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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. Baekeland. 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, transfer molding, 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...
  Bakelite (phenolic resin) is most often actually a product called Catalin. Both, along with Plaskon, are formaldehyde based plastics. Allow me to expand (with liberal borrowing from Dr. Stephen Z. Fadem - a true expert).

  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 New York City. Dr. Baekeland soon realized that a resin that was both insoluble and infusible could have a much wider appeal when used as a molding compound. He obtained a patent and started the Bakelite Corporation around 1910.

  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, irreproducible 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 rigueur 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
Examples of monomers are ethylene, styrene, vinyl chloride and propylene.

Figure 1a
Figure 1a

HOMOPOLYMERS: A-A-A-A-A-A-A-A-A-
(Polymers constructed from a single material)
Examples of polymers built this way are polyethylene, and some acetals.

Figure 1b
Figure 1b

COPOLYMERS:
(Polymers constructed from two different materials)
ALTERNATING TYPES: A-B-A-B-A-B-A-B-A-B-
Some examples of alternating copolymers are ethylene-acrylic and ethylene-ethyl acrylate.

Figure 1c

Some examples of grafted copolymers are styrene-butadiene, styrene-acrylonitrile, and some acetals.

Figure 1d
Figure 1d

TERPOLYMERS: A-B-C-A-B-C-A-B-C-
(Polymers constructed from three different materials)
An example of a terpolymer is acrylonitrile-butadiene-styrene (ABS).

Figure 1e
Figure 1e

  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
  It is important for the chemist to know how long the polymer chains are in a material. Changing the length of the chains in a thermoplastic material will change its final properties and how easily it can be shaped when it is melted.

  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
  Some of the polymers, because of their geometry, pack together very tightly in a regular order when the material is hard and are called "CRYSTALLINE." These polymers usually exhibit a very sharp melting point; that is, they are solid. Then with a small increase in temperature they become liquid or melt. An illustration of a sharp melting point is the melting of ordinary candle wax. Some examples of crystalline plastic materials are nylon, acetal, polyethylene, and polypropylene. The crystalline polymers provide superior properties, but they tend to shrink a considerable amount as they cool and reharden.

  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
  The terms "THERMOSETTING" and "THERMOPLASTIC" have been traditionally used to describe the different types of plastic materials. A "THERMOSET" is like concrete. You only get one chance to liquify and shape it. These materials can be "cured" or polymerized using heat and pressure or as with epoxies a chemical reaction started by a chemical initiator.

  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:
Linear Thermoplastics
PVC
Nylon
Acrylic
Polycarbonate
ABS

Thermoplastics Crosslinked after Processing
PEEK
Polyamide-imide
UHMWPE

Thermosets
Phenolics
Epoxies
Melamines

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)
  Additives are selected to be compatible with the material and the process conditions for shaping the material. The improvement of a specific property of a material by the addition of an additive is usually at the expense of some other property. The chemist attempts to keep all of the other material properties as high as possible while achieving the desired improvement in the specific property, such as improved resistance to burning. Some of the additives that are used in thermosets and thermoplastics are antioxidants to improve high temperature stability, antistatic agents, biocides, flame retardants, impact modifiers, friction reducers, foaming agents, fungicides, and ultraviolet stabilizers.

REINFORCEMENTS (improve strength)
  Other additives enhance the strength of a material. Some reinforcing materials are carbon, glass, mica, and aramids. They may be in the form of short fibers, continuous filaments, mats, spheres, flakes, etc. These reinforcements usually increase the material's strength at the expense of impact resistance. The use of reinforcements in plastics permits them to be used at higher temperatures and loads with greater dimensional stability. The freedom of design, high strength, and light weight of composite materials are permitting significant advances in technology in the aerospace and aviation fields. Reinforcements tend to make stock shapes, such as rods, tubes, slabs, etc., more difficult to machine because of increased tool wear.

COLORANTS (change appearance)
  Another group of additives are colorants that provide the desired color to the material. The colorants may be organic dyes or inorganic powder. The colorant chosen must be compatible with the base plastic, shaping process, and the proposed usages for the finished material. For example, a colorant must also withstand high temperatures and be weatherable if the material is to be extruded and then used outdoors. The type of colorant also affects optical properties of transparent materials, such as acrylics, polycarbonate, and styrene. A colorant can make a clear material transparent, transluscent, or opaque.

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
  A designer or engineer will often use design equations that work with metals while a part is being designed. Metals behave like a spring; that is, the force generated by the spring is proportional to its length. A plot (FIGURE 2) of the force as a function of length is a "straight line."

