Originally published May 1997
TUBING DESIGNMicrobore medical tubing is becoming ever smaller and more complex. Photo: NDH Medical
The use of vacuum as a sizing technique for tubular extrusions has been a common and highly successful practice since the 1950s. In the late 1980s, a modified vacuum sizing principle--generally known as free extrusion with vacuum assist--was developed to enhance the processing of sticky or low-durometer materials such as flexible PVC and urethanes. However, when these same techniques have been tried in the processing of microbore tubing, results have been mediocre at best. One reason is, of course, the size of the tubing: a microbore tube can have a diameter as small as 0.030 in., with a wall as thin as 0.005 in. Given dimensions of this scale, tubing will set up almost immediately upon contact with the cooling water, which makes it very difficult to use vacuum to help with the sizing process.
In this article, we review recent modifications in equipment and processing methods undertaken in order to effectively apply vacuum sizing principles to the production of microbore tubing. Changes such as smaller tank cross sections, specialized low-turbulence features, split platetype vacuum tooling, and the necessity of increased vacuum levels are among the items discussed. The article also explains how--when properly applied--the use of vacuum can enhance the manufacturing process, offering the potential for on-line cutting with no effect on dimensional stability and even minimizing the need for excessive drawdown to ensure roundness on multilumen tubing. We will begin by analyzing current sizing principles used in the processing of both single- and multilumen tubing.
In the free-extrusion process, precision internal airregulation systems are used to induce a specific air pressure/flow through individual or multiple holes in the die and into the molten tube. The belt or wheel puller then pulls the tube from the die and through the cooling medium at a consistent speed. As the polymer exits the die, the process of heat transfer begins, with air as the initial cooling medium, followed by the water within the cooling tank. The speed of the puller and the temperature of the cooling mediums determine how much drawdown occurs. By increasing or decreasing air pressure, product designers can vary the tube diameter along with individual lumen sizes and wall thicknesses. The cooling mediums remove heat from the tube, while the diameter and/or individual lumen configurations are maintained by the internal air pressure.
Given proper die design and precise control of air pressure, water temperature, and belt-puller speed, free extrusion can provide the means to hold extremely tight diameter tolerances. One drawback of free extrusion is that roundness is difficult to maintain. Mutilumen tubing can vary in roundness because of uneven wall thickness and the size and shape of the individual lumens, which cool at different rates. Many single-lumen tubes vary in roundness because of the weight of the water used in the free-extrusion process.
Figure 1. Typical single-lumen microbore tube (left) and a multilumen microbore tube with the design known as double-D (right).
Figure 2. The image on the left shows how ovality can occur when vacuum or internal air pressure is used on a nonconcentric tube. On the right, pressure on a tube with concentric wall thickness results in an even outward force and good roundness.
Figure 3. Irregularities that can arise during free extrusion of a multilumen double-D tube (left) and a three-lumen microbore tube (right).
PHYSICAL PROPERTIES OF TUBE DESIGN
A brief review of the physical properties of single- and multilumen tubing enables one to better understand issues that can affect a processor's ability to make the tube round, especially with free extrusion (see Figure 1).
Single-Lumen Tubing. When a tube is designed as a single lumen, its wall is generally concentric. Roundness is relatively easy to maintain, since any internal pressure will exert an equal force pushing out on all sides. If any section of the wall is thin, however, the same internal pressure can cause an unequal force on the thin side, causing ovality (see Figure 2). With some microbore tubing featuring wall thicknesses as small as 0.003 to 0.005 in., ovality can result when a product is only a few ten-thousandths of an inch out of concentricity.
Multilumen Tubing. If a tube is multilumen in design, it can potentially have a variety of shapes and sizes for the different lumens within the tube, as well as several different wall thicknesses between or surrounding the lumens. Each of these walls will then go through the heat-transfer process at different rates, which can lead to shrinkage variations and ovality. When internal air is used to size the lumens individually, the air pressure will again push out with equal force on all sides. But if the wall thickness varies or the shape of the lumen is something other than round, an unequal force can once more cause ovality.
Because of these problems associated with multilumen tubing, processors have resorted to excessive die drawdown and to the use of minimal air pressure/flow through each lumen. Given the inability to control or limit the growth of the outside diameter with free extrusion, the amount of trial and error involved in fine-tuning the process of multilumen die design can be tedious and expensive (see Figure 3).
