PLASTICS
Plastic materials display
properties that are unique when compared to other materials and have
contributed greatly to quality of our everyday life. Plastics, properly
applied, will perform functions at a cost that other materials cannot match.
Many natural plastics exist, such as shellac, rubber, asphalt, and cellulose ; however, it is man's ability to synthetically
create a broad range of materials demonstrating various useful properties that
have so enhanced our lives. Plastics are used in our clothing, housing,
automobiles, aircraft, packaging, electronics, signs, recreation items, and
medical implants to name but a few of their many applications.
The synthetic plastic industry started in 1909 with the development of a phenol
formaldehyde plastic (Bakelite) by Dr. L. H. Bakeland.
The phenolic materials are, even today, important
engineering plastics. The development of additional materials continued and the
industry really began to blossom in the late 1930's. The chemistry for nylons,
urethanes, and fluorocarbon plastics were developed; the production of
cellulose acetate, melamine, and styrene molding compounds began; and
production of commercial equipment to perform the molding and vacuum forming
processes began.
Acrylic sheet was widely used in aircraft windows and canopies during World War
II. A transparent polyester resin (CR-39), vinylidene
chloride film (Saran), polyethylene, and silicone resins were also developed.
The first polyethylene bottles and cellulose acetate toothpaste tubes were
manufactured during this time period.
The post war era saw the production of vinyl resins started, the use of vinyl
films, molded automotive acrylic taillights and back-lighted signs introduced,
and the first etched circuit boards developed. The injection molding process
entered commercial production. Due to the newness of the materials, the
properties and behavior of the plastic materials were not well understood. Many
products were introduced that failed, creating a negative impression about plastics
in the public's mind.
Chemists continued the development of materials, such as ABS, acetals, polyvinyl fluoride, ionomers,
and polycarbonate. The injection molding, thermoforming, extrusion, transfermolding, and casting processes were all improved.
This allowed the industry to provide an even greater number of cost-effective
products suitable for many, more demanding engineering applications.
In the early days...
Around the turn of the century, the Belgian born scientist Dr. Leo Baekeland, working as an independent chemist, came upon the
compound quite by accident. He sold his rights to Velox
to Eastman Kodak for three quarters of a million dollars and started developing
a less flammable bowling alley floor shellac; bowling
was becoming the latest rage in
Phenolic resin could be produced in a multitude of
colors, commonly yellow, brown, butterscotch, green and red. Omitting the
pigment could produce a transparent or translucent effect. The resin could be
molded or cast, depending on variations in the formula. For molding, the
formula was cooked until resinous, spread out in thin sheets to harden, then
ground to a fine consistency. At this point, powdered fillers and pigment were
added, to enable the resin to be molded and to add color. This mixture was then
put through hot rollers which created large sheets of colored, hardened resin.
These sheets were then ground into a very fine powder which was molded under
high heat and pressure into the final product form. As a molded material the
resin's drawback was the limited range of colors which could be created. For
casting, the formula was modified slightly, enabling the resin to be poured
into lead molds and then cured in ovens until it polymerized into a hard
substance. The liquid resin could be tinted to any color or "marbleized"
by mixing two colors together.
For the first ten years or so after its introduction, the resin was used
primarily to make electrical and automobile insulators and heavy industrial
products. Eventually, uses for the resin spread into the consumer market.
Castings were made in the shape of cylinders or blocks, and then sold to
novelty and jewelry makers. Industrial designers began experimenting with the
new material. Fine craftsmen sculpted the molded products on fast wheels with
razor-like tools to carve out designs that the world has not seen since; after
World War II, most companies switched to creating designs through the use of
patterned molds, instead of hand-carving. Bakelite replaced flammable
celluloid, previously the most popular synthetic material for molded items, as
a major substance for jewelry production.
The process to the collector of today may not be significant, as Bakelite is
now treasured for its unique, unreproducible beauty.
A deeply carved half inch bangle bracelet may sell for $225.00, and a two and
one half inch bangle may command $900.00. Bakelite often acquires a patina
within a few months to a few years of its date of production, and metamorphisizes into a completely different appearing
color. The red, white and blue Bakelite designs of yesterday have mellowed into
lovely yellows, reds and blacks, enhancing further the value of those rare
pieces which have continued to maintain their original color and luster.
