Process Considerations in the Extrusion of Microbore Tubing

Medical Device & Diagnostic Industry Magazine
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An MD&DI January 1999 Column

COVER STORY

Successful processing of small-diameter medical tubing requires careful control over a multitude of variables.

With the evolution of medical science, the demand for smaller-diameter or microbore tubing has increased significantly. More diagnostic as well as therapeutic procedures are being performed using microbore tubing, some of which features complicated lumen geometries. The structural performance and tolerance requirements of many of these components can be difficult to achieve by means of conventional extrusion processes. Polymer extrusion is a highly involved, multivariable process, in which process variables often interact with one another in complex ways.

A statistical process capability index is a measure relating the actual performance of a process to its specified performance. The process is a combination of the manufacturing plant or equipment, the process or method itself, the people, the materials, and the environment. The minimum requirement is that three process standard deviations () on each side of the process mean be contained within the specification limits. This will ensure that 99.7% of the process output will be within the tolerances.

Many processes are found to be out of statistical control when closely examined using established control chart techniques. The root causes may be many, having different origins. Out-of-control conditions are often caused by an excessive number of adjustments made to the process. This behavior, commonly known as hunting, causes an overall increase in variability from the process (Figure 1). If the process is initially set at the target value µ_a and an adjustment is made on the basis of a single test result A, then the mean of the process will be adjusted to µ_b. Subsequently, a single test result at B will result in a second adjustment of the process mean to µ_c. If this behavior continues, the variability or spread of the results from the process will be greatly increased, with a detrimental effect on the ability of the process to meet the specified requirements.

Figure 1. Increase in process variability caused by frequent intentional or nonintentional process adjustments.

When a process is found to be out of control, the first action must be to investigate the assignable cause or special causes of variability. This may require, in some cases, the charting of process parameters other than the product parameters that appear in the specification. For example, it may be that the tubing's dimensions vary because of pressure variations in the die region caused by variations in the polymer's viscosity. A control chart of the die pressure, with recorded changes in the process temperature, may be the first step in breaking down the complexities of the relationships involved. It is important to ensure that all adjustments to a process are recorded, that the relevant data are charted, and that the instruments collecting the data are accurate and calibrated. Bad data are worse than no data!

Extruding small-diameter tubing requires extremely precise process control.

There are many potential assignable causes that can be responsible for a polymer extrusion process being out of control or incapable of producing products to the required dimensional specifications. This article addresses those process variables and their interactions that can adversely affect the variability and quality of a polymer extrusion process as well as the end product that it produces. Opportunities to improve overall process control through the optimization of extruder screw and tooling geometry, process sensors, instrumentation, and control tuning are also discussed.

MATERIALS

Selection and Characterization. Proper selection of the polymer or polymers to be used is imperative. An inappropriate grade of material (in molecular weight, molecular architecture, density, etc.) for an application can result in structural, dimensional, and/or cosmetic deficiencies in the finished part. One should look carefully at the intended use(s) of the finished part and then apply sound engineering skills in selecting those polymers that can meet the targeted performance requirements and be processed with the available equipment. The polymers should also be cost-effective.

Appropriate characterization methods should be applied to ensure uniformity in the molecular-weight distribution (MWD) of the material. Melt-flow-index (MFI) testing may be appropriate to determine or verify the molecular weight (MW) of the polymer; however, it is not the appropriate method for determining the MWD. Analytical gel filtration—that is, gel permeation chromatography (GPC)—serves as a reliable method for determining MWD.

Variations in the geometry of the distribution curve can be responsible for variations in a material's viscosity. A given set of extrusion process parameters, appropriate for one batch of material, may not be optimum for another batch. Variations in the MWD of a polymer may be of no consequence to the manufacturer of garden hoses but can create significant process problems for the manufacturer of microbore tubing. Appropriately administered test methods such as differential scanning calorimetry (DSC) to determine the MFI, GPC, and crystalline melting point (T_m) are extremely helpful not only in troubleshooting process variation caused by changes in the molecular architecture and MW/MWD but also in preventing unwanted process variation.

Formulation and Preparation. Proper compounding techniques are essential for ensuring the polymer's structural integrity. An improper temperature profile, screw geometry, and/or compounding method can easily be the cause of loss in MW from either mechanical degradation (excessive shear) or thermooxidative degradation, which can result from too much shear, an improper temperature profile, or a combination of the two. It is equally important to know that certain mixing or compounding techniques provide for better dispersive or distributive mixing capabilities than others. Not enough shear can result in an undesirable particle-size distribution, whereas too much shear can degrade the carrier resin. Inadequate distributive mixing capabilities can lead to a nonhomogeneous distribution of the additives in the base resin.

