8.1.3 Basic design considerations
In order to avoid unpleasant surprises which can cause a design to fail, it is necessary to know everything possible about the conditions which the product will be exposed to in its lifetime. Armed with that infor- mation, the plastics designer can determine if the design, material, process, and tooling are appropriate for the application. That is, at least to the limits of the available information. A certain degree of risk is inherent in plastics design because the cost in time and resources is too great to permit the accumulation of enough information to elimi- nate that risk. Higher levels of risk are acceptable where tooling investment is low and where product failure results only in very low levels of property loss. As the cost of failure increases, more resources are devoted to risk reduction and greater safety factors are used. When product failure could result in serious injury or loss of life, exhaustive testing and greater safety factors are employed.
Most product structural failures result from conditions the designer did not anticipate. Thus, the first order of business is to establish the design parameters. This is achieved by the rather tedious process of considering all the conditions the product will be exposed to. A check- list is a useful means of reminding the designer of all these conditions
(Table 8.1). This is a general checklist, and the individual designer will no doubt find it necessary to make additions appropriate to his or her specific product area.
Note 2 of the checklist in Table 8.1 serves to remind the designer that not all of the conditions encountered by the product occur in its use. The highest temperature the product may be called upon to endure may actually occur in a boxcar in the desert in midsummer, in cleaning, or in a mandated test procedure. Decorating, cleaning, or assembly may result in unforeseen chemical exposures. The author recalls one project where he was “blindsided” by the discovery, long after production had commenced, that the product was washed in an acid solution which was followed by a base solution out in the field. Fortune smiled on that occasion and the material used happened to be one which could withstand those exposures.
Impacts may also occur under odd circumstances. One time, the author found that the sales department of a company had brought a baseball bat to the product’s introduction and was merrily inviting cus- tomers to take a swing at the housing to demonstrate its superiority over the competitors’ metal housing. The engineers joined in a silent nondenominational prayer that the housing would be able to with- stand 3 days of that kind of treatment.
Apparently their prayer was answered and the product survived the onslaught. However, there is a well-known story in the automobile industry about a bumper mold which had to be rebuilt because the engineers did not account for the heat of the oven which dried the paint. The original mold had been built for a material which would not withstand the oven temperatures and the resin with a higher temper- ature resistance had a significantly different mold shrinkage. The mold could not be salvaged and a new one had to be made.
Fear often strikes the heart of those who are called upon to fill out a checklist, and they find themselves specifying higher levels of per- formance than is really required. Performance has a price and that practice leads to unnecessary cost, particularly when the designer incorporates a safety factor to ensure that the required standard is met. (Safety factors are added in the design stage.) In some cases, requirements are raised to a level which causes a more expensive material or process to be used. Occasionally, they are lifted to a level which cannot be reached using plastics.
Section A of Table 8.1, “Physical Limitations,” sets the basic para- meters of the design. The sizes alone are enough to eliminate some of the processes. Section B, “Mechanical Requirements,” will reveal some interesting aspects of the design not often considered. For example, there is the question of whether it is more important to have a long functional life, a static life, or a shelf life.
Sometimes one type of performance must be sacrificed to another. An example of this type of phenomenon might be a shaft seal in place on a product. Its shelf life would be the length of time the product could remain unused and still have the seal function properly when used. Its static life would be the length of time it would be able to function after it had been first used. Finally, its functional life would be the period of time or number of rotations the seal could withstand while the shaft was rotating. Bear in mind that the temperatures would rise when the shaft was turning and abrasion would also be taking place. It is, there- fore, quite likely that the ideal polymer for one circumstance might be less favorable for another. Decisions must often be made.
Which of the product requirements in this section is most likely to result in a structural failure? Of course any of them can or they would not be listed. However, in the author’s experience, the most common failures (short of gross design errors) occur due to weakening of the material at elevated temperature, impact failure at low temperature, or creep failure over time.
Designers often forget that plastics are, themselves, chemical com- pounds. Hence they are particularly vulnerable to environmental exposures. Section C in Table 8.1, “Environmental Limitations,” attempts to reveal the exposures which could result in failure. The chemical exposures are the most difficult to discover because there are so many of them and they are often hidden from view. Many common cleaning, cosmetic, and food preparations contain unusual and secret ingredients. Nonetheless, we identify what we can and list the com- mercial names for the others. In some cases, resin manufacturers have actually tested their polymers in these commercial preparations.
