with the setting in which extrusion is performed. Extruders can’
t expect to achieve tight and reproducible process control if they don’t have control of the extrusion environment,
says Keith Andrews, director of business development for Upchurch Scientific (Oak Harbor, WA), which extrudes
tubing for medical-device companies. Upchurch controls the extrusion environment by performing the process in a
Class 100000 clean room. Among other things, the clean-room setting allows Upchurch to prevent large variations in
temperature and humidity that could be detrimental to the extrusion process, Andrews notes.

When setting up a new process, extruders can often get help by reviewing data from old processes, according to
Don Centell, general manager of ExtruMed LLC (Placentia, CA), which provides custom extrusions to the medical-
device industry. ExtruMed’s database contains information about almost 14,000 extrusion runs over the last 10
years. Company personnel often tap this extensive database to obtain information that can be applied to a new job,
Centell says.

Particularly important is information about the material used to make the tubing. But companies can’t go to their
databases for information about the many new resins that are always being introduced. So before using a new resin,
Andrews says, manufacturers must test the material to obtain data on its characteristics that will help them set up a
successful and well-controlled process for extruding it.

The material should determine what screw design is chosen for the extrusion process. “If a screw isn’t properly
designed for a particular material, then you typically won’t get consistent output from that screw,” says Mark Saab,
president of Advanced Polymers Inc. (Salem, NH), which manufactures plastic tubing and medical balloons for dilation
procedures. “This means you’ll have a lot of dimensional and temperature fluctuations in your process.”

At Advanced Polymers, the first step in finding the right screw for a particular material is evaluating various in-house
screw designs. If none of these provides consistent output, the company sometimes turns to an outside manufacturer
to produce a new screw for the material. But this can be a lengthy and expensive process, so it’s best to opt for it
when it can be justified by sufficient product volume.

In addition to the screw and other basic equipment, extruders sometimes add a melt pump to their lines. Melt pumps
can provide a more consistent flow of material than the extruder itself, which stabilizes the process and improves
control, says Bob Asam, a senior process engineer at Teel Plastics Inc. (Baraboo, WI), which specializes in custom
manufacturing of close-tolerance plastic tubing.

In some cases, melt pumps are used to reduce pressure fluctuations that can cause dimensional inconsistencies
during the extrusion process. For example, Centell says, pressure on the inlet side of the melt pump might be
fluctuating by 20 or 30 psi, but by less than 5 psi on the discharge side.

On the downside, Saab notes, a melt pump is a costly piece of equipment and expensive to maintain. It also exposes
the extruded material to additional heat, which can cause thermal degradation. For these reasons, he says, most
extruders either don’t use melt pumps at all or only use them when absolutely necessary to meet customer
requirements.

Beneficial Advances

All up-to-date extrusion lines have benefited from advances in motors and drives, which allow manufacturers to tightly
control the speed of screws and other equipment along the line. “When you set the machine to 46 rpm, the speed is
exactly 46 rpm,” says Guy Schultz, a process engineer for Filtrona Extrusion (Athol, MA), which manufactures plastic
tubing. “There’s no variation over time, which allows us to hold tighter tolerances.”

In addition to precise speed regulation, state-of-the-art extrusion lines feature improved temperature control. In the
past, Schultz says, the temperature of the extrusion process “could bounce around a little bit.” Now, however,
temperature controllers on the machines maintain process temperatures within a range of ±1ºF.

According to Saab, most new extruders include high-end PID temperature controllers. But many older extruders that
lack these temperature controllers are still in operation. Such extruders are “something you really want to avoid if you’
re in the medical industry,” Saab says. “In extrusion, being able to control the temperature in your line is
fundamental. But that doesn’t mean everybody does it or does it well.”

Besides temperature controllers, Advanced Polymers uses data-acquisition systems to collect temperature and
pressure information from different points along the line during an extrusion run. This data helps plant personnel
pinpoint production problems and determine their cause.

Inline Measurement

Perhaps the most common and useful process-control tools are devices that perform inline measurement of critical
tubing dimensions. At Filtrona, tubing OD is checked by a high-speed inline monitoring system. The system features
a dual-axis laser micrometer that measures tubing from two different angles 90 degrees apart, the better to catch any
ovality in tube cross sections. Also included is a large LED readout that displays tubing ODs to the fourth decimal
place.

