Gummi Fasern Kunststoffe, No. 10, 2007, pp. 659 663 Machines and plant for tube and profile production T. Pohl and F. Podzelny Process Development, Rubber Machines and Plant, Troester GmbH & Co. KG, Hanover, Germany Selected from International Polymer Science and Technology, 34, No. 12, 2007, reference GK 07/10/659; transl. serial no. 15861 Translated by H. Yesson SUMMARY The continuous production of endless rubber products is a broad and versatile field of application. Profile geometry, choice and composition of compound ingredients as well as process control need to be developed and matched to the application. When it comes to selecting the plant components, various alternatives and concepts are available. For that reason the plant configuration is determined in close cooperation between the plant supplier and product manufacturer. This paper describes the decision-making process in relation to building an extrusion line. 1. EXTRUSION The central processing component of a production line is the extruder, to which is connected the extruder head. An extruder consists in principle of a screw, cylinder, feed unit, drive and a temperature control and lubrication system. The cylinder and screw form a single engineering unit. The screw interacts with the cylinder or a feed roll to draw in the compound, then compacts, conveys and homogenises it before extruding it through a die. The cold-feed extruder, often fitted with a vacuum zone for venting the compound, is conventionally used for producing industrial rubber products. Hot-feed extruders are used for large outputs, as post-compounding extruders for example or in the tyre industry for extruding HGV tyre treads based on natural rubber. The compound is fed to the extruder in the form of pellets or strips. The feed roll in the extruder feed section has the job of supporting the feed of compound by the first screw channels. The feed roll normally rotates at a fixed ratio to the screw speed. It is also possible to design the feed roll with a separate drive, however, which has the advantage of allowing the feed conditions to be adapted to the feed cross-section and compound characteristics. The feed section can be monitored and controlled by means of an optical device. A torque control system in the feed section has proved beneficial for stiff compounds. In order to be able to incorporate a separate feed roll drive in a relatively small footprint, Troester uses a compact gear motor. This drive concept comprises a total of four individual AC asynchronous motors. Three motors transfer their output to a central drive gear connected to the extruder screw, while the fourth motor drives the feed roll (Figure 1). In rubber extrusion, owing to the subsequent pressureless vulcanisation stage, cold-feed extruders are often used as vacuum extruders in order to remove volatile components from the compound. The vacuum extruder screw consists essentially of two feed zones connected in series and divided by a deeper flighted, partially filled, pressureless vent zone (Figure 2). The first screw section is responsible for feeding, conveying and plasticising, while the second screw section operates as a conveying element with pressure build-up to extrude the material. Trouble-free extrusion can only be guaranteed if the vent remains clear during continuous operation and the second section after the vent stage acts against the resistance of the die without surging. This is achieved by giving the second screw section a greater conveying capacity, even with differing die resistances, than the first section, which always acts against the same vent pressure. 2008 Smithers Rapra Technology T/1
die resistance. The volumetric feed characteristics of the gear pump mean that the material throughput is proportional to the speed of the gear pump and is more or less independent of the die pressure. In this way the advantage of a high pressure buildup combined with a low temperature rise offered by the gear pump is optimally matched with the homogenising efficiency of the extruder. 2. STRAINING Figure 1. GS90 VAK with separately driven feed roll Figure 2. Principle of a vacuum extruder The surface of industrial rubber products such as windscreen wipers or seals has to be absolutely perfect. Prior to extrusion, the compounds must be freed from impurities or inadequately mixed ingredients using a straining process, which involves pressing the compound through a finemesh screen. Straining can take place at various process stages: 1. Immediately after the compounding process, with a combined extruder/gear pump unit 2. After storage and prior to extrusion, using a standalone gear extruder 3. During extrusion, ahead of the profile die A cold-feed gear extruder (Figure 4) ahead of the extruder offers the largest processing window combining throughput, melt homogeneity and maximum permissible processing temperature. The rubber compound is strained by means of the volumetric conveying process in the gear pump under minimal shear stress. 