Desing of plastic products

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Vladimirskiy State University named after the Stoletov brothers Vladimir, Russia

Desing of plastic products

Titova E.Yu.

Проектирование изделий из пластмасс

Титова Е.Ю. Владимирский государственный университет имени А.Г. и Н.Г. Столетовых Владимир, Россия

The times we live in are often known as the “Computer Age.” It could also be referred to as the “Plastics Age,” as the production of plastics has exceeded that of steel (by volume) since 1979. In fact, the volume of plastics produced has more than doubled in the last 20 years. Nonetheless, most students who graduate from the major engineering universities are generally unprepared generally unprepared to design to design in plastics. Thus, it is left to the individual engineer to learn plastics engineering on his or her own, often by trial and error.

Unfortunately, this type of education can come at great cost--both to the company and the career of the individual. First, second, and even third and fourth efforts can be disastrous. That is because plastics design is more complicated, and more time consuming, than designing with metals. There are several reasons for this and they center around the processes which are used to manufacture plastic parts, the tooling used for those processes, and the nature of the plastics themselves [1]. plastic quality thickness

Unlike metals, the properties of most plastics vary considerably within normal operating temperatures. A particular acrylonitrile butadiene styrene (ABS) whose tensile strength is 5500 lb/in2 at room temperature can drop to 2800 lb/in2 at 125°F. Other properties are also affected. For example, brittleness increases as the temperature drops, etc.

What does this mean to the design engineer? Basically, it means there will be more work to do. Various pertinent properties will need to be examined at both extremes of the service range. Furthermore, the design parameters must be explored more fully. It cannot be assumed that the product will survive the temperatures endured in cleaning, shipping, or storage unscathed.

Other exposures can cause problems with plastic components as well. Ultraviolet light causes or catalyzes chemical degradation in many resins. Plastics are vulnerable to attack from many chemicals, particularly in heavy concentrations. Some are even affected by water and there is one, polyvinyl acetate, that actually dissolves in water (for example, soap, packets) [3].

Engineers will want to calculate the various performance values in the traditional fashion. Unfortunately, these computations cannot be regarded as valid. It is not that the laws of physics are different for plastics, it is simply that the data employed is not reliable for calculations.

That is not to suggest any skullduggery on the part of the test engineers; it is simply that the standard test sample and conditions are narrowly defined and likely to be significantly different from those to be endured by any specific product. The values obtained for most plastics will vary according to the process, gating, wall thickness, rate of loading, etc. It should be noted that there is some latitude within the test procedures themselves which can affect results. Most plastics engineers use the data sheets principally for the purposes of comparison in material selection[2].

It is important that the design engineer be cognizant of this when employing the data in an empirical fashion. Generous safety factors are customarily used. When greater precision is required, the proposed material may be independently tested under conditions more appropriate to the actual application. When the anticipated market is sufficient, the resin manufacturer may be inclined to perform these tests at the company's expense. Otherwise, it will be the obligation of the product manufacturer to bear the cost. The general stiffness range of most plastics, combined with the general effort to use the thinnest possible wall thickness, means the geometry has a pronounced effect. Other than through comparison to similar constructions, the stiffness of the actual part is difficult to predict in a precise fashion[4]. Although the traditional equations will produce approximate results, stiffness remains a question until the first part is molded. (Finite element analysis results are vulnerable to the many variables involved.) Fortunately, there are so many compound variations available within a given resin, it is usually possible to adjust stiffness within a reasonable range. This has been the saving grace of many an engineer. Also, many plastics engineers withhold the placement of ribbing until the first parts are tested[5].

Even if the material maintained its properties throughout the product's temperature range and the data was perfectly reliable, the product's performance could still vary. That is because the plastics processes are subject to tooling quality and process parameter variations.

Nonetheless, the fact that plastic parts can be successfully designed is attested to by the wide variety of products in the marketplace. It is clearly, however, more work to design in plastic and it is virtually impossible to perfectly predict the initial results. That is the reason prototypes are frequently made.

It is tempting to test a fabricated sample before constructing tooling. However, it should be noted that the final part is likely to produce substantially different test results than one fabricated by some other process. Furthermore, sophisticated plastics engineers will often deliberately under design a product, adding a little material at a time in a prototype mold to determine the thinnest acceptable wall. This approach also permits the testing of the complete assembly whereby the walls of the mating parts reinforce each other to produce a stronger overall structure.

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 information, 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 eliminate 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 checklist is a useful means of reminding the designer of all these conditions. 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[6].

Process selection.