Bakelite's many uses allowed it to become a standard item in the family home of
the 1930s and 1940s. It was frequently found in the kitchen, in the form of
flatware handles, rabbit or chicken napkin holders, salt and pepper shakers, or
serving trays. During the Depression Bakelite sold more than any other
commercial product, and was loved by the public for its brilliant and cheerful
colors and its affordability.
When the Bakelite patent expired in 1927, it was acquired by the Catalin Corporation that same year. They began mass
production under the name "Catalin," using
the cast resin formula which enabled Catalin to add
15 new colors to the original five produced by the Bakelite Corporation, which
used the limited color range molded formula, as well as the now-famous marbleized
effect. One of their most notable products was the Fada
bullet radio. The Catalin Corporation was responsible
for nearly 70% of all phenolic resins that exist
today.
Bakelite-Catalin was sold mostly by Saks Fifth
Avenue, B. Altman and Bonwit Teller, but was also on
the shelves of F.W. Woolworth and Sears. To the wealthy socialites, whose
husbands had fallen on tough times during the Depression, with Tiffany diamonds
and Cartier jewelry now well beyond their means, the vibrantly colorful carved
jewelry adorned with rhinestones became de riguer for
cocktail parties and formal dinners. Yet, Catalin and
Bakelite were within everyone's reach with Depression prices ranging from
twenty cents to three dollars. Diana Vreeland, editor
of Vogue, often spoke of the versatility of Bakelite, as did Elsa Schiaparelli, who was constantly contracting with the
Bakelite and Catalin Corporations for exclusive
buttons for her dress designs.
But in 1942 Bakelite and Catalin suspended sales of
their colorful cylinders to costume jewelry manufacturers in order to
concentrate on the wartime needs of a nation which had totally shifted its
focus. Defense phones and aviator goggles, as well as thousands of other
Bakelite products, found their way to armed forces around the world. The scheme
shifted from the 200 vibrant colors which brightened the dark days of the
Depression to basic black, the no-nonsense symbol of a nation at war. By the
end of the war, new technology had given birth to injection-molded plastics,
and most manufacturers switched to less labor-intensive and more practical
means of developing products. The next generation of plastics had been born -
Acrylic, fiberglass, and vinyl - and they were molded into products commonplace
in our everyday lives today.
Occasionally plastics are still improperly used and draw negative comments. The
thousands of successful applications that contribute to the quality of our life
are seldom noticed and are taken for granted. Remember, MATERIALS DON'T FAIL,
DESIGNS DO.
The number of variations or formulations possible by combining the many
chemical elements is virtually endless. This variety also makes the job of
selecting the best material for a given application a challenge. The plastics
industry provides a dynamic and exciting opportunity.
Plastics encompass a large and varied group of materials consisting of
different combinations or formulations of carbon, oxygen, hydrogen, nitrogen
and other organic and inorganic elements. Most plastics are a solid in finished
form; however, at some stage of their existence, they are a liquid and may be
formed into various shapes. The forming is usually done through the
application, either singly or together, of heat and pressure. There are over
fifty different, unique families of plastics in commercial use today and each
family may have dozens of variations.
How are plastics made? The word "MER" is a Greek word that means
"part." This part of a plastic is a unique combination of molecules
and is called a "MONOMER." It is like a single link in a chain. The
monomers are then fused or joined together, usually using heat and pressure, to
make long chains that result in a material with a useful blend of properties. Using another Greek word "POLY" which means
"many", the long chain of "mers"
forms a "POLYMER." The monomers are held together in a polymer
chain by the strong attractive forces between molecules, while much weaker
forces hold the polymer chains together. The polymer chains can be constructed
in many ways. Some simplified examples of the way polymers are built are shown
in Figure 1: MONOMERS:
A, B, C
HOMOPOLYMERS:
A-A-A-A-A-A-A-A-A-
COPOLYMERS:
Some
examples of grafted copolymers are styrene-butadiene, styrene-acrylonitrile, and some acetals.
TERPOLYMERS:
A-B-C-A-B-C-A-B-C-
The two monomers in a
copolymer are combined during the CHEMICAL REACTION of polymerization.
