Overview

Throughout this book many different properties are reviewed. What follows provides additional information on the properties for different plastics. As a construction material, plastics provide practically unlimited benefits to the design of products, but unfortunately, as with other materials, no one specific plastic exhibits all these positive characteristics. The successful application of their strengths and an understanding of their weaknesses (limitations) will allow designers to produce useful and cost efficient products. With any material (plastic, steel, etc.) products fail not because of the material's disadvantage(s). They fail because someone did not perform their design approach in the proper manner to meet product performance requirements. The design approach includes meeting required performance of material and its fabricating process that operate within material and process controllable variables (Chapter 1, Variables).

There is a wide variation in properties among the over 35,0000 commercially worldwide available materials classified as plastics. They now represent an important, highly versatile group of commodity and engineering plastics. Like steel, wood, and other materials, specific groups of plastics can be characterized as having certain properties.

Many plastics (that are extensively used worldwide) are typically not as strong or as stiff as metals and they are prone to dimensional changes especially under load or heat. They are used instead of metals, glass, etc. (in millions of products) because their performances meet requirements. However there are plastics that have very high properties (Fig. 6.1), meet dimensional tight requirements, dimensional stability, and are stronger or stiffer, based on product shape, than other materials.

Figure 6 Mechanical and physical properties of materials (Courtesy of Plastics FALLO)

Specific Gravity

Modules of Elasticity

Plastic* Rainforcad Pintle* Wood Steal Aluminum Concreto

Plastica Rainforcad Plaatica Wood Stasi Aluminum Concrate-Stona to I 20 I I 100 200

Strength

Thermal Conductivity

Plástica Rainforcad Plastica Wood Staat Aluminum Concreto

Strength

Plástica Rainforcad Plastica Wood Staat Aluminum Concreto

M

m

—1-H-

50 I tOO tt 50 200110s pal 500 1000 MPa

Plastic Foama Rainforcad Plaatica Wood Brick Qlaaa Concreto

50 I tOO tt 50 200110s pal 500 1000 MPa to I 20 I I 100 200

Thermal Conductivity

Plastic Foama Rainforcad Plaatica Wood Brick Qlaaa Concreto

Continuous Service Temperature

"F

Thermal Expansion

Plasties Ratnforcod Plaatlcal Wood Staal & Iron Aluminum ConeretoA Glaaa

Thermal Expansion

Plasties Ratnforcod Plaatlcal Wood Staal & Iron Aluminum ConeretoA Glaaa

Rainforcad Ptaattca WoodChara Allumlnum Coppor Alloys Staal Concreto

100 200umMl*c

Continuous Service Temperature

"F

0 200 400 600 800 1000 1800 I I I I I

Rainforcad Ptaattca WoodChara Allumlnum Coppor Alloys Staal Concreto

100 200umMl*c

0 100 200 300 400 500 900

"C

Highly favorable conditions such as less density, strength through shape, good thermal insulation, high degree of mechanical dampening, high resistance to corrosion and chemical attack, and exceptional electric resistance exist for many plastics. There are also those that will deteriorate when exposed to sunlight, weather, or ultraviolet light, but then there are those that resist such deterioration.

For room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design permanent deformation is considered to be a function only of applied load and can be determined direcdy from the usual tensile stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed.

The plastics material properties information and data presented are provided as comparative guides; readers can obtain the latest and more detailed information from suppliers and/or software programs (Chapter 5). Since new developments in plastic materials are always on the horizon it is important to keep up to date. It is important to ensure that the fabricating process to be used to produce a product provides the properties desired. Much of the market success or failure of a plastic product can be attributed to the initial choices of material, process, and cost.

For many materials (plastics, metals, etc.) it can be a highly complex process if not properly approached particularly when using recycled plastics. As an example, its methodology ranges from a high degree of subjective intuition in some areas to a high degree of sophistication in other areas. It runs the gamut from highly systematic value engineering or failure analysis such as in aerospace to a telephone call for advice from a material supplier in the decorative houseware business. As reviewed at the end of this chapter there are available different publications, seminars, and software programs that can be helpful.

Plastics are families of materials each with their own special advantages and drastically different properties. An example is polyethylene (PE) with its many types that include low density PE (LDPE), high density PE (HDPE), High molecular weight PE (HMWPE), etc. The major consideration for a designer and/or fabricator is to analyze what is required as regards to performances and develop a logical selection procedure from what is available.

Recognize that most of the plastic products produced only have to meet the usual requirements we humans have to endure such as the environment (temperature, etc.). Thus there is no need for someone to identify that most plastics cannot take heat like steels. Also recognize that most plastics in use also do not have a high modulus of elasticity or long creep and fatigue behaviors because they are not required in their many respective designs. However there are plastics with extremely high modulus and very long creep and exceptional high performance fatigue behaviors. These type products have performed in service for long periods of time with some performing well over a half-century. For certain plastic products there are definite properties (modulus of elasticity, temperature, chemical resistance, load, etc.) that have far better performance than steels and other materials.

The designer can use plastics that are available in sheet form, in I-beams, or other forms as is common with many other materials. Although this approach with plastics has its place, the real advantage with plastic lies in the ability to process them to fit the design shape, particularly when it comes to complex shapes. Examples include two or more products with mechanical and electrical connections, living hinges, colors, snap fits that can be combined into one product, and so on.

Designing is the process of devising a product that fulfills as completely as possible the total requirements of the user, and at the same time satisfies the needs of the fabricator in terms of cost-effectiveness (return on investment). The efficient use of the best available material and production process should be the goal of every design effort. Product design is as much an art as a science. Guidelines exist regarding meeting and complying with art and science.

Influencing Factor

Design guidelines for plastics have existed for over a century producing many thousands of parts meeting service requirements, including those subjected to static and dynamic loads requiring long life. Basically design is the mechanism whereby a requirement is converted to a meaningful plan. The basic information involved in designing with plastics concerns the load, temperature, time, and environment. As reviewed throughout this book diere are other important performance requirements that may exist such as aesthetics, non-permeability, and cost.

In evaluating and comparing specific plastics to meet these requirements, past experience and/or the material suppliers are sources of information. It is important to ensure that when making comparisons the data is available where the tests were performed using similar procedures. Where information or data may not be available some type of testing can be performed by the designer's organization, outside laboratory (many around), and/or possible the material supplier if it warrants their participation (technicalwise and/or potential costwise). If little is known about the product or cannot be related to similar products prototype testing is usually required.

When required, plastics permit a greater amount of structural design freedom than any other material (Chapter 4). Products can be small or large, simple or complex, rigid or flexible, solid or hollow, tough or britde, transparent or opaque, black or virtually any color, chemical resist or biodegradable, etc. Materials can be blended to achieve different desired properties. The final product performance is affected by interreladng the plastic with its design and processing method. The designer's knowledge of all these variables can profoundly affect the ultimate success or failure of a consumer or industrial product.

For these reasons design is spoken of as having to be appropriate to the materials of its construction, its methods of manufacture, and the loads (stresses/strains) involved in the product's environment. Where all these aspects can be closely interwoven, plastics are able to solve design problems efficiendy in ways that are economically advantageous. It is important to recognize that these characteristics of plastics exist. This book starting with Chapter 1 provides their characteristics and behavior.

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