Often these exposures take place in combination as in products to be used out of doors. In fact, design for products to be used outdoors is practically a category in itself as ultraviolet light significantly affects most plastics over time. In a few cases, even indoor applications have been severely affected.
Section D in Table 8.1, “Electrical Requirements,” is largely limited to electrical applications. However, Section E, “Appearance Requirements,” affects nearly every application to some degree. Designers often overlook these details in the early stages of the project because they are preoccupied with the structural aspects of the design. Nonetheless, appearance requirements can lead to expensive mold changes. For example, failure to select the proper mold finish can require a change in the draft angle. That can make the part larger, affecting the fitments and so on. Section F in the table deals with assembly requirements.
There are some other design parameters as indicated in Section G. For example, the anticipated volume will often dictate the process to be used. Section H, “List All Tests to Be Performed on This Product,” may reveal test protocols more stringent than the demands on the product in the field, particularly if they are archaic in nature.
8.1.4 Material selection
The early chapters of this book cover the plastics materials in depth. For a thorough discussion of each polymer, the author recommends that the reader refer to these chapters. This section looks at material selection from the designer’s perspective.
It is said that there are some 30,000 to 35,000 plastic compounds on the market as this is being written, with more being added all the time. That number is enough to stagger the mind of the designer try- ing to make a material selection. Fortunately, only a small percentage of them are actually serious contenders for any given application. Some of them were developed specifically for a single product, partic- ularly in the packaging industry. Others became the material of choice for certain applications because of special properties they offer which are required for that product or process. For example, the vast major- ity of roto-molded parts are made of polyethylene, while glass fiber–reinforced polyester is the workhorse of the thermoset industry. A bit of research should reveal if there is a material of choice for any given product application.
First, a bit of a review of the basic categories of plastics materials. In general, they fall into one of two categories: thermosets and ther- moplastics. Thermosets undergo a chemical reaction when heated and cannot return to their original state. Consequently, they are chemical resistant and do not burn. Cross-linked plastics are an example of thermosets. Thermoplastics constitute the bulk of the polymers avail- able. Although some degradation does occur, they can be remelted. Most are readily attacked by chemicals and they burn readily.
Thermoplastics can also be broken down into two basic categories: amorphous and semicrystalline (hereafter referred to as crystalline). The names refer to their structures; amorphous have molecular chains in random fashion and crystalline have molecular chains in a regular structure. Polymers are considered semicrystalline because they are not completely crystalline in nature. Amorphous resins soften over a range of temperatures whereas crystallines have a definite point at which they melt. Amorphous polymers can have greater transparency and lower, more uniform postmolding shrinkage. Chemical resistance is, in general, much greater for crystalline resins than for amorphous resins, which are sufficiently affected to be solvent welded. The trian- gle illustrated in Fig. 8.2 provides an easy way to categorize the ther- mosplastics.
The cost of plastics, generally, increases with a corresponding improve- ment in thermal properties (other properties, typically, go up as well). The lowest cost plastics are the most widely used. The triangle in Fig. 8.2 is organized with the least temperature-resistant plastics at the base and those with the highest temperature resistance at the top. Therefore, the plastics designated “Standard” at the base of the triangle, often referred to as commodity plastics, are the lowest in cost and most wide- ly used. They can be used in applications with temperatures up to 150°F. (Note that these are very loose groupings and the precise properties of a specific resin must be evaluated before specifying it.)
Figure 8.2 Classification of thermoplastics. (Source: Laura Pugliese, Defining Engineering Plastics, Plastics Machining and Fabrication, January-February, 1999, Courtesy DSM Engineering Plastic Products.)
The next level are the “Engineering” plastics which can be used for applications ranging up to 250°F. ABS is often considered an engineer- ing plastic for its other properties although it cannot withstand this temperature level. For applications requiring temperature resistance up to 450°F, the next step, “Advanced Engineering,” is appropriate. The amorphous plastics at this level are often used in steam environ- ments and the crystalline plastics have improved chemical resistance. The top level, the “Imidized” plastics, can withstand temperatures up to 800°F and have excellent stress and wear properties as well.