Materials called "ALLOYS" are manufactured by the SIMPLE MIXING of
two or more POLYMERS with a resulting blending of properties which are often
better than either individual material. There is no chemical reaction in this
process. Some examples of "alloys" are Polyphenylene
Oxide/High Impact Styrene, Polycarbonate/ABS, and ABS/PVC. MOLECULAR
WEIGHT
The "REPEATING UNIT" or molecular group in the homopolymer
(Figure 1) is A-, the group of molecules in the copolymer A-B-, and in the terpolymer A-B-C-. The number of repeating units in the
polymer chain is called the "DEGREE OF POLYMERIZATION." If the
repeating unit has a molecular weight (the combined weight of all of the
molecules in the repeating unit) of 60 and the chain or polymer has 1000 repeating
units, then the polymer has a "MOLECULAR WEIGHT" of 60 x 1000 =
60,000. The molecular weight is a way of measuring how long the polymer chains
are in a given material.
The molecular weight of plastics is usually between 10,000 and 1,000,000. It
becomes increasingly difficult to form or mold the plastic with the application
of heat and pressure as the molecular weight increases. A molecular weight of
about 200,000 is about the maximum for a polymer to still permit reasonable processability. Some higher molecular weight materials,
like Ultra High Molecular Weight Polyethylene (UHMWPE) which has a molecular
weight from 3,000,000 to 6,000,000, can be cast using processes specifically
designed to shape it. CRYSTALLINE/AMORPHOUS
MATERIALS
Materials that do not crystallize upon solidifying are called
"AMORPHOUS." These materials demonstrate a gradual softening as the
temperature is increased. Some examples of amorphous materials are acrylics,
polycarbonate, and ABS. These materials are usually not as easily processed as
the crystalline material since they do not flow as easily during molding.
Polymerchemists may also vary how the polyrnerchains are constructed by grafting as shown in
Figure 1d. This allows the properties of a material to be further tailored to
meet the specific needs of an application. THERMOPLASTIC/THERMOSET
MATERIALS
A "THERMOPLASTIC", in general, is like wax; that is, you can melt it
and shape it several times. The "thermoplastic" materials are either
crystalline or amorphous. Advances in chemistry have made the distinction
between crystalline and amorphous less clear, since some materials like nylon
are formulated both as a crystalline material and as an amorphous material.
Again, the advances in chemistry make it possible for a chemist to construct a
material to be either thermoset or thermoplastic. The
main difference between the two classes of materials is whether the polymer
chains remain "LINEAR" and separate after molding (like spaghetti) or
whether they undergo a chemical change and form a three dimensional network
(like a net) by "CROSSLINKING." Generally a crosslinked
material is thermoset and cannot be reshaped. Due to
recent advances in polymer chemistry, the exceptions to this rule are
continually growing. These materials are actually crosslinked
thermoplastics with the crosslinking occurring either
during the processing or during the annealing cycle. The linear materials are
thermoplastic and are chemically unchanged during molding (except for possible
degradation) and can be reshaped again and again.
As previously discussed, crosslinking can be
initiated by heat, chemical agents, irradiation, or a combination of these.
Theoretically, any linear plastic can be made into a crosslinked
plastic with some modification to the molecule so that the crosslinks
form in orderly positions to maximize properties. It is conceivable that, in
time, all materials could be available in both linear and crosslinked
formulations.
The formulation of a material, crosslinked or linear,
will determine the processes that can be used to successfully shape the
material. Generally, crosslinked materials (thermosets) demonstrate better properties, such as improved
resistance to heat, LESS CREEP, better chemical resistance, etc. than their
linear counterpart: however, they will generally require a more complex process
to produce a part, rod, sheet, or tube. Some
examples of the various types of materials: Thermoplastics Crosslinked after Processing Thermosets ALTERING
THE PROPERTIES OF PLASTICS
As discussed in the previous section, the properties of the various families of
plastics vary from one another and the polymers can be modified to alter the
properties within a family of plastics. Another way that the properties of a
given plastic are changed is the addition of items, such as additives,
colorants, fillers, and/or reinforcement. ADDITIVES
(improve specific properties) REINFORCEMENTS
(improve strength) COLORANTS
(change appearance) MECHANICAL
PROPERTIES OF PLASTICS
This section will acquaint the reader with the technical terms and concepts
used to describe the properties or performance of a material. It is important
to understand these STANDARDIZED terms since they are used by suppliers and
users to communicate how a material behaves under specific conditions. This
allows comparisons of different materials. DESIGN
When a material actually works this way it is called "LINEAR"
behavior. This allows the performance of metals and other materials that work
like a spring to be quite accurately calculated. A problem occurs when the
designer tries to apply these same equations directly to plastics. Plastics DO
NOT BEHAVE LIKE A SPRING (not a straight line), that is they are
"non-linear." Temperature changes the behavior even more. The
equations should be used only with very special input. A material supplier may
have to be consulted for the correct input.