FREE EXTRUSION AND THE COOLING PROCESS
Now that we have defined the two basic types of tubes, let's look at the free-extrusion process and discuss how roundness can be affected by the cooling process itself. When a tube enters the cooling tank, the water used as a heat-transfer cooling medium presents several problems that can lead to ovality or variations in wall concentricity.
Water Turbulence. The initial contact of the tube and the cooling water produces the most dramatic effect on tube diameter, ovality, and wall concentricity. The reason for this is the dramatic temperature differential between the tube and the water. If water is pouring out of the cooling tank--typically in a downward and forward motion--this initial form of water turbulence can cause uneven or variable heat transfer. This drooling of entrance water will generally cause the bottom of the tube to cool faster, with potentially a heavier wall; the top wall will be thinner because it will keep drawing-down longer. With multilumen tubing, this condition can make it almost impossible to fine-tune the individual lumens.
It should be noted that this drooling is not consistent. It generally surges back and forth as much as 0.125 to 0.250 in., depending on the size of the tube and of the clearance hole. Vibration or movement in the tank itself, caused by water pumps or poor mechanical design, can further exaggerate this surging, which in some of the more sensitive materials can result in water marks or even a slight flattening of the tube. Drooling of water can also cause actual movement of the tube--in some cases all the way back to the die. Wall concentricity, especially in very small, thin-walled tubing, will be directly affected by any movement at the die.
Split-Plate Tooling. In horizontal free extrusion of small-diameter tubing, the entrance tooling--which is generally of a split-plate variety--has a clearance hole for the tube to pass through. The only physical contact with the tube is made by the guide rollers, which limit flotation. For this reason, any movement or turbulence can cause the tube to move, allowing the internal air to vary its effect on the tube size and roundness.
The split plate serves as a dam, which enables the tube to enter the tank at a particular level under the water. When properly adjusted, the tube should enter the tank through the clearance hole and never touch the actual plate or tool. The hole should be as small as possible to minimize unwanted drooling; some processors simply drape strips of wet paper or fiber cloth around the hole to reduce the clearance size. Extreme care must be taken not to touch the tube with the paper or cloth so as to avoid blemishes or scratches on the outside.
In certain cases, processors intent on further enhancing free extrusion of their tubing have used an adjustable iris to replace the split plate. They begin with the hole greatly oversized in order to simplify the start-up procedure, then adjust the iris to minimize the clearance hole and consequently the drool.
Both movements of the tube and variations in the tank-temperature gradient can affect tube sizing. It has been observed that when there is no movement of the cooling water--and thus of the tube--sizing tolerances improve by as much as ± 0.001 in.
Heat Exchange. The heat-exchange process itself can potentially be a source of product instability. As heat is pulled from the tube, the surrounding water warms up; this heated water tends to "follow" the tube, like a physical layer. If water turbulence then occurs--whether from water introduction, water removal, rotating product guides, or even vibration--this heated layer is no longer controlled, introducing another variable into the process.
Water Weight. During free extrusion, the actual weight of the water can influence the roundness of the tube. This is especially true with respect to small-diameter, thin-wall tubing. In some cases, the level of the water is kept as little as 0.062 in. above the tube to help maintain roundness. However, the use of this practice to limit flattening can lead to concentricity problems because of water-temperature differentials above and below the tube.
Effects of In-Line Cutting and Coiling. Another occasional complication of free extrusion is the temporary blockage of internally regulated air caused by in-line cutting or flattening in the coiling process. This condition can result in variable flotation and even slight expansion in the outside diameter of the tube as the extrudate first exits the die. Very small annular rings directly related to in-line cutting can sometimes be seen on the tube surface.
With strict roundness, precise dimensional tolerances, and superior appearance becoming ever more critical in both single- and multilumen medical tubing, many processors are now investigating new sizing methods that feature the application of vacuum. The vacuum sizing technique is growing in popularity and offers several recent enhancements designed to facilitate processing difficult materials and smaller-sized tubes that were previously achieved with free extrusion.
