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| Components made from Engineering Polymers for the Manufacture of Screw Conveyors and Material Handling Equipment in General |
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As indicated in the "Characterization of Powder and Granular Materials" section, the handling and transportation characteristics of bulk solid materials are influenced by a number of factors (chemical and physical properties and rheological properties). These properties strongly influence the type of materials that can be used for constructing the handling system.
Handling systems made of traditional materials such as carbon steel or stainless steel are not always suitable and highly abrasive products cause wear on some components in the system.
The wear of a component can be described as the phenomenon that determines a loss of material from the component itself; the activation mechanism is really mechanical and concerns the surface of the component.
The macroscopic effects of the wear phenomenon are significant:
1) reduction in mechanical performance of the machine after a certain period of work;
2) release of material of the worn component into the flow of the handled product;
3) need to carry out special maintenance;
4) frequent need to replace the worn components.
The attempts made to reduce the wear phenomenon have been concentrated above all on the possibility of making the surfaces of the component that is subject to abrasion more resistant. Use of technopolymers as a coating material for those surfaces that come into contact with the handled product has been proved to be the winning solution for some applications.
The characteristics that have the greatest influence on the choice of the technopolymer to be used are mainly the product¿s level of abrasiveness of the handled product and the chemical compatibility between the technopolymer and the product.
The main features required in the field of bulk solids handling are:
- great hardness (65-98 Shore A)
- high resistance to abrasion
- high tensile strength
- high mechanical strain; high elongation at break
- low modulus of elasticity.
As they are used in the food industry, elastomers are subject to very strict national and international standards. In principle it would be necessary to avoid contact of any technopolymer substance used in the manufacture of foodstuffs; but as this contact is technically inevitable, current standards permit the use of substances that do not damage health.
For bulk solids handling using mechanical screw feeder systems, components have been produced which are completely coated in technopolymers: namely conveyor screws and troughs. In view of the excellent results obtained in terms of wear control, the trend is to use technopolymer coating also for other components - such as star-wheel dosing units, centrifuges, rotary valves, screw feeder inlets and outlets - in order to reduce abrasion to the minimum.
OTHER ADVANTAGES OF USING TECHNOPOLYMERS
The production of technopolymer-coated components that come into contact with the handled product (apart from responding to the needs for wear resistance and food compatibility) leads also to:
- the production of non-stick surfaces in contact with the material which, by reducing the quantity of deposits, limits also decomposition of the material;
- lighter structures;
- reduction in power requirements in the handling system, due to the reduction in friction and weight;
- reduction in noise during material handling and transport;
- softer handling of the transported material, and therefore reduced product degradation;
- production of components that have particularly complex shapes that, difficult to produce using welding techniques, are much simpler to produce using the mould casting process.
LIMITS IN USE OF TECHNO-POLYMERS
Limits in use of technopolymers are attributed to:
- the organic matrix of the material;
- the number of additives (pigment, softeners, anti-ageing agents, anti-static agents, plasticizers) that are added during the reaction phase to improve some specific properties of the material.
Technopolymers with a polar organic matrix (e.g. polyurethanes, polyacrylics) are subject to physical and chemical attacks from polar materials (e.g. water, alcoholic solutions); technopolymers with a non-polar organic matrix (e.g. natural rubber, isoprene) are subject to physical and chemical attacks from non-polar materials (e.g. petrol, mineral oils).
The physical effects are basically linked to a variation in volume (expansion or contraction) of the polymer component due to the interaction between the technopolymer and the product being handled. This leads to an alteration in the mechanical characteristics of the elastomer - such as hardness, elasticity, resistance, tensile strenght and elongation at break.
The chemical effects are noted when the handled material has a chemical reaction with the substances contained in the technopolymer, leading to an alteration in the chemical structure, above all in terms of a variation in the grade of cross linking of the polymer.
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Références |
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Nagdi K., "Manuale della gomma"; Tecniche Nuove Milano, 1987
FDA Standard
Schonholzer P., "Wear Control"; Bulk Solids Handling, Vol. 8, n. 2, 1988.
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Auteurs |
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Dr. S. Ghelfi
Dr. A. Zucchelli
University of Bologna
Faculty of Engineering
Mechanical, Nuclear and Aerospace Department (DIEM)
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