Materials formulation is a science in itself, quite often underrated and not seen as an important step in an overall extrusion process. In many instances, the extrusion hardware is unjustifiably blamed for being the cause of either unwanted process variation or structural or cosmetic deficiencies in the finished part. Some time spent up front in selecting the appropriate mixing/compounding equipment and process parameters is usually very cost-effective, since troubleshooting an extrusion process to determine the origin of problems caused by inadequately prepared materials can be a very lengthy and expensive undertaking.

Figure 2. Polymers manufactured by means of a condensation polymerization process must be appropriately dried to avoid the formation of volatile by-product molecules such as water, acetic acid, or hydrochloric acid.

Figure 3. An excessive amount of remaining volatiles in the resin of polymers manufactured by condensation polymerization can result in a loss of molecular weight from chain scission caused by hydrolysis.

Some materials must be appropriately dried prior to processing. These are typically polymers manufactured by means of a condensation polymerization process (e.g., PET, polyamide), in which reaction results in the formation of a small, usually volatile by-product molecule such as water, acetic acid, or hydrochloric acid (Figure 2). An excessive amount of remaining volatiles in the resin can result in a loss of MW caused by chain scission as a result of hydrolysis (Figure 3). This phenomenon may also adversely affect the cosmetic properties of the finished part.

It is strongly recommended that the resin manufacturer's recommended drying parameters be followed closely in order to prevent this type of polymer degradation. It is equally important to maintain the dryness of the material afterward, as some polymers are quite hygroscopic in nature and will easily and rapidly absorb moisture from the ambient atmosphere.

Some of today's high-performance engineering resins (some of them quite expensive) that have come about as a result of innovative polymer chemistry can offer the end-user a variety of outstanding properties only after the material has been properly formulated and processed.

EXTRUSION HARDWARE

Extruder Screw and Tooling Design. The extruder size as well as the L/D (length/diameter) and compression ratios of the extruder screw must be optimized for successful extrusion of microbore tubing. Typical extruder output ranges from 0.5 to 1.5 lb/hr. It is important to keep the residence of the resin inside the extruder within the appropriate limits. Excessive residence time (thermal history) in the extruder barrel can cause some polymers to quickly degrade. Care must be taken in extruder selection and screw design. Extruder screw design must take into consideration the bulk density and rheological properties of the polymer, as well as the required die pressure and polymer melt output. Typical production extruders used in the manufacture of microbore tubing range in diameter from 0.5 to 1.0 in., with L/D ratios from 15 to 24.

The extruder screw design should incorporate appropriate mixing capabilities, if needed. The primary objectives are to deliver a polymer melt of a homogeneous viscosity to the die region at a stable and uniform pressure and to ensure that additives, if required, are properly distributed and of uniform size. The shear rates imposed on the polymer must be appropriate, since excessive shear can lead to polymer degradation, whereas insufficient shear/compression can reduce the melting capacity of the screw and result in inappropriate dispersive mixing.

Figure 4. Leakage-flow mechanism in a single-screw extruder, in which pressure flow acts across the screw-flight gap.

Screw wear on both the major and minor diameters, as well as the inside diameter of the extruder barrel, should be periodically measured. Excessive clearances between the screw flight and barrel will adversely affect the heat-transfer characteristics between the barrel and the polymer melt and will increase leakage flow, which can potentially increase the thermal history of the polymer and reduce extruder output (Figure 4). Extruder output stability will also be adversely affected. Excessive wear of the minor diameter will change the plasticating and melt-conveying characteristics of the screw.

Figure 5. Breaker plate and screens for pressure control.

Tooling used (e.g., tips, dies, breaker plate) in the manufacture of microbore tubing should be designed and manufactured to ensure balance flow. The breaker-plate design should ensure that potential dead spots in front are eliminated (Figure 5). Screen-pack choice should be made carefully. A screen pack that is too dense may impose excessive shear onto the polymer, resulting in mechanical degradation (chain scission). A too-dense screen pack can also cause excessive internal barrel pressure and reduce the extruder's output. A screen pack that is not dense enough may result in inadequate filtering or back pressure. A certain degree of back pressure is desirable, as it provides for additional polymer mixing.

Figure 6. Streamlined profile-extrusion die.

