A better understanding of plastic materials will help you make educated decisions when processing plastics.
This Video is designed to give you a better understanding of what a plastic material is, and how plastics behave during processing.
Plastic Design Engineer need to Learn
- What is Polymers
- Classification of polymers (Based on the structure)
- Classification of polymers (By their application)
- Injection molding process
- Polymer viscosity
- Polymer orientation
- Material handling
- Hygroscopy, etc.
Properties of Polymers
Polymer as strong as Kevlar, which is used to make bullet-proof vests, or a material as easy to rip as a piece of paper. It can be as hard as a bowling ball or as soft as a piece of tissue paper. It can be as brittle as the disposable polystyrene glasses used at parties or as elastic as a Styrofoam coffee cup.
The following list describes some of the important properties of a polymer:
Heat capacity/ Heat conductivity, The extent to which the plastic or polymer acts as an effective insulator against the flow of heat. (The polystyrene in disposable plastic glasses isn’t a very good insulator. However, blowing air through styrene while it is being polymerized gives the Styrofoam used for disposable coffee cups, which is a much better insulator.)
Thermal expansion, The extent to which the polymer expands or contracts when heated or cooled. (Silicone is often used to seal glass windows to their frames because it has a very low coefficient of thermal expansion.) Thermal expansion is also concerned with the question of whether the polymer expands or contracts by the same amount in all directions. (Polymers are usually anisotropic. They contain strong covalent bonds along the polymer chain and much weaker dispersive forces between the polymer chains. As a result, polymers can expand by differing amounts in different directions.)
Crystallinity, The extent to which the polymer chains are arranged in a regular structure instead of a random fashion. (Some polymers, such as Silly Putty and Play Dough, are too amorphous and lack the rigidity needed to make a useful product. Polymers that are too crystalline often are also too brittle.)
Permeability , The tendency of a polymer to pass extraneous materials. (Polyethylene is used to wrap foods because it is 4000 times less permeable to oxygen then polystyrene.)
Elastic modulus, An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance’s resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it..
Tensile strength, maximum load that a material can support without fracture when being stretched, divided by the original cross-sectional area of the material.
High Tensile Strength Plastics
PAI – Polyamideimide (PAI) boasts the highest tensile strength of any plastic at 21,000 psi. This high performance plastic has the highest strength of any unreinforced thermoplastic, good wear and radiation resistance, inherently low flammability and smoke emission, and high thermal stability. PAI parts can be found in engines, valves, gears, electrical connectors, and thrust washers.
Ultem® – Also known as PEI, Ultem® has a tensile strength of 15,200 psi and an excellent combination of mechanical properties. It is easily machined and fabricated, has excellent strength and rigidity, a high dielectric strength, and a continuous use temperature of 340ºF. PEI is often used in medical and chemical instrumentation due to its heat, solvent, and flame resistance.
PEEK – One of the best high performance engineering thermoplastics on the market, PEEK has a superior mix of mechanical properties, including a tensile strength of 14,000 psi. It can withstand high temperatures (up to 480ºF for continuous use), has inherently good wear and abrasion resistance, and offers excellent chemical and hydrolysis resistance. PEEK can be found in the most demanding applications and harshest environments, such as aircraft parts, bearings, pumps, and medical implants.
Nylon – An honorable mention goes to Nylon, with a tensile strength of 12,400 psi. This high tensile strength plastic can often be overlooked, but it has a relatively high melting point (450ºF) and exhibits excellent abrasion resistance. It also has high chemical resistance and is not damaged by oils, solvents or alcohols. Nylon is used in a wide array of applications, from compound bow strings to substituting for low strength metals.
The Glass Transition Temperature (Tg), not to be confused with melting point (Tm),
Is the temperature range where a thermosetting polymer changes from a hard, rigid or “glassy” state to a more pliable, compliant or “rubbery” state.
Resilience, The ability of the plastic to resist abrasion and wear.
Abrasion resistance is the ability of a material to resist material loss when another material is rubbed against it. Due to the molecular makeup of each material, some withstand wear better than others. To reduce abrasion and preserve pipe quality, look to optimize both system conditions and piping materials.
Toughness of a plastic is measured by its resistance to impacts. It is the ability of a material to resist both fracture and deformation. One common way to discuss the toughness of a polymer is to examine the area underneath the stress – strain curve for the particular polymer.
Refractive index, The extent to which the plastic affects light as it passes through the polymer. (Does it pass light the way PMMA does, or does it absorb light like PVC?)
Resistance to electric current— Is the material an insulator, like most polymers, or does it conduct an electric current? (There is a growing interest in conducting polymers, which can be charged and discharged, and photoconducting polymers that can pick up an electric charge when exposed to light.)