How much load or force will the part be required to
carry? How will the part be loaded? What are the direction and size of the
forces in the part? These are but a few of the questions that a designer tries
to answer before a material is selected. STRESS
If Force or Load is in pounds and area is in square inches then the units for
stress are pounds per square inch. STIFFNESS
(Modulus) PROFESSIONAL PLASTICS, INC. TYPICAL
TENSILE MODULUS VALUES (PSI)
STRAIN STRAIN
= (Final Length - Original Length)/Original Length
If the change in size is in inches and the original dimension is in inches,
then the units for strain are inch per inch.
STRESS, STRAIN, and MODULUS are related to each other by the following
equation. The modulus or stiffness of a material can be determined when the
material is loaded in different ways, such as tension, compression, shear, flexural(bending) or torsion (twisting). They will be called
TENSILE MODULUS, also know as plain MODULUS, FLEXURAL MODULUS, TORSIONAL
MODULUS, etc.
Choose the type of modulus in the property sheet that most nearly duplicates
what the customer expects the major load to be, tension, bending (flexural). If
the load is unknown, use the lowest moduli value of
the two. These numbers can be used for short-term loading if the load is to be
applied for only a few days at the most.
The stress/strain equation is the equation used by designers to predict how a
part will distort or change size and shape when loaded. Predicting the stress
and strain within an actual part can become very complex. Fortunately, the
material suppliers use tests that are easy to understand. THE
PERFORMANCE OF A PLASTIC PART IS AFFECTED BY:
THIS IS WHERE PLASTICS DIFFER IN THEIR BEHAVIOR WHEN COMPARED TO OTHER
MATERIALS, SUCH AS METALS AND CERAMICS. CHOOSING STRESS AND/OR MODULI VALUES
THAT ARE TOO HIGH AND DO NOT ACCOUNT FOR TIME AND TEMPERATURE EFFECTS CAN LEAD
TO FAILURE OF THE PART. Some
additional terms that are used to describe material
behavior: YIELD
POINT
To try to further visualize this property, take a piece of wire and slightly
bend it. It will return to its original shape when released. Continue to bend
and release the wire further and further. Finally the wire will bend and not
return to its original shape. The point at which it stays bent is the
"YIELD POINT." The "yield point" is a very important
concept because a part is usually useless after the material has reached that
point. TENSILE
STRENGTH
A good way to visualize this property is to think of pulling a fresh
marshmallow apart and then pulling a piece of taffy apart. The force or pounds
required to pull the taffy apart would be much greater than required to pull
the marshmallow apart. If that force is measured and the taffy and marshmallow
each had a cross-sectional area of one square inch, then the taffy has the
higher "tensile strength" in terms of pounds per square inch.
Plastics may demonstrate tensile strengths from 1000 psi
(pounds per square inch) to 50,000 psi. ELONGATION
COMPRESSIVE STRENGTH
This term becomes less meaningful with some of the softer materials. PTFE, for
example, does not fracture. Consequently, the compressive strength continues to
increase as the sample is deforming more and more. A meaningful
"compressive strength" would be the maximum force required to deform
a material prior to reaching the yield point. The compressive term similar to
"elongation" is "compressive deformation," though it is not
a commonly reported term. It is easy to visualize two identical weights (FIGURE
7), one sitting on a 1" cube of fresh marshmallow and the other on a
1" cube of taffy. The marshmallow would be flattened and deformed more.
SHEAR
STRENGTH
FLEXURAL STRENGTH
TORSIONAL STRENGTH
POISSON'S
RATIO
Figures
12 through 16 show the tensile strain curves for different types of materials.
REMEMBER TO THINK OF PULLING ON DIFFERENT KINDS OF TAFFY; THAT IS, SOFT AND
WEAK, HARD AND BRITTLE, ETC.
Figure
17 shows how a plastic material can appear stiffer and stronger if it is pulled
apart faster. An example of rate sensitivity is when we can't pull a string
apart, but we can snap it apart.
