A Simple Alternative to Rubber-Metal Composites. (photo: Elastogran) Two Substitute Materials, Three Processes

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1 A Simple Alternative to Rubber-Metal Composites Triple Combination. Distinctly shortened cycle times in the production of components with integrated vibration damping allow a new process concept that combines extrusion, injection molding and reactive processing in a manufacturing cell. The process attains the elastic properties and high elastic recovery values of natural rubber and other elastomers by an innovative crosslinking step. Rubber-elastic properties by on-line cross-linking of thermoplastics (photo: Elastogran) PE JOCHEN MITZLER KLAUS HILMER MARKUS SEIDL T he development of new techniques in plastics processing is driven by the classic motivators of innovations: Functional integration, Reduction of the number of components, Lightweight construction, Enhanced product and process quality, Reduced production costs. When a company seeks to strengthen its own competitiveness, acquire new orders or even aspire to technology leadership, these five guiding principles form the basis of success. Anyone aiming to open up new fields of business or applications must always examine all manufacturing processes in a holistic way. A current topic in industry is to produce metal-rubber composites in better quality and at lower cost. These factors play an important role in all sectors especially in the damping of mechanical vibrations, e.g. in machine bearings or in running gear. Two Substitute Materials, Three Processes Performance/cost ratio Metal-rubber composites - Increased assembly costs - Narrow tolerances - Pretreatment of metal (e.g. primer) - Special rubber blends Plastic-rubber composites - Multicomponent technology - Thermal separation in mold - Hot runner/cold runner technology - In-mold assembly, e.g. of seals - Modified rubber blends Costly rubber production, the enormous price hikes for metals and the trend to lightweight construction have finally made the search for alternative component solutions inevitable. In many cases the substitution of heavy metal components by lighter plastics, which also afford considerable advantages in terms of production technology, has already been successful. Steel and aluminum can be replaced, for example, by glass-fiber reinforced polyamides provided the design is suitable. Likewise the rubber component can be replaced by thermoplastic elastomers (TPE) that are processable by injection molding. Here, thermoplastic polyurethane (TPU) is growing in importance on account of its persuasive range of properties covering mechanics, Plastic-TPU-X-composites - Cross-linked thermoplastic TPU having the properties of rubber/elastomers - Multicomponent mold technology - No hot-cold separation - Vulcanization and annealing not needed - Thermoplastic melt and flow properties K Trade Show 1998 K Trade Show 2007 Translated from Kunststoffe 10/2007, pp Fig. 1. When the performance/cost ratio is considered the new process (X-Form) affords significant advantages for many applications (photo detail lower left: Vorwerk Autotec GmbH & Co. KG) 126

2 INJECTION MOLDING Process chain: rubber-metal composites Production of metal Pretreat- ment of materials (e.g. drying) Melting/ direct production (multicomponent technology) Cleaning/ Priming the metal flexibility and adhesion to polyamide. As a result of further material and process developments the elastic properties and high elastic recovery values of natural rubber and other elastomers can be achieved with a thermoplastically crosslinked polyurethane (TPU-X) (Fig. 1). By combining extrusion, injection molding technology and reactive processing in a manufacturing cell it is possible to fulfill these requirements by means of an innovative cross-linking step. Transportation Transportation Production of rubber blend Vulcanization Tolerances, assembly, QA costs, logistics costs, manufacturing coordination, cross-company, etc. Reduced quality and increased production costs Process chain: thermoplastic/tpu-x composites (X-Form) Advantages of injection molding technology Finished Increased quality and reduced production costs Test platen: TPU-X Support: PA 66 GF35 Vibration damping TPU-X All-round seal: TPU-X Finished Sleeve: PA 66 GF35 Fig. 2. Production of the new thermoplastic/tpu-x composites substantially shortens the process chain Fig. 3. The component is produced by a combination of reactive compounding and multicomponent injection molding In this process marketed as X-Form a newly developed thermoplastic polyurethane (TPU, type: Elastollan; manufacturer: Elastogran GmbH) is crosslinked to form TPU-X by the reaction step during cooling (after molding). In this way new highly adherent rigid-flexible composites can be produced with high production quality and considerable economic advantages (Fig. 2). Figure 3 shows a torque converter bearing as an example of such a multicomponent. It consists of a glassfiber reinforced thermoplastic support with sleeve (PA 66-GF35, type: Ultramid A3WG7CR; manufacturer: BASF AG) which gives the component its stability, an adhering rubber-elastic region for vibration damping and an all-round elastic seal (both TPU-X, see above). In addition, a test platen for mechanical characteristics of the material is integrated into the mold. Injection Molding Compounding plus Multicomponent Technology The production system consists of an injection molding compounder (IMC, manufacturer: Krauss Maffei GmbH) which combines continuous extrusion and discontinuous injection molding with one another. A twin-screw extruder uniformly melts the modified TPU and after introduction of the liquid cross-linking agent into the melt produces a homogeneous mixture. The melt then arrives via a heated runner and the injection cylinder in the injection mold (Fig. 4). The combination of the reproducibly controlled processes of compounding and injection molding yields components of consistently high quality over the entire production period. In addition, these are obtained free of flash and without sprue since the thermoplastic processing of rigid and flexible components allows the use of modern mold and hot-runner technology. The holistic challenge of this manufacturing cell (Fig. 5) consists in meshing the injection molding compounding with multicomponent technology to produce composite s possessing integrated V Liquid cross-linker TPU Additives Part with TPU-X properties Fig. 4. The operational principle of the injection molding compounder combines continuous compounding (extrusion) and discontinuous injection molding (unattributed photos: Krauss Maffei) Fig. 5. Injection molding compounding and multicomponent technology form a compact production system for the efficient manufacture of thermoplastic-type rigid-flexible compounds Kunststoffe international 10/

