Strength and lightness of a plastic detail

Transcription

1 MATERIALS ENGINEERING POLYMERS IN THE MECHANICAL INDUSTRY Strength and lightness of a plastic detail In mechanical drives, the use of polymeric materials allows to manufacture gearings, bearings, motor components and details of kinematic units that were previously made of die-cast aluminium or sintered metals. The company TPA Plast, based near Vicenza, cooperates with drives manufacturers to whom it offers its skills in the design of mechanical components made with high-performance engineering polymers. William Pandolfo A useful exercise to understand the role and the importance of plastic matters in mechanical components nowadays consists in analysing the interior of a car, of a motor or of an electric tool. Well, when opening the bonnet of a sedan you will be probably surprised: according to the latest statistics, at present the average amount of plastic components in a European compact car is as much as 105 kilos, 26 of which are located right under the bonnet. If we uncovered the electric motor operating the gate or the horizontally pivoted garage door of our house, we would unveil a varied whole of plastic components including gearings, pulleys, bushes, screws and objects that are typically linked to the world of metal. More familiar indeed are the plastic details inside a drill or another electric tool, since the lightweight external shell has been made of plastic for years. Actually even the inside of electric tools hosts many plastic details such as stators, cooling fans and head insulation, though being a very critical environment because of temperatures (>100 C), of highabrasion components (bearings), of electric conduction (coils and magnets) and of mechanical loads (axial and radial thrusts). Of course we are not talking of traditional plastic but of engineering polymers, i.e. materials with mechanical and thermal performances that give the opportunity to replace aluminium and other alloys more and more often. This is why today all the manufacturers in the automotive, electric tool and industrial component industries are, like novel Pag 1 di 7 March 2006

2 King Midas, trying to turn metal into engineering polymers. The modulus of elasticity of these polymers can be as high as 20-22GPa and their tensile strength can exceed 200 MPa (fig. 1) LCP PAI PEEK PA 6/6T SAN PA MXD6 PPS PS PEI PET PMMA POM PA 6-3-T PES PSU polimero non caricato polimero caricato con fibra di vetro polimero caricato con fibra di carbonio Fig. 1: Tensile strength (ISO 527). Also temperature is not an insurmountable limit for engineering polymers, since many polymers now resist extremely well beyond 100 C as a steady temperature and can stand peaks of C for short periods. Other high-performance engineering polymers can perform very well under continuous working also at temperatures of C (fig. 2). On the basis of these performances, engineering polymers can compete on equal terms with the most common metals and alloys used in the mechanical industry (fig. 3) PEEK PPS PA66 non caricato caricato PEI PBT Fig. 2: Deflection temperature under HDT load at 1.82 MPa (ISO 75). Pag 2 di 7 March 2006

3 Zinco Alluminio Tecnopolimero high performance Fig. 3: Tensile strength at different temperatures (ISO 527). Versatility in design and in manufacture You may now wonder why there is so much interest in the change from metal to polymer. More and more often mechanical designers are faced with the target to manufacture high-quality details with the lowest possible waste of raw materials, of labour and money. This means that products must be lighter (to save on raw materials), require shorter processing cycles (to limit the cost of labour) and, therefore, have good machinability (they must be easy to make, to process and to assemble). Finally also the cancellation of second operations is sought by joining several components within the same body, thus making assembly easier and eliminating all second operations and standard fixing systems (screws, bolts, glue, etc.). To reach these targets the designer can work with engineering polymers that, unlike metals, are more versatile in design and in manufacture. Engineering polymers mean strengthened polyamides, acetal resins, polyesters, polyphenulsulfones, polyetherketones, polyimides, polyamidimides and polyetherimides. The use of high-performance engineering polymers for specific applications allows replacing aluminium, bronze, brass and graphite while preserving the same performances and, sometimes, even improving them. Almost all industrial sectors use engineering polymers, from the Pag 3 di 7 March 2006

4 automotive industry to home equipment, from aeronautics to the industrial sector, from the nautical industry to electronics/electrical sector. In the field of road transports, those parts of vehicles that are exposed to damp, high temperatures, fluids or mechanical stresses as the parts of the engine unit, of the braking system, and electric and electronic components are now made of engineering polymers. In the electric/electronic sector the excellent fluidity of polymeric materials and their good thermal resistance are exploited to make connectors, lamp holders, switches, coils and stator packages. In the industrial sector, polymeric materials allow creating gearings, motor components and details of kinematic units that were traditionally made of die-cast aluminium or sintered metals peso in % idraulica automaz. industr. motore auto oleodinamica settore alluminio tecnopolimeri Fig. 4: Reduction of weight according to the sectors of application. High-performance polymers in mechanical components For ten years now the company TPA Plast, based near Vicenza, has been cooperating with its client companies in the design of mechanical components made with engineering polymers. The company provides its know-how on polymers, together with advanced FEM design software (Moldflow injection-moulding simulator and Ansys structural simulator) and with purpose-made measurement instruments, all aimed at guaranteeing the precision required by our customers products. The work undertaken by TPA Plast in partnership with its customers in Pag 4 di 7 March 2006