Vacuum Calibration. Vacuum calibration refers to the use of differential pressure (vacuum) to force the extrudate against a calibration/sizing tool, during which operation sufficient heat transfer occurs to maintain a specific outside diameter. Once the tube has exited the sizing tool but is still within the vacuum chamber, vacuum is applied to maintain an equal force on all sides of the extrudate to ensure roundness.
Water Rings. In many cases, water rings have been employed both to help create a seal and to lubricate the extrudate so as to minimize sticking problems during initial contact with the calibration tooling. The use of a water ring is called preskinning, which is the process of cooling a very thin (0.0010.005-in.) layer on the outside of the extrudate before it enters the vacuum chamber.
Vacuum Calibration Tooling. Many styles of vacuum sizing tools have been developed to process different kinds of polymers, but most can be divided into plate-type or sleeve-type units. Common to all vacuum calibration is the actual contact of the extrudate with the vacuum sizing tool. Generally, the tool is made slightly larger than the product, which shrinks after exiting the tool according to polymer formulation, cooling-water temperature, and line speed. The force of the vacuum must be kept at a sufficient level to maintain contact between the extrudate and the tool. This process has proven highly successful in sizing most tubular extrusions, with the key issue being the assurance of roundness.
Figure 4. Microbore noncontact vacuum-assist system.
FREE EXTRUSION WITH VACUUM ASSIST
Materials such as flexible PVC, urethanes, and certain thermoplastic rubbers have created problems during vacuum sizing because of their tendency to stick to the sizing tool on contact. Consequently, a new form of sizing has been developed with these materials in mind.
The technique--free extrusion with vacuum assist--is increasingly being used to improve tube roundness and attain higher line speeds with many of these difficult-to-process materials. Typically, a short cooling trough known as a preskinning chamber (from 2 to 8 in. long) is situated in front of the entrance to the vacuum chamber (see Figure 4). Ambient air is introduced as needed through the die, as with vacuum sizing. The short trough is designed to work in concert with the drawdown of the polymer to preskin the extrudate and initiate the sizing process before the tube enters the vacuum chamber.
Vacuum-Assist Tooling. The nature of the materials processed via free extrusion with vacuum assist also dictates a change in the vacuum tooling employed. A glass-filled Teflon or similar material is commonly used to make bushings that are located on both ends of the vacuum chamber and that act as a water and vacuum seal. Typically, from 0.015 to 0.070 in. is allowed for tube clearance through these bushings. In this case, the vacuum is no longer being used to push the tube against the tool, but rather to maintain roundness.
A type of a water ring may also be used in the process, keeping the tube centered in the bushing to avoid contact. Most of the actual sizing is completed within the preskinning chamber, with the vacuum section primarily serving to maintain size and roundness during secondary heat-transfer stages. These vacuum sections generally range from 2 to 6 feet in length, with, in some cases, multiple sections for extruding materials--like nylon--that require differential-temperature processing. Generally, very low vacuum levels--from approximately 0.1 to 135.0 in. water gauge--are maintained in the primary section. Levels must be kept within 0.1 in. to ensure consistent sizing and roundness.
Contoured Rollers and Belt Pullers. It is also necessary to use contoured rollers within the vacuum section to offset flotation and potential ovality. These rollers must be made of a low-coefficient-of-friction material, such as PTFE, to minimize sticking.
It is essential that the belt or wheel puller have very precise speed control. The speed of the tube being drawn from the die and through the cooling medium will directly affect both wall thickness and diameter.
Vacuum Tanks. When processors first looked at the potential of vacuum as a tool to assist in the sizing of microbore tubing, it was assumed that the technique would be an immediate success. But early on, problems surfaced when the only vacuum tanks available to the industry were huge in proportion to the tubing to be sized. The need to minimize water turbulence or vibration was scarcely considered in their design; even roller positions were limited to every one or two feet, which is inadequate for microbore tubing. For these reasons, many medical tubing processors did not attempt the process, but continued to struggle with free extrusion to make their products. Today, however, very rigid small vacuum tanks are being built with features specifically for the processing of microbore tubing. These new tanks are quite similar in size to conventional open free-extrusion tanks, but are far more rigid in design. In some cases, heat-exchange systems are built into the walls of the tank itself to minimize water turbulence.