Table 8.2 lists many of the principle properties and some of the poly- mers which are noted for those properties. While incomplete, this table should at least provide a beginning. They are primarily listed in their natural state without reinforcements, such as glass or carbon fibers. These reinforcements can be used to increase mechanical strength,
TABLE 8.2 Recommended Materials
Abrasion, resistance to (high) Nylon
Cost:weight (low) Urea, phenolics, polystyrene, polyethylene, polypropylene, PVC
Compressive strength Polyphthalamide, phenolic (glass), epoxy, melamine, nylon, thermoplastic polyester (glass), polyimide
Cost:volume (low) Polystyrene, polyethylene, urea, phenolics, polypropylene, PVC
Dielectric constant (high) Phenolic, PVC, fluorocarbon, melamine, alkyd, nylon, polyphthalamide, epoxy
Dielectric strength (high) PVC, fluorocarbon, polypropylene, polyphenylene
ether, phenolic, TP polyester, nylon (glass), polyolefin, polyethylene
Dissipation factor (high) PVC, fluorocarbon, phenolic, TP polyester, nylon, epoxy, diallyl phthalate, polyurethane
Distortion, resistance Thermosetting laminates to under load (high)
Elastic modulus (high) Melamine, urea, phenolics
Elastic modulus (low) Polyethylene, polycarbonate, fluorocarbons Electrical resistivity (high) Polystyrene, fluorocarbons, polypropylene Elongation at break (high) Polyethylene, polypropylene, silicone, ethylene vinyl
Elongation at break (low) Polyether sulfone, polycarbonate (glass), nylon (glass), polypropylene (glass), thermoplastic polyester, polyetherimide, vinyl ester, polyetheretherketone, epoxy, polyimide
Flexural modulus (stiffness) Polyphenylene sulfide, epoxy, phenolic (glass), nylon (glass) polyimide, diallyl phthalate, polyphthalamide, TP polyester
Flexural strength (yield) Polyurethane (glass), epoxy, nylon (carbon fiber) (glass), polyphenylene, sulfide, polyphthalamide, polyetherimide, polyetheretherketone, polycarbonate (carbon fiber)
TABLE 8.2 Recommended Materials (Continued)
Friction, coefficient of (low) Fluorocarbons, nylon, acetal
Hardness (high) Melamine, phenolic (glass) (cellulose), polyimide, epoxy
Impact strength (high) Phenolics, epoxies, polycarbonate, ABS Moisture resistance (high) Polyethylene, polypropylene, fluorocarbon,
polyphenylene sulfide, polyolefin, thermoplastic polyester, polyphenylene ether, polystyrene, polycarbonate (glass or carbon fiber)
Softness Polyethylene, silicone, PVC, thermoplastic elastomer, polyurethane, ethylene vinyl acetate
Tensile strength, break (high) Epoxy, nylon (glass or carbon fiber), polyurethane, thermoplastic polyester (glass), polyphthalamide, polyetheretherketone, polycarbonate (carbon fiber), polyetherimide, polyether-sulfone
Tensile strength, yield (high) Nylon (glass or carbon fiber), polyurethane, thermoplastic polyester (glass), polyetheretherketone, polyetherimide, polyphthalamide, polyphenylene sulfide (glass or carbon fiber)
Temperature, heat deflection Phenolic, epoxies, polysulfone, thermoset polyesters, polyether sulfone, polyimide (glass)
Temperature (maximum use) Fluorocarbons, phenolic (glass), polyphthalamide, polyimide thermoplastic polyester (glass), melamine, epoxy, nylon (glass or carbon fiber), polyetheretherketone, polysulfone, polyphenylene sulfide
Thermal conductivity (low) Polypropylene, PVC, ABS, polyphenylene oxide, polybutylene, acrylic, polycarbonate, thermoplastic polyester, nylon
Thermal expansion, Polycarbonate (carbon fiber or glass), phenolic (glass), coefficient of (low) nylon (carbon fiber or glass), thermoplastic polyester (glass), polyphenylene sulfide (glass or carbon fiber),
polyetherimide, polyetheretherketone, polyphthalamide, alkyd, melamine
Transparency, permanent Acrylic, polycarbonate
Weight (low) Polypropylene, polyethylene, polybutylene, ethylene vinyl acetate, ethylene methyl acrylate
Whiteness retention (high) Melamine, urea
maximum use temperature, impact resistance, stiffness, mold shrink- age, and dimensional stability.