3 functions. With the second injection unit installed in L shape the PA 66 containing 35 % of glass fibers is injected into the mold. In order to protect the screw and cylinder steel against the abrasion produced by short strands a new combination of materials was employed for the construction of the cylinder, screw, nonreturn valve and shut-off nozzle. This measure prolongs the service life of the plasticizing unit by up to a factor of three by comparison with conventional construction (depending on the fiber content, residual moisture content, additives, etc.). A Cycle in Four Phases Using a Rotary Table Mold The damping layer in the component (Fig. 3) should absorb strong vibrations. For that reason it has a wall thickness of 15 mm in this region. Due to the cooling time for such an accumulation of material a long cycle time is to be expected. Since productivity is now the prime requirement in manufacturing systems, component design and mold concept (manufacturer: Mues Products & Moulds GmbH) must be exactly matched to one another. The component together with its integrated functions determines whether the multicomponent mold is designed in sandwich or index technology, with core withdrawal or for transfer molding. The present design suggests rotary table technology. The standard rotary tables for machines of 2,500 kn and more from Krauss Maffei are integrated units. This means that the rotary table and moving platen form a unit. The advantage of this arrangement consists in that up to two thirds of the permissible weight of the mold can be mounted on the rotary table. The rotary table is driven with precision by servomotor independently of the hydraulic system of the machine. Thus, the rotary movement can be combined with that of the mold opening in order to reduce the cycle time. The high precision of the drive and the implementation of the design minimize mold wear. The component is produced in three phases or stations and in the fourth phase it is removed. The support and sleeve are produced from polyamide in the first phase. In each of the following phases the mold is turned by 90. In the second and third station the flexible component is built up in layers with TPU-X (Fig. 6). The mold is constructed in such a way that the robot can move into the fourth station and remove the component while it is closed. The feature that the manufacturing system is not dependent on the removal time increases the productivity of the system by about 10 to 15 %. A contribution to this is made by the fact that the mold concept subdivides the thick TPU-X element into thin layers or star-shaped elements and in this way shortens the cooling time. When the concept of the fourphase rotary table is compared with conventional two-station technology it is found that the cycle time is reduced by more than half. By comparison with classic elastomer processing the cycle time falls to almost a quarter. Ensured processability requires a Moldflow simulation in the design phase in order to optimize the gate points, the position and dimensioning of the hotrunner, flow lines, fiber orientation and the cascade technology. Two separate hot- TPU-X Rubber/Elastomer Processing Properties Flowability ++ Processability ++ + Storage ++ Colorable ++ 0/ Vulcanization not needed essential Subsequent annealing not needed may be necessary Thermoplastic characteristics ++ Remeltable + Part Design and Properties Design freedom as for multicomponent technology ++ Adhesion to engineering plastics, e.g. polyamides ++ 0 High consistency of quality ++ 0 Flash-free ++ 0 Weld line adhesion ++ 0 Mold Technology Rotary table ++ 0 Index ++ 0 Parting slide ++ Transposing + + Runner systems with shut-off nozzles ++ + Multicavity mold ++ + Vulcanization temperatures (different mold temperature ranges hot/cold separation) Cascade control ++ + Factor determining cycle time (degree of cross-linking) no yes Material Properties in Comparison with Natural Rubber Hardness limited adjustability adjustable Stability at low temperatures + ++ Continuous working temperature + 0 Max. thermal load + 0 Resilience + + Ozone resistence ++ 0 Resistance to chemicals no Oils, grease, almost all solvents High constancy of formulation ++ 0 Table 1. Compared to typical rubber materials the new cross-linked TPU-X reveals considerable advantages in processing and properties, design, mold technology and properties of the material Legend: ++ very readily achievable. + readily achievable. 0 achievable with limitations. not achievable yes 0 128