3. SHAPING Figure 3. Compact extruder/gear pump unit A combined extruder and gear pump unit is another common choice for profile extrusion (Figure 3). The extruder is the feed element for the gear pump, which in turn ensures a constant material throughput with a high Owing to the range of functions required of them, modern tubes and profiles are more and more commonly constructed from multiple materials. They include textile linings and metal supports which serve to reinforce the material and as yet cannot usually be adequately replaced by elastomers. An extruder head must be able to combine these various material flows with the desired output rate. Multiple die heads are used for this purpose, the flow channels in which are now optimised by means of computer-aided flow simulations. The design of the heads is chosen with regard to external profile geometry setting options during production, rapid opening for cleaning purposes when the compound is changed and the supply of ancillary media such as calibrating air, lubricants or non-stick agents. Handling of the die heads is simplified by means of manual or pneumatic quickrelease clamping systems on the extruder. T/2 International Polymer Science and Technology, Vol. 35, No. 7, 2008
Figure 4. Rotomex gear extruder performed continuously or discontinuously, in two stages: heating to vulcanisation temperature and holding the temperature until the vulcanisation reaction has ended. The temperature profile along the heating section must be matched to the geometry of the profile and the characteristics of the compound. Continuous processes are generally more efficient than discontinuous processes. However, some profiles have to be vulcanised under pressure to prevent trapped air or moisture from forming cavities and bubbles in the product. In such cases the vulcanising section must be isolated from the environment, something which is normally only done with simple profile geometries such as cables (Figure 6) or tubes. The available vulcanisation methods are restricted by the vulcanising system used for the compound and by the cross-section of the product. In the case of peroxidevulcanising materials vulcanisation must be performed 4. TRANSPORTING THE PRODUCT When the product leaves the die, it has been shaped but is still not dimensionally stable. In the subsequent process steps the product has to be transported smoothly and ideally without contact. If vulcanisation takes place immediately after extrusion, the product is first conveyed vertically downwards or upwards or horizontally by non-contact means in a catenary through an infrared rapid heating section. The rapid heating causes surface crosslinking so that subsequent turnaround conveyors, roller conveyors or conveyor belt supports leave no impression on the product. Figure 5 shows a vertical infrared section for making silicone tubes for medical applications. In tube production vulcanisation is carried out in a subsequent process step outside the production line. After leaving the extrusion die the tube is cooled down to around 30 C so that it can undergo further processing in the line. Depending on the application, the product is either drawn out of the head more thinly using a caterpillar take-off in order to achieve the final dimension of the tube or is transported onwards with as little tension as possible if the axial tensile resistance is low. A lowtension production method requires an appropriate extruder head and die configuration. Caterpillar take-offs, conveyor belts and sets of rollers are used for onward transport. To avoid damaging the surface of the product, the material used for the transport elements and also the contact forces and lengths must be matched to the product. Figure 5. Vertical infrared rapid heating section on a silicone tube line 5. VULCANISATION The extruded rubber compound is vulcanised in a subsequent process step. This process can be Figure 6. Vulcanising section on a cable extrusion line 2008 Smithers Rapra Technology T/3
in a salt bath with oxygen exclusion, since atmospheric oxygen makes the surface of the compound tacky and difficult to transport. In the case of sulfur-vulcanised rubber profiles, small profile volumes are processed by means of hot-air, infrared or salt bath lines. If the product is heated by means of a series of sections connected in sequence, they must be close together to prevent the product from cooling down in the transition zones. It is not cost-effective to heat large-volume profiles by convection. The energy input takes too long. In this case the hot air is assisted by microwave radiation, which penetrates deep into the profile material. Microwaveabsorbing substances have to be incorporated into the rubber. Combined heating with hot air and microwave energy is particularly common in the automotive seal sector. The hot air serves to maintain the surface temperature of the product and to dissipate gas emissions. Microwave radiation provides uniform heating of the profile cross-section, ensuring a uniform vulcanisation of the rubber material and a homogeneous cell structure in the case of foam rubber compounds. Because of the range of possible settings, combined heating and energy input is technically challenging. To minimise the time taken to set up the vulcanising section, Troester has developed a program for calculating the optimum operating parameters. The simulation program takes account of the precise geometry and the material data of the profile structure. This approach allows the temperature distribution and hence the degree of vulcanisation to be calculated even for complex profile cross-sections. Figure 7 shows the temperature and degree of vulcanisation at three characteristic points in the product cross-section as the profile is transported through the plant. 