Generally, the resin prices increase with improved mechanical and thermal properties. When there is no clear-cut material of choice, plas- tics designers generally follow the practice of looking for the lowest- cost material which will meet the product’s requirements. If there is a reason that polymer is not acceptable, they start working up the cost ladder until they find one that will fulfill their needs. In thermoplas- tics, there are the so-called commodity resins. These are the low-cost
resins used in great volume for housewares, packaging, toys, and so on. This group is made up of polyethylene, polypropylene, polystyrene, and PVC. Reinforcements can improve the properties of these resins at moderate additional cost. A lower-priced resin with reinforcement will often provide properties comparable to a more expensive resin. Table
8.3 is a list of the approximate cost of a number of plastics in increas-
TABLE 8.3 Approximate,* in Dollars per
Cubic Inch of Plastics
Polyethylene (HD) 0.010
Polyethylene (LD) 0.014
Thermoplastic elastomer 0.039
Styrene acrylonitrile 0.043
Polyester (TS) 0.043
Polyphenylene ether 0.050
Polyphenylene oxide 0.050
Polyester (PET) 0.055
Styrene maleic anhydride 0.059
Vinyl ester 0.060
Polyurethane (TS) 0.062
Polyurethane (TP) 0.074
Polyphenylene sulfide 0.119
Dially phthalate 0.154
Liquid crystal polymer 0.434
*These values are very approximate. They were arrived at by multiplying the average density by the average price at the time this was being writ- ten. In many cases, the range from which the average was taken was quite wide.
ing order of cost per cubic inch. This is regarded as a more useful fig- ure than cost per pound in selecting a plastic material.
Thermosets usually provide higher mechanical and thermal proper- ties at a lower material cost than do thermoplastics, “more bang for the buck” so to speak. However, most of the processes used to fabricate thermoset parts are slower and more limited in design freedom than the thermoplastic processes. Furthermore, the opportunity to utilize
100% of the material which thermoplastics provide is simply not avail- able with thermosets because the regrind cannot be reused. Recycling possibilities are far more limited for thermosets for the same reason. Nonetheless, glass fiber–reinforced thermoset polyester is the materi- al of choice for many outdoor applications in a severe environment such as boats and truck housings.
About the data. Comparison of resins is usually done with data sheets supplied by the resin manufacturers. It is extremely important that the plastics design engineer understand the limitations of this data. Since the properties of polymers change with temperature, the data sheet does not provide the total picture of a given compound. Instead, think of it as a “snapshot” of the material taken at 72°F. As the tem- perature goes down from this point, the material becomes harder and more brittle. Increasing the temperature makes the polymer softer and more ductile. These are general statements and the effect of tem- perature will vary widely between resins. For one material, tensile strength at 140°F may be only half that at 72°F. For another polymer, it may change only slightly.
The graph depicted in Fig. 8.3 illustrates this phenomenon. The upper curve indicates that the value at 0°F is 14,000 lb/in2. At 72°F, it has dropped to around 12,000 lb/in2. By the time it reaches 140°F, the ten- sile yield strength is approximately 7000 lb/in2. This data is for nylon, a polymer particularly affected by moisture. The lower curve illustrates the effect of 2.5% moisture. In the range of temperatures between 30 and 100°F, the tensile yield strength appears to be about 20% lower for the moist material. Note that the curves begin to run together beyond
150°F as most of the water has been driven off by that point.
Time is also a significant factor. Figure 8.4 illustrates the effect of time on the stress-strain relationship of Delrin, an acetal polymer, at room temperature. Note that the strain rate increases with time. Figures 8.5 and 8.6 demonstrate how the strain rate of this material increases as the temperature rises.
Most of the physical properties, even properties such as the coeffi- cient of friction or the coefficient of linear thermal expansion, can change significantly with changes in temperature, although not nec- essarily to the degree depicted in the graphs for these materials.
Temperature and humidity differences are not the only phenomena which can affect the data. Chemical and ultraviolet light exposure will also affect most resins, in some cases to a very high degree.
Interpreting the test data. This section discusses plastics test data from the point of view of the plastic product designer. For a far deeper dis- cussion of the topic, see Chap. 11.
It is important to recognize that the test data represent a very precise set of circumstances, those established by the test protocol. Product designers must be aware of exactly how the test is performed in order to determine how well the results relate to the conditions experienced by the product under development. The following sections discuss the test procedures for the mechanical and thermal properties most commonly required. Table 8.4 represents a typical property sheet as supplied by the resin manufacturer. The material is Celcon® M90™ acetal copolymer (polyoxymethylene), a high-performance, engineering thermoplastic. For the plastic part designer to use this data effectively, he or she must thoroughly understand exactly what information it provides and the limits of that information. The following discussion will proceed to review the properties provided with a brief description of the test proce- dure and a few comments from the author.
