Plastics Product Design Part 13 potx

x- 460 Plastics Engineered Product Design critical. Any product designed with these guidelines in mind should conduct tests on the products themselves to relate the guidelines to actual performance. With more experience, more-appropriate values will be developed targeting to use 1.5 to 2.5. After field service of the preliminary designed products has been obtained, action should be taken to consider reducing your SF in order to reduce costs. )b * . ’ Safety factors Type of load Safety factor Static short-term loads 1 to 2.5 2 to 5 Static long-term loads Repeated loads 5tO 15 Variable changing loads 4tO 10 Fatigue loads 5tO 15 Impact loads 10 to 15 Realistic SFs are based on personal (or others) experience. The SPs can be related to the probable consequences of failure. To ensure no failure where a product could be damaging to a person (etc.) prototype tests should be run at their most extreme service operating conditions. For instance, the maximum working load should be applied at the maximum temperature and in the presence of any chemicals that might be encountered in the end use. Impact loading should be applied at the lowest temperature expected, including what occurs during shipping and assembly. The effects of variations in plastic lots and manufacturing conditions must also be considered. Safety Factor Exam p I e Due to the unpredictable scheduling and high dollar costs of all weather natural testing, much of the environmental testing has been brought into laboratories or other such testing centers. Artificial conditions are provided to simulate various environmental phenomena and thereby aid in the evaluation of the test item before it goes into service under natural environments. This environmental simulation and testing does require extensive preparation and planning. It is generally desirable to obtain generalizations and comparisons from a few basic tests to avoid prolonged testing and retesting. The type and number of tests to be conducted, natural or simulated, as usual are dependent on such factors as end item performance require- ments, time and cost limitations, past history, performance safety factors, shape of specimens, available testing facilities, and the environment. Specifications, such as ASTMs' provide guidelines. Since GRPs (glass reinforced plastics) tend not to exhibit a fatigue limit, it is necessary to design for a specific endurance, with initial safety factors in the region of 3 to 4 being commonly used. Higher fatigue performance is achieved when the data are for tensile loading with zero mean stress. In other modes of loading, such as flexural, compression, or torsion, the fatigue behavior can be worse than that in tension due to potential abrasion action between fibers if debonding of fiber and matrix occurs. This is generally thought to be caused by the setting up of shear stresses in sections of the matrix that are unprotected by some method such as having properly aligned fibers that can be applied in certain designs. Another technique, which has been used successfilly in products such as high-performance RP aircraft wing structures, incorporates a very thin, high-heat-resistant film such as Mylar between layers of glass fibers. With GRPs this construction significantly reduces the self-destructive action of glass-to-glass abrasion and significantly increases the fatigue endurance limit. With certain plastics, particularly high performance RPs, there can be two or three moduli. Their stress-strain curve starts with a straight line that results in its highest E, followed by another straight line with a lower S, and so forth. To be conservative providing a high safety factor the lowest E is used in a design however the highest E is used in certain designs where load requirements are not critical. In many plastics, particularly the unreinforced TPs, the straight region of the stress-strain curve is not linear or the straight region of this curve is too difficult to locate. It then becomes necessary to construct a straight-line tangent to the initial part of the curve to obtain a modulus called the initial modulus. Designwise, an initial modulus can be misleading, because of the nonlinear elasticity of the material. For this reason, a secant modulus is usually used to identify the material more accurately. Thus, a modulus could represent Young's modulus of elasticity, an initial modulus, or a secant modulus, each having its own meaning and safety factors. The Young's modulus and secant modulus are extensively used in design equations. The example of a building roof structure represents the simplest type of problem in static loading in that the loads are clearly long term and well defined. Creep effects can be easily predicted and the structure can be designed with a sufficiently large SF to avoid the probability of failure. A seating application is a more complicated static load problem than the building example just reviewed because of the loading situation. The 462 Plastics Engineered Product Design self-load on a chair seat is a small fraction of the normal load and can be neglected in the design. The loads are applied for relatively short periods of