6. COOLING Depending on the application, the product is cooled down to between 25 and 40 C so that it can be handled for further processing. Spray cooling is generally used because it is more effective than film or immersion cooling. The jet angle and the bore diameter of the spraying nozzles and hence the spraying width are adjusted to the range of products to be cooled. 7. PROCESSING At the end of the production line the product is either stacked or wound. Stacking systems with cutting devices have their own caterpillar feeder, which is responsible for the accurate feeding of the product. The product can be cut while it is stationary or moving. If the product is stationary, an accumulator is positioned ahead of the caterpillar feeder, generally in the form of a catenary. At higher line speeds the blade moves with the product and cuts it as it moves, giving a better cut quality. A take-off belt behind the cutting device delivers the cut pieces. The pieces are removed by the operator or are sorted into various containers by means of ejectors. An optical check of the product surface is performed at the same time. Winding mechanisms are available for folding reels, drums or winding discs. Figure 8 shows a double folding reel winder with automatic winding function. The product is wound tangentially at the wind-up station to eliminate tension. A take-off conveyor is used to feed the product. When the desired length has been reached, a length counter actuates the cutting mechanism. The wound product is transported to the removal position by a rotary unit and at the same time a new reel is automatically started at the wind-up station. Process steps such as drilling, bonding, bending, coating or longitudinal cutting can also be incorporated in the process. 8. OPEN AND CLOSED-LOOP LINE CONTROL Figure 7. Graph of temperature and vulcanisation profiles showing the heating up, temperature holding and cooling down process at various points on the profile Production lines increasingly feature an automatic process control system. This requires the recording of production data to allow conclusions to be drawn about the consistency of product properties, such as the dimensions and surface temperature of the product, extrusion temperatures and pressures and the temperatures of heat-transfer media. Parameters that are most commonly controlled include extruder output, take-off speeds and heat input. In order for production to run automatically and to ensure reproducible, recipe-controlled machine T/4 International Polymer Science and Technology, Vol. 35, No. 7, 2008
via a data bus rather than by discrete wiring, reducing the amount of cabling required. A connection to the management computer is also increasingly common, so that up-to-the-minute data about the production status is always available. 9. CONCLUSIONS AND PROSPECTS Figure 8. Double folding reel winder settings, all the line settings that affect the dimensional stability or product quality in general must be able to be electronically controlled on a continuous basis. An important factor is the selection of suitable measurement points and sensor types for continuous product measurement. Non-contact systems with optical, capacitive and ultrasonic sensors are increasingly being used. Programmable logic controllers (PLCs) are now used as standard around the world for open and closed-loop line control. The sensors and actuators are connected to the central PLC. The associated operating system ensures that the current status of the sensors is always available to the user program. The user program utilises this information to control the actuators so that the machine or plant operates in the required manner. The start-up process for line production uses a different control concept to ensure that a stable production process is established as quickly as possible (Figure 9). In addition to the core task of open and closedloop control, PLC units also take charge of additional tasks such as visualisation, warning system, recipe management and recording of all status messages. Sensors and actuators are now connected to the PLC Modern machinery in the rubber sector has benefited from many improvements, particularly in terms of the detail design. Separate drives for extruder feed rolls, more efficient vulcanising and cooling sections, lowimpact transport elements and automated winders are just some examples. Since the demand for higher-quality products and lower production costs never goes away, technical tools such as simulation programs or control models for controlling the start-up process will become even more important in future. Their use will give plant operators the means of achieving reliable, reproducible and faster production of their products. Figure 9. Overview of a line control system with access to the line components 2008 Smithers Rapra Technology T/5