Specific gravity—ASTM D792. This is a simple test in which a piece of the material is weighed, submerged in water, and weighed again to determine the difference in its weight. This test is not normally per- formed until 24 h after molding to permit most of the postmolding shrinkage to take place. It is important to consider the specific gravity (or density) in establishing the actual cost of a resin. It is also used to determine the extent of packing in molded parts. The value of this resin, 1.41, is in the upper range for unfilled thermoplastics.
Water absorption: 24 h—ASTM D570. The specimens for this test are 0.125 in thick and 2.00 in in diameter for molding materials. The material is submerged in water and the increase in weight is measured. This test can be performed with several procedures ranging from 1 h in boiling water to 24 h in water at 73.4°F. The physical and electrical properties of plas- tics can be affected by moisture absorption. The value for this resin,
0.22%, is quite low, too low to have a significant effect on properties.
Mold shrinkage—ASTM D955. The mold shrinkage is the difference between the mold dimensions and the molded part. This is established by measuring the part after molding it according to a prescribed set of molding parameters and cooling it for a short period of time. It does not account for all of the shrinkage because shrinkage can continue for up to
48 h for some resins. Furthermore, shrinkage is significantly affected by the wall thickness, shape, and size of the part in addition to the molding temperature, cycle, nozzle size, and packing. The values for this resin,
0.022 in/in in the flow direction and 0.018 in/in in the transverse direc- tion, are typical for unfilled crystalline thermoplastics.
Tensile test—ASTM D638. The first mechanical property most product designers look for in evaluating a potential material is its strength,
and by this they mean its tensile strength at yield or break. Therefore, it is often found at the top of the data sheet. The principal test for this property is ASTM D638; it calls for a “dog bone”–shaped specimen
8.50 in long by 0.50 in wide. The gripping surfaces at the ends are
0.55 in wide, giving it its characteristic shape. The test protocol permits the thickness to range from 0.12 to 0.55 in, and the rate at which the stress is applied from 0.5 to 20 in/min. This test is also used to obtain the percentage elongation at break and produce the stress-strain curve, from which the modulus of elasticity is derived. The values for this resin are upper midrange for unfilled thermoplastics, but much higher values can be achieved with the use of glass or car- bon fiber fillers.
Flexural properties of plastics—ASTM D790. This test is performed by suspending a specimen between supports and applying a downward load at the midpoint between them. The specimen is a 0.50- by 5.00-in rectangular piece. Thickness can vary from 0.06 to 0.25 in, however,
0.125 in is the most commonly used. The distance between the sup- ports is 16 times the specimen thickness. The load is applied at rates defined by the specimen size until fracture occurs or until the strain in the outer fibers reaches 5%. (Most thermoplastics do not break in this test.) The flexural strength is the flexural stress at 5% strain. In the event of failure before that point, the flexural strength is the tensile stress in the outermost fibers at the break point. The flexural modulus is the ratio of stress to strain within the elastic limit of the material. It is the primary means of measuring the stiffness of a material. Acetals have a wide range of flexural stress values. This one is about upper midrange for acetals and for unfilled thermoplastics in general; however, its flexural modulus is on the high side.
Fatigue endurance—ASTM D671. The fatigue endurance of a material is its ability to resist deterioration due to cyclic stress. The test covers determination of the effect of repeated flexural stress of the same mag- nitude with a fixed-cantilever apparatus designed to produce a con- stant amplitude of force on the plastic test specimen. The results are suitable for application in design only when all of the application para- meters are directly comparable to those of the test. Consequently, this test is primarily used for comparison purposes.
Compressive strength—ASTM D695. Except for foams, plastic products rarely fail from compression alone. Consequently, the compressive strength is of limited value. The apparatus for this test resembles a C- clamp with the specimen compressed between the jaws of the appara- tus, which close at the rate of 0.05 in/min until failure occurs. A wide
range of specimen sizes is permitted for this test. The values for this resin are in the upper range for unfilled thermoplastics.
Rockwell hardness—ASTM D785. Hardness of plastics is difficult to establish and compare because there is an enormous range and there is an elastic recovery as well. However, the Rockwell hardness is use- ful in determining the relative indentation hardness between plastics. An indenter is placed on the surface of the test specimen and the depth of the impression is measured as the load on the indenter is increased from a fixed minimum value to a higher value and then returned to the previous value. A number of different diameter steel balls and a dia- mond cone penetrator are used. The Rockwell scale refers to a given combination of indenter and load. M80 places this resin in the upper middle range of hardness for plastics. A number of scales are used within the plastics industry. Figure 8.7 illustrates the relationship between them.