time of the order of 1 to 5 hours, and the economics of the application requires that the product be carefully designed with a small safety factor. Overall, it can be stated that plastic products meet the following criteria: their functional performance meets use requirements, they lend themselves to esthetic treatment at comparatively low cost, and, finally, the finished product is cost competitive. Examples of their desirable behaviors can start with providing high volume production. Plastic conversion into finished products for large volume needs has proven to be one of the most cost-effective methods. Combining bosses, ribs, and retaining means for assembly are easily attained in plastic products, resulting in manufacturing economies that are fiequcntly used for cost reduction. It is a case where the art and technology of plastics has outperformed any other material in growth and prosperity. Their average weight is roughly one-eighth that of steel. In the automotive industry, where lower weight means more miles per gallon of gasoline, the utilization of plastics is increasing with every model- year. For portable appliances and portable tools lower weight helps people to reduce their fatigue factor. Lower weight is beneficial in shipping and handling costwise, and as a SF to humans (no broken glass bottles, etc.). Throughout this book as the viscoelastic behavior of plastics has been described it has been shown that deformations are dependent on such factors as the time under load and the temperature. Therefore, when structural components are to be designed using plastics it must be remembered that the standard equations that are available for designing springs, beams, plates, and cylinders, and so on have all been derived under the assumptions that (1) the strains are small, (2) the modulus is constant, (3) the strains are independent of the loading rate or history and are immediately reversible, (4) the material is isotropic, and (5) the material behaves in the same way in tension and compression. Since these assumptions are not always justifiable when applied to plastics, the classic equations cannot be used indiscriminately. Each case must be considered on its merits, with account being taken of such factors as the time under load, the mode of deformation, the service temperature, the fabrication method, the environment, and others. In particular, it should be noted that the traditional equations are derived using the relationship that stress equals modulus times strain, where the modulus is a constant. From the review in Chapters 2 and 3 it should 7 - Design reliability 463 be clear that the modulus of a plastic is generally not a constant. Several approaches have been used to allow for this condition. The drawback is that these methods can be quite complex, involving numerical techniques that are not attractive to designers. However, one method has been widely accepted, the so-called pseudo-elastic design method. In this method appropriate values of such time-dependent properties as the modulus are selected and substituted into the standard equations. It has been found that this approach is sufficiently accurate if the value chosen for the modulus takes into account the projected service life of the product and/or the limiting strain of the plastic, assuming that the limiting strain for the matcrial is known. Unfortunately, this is not just a straightforward value applicable to all plastics or even to one plastic in all its applications. This type of evaluation takes into consideration the value to use as a SF. If no history exists a high value will be required. In time with service condition inputs, the SF can be reduced if justified. SUMMARY Overview From the initial development of plastics and particularly since the last half of the 20th century one can say it was extremely spectacular based on its growth rate but more important on how they have helped worldwide. The plastic industry is a worldwide multi-billion dollar business. Exciting discoveries and inventions have given the field of plastic products vitality. In a society that never stands still, plastics are vital components in its increased mobility. Plastics surpassed steel on a volume basis about 1983 and by the start of this century plastics surpassed steel on a weight basis (Fig. 8.1). Plastics and a few other materials as shown in Fig. 8.1 represent about 1Owt% of all materials consumed worldwide. The two major and important materials consumed arc wood and construction or nonmetallic earthen (stone, clay, concrete, glass, etc. ). Volumewise wood and construction materials each approach about 70 billion ft3 (2 billion m3). Each represents about 45% of the total consumption of all materials. A continuous flow of new materials, new processing technologies (Chapter l), and product design approaches has led the industry into profitable applications unknown or not possible in the past. What is ahead will be even more spectacular based on the continuous new development programs in materials, processes, and design approaches that are always on the horizon to meet the continuing new worldwide industry product challenges. As an example the University of