Product Design


Plastics offer the opportunity to optimize designs by focusing on material composition as well as product structural geometry to meet different product requirements. In structural applications for plastics, which generally include those in which the product has to resist substandal static and/or dynamic loads, it may appear that one of the problem design areas for many plasdcs is their low modulus of elasticity. Since shape integrity under load is a major consideration for structural products, low modulus type plastic products are designed shapewise and/or thicknesswise for efficient use of the material to afford maximum stiffness and overcome their low modulus. This type of plastics and products represent most of the plastic products produced worldwide.

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 extensive amount of standard equations that are available (Figs 2.31 and 2.32) for designing springs, beams, plates, and cylinders, and so on have all been derived under certain assumptions. They are 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. As reviewed in Chapters 2 and 3 the modulus of a plastic may not be a constant.

There are different design approaches to consider as reviewed in this book and different engineering textbooks concerning specific products. They range from designing a drinking cup to the roof of a house. As an example consider a house to stand up to the forces of a catastrophic hurricane. Low pitch roofs are less vulnerable than steeper roofs because the same aerodynamic factors that make an airplane fly can lift the roof off die house. The roof also requires being properly attached to the building structure.

Example of a product design program approach follows:

1. Define the function of the product with performance requirements.

2. Identify space and load limitations of the product if they exist.

3. Define all of the environmental stresses that the product will be exposed to in its intended function.

4. Select several materials that appear to meet the required environmental requirements and strength behaviors.

5. Do several trial designs using different materials and geometries to perform the required function.

6. Evaluate the trial designs on a cost effectiveness basis. Determine several levels of performance and the specific costs associated with each to the extent that it can be done with available data.

7. Determine the appropriate fabricating process for the design.

8. Based on the preliminary evaluation select the best apparent choices and do a detailed design of the product.

9. Based on the detailed design select the probable final product design, material, and process.

10. Make a model if necessary to test the effectiveness of the product.

11. Build prototype tooling.

12. Make prototype products and test products to determine if they meet the required function.

13. Redesign the product if necessary based on the prototype testing.

14. Retest.

15. Make field tests.

16. Add instructions for use.

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