Figure 8.7 Range of hardness common to plas- tics. (Source: Dominick V. Rosato, Rosato’s Plastics Encyclopedia and Dictionary, Carl Hanser Verlag, Munich, 1993.)
The IZOD impact test—ASTM D256. D256, the IZOD impact test, is the most common variety of impact test. It is not regarded as a reliable indicator of overall toughness or impact strength, however, it is a rea- sonable measure of a plastic’s notch sensitivity. It is a pendulum test with the pendulum dropping from the 12 o’clock position to hit a sam- ple held in a clamp at the 6 o’clock position. The pendulum breaks the sample and the distance it travels beyond the specimen is a measure of the energy absorbed in breaking the sample. The value calculated from this test is usually expressed in foot-pounds per inch of sample width.
The plastics design engineer must be wary of the fact that there are five different methods of performing this test, and the results will vary with each method. The four used by design engineers are as follows:
Method A. The specimen for this method is 2.50 in long by 0.50 in thick. There is a 45° included angle notch at midpoint which is 0.10 in deep and has a 0.01-in radius at the V. The notch faces the pendulum. The impact point is just above the notch.
Method B. This procedure is also known as the “Charpy” test. It is similar to the previous method, except that the bar is laid horizontally and the impact is directly behind the notch. The length of the specimen is increased to 5.0 in for this method.
Method D. The principal difference between this method and method A is that it permits a larger radius at the V of the notch, which substantially affects the results. It is used for highly notch-sensitive polymers.
Method E. In this case, the same size specimen and procedure applied in method A is used, except that the notch faces away from the pendulum.
This resin is notch sensitive and its values are low. The difference between the results of the notched IZOD test and the unnotched IZOD test (when available) can also be used as a measure of notch sensitiv- ity for a given material.
The falling dart (tup) impact test—ASTM D3029. The falling dart test, ASTM D3029, is not on this particular data sheet. However, it may be more appropriate to reveal the behavior of materials on impact for many product applications such as appliance housings and the like. Unfortunately, this test is usually performed only for extrusion grades of resins which are to be made into sheet. Therefore, it may be neces- sary to request that this test be performed on a material under consideration. For testing, a flat specimen is suspended over a circu- lar opening below a graduated column with a cantilever arm attached. A weight, also known as a dart or tup, is attached to the arm, from which it is released to strike the sample. The arm can be
raised or lowered and the weight of the tup varied until 50% of the sample quantity fails the test. Method A of this test calls for an open- ing below the sample which is 5.00 in in diameter. For method B, it is
1.50 in in diameter.
The tensile impact strength—ASTM D1822. The tensile impact test elim- inates the notch-sensitivity aspect of the IZOD impact test and is a more reliable indicator of impact strength for many applications. This test uses an apparatus very similar to that used for the IZOD tests except that, in this case, the specimen is attached to the pendulum on one end and has a T bar attached to the other end. When the pendu- lum drops, the T bar catches on the apparatus at its base causing the specimen to undergo tensile impact. For this test, the specimen is 2.50 in long and necks to 0.125 in at the center. The thickness can vary. The gripping surfaces at the ends are 0.50 in wide. This test is typically performed on materials which are too elastic to fail in the IZOD test and is normally found on data sheets. It can be performed on request if it best represents the product’s performance requirements.
Heat deflection temperature—ASTM D648. This data is dangerous in the respect that it often is the only temperature data provided on a resin data sheet, which leaves the impression that it is a reliable indicator of the limit to which the product can be used (see the section on “Relative Temperature Index” later in this chapter). It is really noth- ing more than the temperature at which a given load (66 or 264 lb/in2) will deflect a specimen an arbitrary amount. Other temperature tests are also used, and some are described in the following sections. Results of those tests are usually available from the resin manufacturer.
The apparatus for this test resembles somewhat that of the flexural test in that the specimen is suspended between two supports 4 in apart with a downward load at the midpoint. However, in this case, the entire structure is immersed in a liquid whose temperature is increased at the rate of 2°C/min. Two loadings are used, 66 and 264 lb/in2. Consequently, the plastics engineer must be careful to compare values for the same loading. The heat deflection temperature is the temperature at which the specimen deflects 0.010 in. The specimen for this test is 0.50 in wide by
5.00 in long. Thicknesses vary from 0.125 to 0.50 in. The values for this resin are fairly high for an unfilled thermoplastic.