OPTIMIZING VACUUM-ASSIST EQUIPMENT AND TECHNIQUES
For single-lumen microbore tubing, free extrusion with vacuum assist was the first process to be tried other than free extrusion. It was assumed, given the uniform wall thickness of the single-lumen design, that noncontact-type tooling would work on the majority of materials. A problem soon arose, however, with the length of the preskinning chamber before the vacuum section. Even with the shortest available preskinning chamber, prior to entering the vacuum section a tube was preskinned from 0.002 to 0.005 in., which in many cases represented the majority of the wall thickness. For this reason, vacuum had little or no effect on tube size or roundness.
Specialized Vacuum Tooling. New tooling has now been developed specifically for the process of free extrusion with vacuum assist of single-lumen microbore tubing. The preskinning chamber is now built into the vacuum tank and used as a low-turbulence area measuring approximately 1.25 in. long. Split plates are used on both ends of the chamber, with clearance holes gauged according to the tube material and size and the line speed. The main innovation in this specialized preskinning chamber is that it is under vacuum. In this system, the water is independently controlled within the preskinning chamber, and is used along with the vacuum to control any water drool and maintain roundness. Depending on the material, the water temperature within the system can be used to vary the effect of vacuum on the actual sizing of the tube. In this way, the vacuum can be employed primarily to maintain roundness of the single-lumen tube.
In the case of multilumen tubing, where internal air would normally be required to size the lumens individually, the die design used for microbore product is such that almost no internal air is needed. If vacuum is applied with the free-extrusion vacuum-assist process, the tube will be affected in the same way as if excess internal air was applied to the lumens: the vacuum will exert an equal force on all sides of the tube and pull where the walls are thinnest, resulting in ovality.
Figure 5. Hybrid vacuum calibrator (patent pending).
For this reason, special vacuum tooling has been developed for multilumen tubing (see Figure 5). To simplify its use, it was designed similar to a split plate, with top and bottom sections to enhance the string-up procedure. As with vacuum calibration, a water ring is used to apply a thin layer of water--in many cases, only 0.001 to 0.005 in. thick--around the tube as it enters the tool. This serves to create a water seal, a lubricant, and an initial skinning of the outer wall of the tube.
Figure 6. Hybrid vacuum calibration tool with water ring.
Because many multilumen tubes are processed from potentially sticky, flexible materials, it was desirable not to have the tube touch the tool. Depending again on the material, product size, and line speed, two or more additional water rings can be installed within the calibrator to maintain this thin water barrier between the tube and the tool (see Figure 6). The tool itself is made of glass-filled Teflon to minimize sticking in case of contact. As vacuum is applied to the vacuum chamber, the tube is actually pushed out against the water barrier, limiting its expansion. Through adjustments in water temperature, water volume within the vacuum tool, and vacuum level, tube roundness can be maintained even with the most difficult multilumen designs (such as the "double D").
Water Weight. An additional benefit of vacuum systems is that, because the vacuum decreases the weight of the cooling water, the water level above the tube becomes less critical than with free extrusion alone. In particular, water turbulence has much less effect on tube size when the multilumen vacuum calibration tool is used, since most of the sizing is completed within the tool itself.
The increasing popularity of specialized vacuum sizing means that many of the unwanted aspects of the use of internal air common to free extrusion can be eliminated, as can most of the negative effects of in-line cutting and coiling. As in the past, die design will continue to be very important in the processing of microbore tubing. However, with the application of vacuum, fine-tuning of the die can be minimized, saving time and money.
As we gain a better understanding of vacuum sizing and the processes are further improved, the production of even smaller microbore tubing, with more lumens and tighter tolerances, will become possible. This will doubtless contribute to the development of increasingly effective miniaturized medical devices in the future.
Robert H. Bessemer is director of development at Conair Jetro/Gatto (Bay City, MI), where his current responsibilities involve developing products for downstream extrusion applications in addition to direct sales and training. His areas of expertise include extrusion technology, vacuum sizing techniques, and tapered-tubing production. David Czarnik is product development manager at Conair Jetro/Gatto. He is also involved in creating downstream extrusion equipment and developing tooling and process improvements. He holds several patents on dies, calibration tooling, and vacuum systems.