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Plastic
Polymers
NPI Plastics is a manufacturer of specialty plastic polymers or customized resins for specialized applications. Using recycled post industrial scrap, our specialty polymers include high density polyethylene: NPI – 04HDPE Mixed Color, linear and low density polyethylene: NPI – 01LL/LD Mixed Color and polypropylene polymers: NPI – 10PP212 Mixed Color. All our polymers are manufactured from recycled plastic and offer injection and extrusion processors a quality alternative to virgin resin. If you are looking for regrind or custom recycled polymers for your injection or extrusion processing business, our products are available in pellet material form. Our polymers can be used to can be used to make new plastic parts, plastic film and sheet and many other rapid prototypes.
Polymer Properties and Polymer Selection
Selecting the right polymer is critical to your successful design of a
new plastic product. Today's plastic design engineer is faced with a myriad
of design considerations, coupled with an ever growing number of available
resins and specialty polymers. The problem lies in fully knowing what
will be required of the product in a wide range of environments along
with understanding the true functional behavior of the polymer. Like many
other materials, plastic has a host of different properties under the
categories of physical, mechanical, thermal, and electrical. There are
also several properties peculiar to plastic that are important for processing
them.
Physical
1. Density is defined as the measure of mass per unit volume of
any material, expressed either as pounds per cubic inch (lb/in.3) or grams
per cubic centimeter (g/cm3). Density is used to calculate the relationship
between weight and volume of plastic in a specific part design. The density
value of a polymer material tells what each cubic inch (or centimeter)
of the part weighs.
2. Specific Gravity is a ration that compares the density of any
material divided by the density of water, both measured at 73° F (23°
C).
3. Shrinkage defines the expected reduction of the plastic part
dimension as the part cools in the mold, compared to the original mold
dimension. Shrinkage rate is usually given as inch per inch (in./in.),
or centimeter per centimeter (cm/cm), and the range is between zero for
low shrinkage materials, and approximately 0.050 in./in. (0.127 cm/cm)
for very high shrinkage materials. Actual shrinkage values depend on many
variables such as temperature, pressure, injection time, and wall thickness
of the part. In general, amorphorus materials have lower shrinkage than
crystalline materials, and glass-reinforced or filled materials have less
shrinkage than unfilled, or neat resins.
4. Water Absorption is expressed as the percentage increase in
weight of a material due to absorption of water. This property determines
the effects on mechanical and electrical properties of a specific plastic.
Plastics with low absorption rates tend to be more dimensionally stable.
5. Transparency (Opacity) are properties that determine the light
transmission abilities of a given plastic and are usually measured as
haze and luminous transmitttance.
6. Modulus (Toughness) is a measure of flexibility and refers to
the material's ability to absorb mechanical energy without fracturing.
Toughness is usually measured by the area under the stress/strain curve
of a plastic text specimen. In general, high impact unfilled plastic materials
have excellent toughness values.
7. Brittleness is a lack of toughness, and is exhibited by lower
impact strength and higher stiffness properties.
8. Elasticity is the ability of a material to return to its original
size and shape after being deformed. Most plastic materials have very
limited elasticity.
9. Plasticity is the inverse of elasticity in so far as it is a
value of the ability of a polymer to remain deformed. Plastic materials
exhibit plasticity when they are stressed beyond their yield point.
10. Ductibility is a polymer's ability to be stretched, pulled,
or rolled into a shape without destroying its structural integrity.
11. Notch Sensitivity is a measure of the ease with which a crack
propagates through a plastic part from a pre-existing notch, crack, or
sharp corner.
12. Lubricity is a function of the load bearing characteristics
of a material under relative motion. It refers to the ability of a plastic
material to slide against itself or other plastic materials.
Mechanical
While mechanical properties are established through the use of individual,
very specific tests performed under critical laboratory conditions, a
finished plastic product is seldom exposed to such rigorous conditions.
Mechanical forces on a plastic part are actually a combination of stresses,
all working with or against each other. These facts must be taken into
consideration when you are trying to decide if a plastic material is right
for you. That being said, the following mechanical properties are at best
guidelines and indicators of a plastic material's ability to withstand
certain conditions.
1. Stress: it is no secret that plastics have very
different structural characteristics than other materials such as metals.
But even a common term like stress suggests several different things for
a plastic part than it would have for a metal part. The real question
is how do the properties of polymers effect the design process? There
are two types of stress: direct and shear. While direct stress is reflected
in forces that act perpendicular to a surface, shear stress is reflected
in forces that act parallel to that same surface. When stress exists,
strain is also present.
2. Strain is a ratio measure between a change in dimension of a
stressed plastic versus the original dimension of the plastic before being
stressed. Strain measurements are usually represented as a percentage.