Tooling geometry should be designed with the polymer's rheological properties in mind. Excessive compression—that is, too long a land length—may result in too much imparted shear. A land length that is too short may not provide for sufficient molecular alignment and can cause excessive extrudate swell (Figure 6). Impregnation of the tooling with selected materials (nickel-Teflon, carbonlike diamond, etc.) has proven to be beneficial in reducing "slip-stick" in the die region. Ultrasonic energy applied to the die can reduce extrudate swell by facilitating molecular alignment.

Figure 7. Draw-down ratios should be calculated carefully.

Figure 8. Careful calculation of tip and die strain will help ensure appropriate molecular alignment and dimensional stability.

Draw-down ratios—typically between 2 and 10 (Figure 7)—and tip and die strain (Figure 8) should be carefully calculated. Any deficiencies in these variables can result in inappropriate molecular alignment and ultimate dimensional instability in the product.

Visual polymer weld or knit lines resulting from the polymer's inability to weld or knit together after being divided in the flow divider or spider (in-line die) should and can be eliminated through improved tool design. Manufacturers extruding chlorinated or fluorinated polymers (e.g., PVC, ETFE, PVDF) should make sure that the wetted parts of their tooling are manufactured from the appropriate base metal in order to withstand the chemical side effects that these polymers produce during processing.

PRODUCT COOLING

Upon the exit of the extrudate from the die, the next step in extrusion typically involves a cooling process. Even cooling at the appropriate rate is paramount to ensuring dimensional stability, concentricity requirements, and proper rate of crystallization. A cascading flow of water at the entrance of the water bath will result in uneven cooling around the circumference of the tubing, which can be responsible for short-term dimensional variations and unwanted ovality. Turbulence in the water reservoir should be eliminated, since this can be a source of unwanted dimensional variation.

Proper control of the temperature of the cooling medium (air or water) is also critical. The polymer's morphology can be greatly influenced—positively or negatively—by the cooling temperature and cooling rate. For some materials, obtaining good dimensional concentricity control is only possible with air cooling. Surface imperfections are often caused by an improper water temperature. Achieving correct alignment of the microbore tubing as it exits the die and goes through the water trough or air tunnel and takeoff system is a must. Any misalignments will adversely affect the product's final geometry. Laser alignment is an inexpensive method to ensure that all components share the same centerline.

VACUUM/VACUUM-ASSISTED SIZING

Although vacuum/vacuum-assisted sizing is not commonly performed in the manufacture of microbore tubing, some manufacturers rely on this process to give them the required dimensional stability and control. Servo control of the sizing tank's vacuum is a must, especially if a feedback loop has been established between the laser scanner and the vacuum pump. Conventional pump drives (other than ac vector drives) may not have the required responsiveness, and tend to over- or undershoot the desired set point. For this technology to be of real benefit, optimum control algorithms must be in place. The algorithms must take into consideration parameters such as the line speed, time delay between the extrudate's exit from the die and measurement by the laser scanner, viscoelastic behavior of the material, and correction rate factor. Any deficiencies in either the hardware or control schemes may cause the process to make improper changes at the wrong time, thus making things even worse than they were initially.

PROCESS CONTROL

Process Sensors. As stated earlier, process control can only be as good as the feedback data (signals) received from the process. If the received data are not representative of the actual process conditions, erroneous process adjustments will be made, usually resulting in the product not meeting performance specifications. Thermocouples (T/Cs) and/or RTDs should be of the proper design and regularly calibrated. Improper location or mounting of these devices will result in an output that may not reflect actual process conditions.

A polymer melt with a uniform viscosity at a uniform and stable pressure in the die region is a must in order to achieve good dimensional control. T/Cs and RTDs are responsible for providing temperature feedback to the individual zone controllers or PLC (programmable logic controller), so that these in turn can apply the proper algorithm and subsequently modulate the heating or cooling medium. Corroded T/Cs or improperly mounted, uncalibrated T/Cs and RTDs will provide wrong information to the controllers, causing them to output erroneous signals to the heating or cooling hardware.

Pressure transducers come in a variety of designs. Several methods are employed to translate the mechanical deflection of the transducer diaphragm to the strain gauge. The use of push rods—capillaries filled with either mercury or a mixture of sodium and potassium—is the most common method. A relatively new transducer design comprises a strain gauge molecularly bonded within a sapphire wafer that is directly exposed to the polymer melt.

Response times and full-range accuracy are the important factors in transducer selection. Typical response times for various transducer types are as follows: 50–100 milliseconds for capillary style, 10–20 milliseconds for push-rod style, and 100–500 microseconds for sapphire wafers. If pressure transducer outputs are to be used in closed-loop process control (feed forward/feedback), it is imperative not only that the most accurate and responsive transducers are used, but that they are also properly sized (operating range) for the process. A 0–10,000-psi transducer used in a process location where the operating pressure range is 0–2500 psi will not provide the optimum resolution needed for proper process control.