5 replacing metal with polymers has yielded satisfactory results, so much that in many cases weight was cut by 50% (see fig. 4) and volume by 20%, plus a 35% curbing of overall costs. For example in the operation mechanism of a civilian automation, a standard bearing was replaced with a bearing made with an engineering polymer (fig. 5). The application sees the low-speed rotation of a 35-diameter shaft that is inserted in a bearing, so as to guarantee movement without friction. Maximum working torque is of 300Nm and the application works within a temperature range of -15 to +60 C. The choice of the polymer, matched with product and mould design, allowed getting centesimal tolerances comparable to those of standard bearings; moreover mechanical and tribologic requirements were met and the cost of the detail was cut by five times, thus guaranteeing the recovery of the cost of the mould. The new bearing was made of acetal resin charged with PTFE: this match achieves the tribologic performances that are typical of polyoxymethylene when strengthened by Teflon, while the lack of anisotropic charges avoids deformation of the moulded piece, guarantees it functioning and allows meeting geometrical tolerances. In another case the mechanical properties of engineering polymers, matched with excellent wear resistance, allowed replacing the gearings previously made of sintered metal and needle bushings. Gearings are part of an epicycloidal system operated by a 25-mm diameter shaft that transmits a 125 Nm torque, placed in between two standard needle bearings interference-inserted in a body made of heat-stabilised, reinforced polyamide. The kinematic chain is inside the reduction unit of a 300W motor, which works at temperatures varying from -20 to +70 C. The design stage is to replace the needle bearings and to manufacture thermoplastic gearings. Bearings were replaced by a single T bush faceted for antirotation, made with an engineering polymer with high temperature resistance and low friction coefficient. The mounting of bearings (which previously was by means of a pneumatic press to overcome a strong interference) was modified to allow manual assembly without purpose-made tools. Thus we curbed the cost of details, the assembly system has become simpler while the mechanical performance of the unit is unchanged (fig. 6). Pag 5 di 7 March 2006

6 Fig. 5: Needle bearing. Fig. 6: Engineering polymer bearing. Gearings are made of polyamide reinforced with fibreglass and charged with molybdenum disulphide. Fibreglass improves the mechanical properties of polyamide considerably, taking the ultimate strength beyond 200MPa, while molybdenum disulphide reduces the friction coefficient and increases the wear resistance of gearing teeth. The injection-moulding technology has allowed complying with dimension tolerances and their repeatability, guaranteeing conformity of gear teeth profile and cutting the cost of components (fig. 7). A further example of the various uses of engineering polymers is the design of the cover of the water pump for a 125 cc motor. The cover has to stand temperatures close to 120 C, it is in contact with the water cooling the motor and it is fixed to the engine bed by means of 3 hexagonal screws, which press down the OR gasket located in the lower part. A vent screw is in the upper part. The internal part of the cover hosts a rotor that makes water flow: for the optimal working of the rotor, as Sinterized Technopolymer Fig. 7: The technology of injection moulding has made it possible to comply with dimension tolerances, repeatability and conformity of gear teeth profile. well as to preserve pump effectiveness, it is important for the coupling profile with the rotor to be precise and continuous. As for the manufacturing side the aluminium cover was die-cast, with consequent deburring. Subsequently Pag 6 di 7 March 2006

7 a machine tool re-worked the base beat that hosts the OR to confer planarity; then a thread was made to screw on the vent screw and, finally, the whole was painted. All these operations, though granting excellent product quality, caused production costs to soar. The engineers of TPA Plast have studied the matter and replaced aluminium with fibreglass-reinforced polyphthalamide, which resists high temperatures and hydrolysis (caused by the circulating water). This is how the use of an engineering polymer together with injection moulding technology made it possible to get a fully finished product in one operation only. The choice of the right engineering polymer allowed meeting dimensional tolerances and centesimal geometries, as well as guaranteeing functionality at temperatures up to 120 C. Moreover the elimination of second processing such as level grinding, threading and painting made it possible to cut the cost of this detail substantially. Success in replacing aluminium (50% reduction in detail weight, and 57% off production costs) was so remarkable that also the internal rotor, as other components of the cooling circuit, is now made of an engineering polymer. February 2006 Organi di Trasmissione Ed. Tecniche Nuove Pag 7 di 7 March 2006