Figure 2

  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
  How does one know if a material will be strong enough for a part? If the loads can be predicted and the part shape is known then the designer can estimate the worst load per unit of cross-sectional area within the part. Load per unit area is called "STRESS" (FIGURE 3).

Figure 3
Figure 3

  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)
  Sometimes a designer knows a part can only bend or deflect a certain amount. If the maximum amount of bending and the shape of the part are known, then the designer can often predict how STIFF a material must be. The measurement of the STIFFNESS of a material is called the "MODULUS" or "MODULUS OF ELASTICITY." The higher the modulus number, the stiffer the material; and conversely, the lower the number, the more flexible the material. The Modulus also changes as the temperature changes. Modulus numbers are also given in pounds per square inch.

TYPICAL TENSILE MODULUS VALUES (PSI)
(at room temperature)
Graphite-epoxy composites 40,000,000
Steel 30,000,000
Aluminum, 1000 series 10,000,000
Epoxy-glass laminates 5,800,000
Polyester-glass reinforced 2,000,000
Nylons, 30% glass reinforced 1,400,000
Acrylics 500,000
Cast epoxy 450,000
Polycarbonate 450,000
Acetal, copolymer 410,000
Polyethylene; high molecular weight 100,000

STRAIN
  The measurement of how much the part bends or changes size under load compared to the original dimension or shape is called "STRAIN." Strain applies to small changes in size.

STRAIN = (Final Length - Original Length)/Original Length
= Change in Length or Deformation/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.
MODULUS = STRESS/STRAIN
or, in other words
MODULUS = Load /change in shape when loaded. (STIFFNESS)

  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:
WHAT KIND OF LOAD THE PART WILL SEE (Tensile, Impact, Fatigue, etc.)
HOW BIG THE LOAD IS
HOW LONG OR OFTEN THAT LOAD WILL BE APPLIED
HOW HIGH AND/OR LOW A TEMPERATURE THE PART WILL SEE
HOW LONG IT WILL SEE THOSE TEMPERATURES
THE KIND OF ENVIRONMENT THE PART WILL BE USED IN. WILL MOISTURE OR OTHER CHEMICALS BE PRESENT?

  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
  The yield point is that point when a material subjected to a load, tensile, compressive, etc. gives (yields) and will no longer return to its original length or shape when the load is removed. Some materials break before reaching a yield point, for example, some glass-filled nylons or die cast aluminum.

  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
  The maximum strength of a material without breaking when the load is trying to pull it apart is shown in Figure 4. This is the system used by the suppliers to report tensile properties in their literature, such as strength and elongation.

Figure 4
Figure 4

  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
  ELONGATION IS ALWAYS ASSOCIATED WITH TENSILE STRENGTH because it is the increase in the original length at fracture and expressed as a percentage. An example would be to pull on a 1 " wide piece of paper that is 4" long. It tears with no visible elongation or nearly 0% elongation. Now do the same thing to a 1" x 4" piece of taffy. It will stretch several times its original 4" length before it fractures. Assume that it is stretched to a 12" length then (12"/4")(100)= 300% elongation (FIGURE 5).

Figure 5
Figure 5

COMPRESSIVE STRENGTH
  The maximum strength of a material without breaking when the material is loaded as shown in Figure 6. Check if the material supplier has the information on compressive strength, since it is not always determined.

  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.

Figure 6
Figure 6

Figure 7
Figure 7

SHEAR STRENGTH
  The strength of a material when the material is loaded as shown in Figure 8. The surfaces of the material are being pulled in opposite directions. Some examples of items that see shear loading are the nail holding a picture on the wall, the cleats of athletic shoes, and tire tread as a car speeds up or slows down.

Figure 8
Figure 8

FLEXURAL STRENGTH
  The strength of a material when a beam of the material is subjected to bending as shown in Figure 9. The material in the top of the beam is in compression (squeezed together), while the bottom of the beam is in tension (stretched). Somewhere in between the stretching and squeezing there is a place with no stress and it is called the neutral plane. A simple beam supported at each end and loaded in the middle is used to determine the flexural modulus given in properties tables. Skis, a fishing pole, a pole vault pole, and a diving board are examples of parts needing high flexural strength.

Figure 9
Figure 9

TORSIONAL STRENGTH
  The strength of a material when a shape is subjected to a twisting load as shown in Figure 10. An example of a part with a torsion load is a screw as it is being screwed in. The drive shaft on a car also requires high torsional strength.