Massachusetts Lowell received patents pertaining to a method of bonding plastic components developed by Avaya, Inc., a Basking Ridge, NJ based provider of corporate net- ~ 7nbi-ay a Estimated plastic consumption through year 2020 1 200 E 3 3 0 10 1 1930 1940 1950 1960 1970 1940 1990 2000 Year working solutions and services. Reportedly valued at about $23 million, the patented technology was developed in the early 1990s for the high- speed bonding of thermoplastic parts, and has been used to assemble millions of telephones, etc. The University plans to license the technology to others for use in a wide range of commercial applications. UMass-Lowell also will commit resources to further develop the technology and incorporate it into the school's curriculum and design solutions. Market Size Plastic product are ranked as the 4th largest USA manufacturing industry with motor vehicles in 1st place, petroleum refining in 2nd place, and automotive parts in 3rd place. Plastic is followed by computers and their peripherals, meat products, drugs, aircraft and parts, industrial organic chemicals, blast furnace and basic steel products, beverages, communications equipment, commercial printing, fabricated structural metal products, grain mill products, and dairy products (in 16th place). At the end of the industry listings are plastic materials and synthetics in 24th place and ending in the 25th ranking is the paper mills. Fig. 8.2 provides a forecast for plastics growth to 2020 year. *_I _I I _*xx-p. -** 466 Plastics Engineered Product Design Figrffr~ 8.2 Weight of plastic and steel worldwide crossed about 2000 (Courtesy of Plastics FALLO) YEAR Customer It is essential to obtain first-hand information on customer likes, dislikes, preferences, and prejudices. Eyeball-to-eyeball discussion, question and answer, and examination of competitors’ trends and specifications are all useful inputs to the product designer. To a great extent, such input will depend on whether there are product line precedents already on the market or whether it is a product breaking new ground. Customer input is, nevertheless, essential to success. The degree of difficulty with which this input is obtained varies enormously from the large on-off turnkey type of project where the designer will interface directly with the customer, to the mass-produced product where one will not. Feedback from the customer or market place should be considered. As an example it is no good incorporating a certain new design in a product that will not be accepted by customers, however when the design is valuable to the customer the skill of the salesperson is required. Examples of exploring new applications that are around us has been the fabrication of tubes, pipes, films, and others on the farm to exploring for oil in the depths of the seas. Constraint The constraints of current company practice should be highlighted and discussed. Is the company constrained by its previous products? If so, it is as well to know about it at the initial design stage. Possible manu- facturing facility constraints (example use is to be a ccrtain plastic and/or process), financial and investment constraints, and attitudes are very 8 - Summary 467 relevant. If needed are there adequate in-house facilities for research, design, development, testing, etc., including quality of personnel; perhaps outside sources will be required or are outside sources reliable. Unfortunately constraints relate to the economic conditions with its upward and downward business trends ranging from within the USA and worldwide. Different industries including the plastics industry are effected by these recessions. Regardless of these recessions the plastics industry always continues to have good growth. As stated by Glenn L. Beall, an outspoken proponent of good plastic product design, the USA plastics industry always continues to ride out the recessions at a growth rate higher then the GDP (Gross Domestic Product). Responsibility The responsibilities of those involved in the World of Plastics encompass all aspects from design to fabrication as well as the functional operation in service of products. Although functional design and fabricating is of paramount importance, a product is not complete if it is functional but cannot easily be manufactured, or functional but not dependable, or if it has a good appearance but poor reliability, or the product will not fail but does not meet safety requirements. Those involved have a broad responsibility to produce products that meet all the objectives of function, durability, appearance, safety, and low cost. As an example the designer should not contend that something is now designed and it is now the manufacturing engineer’s job to determine how to make it at a reasonable cost. The functional design and the production design are too closely interrelated to be handled separately. Product designers must consider the conditions under which fabrication will take place, because these conditions affect product performance and cost. Such factors as production quantity, labor, and material cost are vital. Designers should also visualize how each product is to be fabricated. If they do not or cannot, their designs may not be satisfactory or even feasible fiom a production standpoint. One purpose of this book is to give designers sufficient information about manufacturing pro- cesses (with its references) so that they can design intelligently from a productivity standpoint. Responsibility Commensurate with Ability Recognize that people have certain capabilities; the law says that people have equal rights (so it reads that we were all equal since 1776) but some interpret it to mean equal capabilities. So it has been said via Sun Tzu, The Art of War, about 500 BC “Now the method of employing people is to use the avaricious and the stupid, the wise and the brave, 468 Plastics Engineered Product Design , I ”.