How Plastics are Made… Understanding the Physical Properties of Plastics

The International Association of Plastics DistributorsThe International Association of Plastics Distributors

The IAPD Plastics Primer, Module 1The IAPD Plastics Primer, Module 1

Prepared by the IAPD Education Committee (Module 1)

Presented courtesy of Modern Plastics, Inc.

How Plastics are Made…

Understanding the Physical Properties of Plastics

Why use plastics• Plastic are easily formed materials.

• The advantage to the manufacturer is that plastic products can be mass-produced and require less skilled staff.

• Plastics require little or no finishing, painting, polishing etc. Plastic is referred to as a self-finishing material. Particular finishes can be achieved at relatively low cost.

• Plastics can be easily printed, decorated or painted.

• Plastics are corrosion resistant, and generally waterproof although certain types of plastics such as UPVC can become brittle and it is possible for the sun’s rays to cause the colour of the plastic to fade. It becomes bleached.

• Plastics are lighter than metals, giving deeper sections for a given weight, and hence stronger sections.

Origins of Plastics - synthetic plastics.

• The main source of synthetic plastics is crude oil.

• These gases are broken down into monomers. Monomers are chemical substances consisting of a single molecule.

• A process called Polymerisation occurs when thousands of monomers are linked together. The compounds formed as called polymers.

• Combining the element carbon with one or more other elements such as oxygen, hydrogen, chlorine, fluorine and nitrogen makes most polymers.

Thermoplastics• There are a wide range of

thermoplastics, some that are rigid and some that are extremely flexible.

• The molecules of thermoplastics are in lines or long chains with very few entanglements. When heat is applied the molecules move apart, which increases the distance between them, causing them to become untangled. This allows them to become soft when heated so that they can be bent into all sorts of shapes.

• When they are left to cool the chains of molecules cool, take their former position and the plastic becomes stiff and hard again. The process of heating, shaping, reheating and reforming can be repeated many times.

Long chain molecules

General properties: low melting point, softer, flexible.

Typical uses: bottles, food wrappers, toys, …

Examples:

Polyethylene: packaging, electrical insulation, milk and water bottles, packaging film

Polypropylene: carpet fibers, automotive bumpers, microwave containers, prosthetics

Polyvinyl chloride (PVC): electrical cables cover, credit cards, car instrument panels

Polystyrene: disposable spoons, forks, Styrofoam™

Acrylics (PMMA: polymethyl methacrylate): paints, fake fur, plexiglass

Polyamide (nylon): textiles and fabrics, gears, bushing and washers, bearings

PET (polyethylene terephthalate): bottles for acidic foods like juices, food trays

PTFE (polytetrafluoroethylene): non-stick coating, Gore-Tex™ (raincoats), dental floss

Plastic types: Thermoplastics

General properties: more durable, harder, tough, light.

Typical uses: automobile parts, construction materials.

Plastic types: Thermosets

Examples:

Unsaturated Polyesters: lacquers, varnishes, boat hulls, furniture

Epoxies and Resins: glues, coating of electrical circuits,composites: fiberglass in helicopter blades, boats, …

Plastic types: Elastomers

General properties: these are thermosets, and have rubber-like properties.

Typical uses: medical masks, gloves, rubber-substitutes

Examples:

Polyurethanes: mattress, cushion, insulation, toys

Silicones: surgical gloves, oxygen masks in medical applicationsjoint seals

Clear Acrylic (Perspex)

• It was first used to make aircraft canopies. It is ten times more impact resistant than glass.

Polystyrene

• Polystyrene is used to make plates, cutlery and model kits.

• It is stiff hard and comes in a wide range of colours.

Nylon • Nylon is hard, tough, self-

lubricating, has a high melting point and has very good resistance to wear and tear.

• It has been used to make fibres, clothing, bearings and propellers.

PVC • The rigid type is used to

make pipes, guttering and roofing. It is very lightweight and is resistant to acids, alkalis & the weather.

• The plasticised type is used for suitcases, hosepipes, electrical wiring and floor coverings.

Polythene

• High-density polythene has been used to manufacture milk crates, bottles, buckets, bowl and gear wheels.

• It is stiff, hard, can be sterilised and is dense.

The Performance of a Plastic Part is Affected By:

• Type of load • Size and application of load • Frequency of application of load • Speed of load • Temperature the part will see, and for how long• Use and environment of load

Mechanical Properties

• Tensile strength• Elongation• Compressive strength• Creep• Shear strength• Flexural strength

• Torsional strength• Modulus• Impact strength• Specific gravity• Water absorbtion• Coefficient of Friction (COF)

Tensile Strength

Elongation

Compressive Strength• Measured in Kg/m2• Higher Kg/m2 = harder to compress

Creep• Associated with compressive strength• Creep at room temperature is called “cold

flow”

Shear Strength

Flexural Strength

Torsional Strength

Modulus

• Also referred to as “stiffness”

• Used in conjunction with strengths (flexural modulus, tensile modulus, etc.)