Figure 10
Figure 10

POISSON'S RATIO
  Sometimes a designer will need a value for Poisson's Ratio. This ratio occurs in some of the more complex stress/strain equations. It sounds complicated, but it is simply a way of saying how much the taffy (material) necks down or gets thinner in the middle when it is streched (FIGURE 11). Its value is most often between .3 to .4 for plastic materials. Check supplier literature for specific information.

Figure 11
Figure 11

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 12

Figure 13

Figure 14

Figure 15

Figure 16

  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

 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

  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)
Low alloy hardening steels; wrought, quenched, and tempered 288,000
High strength low alloy steels; wrought, as rolled 80,000
Aluminum casting alloys 55,000
Aluminum alloys, 1000 series 24,000
Polyphenylene Sulfide, 40% glass reinforced 21,000
Acetal, copolymer, 25% glass reinforced 18,500
Nylons, general purpose 12,600
Acetal, homopolymer 10,000
Acrylics 10,000
Acetal, copolymer 8,800
ABS/Polycarbonate 8,000
Polypropylene, general purpose 5,200
Polypropylene, high impact 4,300

Creep
  Visualize large weights being hung on bars of different materials. All materials will experience some initial and immediate deformation or stretching when the load is first applied. As long as the yield point has not been exceeded, a metal sample which acts like a spring will not stretch any more regardless of how long the weight is left on. When the weight is removed, the metal bar will return to its original shape. The length of a "thermoplastic" bar will continue to slowly increase as long as the load is applied. This is called CREEP. The amount of creep increases as the load and/or temperature are increased. Some thermoplastics like nylons will creep more when they have softened because of the presence of moisture. The "crosslinked" or "3D net" structure in "thermosets" resists creep better than thermoplastics. Reinforcements like glass and carbon, which do not creep, greatly reduce the creep of the composite material when mixed with a plastic.
Remember the relationship between stress/strain/modul is:
Modulus = Stress/Strain
  The initial strain or change in length with the weight will give a value for the modulus (this is usually the short term value reported in the property tables for the tensile modulus or flexural modulus). If the weight (stress) is left on over a period of time, the amount of bending or elongation continues to increase and the value for the modulus will decrease with time as shown in Figure 16. This decreasing modulus that is a function of time (and even temperature) is called the "CREEP MODULUS" or "APPARENT MODULUS."
  THIS IS THE MODULUS THAT THE DESIGNER SHOULD BE USING TO MORE ACCURATELY PREDICT THE BEHAVIOR OF THE PLASTIC MATERIALS. THE VALUE CHOSEN FROM THE SUPPLIER'S LITERATURE WILL BE BASED ON THE ESTIMATED TIME THE LOAD WILL BE APPLIED, THE AMOUNT OF THE LOAD, AND THE TEMPERATURE CONDITIONS PRESENT WHEN THE LOAD IS TO BE APPLIED.


Figure 19

REMEMBER THAT CREEP IS AFFECTED BY:
LOAD (STRESS)
TEMPERATURE
LENGTH OF TIME THE LOAD IS APPLIED
OTHER ENVIRONMENTALS, SUCH AS MOISTURE OR CHEMICALS

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 20

Figure 21

  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.
THESE VALUES WILL BE THE STRESS LEVELS RECOMMENDED AS DESIGN CRITERIA.


Figure 22

  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.


Figure 23

FATIGUE STRENGTH
  Plastics, as well as other materials, subjected to cyclic loading will fail at stress levels well below their tensile or compressive strengths. The combination of tension and compression is the most severe condition. This information will be presented in S-N Curves or tables. The S-N stand for Stress-Number of cycles. A PART WILL SURVIVE MORE CYCLES IF THE STRESS IS REDUCED. The stress can be reduced by reducing the deflection and/or decreasing the thickness of a part.

  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.


Figure 24

IMPACT STRENGTH
  Many plastics demonstrate excellent impact strength. Impact strength is the ability to withstand a suddenly applied load. Toughness is usually used to describe the material's ability to withstand an impact or sudden deformation without breaking. No single test has yet been devised that can predict the impact behavior of a plastic material under the variety of conditions to which a part can be subjected. Many materials display reduced impact strength as the temperature is lowered. Thermosets and reinforced thermoplastics may change less with changes in temperature. Check the supplier literature for any unusual factors that may affect the impact performance of a part.

  Some of the impact tests commonly used in supplier literature are:


Figure 25

  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.

