From an engineering point of view, plastics differ from metallic materials in the following ways:
Plastics tend to have pronounced non-linear time-dependent stress/strain behavior. Below the yield point, metals typically have elastic behavior which is nearly linear.
Instead of a single melting point, plastics typically have a
Glass Transition Temperature (TG).Metals typically have a clearly-defined melting point.
Plastics in general exhibit pronounced creep. Metals undergo creep to a certain extent, but for most engineering configurations their creep is insignificant.
Plastics tend to degrade or denature (due to heat) rather than corrode within a typical atmosphere. Of course, chemical degradation can occur when reactive chemicals are present.Plastics impregnated with organic fillers can be subject to bacterial infestation. Metals can corrode even in a benign atmosphere from reaction with oxygen and water, but are not affected by bacteria.
PLASTICOS vs PLASTICOS
AMORPHOUS POLYMERS: e.g., PVC, Polystyrene, Acrylic, ABS, PPO, PC, Polyetherimide have
a wide melting range,
low shrinkage after molding,
better impact and lowerchemical resistance than crystalline polymers,
moderate heat resistance,
and superior cosmetics of outer surfaces.
CRYSTALLINE POLYMERS: e.g., PPS, PBT (Valox), Polypropylene (PP), have
a sharply-defined melting point,
high shrinkage after molding,
low impact strength,
high heat resistance,
good fatigue endurance,
good lubricity; wear resistance,good chemical resistance,
and the ability to flow in thin-walled sections.
Because of the cross-linked chain structure of polymers, the tensile strength of polymers tends to degrade with increasing temperature.
As the temperature rises, the polymer's
Modulus (tensile, flexural) values drop.
Tensile strength drops.
Creep effects increase.
Impact strength (toughness) increases.
Skin typically formed because of thermal and viscous boundary layer effects upon injection of molten material into a mold.
Skin consists of highly directional, often fiber-depleted and sometimes crystalline material.
Skin is typically highly influential to the properties of a molded part. This fact makesprototyping (prior to hard tooling) difficult.
Effects of Additives
Flame-retardants are added to minimize the chance of a material igniting. For a material to be considered flame-retardant, it needs to meet the UL flammability standards, UL 94. The standards, listed in descending order from the highest flame resistance, are V-0, V-1, V-2, and HB.
Flammability requirements could dictatewall thickness.
Increasing flammability requirements cuts down on available material choices.
The end-product must be tested to get UL approval.
Stabilizers are added to inhibit degradation caused due to exposure to Oxygen, sunlight (UV exposure), heat, and water.
Colorants are added to give certain desired colors to the polymer.
Flowing agents are added to improve flow characteristicsduring processing.
Release agents are added to improve mold release characteristics.
Lubricity agents are added to lower the surface coefficient of friction of the finished product.
Polymeric materials are characterized by long chains of repeated molecule units known as "mers". These long chains intertwine to form the bulk of the plastic. The nature by which the chainsintertwine determine the plastic's macroscopic properties.
Typically, the polymer chain orientations are random and give the plastic an amorphous structure. Amorphous plastics have good impact strength and toughness. Examples include acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile copolymer (SAN), polyvinyl chloride (PVC), polycarbonate (PC), and polystyrene (PS).
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