3. Modulus of Elasticity (or Young's Modulus) is a constant in
Hooke's Law which measures the "slope" of the original portion
of the stress/strain curve.
4. Poisson's Ratio is a constant used for determining the stress
and deflection properties of plastic structures such as rotating discs
or plates.
4. Ultimate Strength is the maximum stress a material can withstand
when subjected to a load.
5 . Flexural Strength is a property from which we can determine
the bending capabilities of a plastic. A specimen is placed across two
beams and a force is directed from the opposing side.
6. Yield Strength represents the amount of stress required to
begin to permanently deform the material.
7. Creep is defined as increased strain over a period of time in
the presence of constant stress occurring from a constant load.
8. Impact Strength measures the ability of a plastic part to absorb
energy from impact. Impact strength is determined by a plastic specimen's
basic shape, size, thickness, as well as the type of plastic used to make
the specimen.
9. Fatigue Endurance is the ability of a polymer to endure repeated
stress as the result of cyclic action, i.e., the snap action latch device,
or any component subjected to vibration.
10. Tear Strength measures the tear resistance of a plastic material.
Thermal
Anyone involved with selecting a plastic material, such as the design
engineer, should be aware of the operating environment to which the final
product will be subjected. Thermoplastic polymers exhibit varying characteristics
dependent on their properties. The following properties represent some
of the thermal properties used for characterizing polymers.
1. Melting Point is the temperature a plastic material
must rise to before it can become fluid enough to flow.
2. Glass transition Temperature is the temperature at which
a plastic material will undergo a significant change in properties. Generally,
at temperatures below the glass transition point, the polymer will be
stiff, brittle and glassy, while above that point it is more ductile and
has a rubbery response to impact loads.
3. Melt Index is a value that defines the ability of the plastic
material to flow. It is usually listed in grams per 10 minutes and reflects
how much material flow through a pre-defined orifice, at a predefined
temperature, with a predefined load applied. The melt index is intended
to represent the actual injection molding process. Generally, the higher
the melt index the easier the material will flow. However, melt index
has a direct influence on material properties, with the lower melt index
numbers exhibiting greater physical properties than the higher numbers
for a given plastic.
4. Heat Deflection Temperature is the temperature at which a plastic
test bar, loaded to a specified bending stress, deflects by 0.010 in.
(0.254 mm). This value tells you how a plastic material will react to
elevated temperatures while supporting loads.
5. Coefficient of Linear Thermal Expansion is the ratio of the
change of a linear dimension to the original dimension, for a unit change
of temperature. This value is useful when assembling two different materials
to determine how each will expand and contract when exposed to elevated
temperatures during use.
6. Thermal Conductivity is the rate at which a plastic conducts
heat along its length or through its thickness. This is an important property
to understand if the final product is to be used as a heat insulator.
Electrical and Optical
Polymers, by nature, are excellent electrical insulators and have found
widespread use in a variety of electrical and electronic applications
throughout many industries. Besides insulation, there are many electrical
properties that must be considered when using plastics for product designs.
1. Volume Resistivity is defined as the resistance
of a polymer to an applied electrical current. It can be construed as
the inability of a polymer to conduct electricity. Materials with values
above 108 ohm-cm are considered to be insulators, while those with values
between 108 and 103 ohm-cm are considered to be partial conductors.
2. Surface Resistivity defines the ability of a polymer to resist
the flow of electrical current across the surface of a plastic specimen.
The rating of this property can be influenced by surface contamination,
i.e., moisture.
3. Dielectric Strength is a measurement of the amount of voltage
required to break down the insulation properties of a plastic polymer
at a specific thickness.
4. Dielectric Constant (Permittivity) is the value of a constant,
a ration between the permittivity of the plastic and the permittivity
of air. Permittivity is best described as the ease with which plastic
molecules can be polarized.
5. Dissipation Factor is a measure of heat dissipation and is a
constant ratio of the energy lost as heat compared to energy originally
transmitted.
6. Arc Resistance is a time-rated value that measures the ability
of a polymer to withstand exposure to an electrical current across its
surface before a conductive path of carbon is created.
7. Comparative Tracking Index (CTI) is similar to arc resistance
except that an electrolyte (a solution of ammonium chloride) is placed
on the surface of the plastic being tested.
Processing
The most important processing and physical properties for specialty polymers
include viscosity, shrinkage, and melt flow index.
Blow molding and extrusion processes tend to use lower melt flow index,
while injection molding typically uses a higher melt flow index.
1. Viscosity, a measurement of a fluid's resistance to flow, controls
how well a resin fills the cavities or voids in a mold.
2. Shrinkage is the maximum percentage of linear shrinkage that
occurs after drying, setting, and/or curing.
3. Melt flow index (MFI) is the output flow rate in grams that
occurs during a 10 minute period.
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Nexcycle Plastics Inc.
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