~ **_ ” * “X *. x . x and to give responsibilities to each in situations that suit the person. Do not charge people to do what they cannot do. Select them and give them responsibilities commensurate with their abilities.” Risk Designers and others in the plastics and other industries have the responsibility to ensure that all products produced will be safe and not contaminate the environment, etc. Recognize that when you encounter a potential problem, you are guilty until proven innocent (or is it supposed to be the reverse). So keep the records you need to survive the legal actions that can develop. There are many risks people are subjected to in the plant, at home, and clsewherc that can cause harm, health problems, and/or death. Precautions should be taken and enforced based on what is practical, logical, and useful. However, those involved in laws and regulations, as well as the public and, particularly the news media should recognize there is acceptable risk. Acceptable Risk This is the concept that was developed decades ago in connection with toxic substances, food additives, air and water pollution, fire and related environmental concerns, and so on. It can be defined as a level of risk at which a seriously advcrsc result is highly unlikely to occur but it cannot be proven whether or not there is 100% safety. In these cases, it means living with reasonable assurance of safety and acccptable uncertainty. Examples of this concept exists all around us such as the use of automobiles, aircraft, boats, lawnmowers, foods, medical pills and devices, water, air we breathe, news reports, and so on. Practically all elements around us encompass some level of uncertainty and risk. Otherwise as we know it would not exist. Interesting that about 1995 a young intern at FDA made some interesting calculations. If they permitted the packaging of Coca Cola in acrylic barrier plastic bottles, and if you drank 37,000 gallons of coke per day for a lifetime, you would have a 10% risk of getting cancer. Since normal people have a 25% risk of getting cancer, reducing it to 10% was a real plus for coke (and the acrylic barrier plastic bottle). So perhaps a law should be enacted requiring that the public should drink lots of coke. People are exposed to many risks. Some pose a greater threat than others. The following data concerns the probability over a lifetime of premature death per 100,000 people. In USA 290 hit by a car while 8-Summary 469 -"-" being a pedestrian, 200 tobacco smoke, 75 diagnostic X-ray, 75 bicycling, 16 passengers in a car, 7 Miami/New Orleans drinking water, 3 lightning, 3 hurricane, and 2 fire. DVR personal statistic (for real) based on personal knowledge of my large family, those that smoke and drank wine died close to 100 years of age. Those that did not smoke or drink died in their 60s (personal genies probably involved), Of course there were/are exceptions. So let the smokers continue to smoke and sue someone; regardless best not to smoke. Then there are other dilemmas such as exposure to asbestos, etc. that provide for interesting legal cases in USA. [After working with asbestos most of my life (now DVR at age 82) it never bothered me; however asthma has been with me since I was born except when I was in the Air Force.] Predicting Performance Avoiding nonstructural or structural failure can depend in part on the ability to predict performance of materials. When required, designers have developed sophisticated computer methods for calculating stresses in complex structures using different materials. These computational methods have replaced the oversimplified models of materials behavior relied upon previously. The result is early comprehensive analysis of the effects of temperature, loading rate, environment, and material defects on structural reliability. This information is supported by stress-strain behavior data collected in actual materials evaluations. With computers the finite element analysis (Chapter 5) method has greatly enhanced the capability of the structural analyst to calculate displacement, strain, and stress values in complicated plastic structures subjected to arbitrary loading conditions. Nondestructive testing (NDT) is used to assess a component or structure during its operational lifetime. Radiography, ultrasonics, eddy currents, acoustic emissions, and other methods are used to dctcct and monitor flaws that develop during operation (Chapter 7). The selection of the evaluation method(s) depends on the specific type of plastic, the environment of the evaluation, the effectiveness of the evaluation method, the size of the structure, the fabricating process to be used, and the economic consequences of structural failure. Conventional evaluation methods are often adequate for baseline and acceptance inspections. However, there are increasing demands for more accurate characterization of the size and shape of defects that may require advanced techniques and procedures and involve the use of several methods. [...]