PSI - Issue 13

476 R Mitrović et al. / Procedia Structural Integrity 13 (2018) 475– 482 2 R Mitrovi ć , Ž Miškovi ć , M Ristivojevi ć , A Dimi ć , J Danko, J Bucha, T Milesich/ Structural Integrity Procedia 00 (2018) 000–000

Polymers, as a relatively new group of materials, require special attention because their domain of application in engineering practice is very diverse, Table 1, Dowling (2013). A polymer is a large molecule (macromolecule) composed of repeating structural units. Although the term polymer sometimes refers to plastics, it actually encompasses a large class comprising both natural and synthetic materials with a wide variety of properties. Polymer materials applied in mechanical engineering are divided into two main groups: duroplasts (duroplastics) and thermoplastics, Jelaska (2012). Polymers most commonly used in the manufacture of machine parts are from the thermoplastic group: polyamide (PA or Nylon), polyoxymethylene (POM) and acrylonitrile butadiene styrene (ABS).

Table 1. Classes, Examples, and Uses of Representative Polymers. Polymer

Typical usage

Thermoplastics: ethylene structure Polyethylene (PE) Polyvinyl chloride (PVC) Polypropylene (PP) Polystyrene (PS)

Packaging, bottles, piping Upholstery, tubing, electrical insulation Hinges, boxes, ropes Toys, appliance housings, foams Windows, lenses, clear shields, bone cement Tubing, bottles, seals Telephone and appliance housings, toys

Polymethyl methacrylate (PMMA, Plexiglas, acrylic) Polytetrafluoroethylene (PTFE, Teflon) Acrylonitrile butadiene styrene (ABS)

Thermoplastics: others Nylon

Gears, tire cords, tool housings High-strength fibers Gears, fan blades, pipe fittings Coatings, fans, impellers afety helmets and lenses

Aramids (Kevlar, Nomex) Polyoxymethylene (POM, acetal) Polyetheretherketone (PEEK) Polycarbonate (PC)

Thermosetting plastics

Electrical plugs and switches, pot handles Plastic dishes, tabletops Buttons, bottle caps, toilet seats Matrix for composites Fiberglass resin

Phenol formaldehyde (phenolic, Bakelite) Melamine formaldehyde Urea formaldehyde Epoxies Unsaturated polyesters

Elastomers

Shock absorbers, tires Tires, hoses, belts Shoe soles, electrical insulation O-rings, oil seals, hoses Wet suits, gaskets

Natural rubber; Styrene-butadiene rubber (SBR) Polyurethane elastomers Nitrile rubber Polychloroprene (Neoprene)

In the past, plastic machine parts were considered unworthy substitutions for parts made of metal because they did not have the ability to work under the same operating conditions due to limited strength. However, the development of plastic materials of increased load capacity, the advancement in the technologies for the production of plastic parts and the development of reliable engineering databases have led to successful and increased use of plastic machine parts, EY (2016). Some of the advantages of plastic mechanical parts regarding to metal are: lower density (light mass and low inertia), ability to work without or with minimal lubrication, low friction coefficient, corrosion resistance, etc. Because engineering plastics, as a family, are much younger than engineering metals, their database is not yet complete. In addition, their rapid evolution makes the material selection process more difficult, as it was pointed by Davis (2005). Due to the increasing popularity of polymer i.e. plastic use in engineering a wide range of plastic manufacturing technologies has been developed aimed at increasing the accuracy of a printed object, and accelerating the production process with the acquisition of appropriate mechanical properties and working on the principle of additive manufacturing (AM). In contrast to conventional subtractive manufacturing methods (removing layers of material to reach the desired shape), additive manufacturing is the technology of making objects directly from a Computer Aided Design (CAD) model by adding a layer of material at a time as it is stated in work of the authors Letcher et al. (2015). An overview of the division of additive technologies most commonly applied in rapid prototyping is given in the table 2, www.3dhubs.com. The additive production procedure usually consists of four activities, which is emphasized by Mitrović and Mišković (2017): 1. Creating a 3D model in one of the available commercial softwares (Catia, Inventor, Solidworks); 2. Conversion of 3D models into STL (Stereo-lithographic) format - a compact 3D model is divided into parallel layers of a certain thickness; 3. Defining the 3D printing parameters and 3D model orientation in the 3D printer workspace; 4. Physical prototype production.