4 INJECTION MOLDING Phase 3 TPU components 2.1 & 2.2 runner systems (for PA 66 and TPU-X) equipped with hydraulic needle valve nozzles are employed. In this fashion the filling behavior during the injection process and the holding pressure in each cavity can be controlled. The core inserts were built up layer by layer ( lasercusing ). With this method a cooling channel can be taken into the tips of the insert and along a contour (Fig. 7). The contour cooling shortens the cycle time because especially in the region of the star-shaped elements it is substantially more effective than classical temperature control. Furthermore, the homogeneous Phase 2 TPU component 1 Phase 1 PA 66 components Fig. 6. The mold design is based on the principle of a four-phase rotary table: Phase 1 to 3 are needed for producing the, phase 4 is used for removal cooling conveyor belt located inside the protective housing of the machine behind the movable platen. Since the robot moves exclusively within this security zone no other protection is needed (Fig. 8). This design, depending on removal position, reduces the floor space of the installation by 20 % (longitudinal removal) to 40 % (removal on the operating side). Thermoplastic with Rubber Properties The new TPU-X is characterized by an interesting combination of properties. The improved compression deformation and vibration characteristics of the crosslinked material opens up potential uses in automotive and mechanical engineering. The desired low hardness of 60 Shore A is achieved without the addition of plasticizers. In continuous use the material withstands temperatures up to 120 C and the maximum operating temperature is 150 C. Since due to the cross-linking the material is more resistant to chemicals new applications can be found in which traditional rubbers would never have been considered due to their vulnerability to certain oils and greases. Other advantages relative to traditional rubber blends arise from its good resistance to ozone, the possibility of using many different color systems and its neutral odor (Table 1). TPU-X Elastollan adheres very well to the likewise thermoplastic polyamides of the Ultramid CR family (Fig. 9). By combining both materials multicomponent s can be produced in one process without the need for a bonding agent (e.g. primer) or on-line treatment (plasma). The cross-linking of the TPU takes place at the mold temperatures and within the cycle times customary in injection molding. Due to the continuous direct melting and constant process conditions high consistency of quality is achieved. In comparison with the manufacture of conventional rubber-metal composites the new process affords considerable advantages for a from the primers, vulcanization and other downstream steps (e.g. aftertreatment and calibration) are dispensed with and on top of that the product developers enjoy greater design freedom (Table 1). A detailed description of the properties of the new TPU-X will follow in a later issue as Part 2 of this article. Conclusion Fig. 7. By means of the laser cusing process core inserts in the mold can be built with precise and complex contour cooling (photo: Mues Product & Moulds) With the injection molding compounder new rigid-flexible composites based on thermoplastics having the properties of V cooling improves the quality of the molding. The two-plate technology of the injection molding machine allows icularly space-saving automation solutions. A linear robot of the LR-S series mounted on the machine in the longitudinal axis removes the finished s and after separation of the sprue sets them down on a i Development Associates Elastogran GmbH Elastogranstr. 60 D Lemförde Tel. +49 (0) 54 43/12-0 Fax +49 (0) 54 43/ elastogran@elastogran.de Mues Products & Moulds GmbH Gewerbepark Conradty 1 D Kolbermoor Tel. +49 (0) 80 31/ Fax +49 (0) 80 31/ Krauss Maffei GmbH Krauss-Maffei-Str. 2 D Munich Tel. +49 (0) 89/ Fax +49 (0) 89/ info@kraussmaffei.com Kunststoffe international 10/

5 rubber or elastomers can be produced. In doing so the holistic matching of the material, cross-linking formulation, design, mold technology, automation and injection molding machine affords considerable economic advantages. As a result of a cooperative project in the field of plastics technology the new composite technology was developed as an innovative manufacturing system. The torque converter bearing presented as example is about 40 % lighter than an analogous steel-rubber composite and due to the short cycle time of around 60 s process costs can be reduced greatly while product quality is substantially improved. The development associates (see box on previous page) will present the project at the K 2007 exhibition on the Krauss Maffei stand in Hall 15. Conceivable future applications include housing lids with an integrated seal, motor Setting down in separate robot containment area - Longitudinal removal Setting down in separate robot containment area - Transverse removal on operating side Setting down in the machine housing Fig. 8. By setting down inside the machine s protective housing the production area can be reduced by up to 40 % Fig. 9. Outstanding composite adhesion between Elastollan TPU-X and Ultramid CR (PA 66-GF) without additional bonding agent (photo: Elastogran) mountings, vibration blockers and substitutes for other metal-rubber composites. ACKNOWLEDGEMENTS The authors thank all project icipants, namely Dr. Anka Bernnat and Dipl.-Ing. Bernhard Aumer, BASF AG, Ludwigshafen, Dipl.-Ing. Herbert Günther, Günther Heisskanaltechnik GmbH, Frankenberg, Prof. Dr.-Ing. Hans-Gerhard Fritz, Institut für Kunststofftechnik (IKT; Institute of Plastics Technology) of the University of Stuttgart,. THE AUTHORS DIPL.-ING. (FH) JOCHEN MITZLER, born in 1973, is Technology Manager for Multicomponent Technology at Krauss Maffei GmbH, Munich,. DIPL.-CHEM. ING. KLAUS HILMER, born in 1958, is Head of the Applications Pilot Plant at Elastogran GmbH, Lemförde,. MASTER OF MOLD MAKING MARKUS SEIDL, born in 1969, is Managing Director of Mues Products & Moulds GmbH, Kolbermoor, ; markus.seidl@mues-pm.de 130