MANUFACTURING
The tubing process
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The process for manufacturing bump tubing is similar to any extruded tubing process. An extrudable compound or resin must be brought to a desirable moisture level. This is accomplished by holding the material in a hopper with heated, desiccant dried air for a specific time. This is necessary for hygroscopic materials such as nylon, polyurethane and polyethylene terephthalate, among others. After the material is prepared, it is fed, usually by gravity, to the rotating screw of an extruder. In the extruder, the polymer is heated, fed, compressed and metered. The homogeneous melt is fed to a melt metering pump or directly into an extrusion die inline (extrusion is inline with the downstream) or crosshead (extrusion is perpendicular to the downstream equipment); this will give the thermoplastic mass its initial shape. After this initial shape, the extrudate must be brought to its final size as it is cooled. This final size can be accomplished by
• free extrusion whereby the pressure on the inside of the tube is held at atmospheric level and the size of the part by the speed of the puller compared with the speed of the extruder sizes the part
• applying a controlled source of compressed air to the inside of the tube as it is cooled
• vacuum sizing by controlling the pressure outside the tube compared with the atmospheric pressure on the inside of the product as it is being cooled.
The tube can be measured inline using ultrasonic sensors to measure the wall thickness. The sensors are usually positioned in the vacuum or cooling tank close (approximately 0.5 m)to the die and a dual scanner laser positioned before the puller. The puller in the process draws the product from the extruder through the cooling and sizing apparatus at a constant rate of speed. At this point, the part can be cut to length or coiled. The coil can be shipped or fed into an offline process.
Bump or tapered tubing adds to the complexity of the process because of forming and sizing more than one diameter as the part is being continuously extruded. The larger diameter is formed as the extrudate leaves the die in the gap between the die and the cooling device. This is done, as discussed above, by controlling the internal air pressure while varying the speed at which the product is being extruded. Simply put, the puller speed is lowered while the internal air pressure is increased to create the larger diameter; then the reverse brings the tube back to the smaller diameter. It is evident that to maintain a consistent size of either or both diameters, the rate at which this is done must be accurately controlled.
If the part is to be coiled for shipment or to be fed to an offline operation, a cutter may or may not be used. For a cut-to-length product, the cutter most commonly used is servo controlled by a microprocessor. In a bump-tubing line, the cutter must also be a part of the control system so that it is positioned to cut at the precise part of the bump and thereby create the required shape, and then the overall length.
Main features of the system components
Extruder. This is the main component in a tubing system. It is important that the extruder has a sturdy base and a well-designed barrel support to eliminate any vibration transferring to the polymer and finished product.
Drive and motor. These components must be sized properly to ensure constant speed and the right amount of torque for processing a particular polymer. Most machines are equipped with AC vector drives.
Gearbox. Serviceability, efficiency, lubrication and gear design are of the utmost importance. Double reduction gearboxes with carburised or hardened gears and internal lubrication systems are most desirable.
Feed section. The two main functions here are to isolate the polymer from the barrel heat until it is fed to the screw, and allow the polymer to free flow to the screw. A separate machined casting that is cored to allow cooling with a feed opening more than one diameter long will accomplish both.
Screw and barrel. This combination must take the polymer (which for tubing is normally in pellet form) and convey, melt, mix and meter it into the die. A barrel made from tool steel, with a liner cast into it that has the proper composition so that it will withstand the pressure, corrosiveness and abrasiveness of the melting process, is required. A barrier screw that is properly designed to separate the solid from the melted polymer and mix it properly functions best.
Temperature control. Although 80% of the required heat should be put into the polymer by using the proper screw design, the temperature control system must accurately maintain the level. Discreet temperature controllers with proportional integral derivative settings to control induction heating of the barrel through finned castings with built-in rod heaters are commonly used. Microprocessor and programmable logic controllers are also available, which allow recipe storage and other advantages, will be discussed later. In most medical tubing applications, cooling is co-ordinated with air blowers for each zone.
Melt pump. Melt pumps are necessary in some applications, however, they do put into the system another component that must be controlled and maintained. They are necessary when part tolerance is closer than +/- 1% or when batch-to-batch material consistency cannot be guaranteed.
Die. This gives the product its initial shape. The transition from the extruder to the die set must be streamlined with no obstacles or hang-up points for material to stagnate on. The quantity of material in the transition must also be held to a minimum. Calculation for the die size, the pin size and the land length must be made in conjunction with the material being extruded, the sizing method to be used, and the final part dimensions.
Internal air control. This becomes necessary when used as a sizing method for multilumen or bump tube products. The die must have a free passage for air to the inner diameter(s) of the tubing. This passage is either through the spider leg(s) in an inline die or through the core tube in a crosshead die. High-pressure plant air must be decreased to useable pressure (usually 5 psi or less) and maintained accurately. In a bump tube system, the exhaust of the applied air is equally important.
Sizing and cooling. There are two methods of vacuum sizing. Contact sizing is where the extrudate expands to contact a sizing tool that is calculated to produce the desired diameter, or for soft sticky materials. Noncontact sizing is where the vacuum causes the tube to be surrounded by a layer of water to lubricate and keep the tube a calculated distance from the sizing tool. Both allow the producer to establish a repeatable set of process parameters.
Puller, cutter and control. The puller in a tubing line must grip the product tight enough to ensure there is no slippage between the product and the belts and convey it at a constant rate of speed through the sizing tank. Any deviation in the speed, either from an inaccurate drive or from slippage between the belts, will show up as an inaccuracy in the final product. To ensure there is no slippage, belt material must establish a high coefficient of friction between the belt and the product. Pullers that use rollers in place of belts introduce distortion to the product. Therefore, a puller with a set of belts having a contact length from 12–18 in (approximately 30–45 cm) is most desirable. Puller drives should have a speed regulation of 0.5% or better.
Advances in the process
Inline wall measuring of the outside diameter and control units that utilise laser and ultrasonic technology have made it possible to automatically control important dimensions. In addition, there are process controls available that can integrate a system to the point where all process parameters, once set, can be downloaded from one recipe command. All of the component parts of the system can be monitored and controlled from one operator control station. Process control of this type will also include alarm and event logging, networking capability, process and equipment troubleshooting via real time and historical trending, as well as statistical quality control.
For many years, the extrusion of tubing in general and specifically bump tubing has been considered more of a black art than an engineering science. With the introduction of the microprocessor, reliable drive control, and the vacuum sizer, tube extrusion has evolved into a determinable, validatable process.
Charles Sparacino is System Sales Consultant at Davis Standard LLC, 1 Extrusion Drive, Pawcatuck, Connecticut 06379, USA, tel. +1 86599 1010. e-mail: csparacino@davis-standard.com [6] www.davis-standard.com [7]