Now, some commercially available laser gauges add a third axis to the measurement process. These three-axis
gauges are more likely to catch tubes that aren’t perfectly round than their two-axis counterparts, according to Saab.

On Filtrona’s extrusion lines as well as others, laser gauges that measure tube OD are joined by ultrasonic devices
that can determine the thickness of tube walls. With a tube’s OD and wall thickness known, Filtrona’s system can
calculate the ID to the fourth decimal place, Schultz says.

Inline measuring devices make it possible for tube manufacturers to set up feedback loops that operate during the
extrusion process. For example, ExtruMed engages feedback loops based on the tube OD readings taken by a laser
micrometer. If the OD readings are moving toward the high end of the allowable range, the system will remove a little
air from the extrusion process and then take more readings to see the results of that action, Centell explains. Then, if
necessary, the system can remove more air to further adjust the process.

Since feedback loops introduce another variable into the extrusion process, ExtruMed uses them only when
absolutely necessary. “One rule of thumb is that if you’ve got a real slow drift, go ahead and engage the feedback
loop. It will help you out,” Centell says. “But if [the process] is running stable on its own, keep it manual.”

ExtruMed’s database of information from past extrusion runs can alert operators to situations that might require a
feedback loop. For example, Centell says, data from previous runs may show that a pressure feedback loop is
usually engaged when extruding a certain material. In a case like this, the feedback loop may not be engaged to
begin with, but the operator won’t hesitate to do so if run data indicate that it can help solve a control problem.

Though it can be a useful tool in many cases, closed-loop control has significant limitations. “It can only adjust what it’
s set up to adjust,” Saab notes. “So if you have another problem, it’s not going to correct for that.” In fact, it can
actually make matters worse by repeatedly making adjustments in an effort to solve a problem that it hasn’t been set
up to solve.

Therefore, Saab says, a better control system is one that involves a trained operator. In such a system, a drifting
process sets off an alarm that alerts the operator, who can then assess the situation and decide on a course of
action.

Advantages of SPC

Filtrona’s monitoring system prints out quality-control reports during the extrusion process, when they can be used to
make any necessary process adjustments. “Once the job is done, it’s too late” to get this information, Schultz says.
“We need real-time quality control information, and that’s what the system gives us.”

At Teel, computerized statistical process control (SPC) makes it easy for operators and quality personnel to access
process data. “I can just go to my computer and pull up a run to see if we were in control or out of control,” say
Rachell Bainbridge, the company’s director of quality.

Before SPC was computerized at Teel, operators wrote process information on paper control charts. In addition to
plotting data points on the charts, they had to calculate ranges and averages. Now, though, operators simply enter
measurements into the system and it produces process information at the press of a button, speeding up data
analysis and eliminating the possibility of human error in the calculations.

Like the Teel system, ExtruMed’s process provides real-time data in chart form that allows plant personnel to spot
trends. “It’s visual, so you can very quickly see if you’re running stable or experiencing very slow fluctuation,” says
Centell.

Most of ExtruMed’s customers don’t ask the company to provide SPC data, but the company still gathers the
information for its own purposes. “We use it to find out if the current run is similar to the other runs we’ve done for the
customer in recent years, so we know if we’re consistent,” Centell explains.

Other Factors

Another factor impacting process control is the caliber of an extruder’s engineering and quality teams. At Teel, an
engineer is always present for the initial run of a material in order to optimize the process, according to Chris O’
Connor, a senior product development engineer at the company. In addition, Saab says, operators his company who
can’t get a process to run within specifications will call in an engineer, who will to look at factors such as tooling,
temperature, and screw design. And if a number of recent extrusion runs produce tubes that are rejected for the
same reason, manufacturing engineers and the quality team at ExtruMed will get together to try to pinpoint the cause
of the problem.

Then there are the operators. Has the extruder established effective training programs for them? In many cases,
operators aren’t trained well enough to take full advantage of the latest extrusion equipment, Centell says.

In addition to training their operators, companies concerned about process control should make sure they follow
standard work procedures. Examples of standard work are the steps established by a company for starting up an
extrusion line and calibrating gauges. “We don’t want five different operators to be doing five different things,”
Andrews says. “We want all the operators to follow a proceduralized process that will help us get consistent results.”

