TECNICA board. It is therefore not surprising that many manufacturers have models in their catalogue with mouldboards built (or covered) with plastic material, achieving some fuel savings during processing and being able to maintain slightly higher working speeds than equivalent traditional ploughs. The guards of almost all cardan shafts are also made of polyethylene, which in this case is well suited for its elasticity and is also very economical. However, in relation to this specific application, it is a material that does not have optimal characteristics in relation to various attacks, both mechanical and chemical shocks, but also in relation to UV aging, which over time cause cuts, fissures, deformations, browning, cracks and crumbling of different parts of the protection, compromising the effectiveness of segregating the serious danger posed by the rotating cardan shaft. Polypropylene or polyurethane can improve the situation, but there is no doubt that a steel sheet protection, although more expensive and less flexible, would guarantee a much longer lifespan. Polyethylene (PE), but also fiberglass (GRP, Glass-Reinforced Polyester) are the most common materials used for the construction of mixture tanks on sprayers. Both involve numerous positive qualities and obviously some critical issues, such as to make it possible to opt for one or the other solution depending on the needs. Polyethylene has several advantages: good resistance to shocks, chemical corrosion (due to contact with acids, alkalis, alcohol, gasoline, salt solutions, etc.), high temperatures (up to 130 °C for high-density PE), good light weight (0.9-0.95 kg/m³). Another very favourable aspect of polyethylene in the construction of tanks lies in the possibility of obtaining models with even very elaborate shapes through rotational moulding, with which robust, seamless, lightweight, monolithic containers without internal tensions are obtained. Basically, in a mould made of shaped and welded steel sheet, or in cast aluminium, polymer powder is loaded, which is heated and rotated along one or more axes, so that it can liquefy and adhere to the walls of the mould homogeneously, taking its shape. Fiberglass shows high resistance to compression, thanks to plastic resins, but also to traction, due to the presence of glass fibers. In addition, it has a limited volume mass, a valid inertia to the aggression of most chemical substances and to corrosion (even of the electrolytic type), it is stable to UV radiation and temperature changes. Finally, it has a pretty long lifespan. If damaged, it can be easily repaired, but with the same capacity, the cost of the fiberglass tank is higher than the polyethylene equivalent. Ceramics. From a chemical-physical point of view, the main advantage of ceramic is its high resistance to wear (both due to chemical corrosion and friction), conferred by its typical hardness. However, it is necessary to consider its notable fragility, which is ill-suited to uses involving mechanical shocks. In general, these are favourable conditions for its successful adoption in the construction, for example, of the nozzles of sprayers, in particular the spray tips. In this case, for the production of drops of a constant diameter over time, it is important that the diameter of the hole does not undergo undesirable increases, due precisely to wear. Moreover, other materials are commonly used for the manufacture of nozzles: brass, for slit and mirror spray tips, and some plastic materials (such as polyvinyl chloride or polypropylene) especially for slit nozzles, which are cheaper, but with limited resistance to wear; stainless steel is in an intermediate condition. Concrete. It may seem rather strange, but one of the most common materials suitable for building constructions has been used for some time now also for the manufacture of tractor ballasts which, as is known, have the dual purpose of increasing the shooting capacity of the machine, through an appropriate increase in its adherent weight and, if placed at strategic points in the vehicle, to rebalance the distribution of the masses of the tractor-operator complex. In this case, as an alternative to classic cast iron 40 ne, ma è indubbio che una protezione in lamiera d’acciaio, quantunque più costosa e meno flessibile, garantirebbe una durata decisamente più lunga. Sempre il polietilene (PE), ma anche la vetroresina (PRFV, Poliestere Rinforzato con Fibra di Vetro) sono i materiali più comuni impiegati per la costruzione dei serbatoi della miscela sulle macchine irroratrici. Entrambi comportano numerose qualità positive e ovviamente alcune criticità, tali da far optare per l’una o l’altra soluzione in funzione delle esigenze. Il polietilene, evidenzia diversi vantaggi: buona resistenza agli urti, alla corrosione chimica (dovuta al contatto con acidi, alcali, alcool, benzina, soluzioni saline, ecc.), alle alte temperature (fino a 130 °C per il PE ad alta densità), buona leggerezza (0,9-0,95 kg/m³). Un ulteriore aspetto molto favorevole del polietilene nella costruzione dei serbatoi risiede nella possibilità di ottenere modelli con conformazioni anche molto elaborate mediante stampaggio rotazionale, con il quale si ottengono contenitori robusti, senza saldature, leggeri, monolitici e privi di tensioni interne. In sostanza, in uno stampo in lamiera di acciaio sagomata e saldata, oppure in fusione di alluminio, viene caricata polvere polimerica, che viene riscaldata e fatta ruotare lungo uno o più assi, in modo che possa liquefarsi e aderire alle pareti dello stampo in modo omogeneo, assumendone la forma. La vetroresina evidenzia un’elevata resistenza alla compressione, grazie alle resine plastiche, ma anche alla trazione, per la presenza di fibre di vetro. Inoltre, ha una limitata massa volumica, una valida inerzia all’aggressione della maggior parte delle sostanze chimiche e alla corrosione (anche di tipo elettrolitico), è stabile alle radiazioni UV e alle variazioni di temperatura. Infine, ha una vita utile piuttosto lunga. Se danneggiata, può essere facilmente riparata, ma a parità di capacità, il costo del serbatoio in vetroresina è superiore riTECNICA La miglior resistenza meccanica dell’aratro può essere ottenuta con trattamenti termici dell’acciaio (sopra), ma anche con la costruzione a strati differenziati e l’aggiunta di placchette al carburo di tungsteno per la parti maggiormente sollecitate (sotto) The best mechanical strength of the plough can be obtained with heat treatments of the steel (above), but also with the construction with differentiated layers and the addition of tungsten carbide plates for the most stressed parts (below)
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