Figure
17 also shows how the material is softer and weaker at higher temperatures,
like wax. Plastics are also affected by low temperatures and many become more
brittle as the temperature goes down.
Figure 18 shows the effect of moisture in the atmosphere on the properties of a
material like nylon. The dry material is hard and brittle while the wet
material is softer and tougher. This is like comparing uncooked spaghetti to
cooked spaghetti. Typical
tensile yield strengths of some materials (psi)
Creep
REMEMBER
THAT CREEP IS AFFECTED BY: Since
the STRESS is kept constant, i.e., the weight or load is not changed or
removed, the equation becomes: Apparent
Modulus x Total Strain = Constant (Stress) or in other words, if the strain
goes up, then the Apparent Modulus must come down. Since the strain increases
with time and temperature, the Apparent Modulus decreases with time and
temperature.
The data is sometimes presented in supplier literature in terms of Stress
Relaxation. This means that the STRAIN is held constant and the decrease in the
load (stress) is measured over time. This is called "STRESS RELAXATION''.
This information is important for applications, such as gaskets, snap fits,
press fits, and parts joined with screws or bolts. The equation becomes: Apparent
Modulus / Stress = Constant (Strain) or in other words, as the stress
goes down because the material moves, then the apparent modulus also goes down.
Sometimes a supplier will recommend a maximum design stress. This has a similar
effect to using the apparent modulus. The recommended design stress for some
acrylic injection molded parts is 500 psi and yet its
tensile strength could be reported to be as much as 10,000 psi
in the property chart. Designers will often look at the 10,000 psi value and cut it in half to be safe; however, it is not
really enough and could lead to failure of the part.
Figure 22 shows the Tensile Elongation of a Material as a function of Time at
Various Stress Levels. Think about pulling a piece of taffy to help visualize
what is happening. The X indicates that the test bar broke. Notice how the
elongation is significantly reduced as the stress level is reduced. A stress
level is finally reached where the creep is nearly negligible.
Figure 23 shows one of the ways the creep data is often presented in
literature. The time scale is usually over a very long time, hundreds and more
often thousands of hours. Most of the literature will compress the time scale
for ease of reading with the use of a logarithmic scale along that axis.
FATIGUE
STRENGTH
Some examples of cyclic loading are a motor valve spring or a washing machine
agitator. With time, parts under cyclic loading will fail; however, properly
designed and tested they will not fail before several million loadings have
been completed. Figure
24 shows a typical S-N curve.
IMPACT
STRENGTH Some of the impact tests commonly used in
supplier literature are:
Izod Test: designed to measure the effect of a sharp
notch on toughness when the test specimen is suddenly impacted. Tensile
Impact Test: designed to measure the toughness of a small specimen without a
notch when subjected to a sudden tensile stress or load. Brittleness
Temperature Test: determines ability of the material to continue to absorb
impacts as the temperature is decreased. Special
tests may need to be devised to more nearly duplicate the actual application.
INFORMATION
PROVIDED BY THESE TESTS WILL AID IN CHOOSING MATERIAL CANDIDATES; HOWEVER, THE
DESIGNER MUST STILL TEST THE ACTUAL PART UNDER CONDITIONS AS NEAR AS POSSIBLE
TO ACTUAL USE CONDITIONS BEFORE BEING CONFIDENT THAT THE MATERIAL SELECTION IS
ADEQUATE.
NOTCH
SENSITIVITY Some
plastic materials have exceptional impact performance and very good load
carrying capability; however, the performance of a material can be greatly
reduced by having sharp corners on the part. The sharp corners can be part of
the design or from machining operations. A SHARP CORNER IS A GREAT PLACE FOR A
CRACK TO START. The Izod impact strength of a tough
material like polycarbonate is reduced from 20 to 2 as the radius of the notch
is reduced from 0.020"R to 0.005"R respectively. The
sharp corners not only reduce the impact resistance of a part, but also allow
for a stress concentration to occur and encourage the premature failure of a
load carrying part.
