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:
A, B, C
Some examples of grafted copolymers are styrene-butadiene, styrene-acrylonitrile, and some acetals.
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.
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.
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.
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.
examples of the various types of materials:
Thermoplastics Crosslinked after Processing
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.
(improve specific properties)
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.
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.
If Force or Load is in pounds and area is in square inches then the units for stress are pounds per square inch.
PROFESSIONAL PLASTICS, INC.
TENSILE MODULUS VALUES (PSI)
= (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
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.
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:
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.
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.
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.
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)
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.
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.
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.
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.
TYPICAL COEFFICIENTS OF EXPANSION (in/in/F)
change in length = Original length x the coefficient of expansion x the change
TEMPERATURE UNDER LOAD
TYPICAL DEFLECTION TEMPERATURES, LOADED TO 264 psi (F)
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.
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:
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
(OR SPRAY) LAY-UP
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.
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.
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.
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.
THE PART SEE CYCLIC LOADING (FATIGUE)?
Note: Materials like acetal and nylon are generally good candidates for cyclic loading.
TEMPERATURES WILL THE PART SEE AND FOR HOW LONG?
Note: The temperature extremes could occur during shipping.
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.
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..
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.
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.
THE PART BE EXPOSED TO ANY RADIATION?
Note: This requirement could occur for military, utility (atomic power plants), or medical applications.
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.
THE PART HAVE ANY OPTICAL REQUIREMENTS?
Note: Acrylics and polycarbonates have excellent optical properties.
THE PART BE USED OUTDOORS?
ANY VOLATILES BE GIVEN OFF BY THE MATERIAL?