Extrusion of single- and multi-lumen catheter tubes with thermoplastic materials

Transcription

1 6 Komplette Extrusionsanlagen Einschnecken-Extruder Labor-Extruder Beispritz-Extruder Schmelzkleber-Extruder Nachfolgeanlagen complete extrusion lines single-screw-extruders laboratory-extruders co-extruders hot-melt extruders downstream-equipment EXTRUDEX Kunststoffmaschinen GmbH In den Waldäckern 16 D Mühlacker/Germany Tel.: +49 (0) Fax: +49 (0) Mail: Extrusion of single- and multi-lumen catheter tubes with EXTRUDEX extrusion plant for the production of single- and multi-lumen catheter tubes Publication: 30 th September 2008 Helmut Wahl Photographs: EXTRUDEX Kunststoffmaschinen GmbH Sparkasse Pforzheim Deutsche Bank Pforzheim Postbank Stuttgart Volksbank Pforzheim Registergericht Mannheim BLZ BLZ BLZ BLZ HRB Konto Nr Konto.Nr Konto.Nr Konto Nr.: Geschäftsführer Helmut Wahl Swiftcode No. PZHS DE 66 Sitz der Gesellschaft : Mühlacker IBAN: DE Ust-Id.-Nr: DE

2 Raw material Thermoplastic materials as for example Pebax, polyamide, polyurethane, polyethylene or soft PVC are used for the production of medical catheter tubes. All used materials must be approved for medical use. Their composition must be physiologically harmless. Material mixtures, the so-called compounds, consisting for example of Pebax and barium sulfate, are often used to make the catheter tube visible during x-raying. The manufacturing company receives the raw materials from the supplier in plastic bags of 25 kg each. Each type of material has to be tested if being hygroscopic or not. Not hygroscopic materials can be processed at room temperature immediately after delivery. Material dryer with conveyor Material conveying Dimensions of multi-lumen tubes The hygroscopic materials have to be dried in a drying device according to their material data sheet. Insufficient drying of these materials will result in considerable problems during extruder processing (screw press), as for example foaming of the thermoplastics when exiting the nozzle, because of too much moisture in the raw material. This makes production impossible. With the help of a suction conveyor, the commercial granulate raw material in the size of 3 x 3 mm is conveyed from the reservoir or the dryer into the material hopper of the extruder. The level of the hopper is either controlled by the conical pile or by minimum and maximum measuring heads so that sufficient material will always be available at the feeding zone of the extruder and the extrusion process will not be interrupted due to a lack of material. Page 2

3 The granulate falls from the hopper into the feeding zone of the extruder, i. e. the cylinder wall in the feeding zone is provided with grooves. This grooved feeding cylinder (the grooves can have different sizes and shapes, depending on the type of material) contains the screw, the so-called conveyor and plastification unit of a single-screw extruder. Due to the rotation of the screw and the grooves in the feeding cylinder, a so-called bolt and nut effect is achieved, i. e. the granulate can only move into one direction as if turning a bolt on a nut. Regarding extrusion with groove extruders, this is therefore called a forced feeding. This forced feeding guarantees a much more constant feeding of the granulate than with a smooth cylinder and therefore leads to a more constant discharge of the thermoplastics in the forming tool which results in lower product tolerances. Thus, another feature is the reduction of pulsations in the thermoplastics stream to an absolute minimum. Due to the heating of the whole screw cylinder, except the grooved feeding zone, the polymer is fed with energy and the extrusion process starts in different zones. The first zone is the feeding or solid zone. This zone shows the most important wear as the granulate is still firm. The second zone is called melting zone. The first heating belt is attached directly after the feeding zone so that the granulate starts to coagulate and is then completely melting. The third zone is a short compression zone between zones 2 and 4. Compression develops as the working depth towards the screw nose and the forming tool decreases. The fourth zone is called metering or discharge zone. Conveyor on the material hopper of the extruder Extrusion Extruder EN D Medtech Due to the screw geometry adapted to the respective material, the polymer granulate in the extruder cylinder is kneaded to a plastic and homogeneous mass so that inhomogeneous particles will no longer be available. It is thus very important that the respective material will be processed within the temperature range mentioned in the data sheet in order to avoid any thermal damages or a decline of properties. Page 3