Plastics product designers are primarily interested in the ability of a given process to produce the shape they require. Therefore, the processes have been grouped according to their ability to produce a given shape. The groupings are 1)thermoplastic open shapes; 2)thermoset open shapes; 3)hollow parts; 4)profiles; 5)ultra high strength.

Beyond the fundamental design requirements, cost becomes the most significant factor in selecting the optimum process for the application. Product cost has three interrelated components: part cost, labor cost, and tooling amortization. Labor is related to process selection because some plastics processes permit the combining of parts to eliminate labor cost. For example, the cost of blow molding a hollow container must be compared to the cost of injection molding two halves and assembling them. Tooling amortization and piece part cost are directly related to anticipated annual volume, which is often difficult to forecast for new products[7].

As a broad statement, processes that require a higher initial investment in tooling produce parts at a lower cost. This is largely due to the fact that the reduced part cost is the product of faster molding cycles. Faster cycles require pressure on the plastic to reduce the time required to fill the mold cavity. The greater the pressure, the stronger the tooling and the more sophisticated the processing equipment must be. Both of these are factors which increase the initial investment.

The product design engineer is, therefore, keenly interested in the volume at which the additional investment would be justified by reduced part cost. It would simplify the decision making tremendously if it were possible to determine that point in terms of a given volume for each process. That might be feasible if all parts were identical in shape and size. One may presume that the larger and/or more complex the part, the greater the investment will be. As the investment grows, the production volume must be greater in order to pay the difference with lower piece part costs within an acceptable time period. That period also varies considerably between companies. In the following discussion, the processes are ranked in order of increased tooling expense, which is usually commensurate with decreasing piece part cost.

Design for multiple part assemblies.

Dimensional control. The next major consideration is the fact that plastic part quality is “process sensitive.” That means that the part's size and shape can vary Design for multiple part assemblies. Thus far, the design discussion has centered around the design of individual piece parts. However, most products require multiple part assemblies, often consisting of parts made of different materials. The first step is to ensure that the parts fit together properly--not merely at room temperature, but at the temperature extremes of what the product may be expected to encounter. (For example, a force fitment that works perfectly at room temperature may loosen at elevated temperatures or fracture at low temperatures.) That involves the determination of the fitments after the relationship of the parts to each Design of Plastic Products other has changed due to differences in the coefficient of linear thermal expansion. Thus, the establishment of acceptable dimensional limits, generally known as tolerances, for the fitment dimensions is of critical importance according to variations in process parameters. The thermoplastic processes generally operate with a cool mold, with the moldment remaining in the tool until the part is rigid enough to withstand the forces of ejection[9]. If the part is ejected while it is too hot, it can be distorted and dimensional control lost. Furthermore, a mold core can act as a shrink fixture if the part is left in the tool beyond that point. Also, the temperature of the melt has an effect as a hotter melt is less viscous and can be injected into a mold at a higher speed. However, a hotter melt can lead to greater shrinkage, more distortion, and take longer to cool[8].

Deviations from drawing tolerances can be approved if they turn out to be excessively tight when the actual parts are available. When parts are accepted with deviations from the contract drawing, a written record should be retained and the drawing should be altered accordingly to reflect the newly approved tolerance. Regardless of how it is specified, the objective remains the same, namely, that the parts must fit together readily and stay together within acceptable parameters.

References

1. Cadillac Plastic & Chemical Co., Troy, Mich.

2. Terry A. Richardson, "Machining and Finishing," Modern Industrial Plastics, Howard W. Sams & Co., New York, 1999. - P. 69.

3. John L. Hull, "Design and Processing of Plastic Parts," Handbook of Plastics Elastomers and Composites, 2d ed., Charles A. Harper, ed., McGraw-Hill, New York, 2001. - P.83.

4. J. 0. Trauernicht, "Bonding and Joining, Weigh the Alternatives, Part 1, Solvent Cements, Thermal Welding," Plastics Technology, August 1999. - P.147.

5. "Engineer's Guide to Plastics," Materials Engineering, May 2002. - P. 369.

6. "Mechanical Fastening," Handbook of Plastics Joining, Plastics Design Library, Norwich, NY, 2001. - P.259.

7. "Joining of Composites," in A. Kelley, ed., Concise Encyclopedia of Composite Materials, The MIT Press, Cambridge, 1999. - P. 247-269.

8. D. K Rider, "Which Adhesives for Bonded Metal Assembly," Product Engineering, May 25, 2001. - P. 299-300.

9. "Surface Preparation of Plastics," in Adhesives and Sealants, vol. 3, Engineered Materials Handbook, H. F. Binson, ed., ASM International, Materials Park, Ohio, 2004. - P. 169.

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