... The fabricator and the mold or die designer should advise the product designer on plastic materials behavior and how to simplifjr the design to permit easier processability Design Verification DV refers to the series of procedures used by the product development group to ensure that a product design output meets its design input It focuses primarily on the end of the product development cycle It is routinely... have to design the human body The human body is the most complex structure ever “designed” with its so-called 2,000 parts (with certain parts being replaced with plastics) Can you imagine designing the heart (now occurring) that recirculates all the blood in the body every 20 minutes, pumping it through 60,000 miles of blood vessels, etc Thus the designer of the 477 478 Plastics Engineered Product Design. ..470 Plastics Engineered Product Design Designing a good product requires a knowledge of plastics that includes their advantages and disadvantages (limitations) with some familiarity of the processing methods (Chapter 1).Until the designer becomes familiar with processing, a fabricator must be taken into the designer’s confidence early in the development stage... more comfortable, safe, and productive design solutions and a better overall product for the end-user Costing ilslllRls A major cost advantage for fabricating plastic products has been and will continue to be their usual relatively low processing cost The most expensive part of practically all products is the cost of plastic materials Since the material value in a plastic product is roughly up to one-half... skilled designer who blends a knowledge of materials, an understanding of manufacturing processes, and imagination into successhl new designs Recognizing the limits of design with traditional materials is the first step in exploring the possibilities for innovative design with plastics What is important when analyzing plastic designs is the ease to incorporate ergonomics and empathy that results in products... equipmentwise, particularly the military Their work in turn expands into the industrial industry 8 Summary Desian demand It can be said that the challenge of design is to make existing products obsolete or at least offer significant improvements Despite this level of activity there are always new fields of products to explore Plastics will continue to change the shape of worldwide business rapidly Today’s plastics. .. temperature-timeload and environment Unfortunately sometimes a new design concept is not accepted or may simply be ahead of its time In 1483 Leonard0 da Vinci designed what he called a spiral screw flying machine In 1942 Igor Sikorsky developed the R4B helicopter (included plastics parts) One could say, in a joluiig manner, that it took 459 years to bring a designed product to market; seems a failure in materials/or... with engineers and designers in the early stages of product development When ergonomists are called in to fix a product that has already been sent to market and failed, costs will escalate A manufacturer’s decision to adopt an ergonomic orientation will serve to reposition its products from a commodity-based supplier to a supplier of high-value products Integrating ergonomics into a design program ensures... generally for specific products by specific processes that principally include many thousands of products Plastics are now among the nations and world’s most widely used materials, having surpassed steel on a volume and weight basis Plastic materials and products cover the entire spectrum of the world’s economy, so that their fortunes are not tied to any particular business segment Designers are in a good... configurations, etc.) issues 472 Plastics Engineered Product Design Ergonomics includes concept modeling and product design, job performance analysis, functional analysis, workspace and equipment design, computer interfaces, environment design, and so forth The true basis of ergonomics understands the limitations of human performance capabilities relative to product interaction These limitations are either physical . the designer of the 478 Plastics Engineered Product Design human body had to be extremely creative; some of us know who designed the human body. The past events in designing plastic products. procedures and involve the use of several methods. 470 Plastics Engineered Product Design Designing a good product requires a knowledge of plastics that includes their advantages and disadvantages. to design the human body. The human body is the most complex structure ever “designed” with its so-called 2,000 parts (with certain parts being replaced with plastics) . Can you imagine designing