Process control also depends in part on how well extrusion personnel maintain their equipment. If it’s not properly
maintained, “the process can drift over time because of equipment wear,” Saab says. “As a result, what you make
this year can be significantly different from what you made two years ago.”

No equipment is more important to the process than the screw and barrel. “As these components wear over time,
your process will change,” Andrews says. “So it’s crucial to keep track of wear of the screw and barrel—and to know
when you have to repair or replace them.”

In addition to checking for wear, Saab says, extrusion personnel should make sure that their equipment is properly
calibrated. Consider, for example, the temperature controller on an extrusion line, which could “very easily” be off by
5ºF, Saab says. While that may seem like a small amount, it can have an impact on the properties of tubing made on
that line.

The Importance of Inspection

To ensure that an extrusion process remains in control, no job is more important than inspection. At ExtruMed,
technicians are constantly checking their products and filling out in-process inspection sheets. Depending on what
data a customer wants, technicians will record measurements of tube OD, ID, and/or wall thickness. If multi-lumen
tubing is being extruded, inspectors will also record all lumen dimensions.

Once every half hour during the extrusion process, Filtrona personnel cut a cross section out of the tubing and
inspect it using an optical comparator. This device magnifies the cross section 20 times, making it easier for
inspectors to examine tubing and check critical dimensions.

Vision systems are crucial tools at Molded Rubber and Plastic Corp. (Butler, WI), which extrudes silicone tubing for
the medical industry. These elastomeric products can’t be measured with a touch probe because contact with the
measuring device can cause deformation of the soft material, explains Greg Riemer, MRPC’s vice president of sales
and marketing.

But measurement using a vision system doesn’t require any contact with the product. These systems put a magnified
color image of a tube cross section on a computer monitor, allowing inspectors to manually take dimensions such as
OD, ID, and wall thickness. Since the systems can “see” dark and light, they can also locate the edges of a tube for
an inspector, notes Mark Tesch, MRPC’s engineering manager.

Plant personnel certainly need help in meeting today’s greatest inspection challenges. For example, studies done by
Advanced Polymers have found that inspectors simply can’t measure tubing with the super-tight tolerances now
being specified by many medical-device firms. So the company is working with Lumetrics Inc. (West Henrietta, NY) to
develop a non-contact optical gauging system that will be used offline to measure tubing. An operator will set up the
system and load tubing into it, but the system automatically takes measurements without operator involvement,
thereby freeing the measurement process from human error and limitations. In addition to providing greater
accuracy, Saab hopes the system will also be faster than manual measurement.

Customers and Control

How much inspection is necessary? That’s ultimately up to the customer. ExtruMed can show customers a year’s
worth of inline measurement data to statistically prove that an extrusion process is under tight control—and,
therefore, that costly inspections can be reduced. But if a customer still insists on frequent inspections to check
critical tube dimensions, “then that’s part of the price that the customer is willing to pay,” says Brandon Gosiengfiao,
ExtruMed’s senior quality engineer.

ExtruMed makes several different tubing products for Spectranetics Corp., a medical-device firm in Colorado Springs,
CO. These include three sizes of bump tubing made of HDPE, which is a very difficult material to extrude, according
to Jamie Fearing, the company’s purchasing supervisor. Adding to the difficulty of the job are the tolerances, which in
one case are as tight as ±0.0001 inch, Fearing notes.

To help ensure that ExtruMed’s processes are up to the job, Fearing’s firm conducts regular audits at the extruder’s
facility. During these audits, Spectranetics personnel look for:
• Quality systems that are ISO and/or FDA certified.
• Records showing that operators are properly trained.
• Evidence that processes are in control. This is obtained by watching production runs and reviewing inspection data
from past runs.

Of course, the evidence that will satisfy Spectranetics is different from the evidence that will satisfy another medical-
device firm. So the quality plan for an extrusion job will be more or less demanding, depending on customer
requirements. “Some of our very stringent customers want an ID, OD, wall, and concentricity measurement for every
20 pieces we run,” Tesch reports.

The price paid by these customers for their stringent requirements can be very high. According to Saab, process-
control equipment and personnel can easily double or triple the cost of an extrusion run. “There’s a huge difference
between what we charge a customer who has broader tolerances and specs and a customer with tight tolerances
and specs.”

This article appeared in Medical Device & Diagnostic Industry magazine.