MINIMIZING
SHARP CORNERS MAY MAKE THE MACHINING OPERATION MORE DIFFICULT; HOWEVER, IT MAY
BE CRUCIAL TO THE PART'S SUCCESS. Edges
of sheet being used in impact applications like glazing must also be finished
to be free of sharp notches. This is a concern with acrylics and even tough
materials like polycarbonate. THERMAL
PROPERTIES COEFFICIENT
OF EXPANSION TYPICAL
COEFFICIENTS OF EXPANSION (in/in/F)
The
change in length = Original length x the coefficient of expansion x the change
in temperature DEFLECTION
TEMPERATURE UNDER LOAD
TYPICAL DEFLECTION TEMPERATURES,
LOADED TO 264 psi (F)
THERMAL
CONDUCTIVITY EFFECTS
OF THE ENVIRONMENT ON PLASTICS
Plastic materials do not rust or corrode and many plastics perform
significantly better than metals in corrosive environments. Also understand
that the MORE CHEMICALLY RESISTANT a plastic is, the MORE DIFFICULT it is to
bond to since bonding generally requires some chemical attack.
Chemical resistance is also a critical factor if the part is to be PAINTED. The
solvents in the paint must be compatible with the material to be painted. It is
best to use paints recommended by the material supplier.
Gaskets, "0" rings, or other dissimilar materials that will be in
intimate contact with a plastic over a long period of time MUST not contain chemicals,solvents, or
plasticizers that will leach out and attack the base material. Flexible vinyl
is an example of a material softened by a chemical additive. This vinyl is also
a good example of plasticizer migration (outgassing)
from pieces inside a car and it ends up fogging the windows.
The outgassing of volatiles is accelerated when the
material is exposed to high temperatures and/or vacuum. In critical
applications requiring no outgassing, a material must
be selected that does not contain any plasticizers or other additives that can
outgas. Often, pre-baking the material at a temperature slightly above the
application temperature will drive out most of the volatiles. Check with the
material suppliers. Materials such as polycarbonate, acetals,
nylons, and acrylics have been used in these applications. ELECTRICAL
PROPERTIES OF PLASTICS
A BASIC CONCEPT TO REMEMBER is that electrons must be exchanged between
molecules for electric current to flow through a material. Plastic molecules
hold on to their electrons and do not permit the electrons to flow easily; thus
plastics are insulators.
The molecules in plastics are also "polar" which means that they will
tend to act like little magnets and align themselves in the presence of a
voltage or field, the same as the needle in a compass trying to point North. The
electrical properties of plastics are usually described by the following
properties: VOLUME
RESISTIVITY
Visualize putting DC electrodes on opposite faces of a one centimeter (.394
inch) cube of a plastic material. When a voltage is applied, some current will
flow in time as the molecules align themselves (Figure 30).
Ohm's Law tells us that a voltage (volts) divided by the current (amps) is
equal to a resistance (ohms) or V/I = R. When the voltage applied to the cube
is divided by the current, the resistance for 1 cm of the plastic is determined
or ohm per cm. Generally
plastics are naturally good insulators and have very high resistance. The
Volume Resistivity can change with temperature and
the presence of moisture or humidity. SURFACE
RESISTIVITY
Again refering to Ohm's Law, The Surface Resistivity is a measure of how much the surface of the
material resists the flow of current.
DIELECTRIC
CONSTANT
The dielectric constant is a measure of how good a material works to separate
the plates in a capacitor. Remember that the molecules are like little magnets
and are trying to realign themselves every time the
voltage (current) changes direction. Some materials do it better than others.
The dielectric constant for a vacuum has a value of 1. Dry air is very nearly
1. All other materials have "dielectric constants" that are greater
than 1. The "dielectric constant" for a plastic material can vary
with the presence of moisture, temperature, and the frequency of the
alternating current (and voltage) across the plates.
The units for frequency are usually "HERTZ (Hz)" which means cycles
per second. 3 kilohertz is the same as 3,000 hz and 3 megahertz is the same as 3,000,000 hz. DIELECTRIC
STRENGTH Note:
One mil is another way of saying .001 of an inch, so a piece of plastic film 5
mils thick is .005 inch thick.
The test is similar to that used for "Volume Resistivity"
except the voltage is increased until there is an are
across the plates. This means that the voltage was strong enough to break down
the material and allow a large current to flow through it. Again this property
can be affected by the presence of moisture and temperature. Frequency may also
affect this property when the material is subjected to an Alternating Current.
See Figure 30.