Gardner Impact Test: drops a shaped weight and determines the energy required to break the test sample.

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.


Figure 26

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.


Figure 27

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.


Figure 28

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
  With a change in temperature, plastics materials tend to change size considerably more than other materials, such as steel, ceramics, and even aluminum. A designer must consider these differences in the sizes. In fact, the shipping environment may expose the part to a much greater temperature variation than the part will ever see in use. The measure of how much a part changes size as the temperature changes is called the "THERMAL COEFFICIENT OF EXPANSION".

COEFFICIENT OF EXPANSION
  The units are usually given in inches per degree Fahrenheit. It is the change in length (inches) of one inch of a part caused by changing the temperature one degree.

TYPICAL COEFFICIENTS OF EXPANSION (in/in/F)
Polyethylene .000140
Acrylics .000060
Acetal, copolymer .000047
Polycarbonate .000037
Aluminum, 1000 series .000013
Polycarbonate, 30% glass reinforced .000009
Steels .000008
Glass .000004

Example: assuming an acrylic material, how much will a 10 inch dimension change if the temperature changes 40°F?

The change in length = Original length x the coefficient of expansion x the change in temperature
= 10 x .00006 x 40 = .024 inches

DEFLECTION TEMPERATURE UNDER LOAD
  In addition to changing size, the strength and modulus of elasticity of plastic materials tend to decrease as the ambient temperature increases. The standard test for determining the DEFLECTION TEMPERATURE UNDER LOAD (DTUL) at 66 and 264 psi provides information on the ability of a material to carry a load at higher temperatures. The 66 psi means a light load and the 264 psi means a heavy load on a beam. The temperature of the loaded beam is raised until a certain amount of deflection is observed. The temperature when that deflection is reached is called the DTUL. Plastics usually have a higher DTUL at 66 psi than 264 psi because of the lower load.
Note: The DTUL is sometimes referred to as the Heat Distortion Temperature or HDT.


Figure 29

TYPICAL DEFLECTION TEMPERATURES, LOADED TO 264 psi (F)
Silicon materials 850
Nylons, 30% glass reinforced 495
Epoxy, mineral, glass reinforced 400
Acetals, glass reinforced 325
Polycarbonates 295
Nylons, general purpose 220
Acrylics 180
Propylene, general purpose 140


  Impact strength is also affected by changes in temperature in most plastic materials. The changes in strength can be significant, especially as the temperature is lowered. Check the supplier literature carefully.

THERMAL CONDUCTIVITY
  Plastics are good thermal insulators; that is, heat does not travel through them easily. We experience this every time we pick up a hot pan by its plastic handle. The "CONDUCTIVITY" of plastics is 300 to 2500 times poorer than most metals. This property shows why it takes a long time for a casting or other molded parts to cool down in the middle. Internal stress can be set up in a material because of the differences in the cooling rates between the outside of a part and the core.

EFFECTS OF THE ENVIRONMENT ON PLASTICS
  Environmental factors, such as ambient moisture, chemicals (liquid or vapor), exposure to sunlight, high temperatures, hot water and/or steam, bacterial/fungi (underground conditions), and irradiation all tend to attack plastic materials. Materials may not only change appearance, but have a significant decrease in properties, such as impact and tensile strength. Again check the supplier's literature carefully.

  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
  Commercial plastics are generally very good electrical insulators and offer freedom of design in electrical products. Electrical properties may also be changed by environmental conditions, such as moisture and/or temperature.

  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
  The Volume Resistivity is defined as the ratio between the voltage (Direct Current or DC), which is like the voltage supplied by a battery, and that portion of current which flows through a specific volume of the specimen. Units are generally ohm per cubic centimeter.

  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).


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
  The Surface Resistivity is the ratio between the direct voltage (DC) and current along the surface per unit width. Units are generally ohms.

  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.


Figure 31

DIELECTRIC CONSTANT
  The Dielectric Constant is the ratio of the capacitance (AC voltage) of electrodes with the insulating material between them to the capacitance of the same electrodes with a vacuum or dry air in between.

  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
  Dielectric Strength is the voltage difference (DC) between two electrodes at which electrical breakdown occurs and is measured as volts per mil of thickness. This is an indication of how effective an "insulator" the material is.

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.


Figure 32

DISSIPATION FACTOR
  The Dissipation Factor (AC) is the tangent of the loss angle of the insulating material. It can also be described as the ratio of the true in-phase power to the reactive power, measured with voltage and current 90 degrees out of phase.

