Plastics are a group of organic materials that can be shaped into various forms by applying heat and pressure. They are widely used in engineering applications because of their versatility, durability, and low cost. There are two main types of plastics for engineering purposes: thermoplastics and thermosets.

Thermoplastics are plastics that can be melted and remolded repeatedly without losing their properties. They are solid at room temperature but become soft and pliable when heated. Thermoplastics are suitable for applications that require flexibility, recyclability, and resistance to corrosion. Some common thermoplastics used in engineering are:

• Polyethylene (PE): The most widely used plastic in the world, PE is a lightweight, tough, and flexible material that can be molded into various shapes. PE is used for packaging, pipes, bottles, containers, films, and toys.
• Polypropylene (PP): A similar plastic to PE, but with higher melting point, stiffness, and strength. PP is used for automotive parts, furniture, carpets, medical devices, and textiles.
• Polyvinyl Chloride (PVC): A rigid or flexible plastic that can be blended with other additives to enhance its properties. PVC is used for pipes, fittings, cables, flooring, roofing, and window frames.
• Polystyrene (PS): A hard and brittle plastic that can be expanded into foam or extruded into sheets. PS is used for insulation, packaging, cups, plates, and disposable cutlery.
• Polyethylene Terephthalate (PET): A clear and strong plastic that can be formed into fibers or bottles. PET is used for clothing, carpets, food and beverage containers, and medical implants.
• Nylon: A synthetic polymer that can be spun into fibers or molded into shapes. Nylon is used for clothing, ropes, gears, bearings, and fasteners.
• Acetal: A strong and rigid plastic that has good resistance to wear, friction, and chemicals. Acetal is used for gears, bearings, valves, pumps, and electrical components.
• Polycarbonate (PC): A transparent and impact-resistant plastic that can withstand high temperatures and UV rays. PC is used for lenses, helmets, shields, lighting fixtures, and CDs.

Thermosets are plastics that undergo a chemical reaction during processing that makes them hard and infusible. They cannot be melted or remolded once they are cured. Thermosets are suitable for applications that require high strength, stability, and resistance to heat and chemicals. Some common thermosets used in engineering are:

• Phenolic: A dark-colored plastic that is made from phenol and formaldehyde. Phenolic is used for electrical insulation, laminates, adhesives, and brake pads.
• Urea-formaldehydes: A white-colored plastic that is made from urea and formaldehyde. Urea-formaldehydes are used for adhesives, coatings, molding compounds, and particle boards.
• Epoxides: A plastic that is made from epoxy resin and a hardener. Epoxides are used for adhesives, coatings, composites, and electronic components.
• Polyester: A plastic that is made from polyester resin and a catalyst. Polyester is used for fiberglass reinforced plastics (FRP), boat hulls, tanks, pipes, and roofing materials.

Selecting the “Right” Plastic Material

The two things that drive the choice of plastic material for a part are 1) the desired material properties, and 2) the feasibility of manufacturing.

To start, decide on the range of properties you want and select a subset of material types that will satisfy. Consider the potential negative aspects of those properties as well. For example, resistance to chemical wear and potential for degradation in sunlight. Plastic suppliers have property tables that can help you narrow down your selection.

Try it yourself and explore the table here: Plastic Material Properties Table | Sort & Compare | Curbell Plastics

Not all manufacturing process are possible (or commonly available) with all types of plastics. You may be limited by the manufacturing equipment you have available. This can further restrict the type of plastic you select. You can find tables in the Machinist Handbook and similar resources to find manufacturing methods and the common types of plastics available.

Look through the list of manufacturing methods and applicable plastic here: Guide to Manufacturing Processes for Plastics | Formlabs