4 Plant control The microprocessor-controlled extruder control Medtec has been developed for extrusion lines with different applications. The separate management of the drive, temperature and ramp parameters enables a fast configuration and start-up of the drives and temperature zones. The respective heating zone parameters are indicated in a tabular display, the temperature variation is shown by graphic display. Different password levels for the management of passwords, weekly timer and an extensive alarm management are already included. For the alarm signals, switch-off conditions etc. 6 optionally configurable digital inputs are available. Help texts are displayed in PDF format and can be booted into the control via the USB connection on the front side of the ETV. Control features ETV VARAN CPU with colour display and touch screen Configuration for up to 7 extruders with up to 16 temperature zones each Integrated ramp operation for the start-up, threading and production point Data logging: up to 8 channels with oscilloscope Separated management for the drive, temperature and ramp parameters Help texts in PDF format Easy programming with the object-oriented programming tool LASAL An ETV control panel with touch screen and integrated soft SPS serves as CPU. The realtime and Ethernet-based VARAN bus enables a fast and actually jitter-free communication. Additional interfaces like CAN, USB etc. are available for the communication with external sensors (for example Zumbach or similar). The basic version consists of up to 7 optional extruders with up to 16 heating zones each. In addition, a melt temperature and melt pressure measuring unit are available for each extruder, as well as a ramp operation for the start-up, threading and production point start-up. With the corresponding adjustments, all drives can be used synchronously. Subsequent extruder solutions like haul-offs or saws can be integrated into the control. Forming tool When leaving the metering zone the plasticized and homogeneous mass is led to the forming tool. In order to produce multi-lumen tubes, the tool is a cross head being attached at right angles (90 ) towards the extrusion direction. This is necessary in order to use backside air connections for the respective hollows (lumen) in the tube. Regarding small tubes for medial products it is of great importance that the forming tool is rheologically impeccable, i. e. that a too large mass volume is not allowed in the tool, but in any case only the quantity required for the product to be manufactured. If the mass volume is bigger than the quantity for the product, this will result in a degradation of the material and possibly thermal damages which are absolutely bad for medical tubes. Page 4

5 Calibration Cross head for single- and multi-lumen When the product leaves the forming tool in a still plastic and smooth condition, the tube is led to the calibration tool in the vacuum calibration bath as well as to the haul-off-cutting device through the vacuum calibration bath so that adjustment and optimisation of the process can be made through constant haul-off. In the twopart calibration tool the tube is vacuumed, adjusted to the necessary outside dimension and at the same time cooled in order to keep shape. The thermoplastics being diverted in a 90 angle in the cross head, an optimum melt distribution system is of great importance to achieve a consistent distribution and thus a perfect product. When adjusting bolt and nozzle, finally resulting in the form of the product to be manufactured, the so-called down draw ratio must be considered with regard to the extrusion speed. This down draw ratio is different for each type of material and is mainly based on a long-year experience. Regarding the down draw ratio, the shrinking factor for the respective material has to be considered so that the required dimension remains after a state of rest of the product. Air control unit The air necessary for single- and multi-lumen tubes is realized in 4 independently adjustable air units. A proportional valve guarantees an absolutely precise air control for each tube hollow, even in case of minimum performance. The air is controlled through precision potentiometers. Two-part V2A block calibration in the vacuum chamber Vacuum calibration bath The vacuum calibration bath consists of 1 to 2 vacuum chambers as well as a full bath for the subsequent cooling of the product. According to the requirements and the product, both vacuum chambers can be operated with different vacuum settings. The calibration length and the cooling length considerably influence the extrusion process speed. Air modules for 1 to 4 lumen Page 5

6 Besides the two vacuum chambers and the full bath, the vacuum calibration bath consists of two additional chambers which are the measuring chamber for an ultrasonic wall thickness measuring as well as a drying chamber containing a blow-off nozzle to blow off the water on the product. Laser measuring head for x- and y-axis with display and control unit Haul-off-cutting device A belt haul-off with two electronically synchronized servo motors, operating the respective belt carrier independently from each other, guarantees an absolutely constant haul-off of the tube. The used belts must be approved for medical products. Different coating types as well as the hardness can be chosen depending on the product. In any case the belts have to be absolutely abrasion-proof so that no abrasion particles may settle on the product. Vacuum calibration bath VB 150 Diameter measuring and control device At the exit of the drying chamber of the vacuum calibration bath a laser measuring head is attached for measuring the outside diameter through the x- and y-axis of the tube. This is not only a measuring device, but when measuring the outside diameter the haul-off speed of the next belt haul-off is increased or reduced in case of undersize or oversize in order to achieve the necessary dimension. Page 6

7 Belt haul-off BA 030 Medtec Cutting unit with roll haul-off The cutting unit directly mounted to the belt haul-off enables cutting of the tubes to a preselectable length. A knife directly mounted to the drive shaft of a servo motor enables high cutting frequencies and a perfect and burr-free cut of the product. A small roll haul-off directly mounted after the cutting zone leads the cut tubes on a subsequent haul-off belt with integrated stacking device. This roll haul-off is especially necessary for very soft materials as for example PUR, soft PVC and small diameters, in order to transport the tubes out of the cutting zone. The following conveyor belt with stacking device transports the cut tubes to the stacking place towards the extrusion direction and those are blown into the receiver through air nozzles mounted at the side. The tube bundles can be taken there from the operator for packing or further processing. Bad products (not meeting tolerances) are recognized by the laser head and are transported to a backside receiver by the control device. Conveyor belt with blow-off device and receiver Page 7