Different sample thicknesses and processes can produce significant differences in values. The author recalls a project where the field had been narrowed to two competing materials. One of them had a 15% heat deflection temperature advantage at a slightly higher cost. However, the other was produced by a long-standing supplier and, before taking the business away from that vendor, it seemed only fair to call and ask if a
comparable resin were available which was not in the current brochure. The discussion revealed that the competing material was, in fact, the very same resin which the competitor bought from our supplier and resold as the company’s own brand. Why then the difference in test values? Further research revealed that one supplier had used an injec- tion-molded sample 0.125 in thick and the other had tested an extruded sample 0.50 in thick, which resulted in higher values.
Vicat softening point—ASTM D1525. The Vicat softening point is not provided on this particular data sheet, however, it is a method of deter- mining the softening point of plastics which have no definite melting point. A 1000-g load is placed on a needle with a 0.0015-in2 circular or square cross section. The softening point is taken as the point where the needle penetrates the specimen to a depth of 1 mm.
Glass transition temperature—ASTM D3418. Thermoplastics exhibit a characteristic whereby they change from a material which behaves like glass (strong, rigid, but brittle) to one with generally reduced physical properties (weaker and more ductile). This is known as the glass transition temperature (Tg) and is actually a range of tempera- tures as the value is different for each property and is significantly affected by variations in the test protocol. Usually a single value is provided, therefore, it should be treated as an approximation.
Relative temperature index—UL746B. Underwriter’s Laboratories’ Inc. (UL) has devised a thermal aging test protocol whereby a subject material is tested in comparison with a material with an acceptable service experience and correlates numerically with the temperatures above which the material is likely to degrade prematurely. The end of life of a material is regarded as the point where the value of the criti- cal properties have dropped to half their original values. The resin manufacturer must submit his material to UL to have it tested. When this has not been done, the designer can use the generic value for the polymer, which is usually regarded as conservative.
Shear strength—ASTM D732. Shear strength is rarely a factor in mold- ed and extruded plastic products due to their relatively thick wall sections, however, it can be important in film and sheet products. In this test, 2-in-diameter or 2-in-square specimens, ranging in thickness from 0.005 to 0.500 in are placed in a punch-type shear fixture. Pressure is applied to the punch at the rate of 0.005 in/min until the moving part of the sample clears the stationary part. The force divid- ed by the area sheared determines the shear strength.
There are a number of other properties commonly used to evaluate plastic materials. Space limitations prevent a detailed description of the tests used to establish values for these properties. However, a list- ing of them is provided for the reader to research independently in Table 8.5, and data is often available from the resin manufacturer. More information about plastics tests can be found in the Plastics Engineering Handbook and can be obtained from the American Society
TABLE 8.5 Other Tests of Interest
Coefficient of linear thermal expansion ASTM D696, E228
Stress relaxation ASTM D2991
Tensile, compressive, and
flexural creep and creep-rupture ASTM D2990
Crystallization, heat of ASTM D3417
Cure kinetics of thermosets ASTM D4473
Deformation under load ASTM D621
Dimensional stability ASTM D756
Arc resistance ASTM D495
Dielectric constant ASTM D150
Insulation resistance ASTM D257
Friction, coefficient of ASTM D1894
Flexural fatigue ASTM D671
Tension-tension fatigue ASTM D3479
Density of smoke from burning ASTM D2843
Flooring radiant panel test ASTM E684
Oxygen index ASTM D2863
Smoke emission ASTM E662
Steiner 25-ft tunnel test ASTM E84
Vertical test for cellular plastics ASTM D3014
Flowability of thermosets ASTM D3123
Fracture toughness ASTM D5045
Moisture vapor transmission ASTM D675
Color ASTM E308
Haze ASTM D1003
Specular gloss ASTM D253
Shear modulus ASTM D5279
Viscosity-shear rate ASTM D3835
Abrasion resistance ASTM D1242
Mar resistance ASTM D673
Accelerated ASTM G23, G26, G53
Light and water exposure ASTM D1499, D2565
Outdoor ASTM D1435, E838
for Testing Materials, 100 Barr Harbor Drive, West Conshohoken, PA