DISSIPATION
FACTOR
This is an indication of the energy lost within the material trying to realign
the molecules every time the current (voltage) changes direction in alternating
current. The property varies with moisture, temperature, and frequency.
ARC
RESISTANCE
EMI/RFI
EMI and RFI are electromagnetic energy that can be emitted by an electronic
product and affect the operation of other electronic equipment near it.
Conversely, energy from the other products could interfere with the operation
of a given product. FCC regulations control the amount of energy that can be
emitted by a product.
Examples of EMI and RFI interference are: when you hear other noise and/or
stations on your car radio; when a CB broadcast is heard on your FM receiver;
when you see snow on your TV set when an appliance is run; warnings in
restaurants that a microwave is being used.
The screen or perforated metal seen in your microwave door is an example of
EMI/RFI shielding. Coaxial cable for your TV antenna is a wire surrounded by a
woven metal shield that is to be grounded. The shield absorbs energy coming in
from outside sources and keeps the signal in the wire pure while preventing
that signal from escaping and interfering with some other electronic product.
Another serious potential problem is the static charge that can be picked up
walking across a room and zap an electronic product.
The charge can often be harmlessly dissipated by correctly grounding the
equipment. The application of an anti-static may also
be used to provide a temporary solution. OPTICAL/COLORABILITY
PROPERTIES OF PLASTICS
Transparent colored materials transmit that portion of the visible spectrum
that allows the eye to see the desired color. Most plastic materials are not
transparent and the color of the base material may limit the selection of
colors available. WEAR
CHARACTERISTICS OF PLASTICS
A material like glass may be very resistant to scratching yet can be readily
abraded by sand blasting, as evidenced by the pits in a windshield. Conversely,
another material like acrylic is easily scratched when wiped and yet is much
more resistant than glass to abrasion from sand blasting. It is usually best to
devise a test that will duplicate actual use conditions to accurately determine
a material's suitability for an application.
Many plastics are specifically formulated for running against surfaces. The
base polymer may exhibit self-lubricating properties. Additives such as TFE,
silicone oil, molybdenumdisulfide, and carbon are
used to further enhance the bearing capabilities of some materials. Materials
have their bearing properties even further enhanced by the addition of
additives, such as TFE. MACHINABILITY
Annealing is the baking of a material, without melting or distorting the part,
for a time to relax the internal stresses. The internal stresses are usually
caused by uneven cooling, that is the outside of the part cools much faster
than the inside when the blank is made. This uneven cooling can also cause
variations in the properties from the outside to the inside.
The poor thermal conductivity of plastics requires that care is taken to
prevent the area being machined from getting too hot. The type of tool, depth
of cut, rate of feed, and coolant flow may have to be adjusted. If a coolant is
used, MAKE SURE IT DOES NOT CHEMICALLY ATTACK THE
PLASTIC BLANK. Check
the supplier literature for specific recommendations on the types of tools,
speeds, etc., to be used with a particular material. TOLERANCES PROCESSING INJECTION
MOLDING
Once melted the material is forced, under pressure, into the mold where it
conforms to the shape of the cavity. The mold is temperature controlled,
usually by circulating temperature controlled water through it. Once the part
is cooled, the mold is opened and the part removed. The mold is then closed and
ready for the next shot. The mold is clamped shut while the material is being
injected in to the cavity since the cavity pressure may be as much as 5,000 psi. The clamp is sized by the "Tonnage" it
holds. Injection molding machines will be referred to by its shot size in
ounces and its tons of clamping ability. An example would be a 6 oz, 80 Ton
machine.
The molds are most often made out of hardened steel and carefully finished.
They may also be made out of prehard steel, aluminum,
epoxy, etc. The type of mold material selected depends on the number of parts
to be made and the plastic material to be used. Parts are often machined to
test the shape and function of a part before a mold is built. EXTRUSION
The extrusion process is also used with a system of molds and called "Blow
Molding." This is how bottles, such as the gallon milk bottle, are
produced. THERMOFORMING CALENDERING CASTING THERMOSETS
The injection molding of thermosets is similar to the
injection molding of thermoplastics except the material is kept cool until it
is pushed into the heated mold where it is crosslinked.