  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.


Figure 34

ARC RESISTANCE
  The Arc Resistance is the elapsed time in which the surface of the material will resist the formation of a continuous conductive path when subjected to a high-voltage (DC), low-current arc under rigidly controlled conditions.


Figure 35

EMI/RFI
  There is also considerable effort being expended by material suppliers to try and improve the conductivity of plastics for applications requiring EMI (electromagnetic interference) and RFI (radio frequency interference) shielding. This becomes more and more critical as circuitry is getting smaller and denser. The improvement in conductivity is currently achieved by adding carbon fibers, metal fiber, and/or metal flakes as a filler in the material or coating the plastic part with conductive paint.

  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
  Many plastic materials are transparent and used in optical applications. Some of these materials are acrylics, styrene, PVC, polycarbonate, ABS, and Epoxy. The properties measured and presented in the material suppliers literature are concerned with items, such as the % Haze (cloudiness) in a material, the transmittance capability (how much light gets through the material), yellowness index (appearance), and the index of refraction (how much light is bent as it goes into and out of the material)

  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
  Wear characteristics of a material are very difficult to define. It can mean being resistant to scratching when the part is cleaned. It might mean being resistant to abrasion when the wind blows sand against it. It might mean running another part against it. It might mean being able to maintain its appearance after considerable handling.

  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
  Plastic stock shapes may be easily machined; however, the tool geometry and speed must be adjusted for optimum performance with a specific material. The tolerances for machining plastics usually should be larger than applied to metals. The tolerances must be larger because of thermal expansion and the shape changing from the relaxation of internal stresses within the material. In critical applications, it may be necessary to premachine the part slightly oversize and STRESS RELIEVE or ANNEAL the part before taking the final cuts.

  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
  Many designers will ARBITRARILY put a +/-.005 tolerance on a part if it is to be machined. Quiz the designer if the tolerances can't be increased. Remember that a piece of paper is about .003 inch thick, +/- .06 is equal to 1/16 of an inch, and +/- .13 equals 1/8 of an inch. Look at a ruler to visualize the size of the tolerance and think about the tools available to make the cut. Work with the designer to specify the tolerances really needed to make his part work and that can really be produced with the equipment available.

PROCESSING
  Plastics are changed into useful shapes by using many different processes. The processes that are used to mold or shape thermoplastics basically soften the plastic material so it can be injected into a mold, flowed through a die, formed in or over a mold, etc. The processes usually allow any scrap parts or material to be ground up and reused. Some of the more common processes are injection molding, extrusion, blow molding, rotational molding, calendering, thermoforming (which includes vacuum forming), and casting.

INJECTION MOLDING
  "Injection Molding" is used to make three dimensional shapes with great detail. The material is placed in the hopper of an injection molding machine where it is fed into a chamber to be melted. The melting is achieved by conducting heat into the material in a "Plunger" machine, while the material is primarily heated by shearing or mechanically working the material in a "Screw" machine. Several shots of material are being heated and held in the injection unit. The maximum volume of material a machine can inject in a single shot determines its shot capacity. The capacity is given in ounces of a material.

  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
  "Extrusion" is like squeezing toothpaste out of its tube. The process produces continuous two dimensional shapes like sheet, pipe, film, tubing, gasketing, etc. The material is fed into the extruder where it is melted and pumped out of the extrusion die. The die and the take-off line shape the material as it cools and control the final dimensions of the cross-section of the shape. The equipment is designed and controlled to produce melted plastic at a very uniform temperature and pressure which control the size and quality of the extruded product.

  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
  An extruded or cast sheet can be heated, draped over a mold, and allowed to cool to produce a part. This process is called thermoforming. The material can be made to better conform to the shape of a mold by using a vacuum to pull the material down. A bubble or shape can also be blown up with air pressure. These are but two of the techniques that can be used to push the material into some desired shape. They basically require that the material be softened so a low force can be applied to shape the part. Signs, skylights, bubble packaging, boat and motorcycle windshields are some examples of parts made using this process.

CALENDERING
  Calendering is a process that usually uses four heated rolls rotating at slightly different speeds. Again the material is fed into the rolls, heated and melted, and then shaped in sheet or film. PVC is the most commonly calendered material.

CASTING
  Acrylic and nylons can also be cast. Just as the name implies, the material in a liquid form is poured into a mold and hardened. The process requires considerable process control to obtain high quality parts. Tubing, rods, sheets, and slabs are often made this way.