• Higher modulus = stiffer material

• Measured in N/m2 (Kg/m2)

Impact Strength

Izod Impact

Specific Gravity

• Related to the density of material• Can be used to determine the weight of material• Specific gravity of less than 1.0 will float in water

Water Absorption

• Measured by the percentage of swell

• Think of a sponge as having high percentage absorption

Chemical Resistance

• Typically expressed in terms of alkali or acid resistance

• Inertness; or the ability not to dissolve or react to chemicals

Coefficient of Friction (COF)

• Resistance to sliding (slickness)

• Low COF = more slippery (think of “wet ice” as having lowest COF)

• Static COF refers to initial movement from rest

• Dynamic COF refers to being already in motion

Thermal Properties

• Coefficient of thermal expansion

• Heat deflection temperature

• Continuous service temperature

• Melting point

• Thermal conductivity

Coefficient of Thermal Expansion (CTE)

• Change in size as temperature changes

• Lower value = less change with temperature

Continuous Service Temperature (CST)

• Highest temperature a material can withstand and

still retain at least 50% of its properties

• Measured in degrees Fahrenheit, in air

• In high temperatures, both CST and HDT must be

considered

Melting Point

Temperature at which a crystalline thermoplastic

changes from solid to liquid

Thermal Conductivity

• How much heat a material will conduct

• Most plastics are good “insulators” (do not conduct

heat well)

• Higher value = more heat conducted

• Thermal conductivity of plastics is 300 to 2,500 times

less than most metals

Mouldability

• The ability of a plastic to flow into a complex mould

• Determined by its liquid viscocity, &

• Rate of cooling

Electrical Properties

• Volume resistivity

• Surface resistivity

• Dielectric constant

• Dielectric strength

• Dissipation factor

• Arc resistance

• Flammability

• Most favorable ratings are given to materials that extinguish themselves rapidly, and do not drip flaming particles

• Scale from highest burn rate => most flame retardant is HB, V-2, V-1, V-0, 5V

Flammability

• Ranging from PP & PE @ ~ £0.90/kg

• & Nylon @ £3 - £4/kg

• to PTFE @ £10/kg …

Cost…

Processing• Injection molding• Extrusion

– Ram extrusion– Screw extrusion– Coextrusion

• Casting• Compression molding• Rotational molding• Transfer molding• Calendering

• Hand (or spray) lay up• Laminating• Filament winding• Polymer orientation

Injection Moulding

Blow mouldingheated glass

3-piece mold

(a) The hollow piece of heated glass (parison)is first created by a blow mold(see text-book Fig 17.25)

(b) The mold is put together

(c) Plunger and hot air push theglass up

(d) Hot air blows the glass out towardsthe mold surface

(e) Mold comes apart, bottle is removed

heated glass

3-piece mold

(a) The hollow piece of heated glass (parison)is first created by a blow mold(see text-book Fig 17.25)

(b) The mold is put together

(c) Plunger and hot air push theglass up

(d) Hot air blows the glass out towardsthe mold surface

(e) Mold comes apart, bottle is removed

Extrusion• Ram• Screw• Coextrusion

Casting

Base Material Reactive Additive

Casting

Compression Moulding

• Sheet and block moulding• Parts moulding

Mold Lid

Mold Resin

Rotational Moulding• Low cost• Low pressure• Used in many markets• Easily adapted for short production runs

Transfer Moulding

Widely used in the semiconductor industry…

Ideal for thermosets but slow…

Calendering

for films & sheets…

Hand (or Spray) Lay Up

• Used to make large parts• Used to produce fiberglass boats and camper

shells

Laminating

Heat and pressure applied to the top and bottom of the material

Resin Binder

Reinforcing Substrate

Filament Winding

• Used to make containers and tubes• Items used for transportation of liquids or

gasses

Polymer Orientation

Used to manufacture polymer fibers, strapping, webbing, film, sheet and profiles

Plastics Fabrication • CNC machining centers• CNC lathes• Cutting and drilling• Profiling• Routing and milling• Thermoforming• Forging • Milling• Welding• Bending• Bonding

Thermoforming• Single station

thermoforming• Rotary station

thermoformers• Pressure forming• Twin sheet

thermoforming

Welding• Extrusion welding• Hot gas welding• Butt welding• Spin welding• Solvent welding

Bonding

The union of materials by adhesives; to unite materials by means of an adhesive

How Plastics are Made… Understanding the Physical Properties of Plastics

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