Pressure transducers, like T/Cs, need to be calibrated regularly at operating temperature. They should be periodically removed from their location so that degraded material can be cleaned from the mounting well. Extreme care should be taken when installing and removing transducers, and the manufacturer's instructions should always be followed.

Noncontact Dimensional Gauging. Statistical rigor must be applied when evaluating the performance of a noncontact laser gauge. Quite often, gauging systems are placed in-line with an extrusion process that do not possess the resolution, repeatability, reproducibility, and thermal stability needed to perform the required tasks. Often, the laser gauge is an integral part of a process-control loop used to modulate factors such as vacuum, lumen air, and takeoff speed, so as to maintain dimensional stability. Important criteria in selecting a noncontact gauging system include the following:

Controllers. Most small extruders used in the manufacture of microbore tubing incorporate single-loop controllers. Today's single-loop controller technology is excellent: most current controllers feature PID (proportional-integral-derivative) control and have self-tuning capabilities, and some are capable of "adaptive control" as well. Several manufacturers market controllers that not only provide for PID control but also make use of fuzzy logic (a form of artificial intelligence) algorithms. As always, the controller's output is only as good as the quality of the input signal and the applied control algorithm. Quite often, considerable process improvement can be realized by optimizing the feedback/feed-forward control algorithms.

Control Tuning. Long-term process variations (in pressure or dimensions) often come as a result of improperly tuned controllers or hardware that is incapable of responding to the controller's output signals. Finding the optimum P, PI, or PID algorithms for a temperature or pressure controller is a lengthy exercise that requires considerable expertise. It is imperative to allow the process to come to steady-state conditions after an algorithm change. This can sometimes take 20 minutes or more.

Several manufacturers make self-tuning temperature/pressure controllers that work quite well. It should be noted, however, that an optimum set of algorithms for one particular material or process may be far from optimum under different conditions. Some processors find it of benefit to first develop optimized control algorithms for each of their processes, store these on disk or on paper, and then download them to individual controllers either by hand or through the appropriate computer interface. All the resources spent on hardware, screw and downstream equipment design, and materials optimization will be of no benefit unless the proper process control methods are in place.

Process-Variable Interactions. Polymer extrusion consists of a significant number of variables. It is mathematically complex and often not fully understood. Some of the variables interact in a nonlinear fashion, which makes controlling them a challenge. Several important variables and their interactions either with another variable or with the product's physical properties or quality are listed in Table I.

Table I. Polymer extrusion variables.

Advanced Multivariable Control. PID control is effective for single-variable control but cannot control more than one variable at a time, is incapable of nonlinear control, and is not adaptive to changing process conditions. For applications that require multivariable control, fuzzy logic is becoming increasingly popular. The flexibility and versatility that come with fuzzy logic are due to the fact that it does not make use of Boolean logic ("on-off" or "0-1") but allows for approximation of process values. Process interactions can be geometrically described and weighed (as in a neural network), and control algorithms can be designed that precisely match the magnitude of the prevailing process values and their respective interactions.

CONCLUSION

Precision polymer extrusion is often referred to as an art. Achieving successful fabrication of a product as complex as microbore tubing, however, requires a scientific process in which every process component and interaction can be mathematically quantified and explained. This article has presented the variables involved and the necessity for attention to details—from raw materials to extruder hardware, screw and tooling design to control schemes and instrumentation. Less than optimum conditions in any of these areas will adversely affect a microbore tubing extrusion process. Although variations in the polymer's MW or MWD are often unavoidable, they can be effectively handled by incorporating adaptive techniques in the control of the melt pressure. Careful advance planning is critical, and resources spent up front in developing a robust extrusion process are always cost-effective.

BIBLIOGRAPHY

Cheremisinoff, Nicholas. Polymer Mixing and Extrusion Technology. New York: Marcel Dekker, 1987.

Murrill, Paul. Fundamentals of Process Control. Research Triangle Park, NC: Instrument Society of America, 1988.

Rodriguez, Ferdinand. Principles of Polymer Systems. New York: Hemisphere Publishing, 1970.

Hans W. Kramer, PhD, is a research scientist at Medtronic Interventional Vascular (San Diego), where he specializes in the development of novel polymer formulations and processing technologies as well as product R&D. He also serves as an adjunct professor at San Diego State University, where he is presently involved in establishing a graduate-level polymer science program.

Photo by Roni Ramos

Copyright ©1999 Medical Device & Diagnostic Industry