The mold is then opened and the hot, but rigid, part is removed. TRANSFER
MOLDING COMPRESSION
MOLDING HAND
(OR SPRAY) LAY-UP LAMINATING FILAMENT
WINDING
There are many other processes, too numerous to mention in this text. It is
suggested that the reader obtain other literature that can provide more
information, in greater depth, on the various processes. MATERIAL
SELECTION
As a plastic materials professional, one must be alert to those applications
that are not correct for plastics. Sometimes a designer or customer becomes
enamored with using a plastic without understanding the properties of plastics
and if a plastic material is even suitable for the application. One must also
be careful of a design that is worked in aluminum or steel and is to be
converted to plastic. A metal part may not work in plastic. THIS IS WHERE IT IS
IMPORTANT TO UNDERSTAND WHAT THE CUSTOMER EXPECTS THE PART TO DO. A material
supplier may have to be consulted before the customer can be given a
suggestion.
The first and most important step in selecting a material from the broad
spectrum of materials (steel, aluminum, brass, polycarbonate, acrylic, nylon,
etc.) is to carefully define the requirements of the application. The second
step is to try and match those requirements to the properties of the available
materials.
It may be necessary to ask some or all of the following questions to define the
application. One will develop expertise in how to ask questions with
experience. The more completely the application is defined, the better the
chance of selecting the best material for the job. WHAT
LOAD WILL THE PART HAVE TO CARRY? Note:
Thermosets often perform well under high continuous
loads. Reinforced thermoplastics, such as a thermoplastic
polyester, may also perform satisfactorily. WILL
THE PART HAVE TO WITHSTAND IMPACT? Note:
Laminated plastics, such as glass-reinforced epoxy, melamine, or phenolic generally have good impact strength. Polycarbonate
and UHMW polyethylene also exhibit excellent impact resistance. WILL
THE PART SEE CYCLIC LOADING (FATIGUE)? Note:
Materials like acetal and nylon are generally good
candidates for cyclic loading. WHAT
TEMPERATURES WILL THE PART SEE AND FOR HOW LONG? Note:
The temperature extremes could occur during shipping. WILL
THE MATERIAL BE EXPOSED TO CHEMICALS OR MOISTURE? Note:
Crystalline and thermoset materials generally exhibit
good chemical resistance. WILL
THE MATERIAL BE USED IN AN ELECTRICAL DESIGN? What voltages will the part be
exposed to? Alternating (AC) or direct (DC) current? If AC, what frequencies?
Where will the voltage be applied (opposite side of the material, on one
surface of the material, etc.). Note:
Enough carbon reinforcement can make a plastic conductive. WILL
THE MATERIAL BE USED AS A BEARING OR NEED TO RESIST WEAR?
Note: Materials with friction reducers added, such as TFE, molybdenumdisulfide,
or graphite, generally exhibit less wear in rubbing applications. DOES
THE PART HAVE TO RETAIN ITS DIMENSIONAL SHAPE? What
kind of dimensional stability is required?
Note: An application requiring a very high level of dimensional stability may
not be suitable for plastic materials. Remember that the plastic materials move
more with changes in temperature than do metals. The
most stable plastics are reinforced with glass, minerals, etc..
WILL
THE MATERIAL HAVE TO STRETCH OR BEND A LOT?
Note: A flexible material like flexible vinyls,
urethanes, rubber, or a thermoplastic elastomer may
be used. WILL
THE PART HAVE TO MEET ANY REGULATORY REQUIREMENTS?
Note: Make sure the supplier has approval from the desired agency and not just
its own lab. The customer may require proof of approval. DOES
THE MATERIAL OR FILM HAVE TO PREVENT CERTAIN GASES OR LIQUIDS FROM PASSING
THROUGH? Does
the material have to be impermeable to gases or liquids? If so, which ones? Note:
This is important for packaging foods and some medical applications. WILL
THE PART BE EXPOSED TO ANY RADIATION?
Note: This requirement could occur for military, utility (atomic power plants),
or medical applications. DOES
THE MATERIAL HAVE TO HAVE A SPECIAL COLOR AND/OR APPEARANCE?
Note: Direct customers toward the colors that are
readily available from the suppliers. Special colors can be more costly, expecially in small quantities. DOES
THE PART HAVE ANY OPTICAL REQUIREMENTS?
Note: Acrylics and polycarbonates have excellent optical properties. WILL
THE PART BE USED OUTDOORS? CAN
ANY VOLATILES BE GIVEN OFF BY THE MATERIAL? |
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