THERMOSETS
  Thermosets must use a process that allows the material to flow to the desired shape and then become crosslinked and rigid. The material cannot be remelted or reused after crosslinking occurs. Some of the processes commonly used to process thermoset materials are injection molding, transfer molding, compression molding, hand (or spray) lay-up, lamination, and filament winding.

  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
  In transfer molding, only enough material for one shot is placed in a separate chamber or pot. The material is then pushed from the pot into the hot mold and crosslinked. All of the "cured" material is removed from the machine and another charge loaded for the next shot.

COMPRESSION MOLDING
  A single charge of material is placed directly into the cavity of the heated mold. The material flows and fills the cavity as the mold closes. The mold is kept closed until the material crosslinks. All of the cured material is removed from the mold prior to recharging the cavity.

HAND (OR SPRAY) LAY-UP
  Hand lay-up is used to produce products, such as fiberglass boats and camper shells. The plastic resin, usually a polyester, is rolled or sprayed with glass reinforcement into a mold. A catalyst is added to the material to cause the material to crosslink or harden at room temperature. This process lends itself to making large and strong parts.

LAMINATING
  Thermosets are also used in making laminates. The materials to be laminated are stacked in a press, clamped, and heated. Some examples of laminates using thermosets are plywood (the adhesive), electronic circuit boards, cloth reinforced phenolic sheet, and counter top laminates.

FILAMENT WINDING
  Filament winding is an automated version of the hand lay-up process. Reinforcing filaments are covered with a resin and then wound over a mandrel. The number of layers and orientation can be varied depending on the load that the part is to carry. A strong thin hollow part is left after the mandrel is removed. Storage tanks and street lighting poles are some examples of filament wound parts.

  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
  The selection of a material for an application is a very difficult task. Usually one is only able to narrow the selection down to two or three candidates and the final selection is then determined by testing. SOMETIMES THE SELECTION IS DETERMINED BY THE BEST MATERIAL IMMEDIATELY AVAILABLE SO THE SCHEDULE CAN BE MET OR THE LEAST EXPENSIVE MATERIAL. This DOES NOT always lead to a successful application or a satisfied customer.

  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?
  Will the design carry high loads? What will the highest load be? What is the maximum stress in the part?What kind of stress is it (tensile, flexural, etc.)? How long will the load be applied? What is the projected life of the part or design?

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?
  Will the part be subjected to impact? Which impact test/data more nearly duplicates the projected application?

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)?
  Will the part be subjected to a variable load? Is the load alternating compressive/ tensile? What will the stress levels be? What is the thickness of the part being flexed?How much will the part be deflected?

Note: Materials like acetal and nylon are generally good candidates for cyclic loading.

WHAT TEMPERATURES WILL THE PART SEE AND FOR HOW LONG?
  What is the maximum temperature the material will see in use? What is the minimum temperature the material will see in use? How long will the material be at these temperatures? Will the material have to withstand impact at the low temperature?

Note: The temperature extremes could occur during shipping.

WILL THE MATERIAL BE EXPOSED TO CHEMICALS OR MOISTURE?
  Will the material be exposed to normal relative humidity? Will the material be submerged in water? If so, at what temperature? Will the material be exposed to steam? Will the material be painted? Will the material be submerged or wiped with solvents or other chemicals? If so, which ones? Will the material be exposed to chemical or solvent vapors? If so, which ones? Will the material be exposed to other materials that can outgas or leach detrimental materials, such as plasticizers?

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?
  Will the material be expected to perform as a bearing? If so, what will the load, shaft diameter, shaft material, shaft finish, and rpm be? What wear or abrasion condition will the material see?

  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?
Are rubberlike properties needed?Does the material have to stretch?

  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?
  Is an Underwriter's Laboratories (UL) listed material required? If so, which rating? Is a UL yellow card required? Is a low smoke generating material required (FAA)? Is an FDA approved material required (taste/odor)?

  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?
  Will the material be exposed to radiation? If so, how much and how long?

  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?
  What color material is desired? Does it have to match anything else? Is a textured surface needed?

  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?
  Does the material need to be transparent? Does the material need to transmit any particular wavelengths? If so, which ones?

  Note: Acrylics and polycarbonates have excellent optical properties.

WILL THE PART BE USED OUTDOORS?
Note: Acrylics have excellent weatherability.

CAN ANY VOLATILES BE GIVEN OFF BY THE MATERIAL?
Note: This is often referred to as outgassing.

Nobody

 



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